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The China Geological Survey successfully completed the second trial of natural gas hydrate (NGH) production in 2020, and the daily gas production and total gas production were greater than those in the first trial, but the evaluation of NGH resources in the South China Sea (SCS) is still in the evaluation stage, specifically for the gas content. In 2021, some scholars used different methods to more accurately evaluate NGHs in the SCS to aid in NGH exploration and evaluation in the SCS; some of them analysed the main factors controlling NGH resources, added several factors to the volume method for optimization, studied the sensitivity of the evaluation of resources to each of the main controlling factors (NGH stability zone area, NGH area coefficient, NGH stability zone thickness, NGH thickness coefficient, NGH reservoir porosity, NGH reservoir saturation, NGH volumetric ratio, NGH resource ratio coefficient and NGH recovery coefficient) and found that the effective thickness coefficient, effective thickness and effective area coefficient are the most sensitive; these values are 23.15%, 15.66%, 13.98%, and 13.95%, respectively, while the other values are 0%, 15.66%, 8.42%, 8.41%, 2.50%, and 13.92%; taking into account the influence of the main controlling factors, the range of the water resource potential evaluation results in the SCS is 2.4 × 107 m3–5.9 × 1014 m3, the modal value is 7.25 × 1011 m3, and the confidence level of the modal ±25% interval is 77.8%; these values provide a higher evaluation accuracy and higher resource level, which is consistent with the current results of NGH research in the SCS. Although the development of NGHs in the SCS is progressing rapidly, basic exploration is relatively scarce, and the number of samples of effective thickness obtained through dozens of exploratory wells is too few. It is recommended that China carry out more exploration and research projects in the Shenhu area and the entire SCS to obtain more accurate thickness data. [ABSTRACT FROM AUTHOR]

Hydrocarbon potential evaluation is a high priority in oil/gas exploration which is significantly influenced by evaluation methods and subjective cognition. With the aim of quantitatively establishing a hydrocarbon generation and expulsion (HGE) model of source rocks, a data-driven approach is proposed based on abundant and readily available Rock-Eval/total organic carbon content (TOC)/vitrinite reflectance (VRo) datasets. This approach takes into account the original hydrocarbon generation potential (GPIo) and the loss of TOC. Hydrocarbon generation simulation was also adopted in this study. This data-driven method was applied to the Lower Paleogene Shahejie Formation (Es4), which has three sets of superior source rocks (average thickness > 250 m, TOC > 3%). The GPIo of the Es4 low-maturity source rock was 600 mg HC/g TOC. The initial maturity values of hydrocarbon generation and expulsion were 0.36% and 0.46%, respectively. The Gaosheng sub-member of the Es4 source rock had the largest amount of hydrocarbon generation but a limited amount of hydrocarbon expulsion, implying a favorable exploration of shale oil resources. By contrast, the values of the conventional, unconventional tight, and shale oil resource potentials of the Niuxintuo sub-member were 3.20 × 107, 7.70 × 107, and 5.93 × 108 t, respectively, indicating good prospects for tight oil and shale oil exploration. A comparison between the previous method and the data-driven method showed considerable differences in the restoration of GPIo and TOC, and both had their own limitations. This data-driven approach provides a quick and valid source rock evaluation method and can greatly enhance the accuracy of resource assessment. [ABSTRACT FROM AUTHOR]

Gas-bearing property evaluation and main controlling factor analysis have remained a concern in shale gas research. The application of principal component analysis, an important mathematical statistical method, in gas-bearing property evaluation and main controlling factor analysis of the Longmaxi shale in the Weirong area, Sichuan Basin, was examined. The Longmaxi shale exhibits high heterogeneity, manifested in the organic matter abundance, mineral composition, and pore structure. Seven geological factors, including the temperature, pressure, TOC content, clay content, brittle mineral content, pore volume, and specific surface area (SSA), were selected in principal component analysis. Four principal components with geological significance, such as mineral composition, formation condition, pore structure, and organic matter abundance, were extracted through principal component analysis, and further constituted a comprehensive factor. Shale gas-bearing properties were evaluated according to the score of the comprehensive factor. The Longmaxi shale could be identified as exhibiting good, medium, and poor gas-bearing properties based on the comprehensive factor scores of these samples. According to each geological factor's contribution to the comprehensive factor, combined with geological analysis, it could be considered that gas-bearing properties are primarily controlled by pore volume, SSA, and clay content, followed by TOC content, brittle mineral content, temperature and pressure. [ABSTRACT FROM AUTHOR]

In recent years, exploration of tight oil in the Nanpu Sag has made significant progress in the third member of the Upper Palaeogene Shahejie Formation (Es3) in the Nanpu Sag, Bohai Bay Basin, Northeast China, showing excellent prospects. However, owing to insufficient understanding of the hydrocarbon generation and expulsion characteristics of Es3 source rocks and their matching relationship with the reservoir tightening time, the genetic type and resource potential of tight oil systems are not precise, which limits the further exploration of tight oil. In this study, the geological and geochemical characteristics of Es3 source rock are identified through a comprehensive geological and organic geochemistry analysis. In addition, hydrocarbon generation and expulsion patterns are established using a hydrocarbon generation potential model. On the basis of these results, combined with sandstone reservoir evolution histories, we have identified the genetic type and resource potential of the tight oil. The results show that the kerogen types of the Es3 source rocks of the Nanpu Sag, are widely distributed in an area of nearly 1,000 km2 and have a large thickness of more than 700 m, are mainly Types II and III with a high average total organic carbon (TOC) value of 1.63%. Moreover, the source rocks are primarily in the mature stage, with an average vitrinite reflectance (Ro) of 1.06%. The Es3 source rock reached the hydrocarbon expulsion threshold at Ro = 0.7%, and the hydrocarbon expulsion efficiency was approximately 70%. Two hydrocarbon generation and expulsion centres are identified, the Gaoshangpu and Laoyemiao structural belts, with maximum hydrocarbon generation and expulsion intensities of 100 × 106 t/km2 and 80 × 106 t/km2, respectively. In addition, two genetic types of tight oil are identified with accumulation before and after tightening, respectively. The resources of these types are 2.3 × 108 t and 20.4 × 108 t, respectively, with a total of 22.7 × 108 t, which indicates significant prospects for tight oil exploration. [ABSTRACT FROM AUTHOR]

Significant achievements have been reported for deep oil and gas exploration in the Bohai Bay Basin. Among them, the Nanpu Sag has achieved good deep (greater than 3.5 km) hydrocarbon exploration results. However, the hydrocarbon formation mechanisms and the diagenetic evolution of these high‐quality sandstone reservoirs remain unclear. Mineralogical, petrographic, and fluid inclusion data were selected to investigate the deeply buried sandstone reservoir physical properties, pore systems, and diagenetic features and the main formation mechanisms of the deep high‐quality reservoirs of the Es1 (the top member of the Paleogene Shahejie Formation) sandstones in the Nanpu Sag, eastern China. The results show that the Es1 sandstone reservoirs are dominated by lithic arkoses and have low porosities (average porosity 13.4%) but medium to high permeability (average permeability 188 mD). The pore systems consist of primary intergranular pores, secondary dissolution pores, and microfractures. Loss of porosity is greater due to compaction (average 20.89%) than due to cementation (average 4.82%), and dissolution contributes an improvement in porosity of approximately 5.85%. Two peaks in the inclusion homogenization temperatures were observed: The first fluid inclusion homogenization temperature term ranges from 130°C to 145°C and exhibits yellow fluorescence; the other peak occurs from 155°C to 165°C, showing green fluorescence. Eodiagenesis is due to mechanical compaction, precipitation of early calcite cements and quartz overgrowths, and dissolution of feldspar and carbonate cements. Mesodiagenesis occurs due to mechanical compaction, framework grain (mainly feldspar) dissolution, and precipitation of quartz and late carbonate cements. The main factors controlling the formation of the deep high‐quality reservoirs are the presence of a high hydrodynamic depositional environment (subaqueous distributary channel), the type of provenance (Archean granites), and the creation of secondary porosity by feldspar dissolution. [ABSTRACT FROM AUTHOR]

Fossil fuel resources are invaluable to economic growth and social development. Understanding the formation and distribution of fossil fuel resources is critical for the search and exploration of them. Until now, the vertical distribution depth of fossil fuel resources has not been confirmed due to different understandings of their origins and the substantial variation in reservoir depths from basin to basin. Geological and geochemical data of 13 634 source rock samples from 1286 exploration wells in six representative petroliferous basins were examined to identify the maximum burial depth of active source rocks in each basin, which is referred to in this study as the active source rock depth limit (ASDL). Beyond the ASDL, source rocks no longer generate or expel hydrocarbons and become inactive. Therefore, the ASDL also sets the maximum depth for fossil fuel resources. The ASDLs of basins around the world are found to range from 3000 to 16 000 m, while the thermal maturities (Ro) of source rocks at the ASDLs are almost the same, with Ro ≈3.5±0.5 %. The Ro of 3.5 % can be regarded as a general criterion to identify ASDLs. High heat flow and more oil-prone kerogen are associated with shallow ASDLs. In addition, tectonic uplift of source rocks can significantly affect ASDLs; 21.6 billion tons of reserves in six representative basins in China and 52 926 documented oil and gas reservoirs in 1186 basins around the world are all located above ASDLs, demonstrating the universal presence of ASDLs in petroliferous basins and their control on the vertical distribution of fossil fuel resources. The data used in this study are deposited in the repository of the PANGAEA database at: 10.1594/PANGAEA.900865 (Pang et al., 2019). [ABSTRACT FROM AUTHOR]

The third member of the Upper Triassic Xujiahe Formation (T 3 x3) is a typical source-reservoir-cap unit for natural gas exploration in the Sichuan Basin. However, most previous studies mainly focused on the T 3 x3 as a source rock, ignoring its role as a reservoir. As a result, the evaluation of hydrocarbon resource potential was not accurate. This study established a hydrocarbon generation and expulsion model through analyzing the geological and geochemical characteristics of the T 3 x3 source rocks, and calculated the resource potential of conventional gas, tight gas and shale gas systematically with combining the lower boundary of buoyancy-dominated accumulation (LBBA). The results indicated that the thickness of the source rocks dominated by mudstone was distributed widely, with an average value of more than 45 m. The sedimentary environment was salty lacustrine deposits. They have a high content of total organic carbon (TOC) with an average value of 4.80 wt%, the kerogen is mainly type III with minor type II, and the thermal evolution has entered the high to over mature stage. The hydrocarbon generation threshold was determined to be 0.5%Ro, while the hydrocarbon expulsion threshold and the LBBA were determined to be 0.83%Ro and 1.12%Ro, respectively. The efficiency of hydrocarbon expulsion was relatively high, with an average of about 56%. Also, the maximum hydrocarbon generation and expulsion intensity reached to 110 × 108 m3/km2 and 53 × 108 m3/km2. The amounts of hydrocarbon generation, expulsion and residual associated with the T 3 x3 source rocks were 125.68 × 1012m3, 70.38 × 1012m3, and 55.30 × 1012m3, respectively. Under the criterion condition of the LBBA, the conventional gas resource was 7.21 × 1010m3, the tight gas resource was 294.34 × 1010m3, and the shale gas resource was 2488.50 × 1010m3. It indicated good prospects for exploration. • The geological and geochemical characteristics of source rocks were described. • The hydrocarbon generation and expulsion characteristics were studied by hydrocarbon generation potential method. • The resource potential of conventional and unconventional gas was calculated systemically. [ABSTRACT FROM AUTHOR]

Reservoir quality is critical in tight gas exploration. A systematic study of the petrological, petrophysical, and diagenetic characteristics of the Shanxi Formation sandstones in the north‐eastern Ordos Basin (China) was undertaken to characterize the pore system of these tight sandstones and recognize the distribution of reservoirs favourable for hydrocarbon accumulation. The results suggest that the Shanxi Formation sandstone has low compositional and moderate textural maturity, with its porosity ranging from 0.7% to 11.8% (average 5.97%) and permeability ranging from 0.001 to 2.77 mD (average 0.36 mD). In addition, thin section and SEM observations suggest that the sandstone is dominated by secondary dissolution pores, primary intergranular pores as well as microfractures, and has undergone compaction, cementation, and dissolution. Diagenetic minerals, such as carbonate cements, authigenic quartz, clay minerals, and dissolved feldspar, are identified. Fluid inclusions are observed in healed microfractures of quartz grains and in quartz overgrowths, while the homogenization temperatures of fluid inclusions in healed microfractures and in quartz overgrowth are in the ranges of 92.8–179.1°C (average 134.8°C) and 104.7–169.1°C (average 145.7°C), respectively. Authigenic kaolinite is sourced from the process of K‐feldspar dissolution, authigenic illite is sourced from transformation of smectite and kaolinite, while sources for quartz cements include mineral alteration and pressure dissolution. Selective dissolution of K‐feldspar in the presence of carbonate minerals is observed in the Shanxi Formation tight sandstones due to the different equilibrium constants of carbonate minerals and K‐feldspar leaching, resulting in dissolution pores associated with K‐feldspar. Reservoir quality in the Shanxi Formation tight sandstones is greatly influenced by diagenesis, as compaction and cementation are responsible for the loss of porosity while dissolution accounts for the enhancement of secondary porosity. [ABSTRACT FROM AUTHOR]

The north‐eastern Ordos Basin in Central China has become an exploration target in recent years due to the potential of natural gas resources in the Upper Palaeozoic strata. In this study, by analysing the geochemical characteristics of the source rocks and the natural gas, the genesis of the Upper Palaeozoic natural gas was identified. In addition, the accumulation process was discussed by combining studies of hydrocarbon generation and burial/thermal history. The results reveal three sets of source rocks developed in the study area—Carboniferous Benxi (C2b), Permian Taiyuan (P1t), and Shanxi (P1s) formations—consisting of mudstones, carbonaceous mudstones, and coals. The source rocks contain abundant Type III organic matter and have entered mature to high‐mature stage, while source rocks in the south‐eastern part of the study area have entered the overmature stage due to the activity of Zijinshan magmatic rocks. The natural gas has a high content of methane (average 93.99%). The stable carbon isotope of methane (δ13C1) value (average −37.05‰) and that of ethane (δ13C2) value (average −26.4‰) and the composition of the light hydrocarbons reflect the idea that the natural gas is organic in origin, thermogenic, and mainly terrigenous‐sourced. The evolution of a transitional to continental depositional environment of the source rocks results in the variation in the geochemical characteristics of the Upper Palaeozoic gas in the study area. Analysis of fluid inclusions indicates a long and continuous period of natural gas charging in the study area from 165 to 65 Ma. Sandstone bodies close to or interbedded with the source rocks made the gas charging from the source rocks to the reservoirs over a short distance with high efficiency, while faults and fractures provided paths for the gas migrating to reservoirs vertically far from the source rocks. The huge gas generation potential of the source rocks and continuous distribution pattern of sandstone bodies provides favourable conditions for the development of a large‐scale natural gas reservoir. [ABSTRACT FROM AUTHOR]

The genetic type and accumulation model of the Devonian reservoirs in the Marsel Block remain unclear, despite decades of exploration history. According to the well testing, logging interpretations and sample testing results, the Devonian natural gas reservoir in the Marsel Block has five typical characteristics: (1) It is obvious that the traps contain continuous gas accumulations. Not only the apexes of the structures are enriched in natural gas, but also the slopes and depressions contain gas accumulations. (2) The gas reservoirs are classified as tight reservoirs, but there are also reservoirs with high porosity and permeability in some areas. (3) The general negative or low-pressure in the gas reservoir is obvious, although the pressure in the target layers of some wells is close to normal. (4) The yields of single wells in the Devonian reservoir are quite different: some wells have low yields or are dry, whereas the gas production from high-yield wells has reached 700 000 m3/day. (5) The gas-water relationship is complicated: there is no obvious gas-water interface, but the water-producing layer is generally located at the apexes of structures. Research and analysis have shown that using the model of the conventional gas reservoirs genetic type can only explain the characteristics of parts of the gas reservoir, while the model of accumulation in a deep-basin gas reservoir cannot fully explain the distribution characteristics of the Devonian reservoir. However, the model of accumulation in a stacked complex continuous oil and gas reservoir can reasonably explain the geological and distribution characteristics of the Devonian reservoir. Moreover, the predicted gas distribution along a cross-section of the reservoir is also in agreement with the geological background and tectonic environment of the Marsel Block, therefore, the genetic type of the Devonian natural gas reservoir in the Marsel Block is a stacked complex continuous tight-gas reservoir. Finally, by comprehensively analyzing the source rock, reservoir and cap rocks, as well as the structural characteristics, it is verified that Devonian in the Marsel Block has favorable geological conditions for formation of a superimposed continuous tight gas reservoir. [ABSTRACT FROM AUTHOR]

Superimposed basins are commonly characterized by a complex distribution of oil and gas accumulations. Because they have generally experienced multistage hydrocarbon accumulation and tectonic movement, it is challenging to predict favorable exploration zones in these basins. The exploration of the deep strata of the Sichuan Basin (i.e., a typical superimposed basin), West China, has been unsatisfactory for petroleum prospectors. Therefore, we chose the Cambrian Longwangmiao Formation in the Sichuan Basin as an example to demonstrate the method, i.e., combination of petroleum system elements to quantitatively predict the petroleum favorable exploration zones. Based on the detailed analysis of the hydrocarbon accumulation process, the source rock (S), migration (M), reservoir depositional facies (D), and regional cap rock (C) are the most important elements controlling the formation and distribution of oil and gas accumulations in the studied basin. By studying the functional relationships among these four elements and the distribution of the petroleum accumulations, we determined the accumulation probability related to each element and quantitatively predicted the probability of a favorable hydrocarbon accumulation zone. We built a quantitative model of the reservoir preservation probability based on the statistical relationship between the intensity of the tectonic movement and subsequent destruction of existing oil and gas accumulations. Favorable exploration zones can be determined by overlaying favorable accumulation zones with disturbances associated with the tectonic evolution. The results suggest that the most favorable exploration zones of the Longwangmiao Formation in the Sichuan Basin are distributed in the regions west of Weiyuan and east of Gaoshiti–Moxi, Pengxi–Yilong, and Dazhou–Kaijiang. The production data for existing fields in the Sichuan Basin show that 87% of the successful wells are in the predicted favorable exploration zones and that the success rate with respect to the prediction of failure wells is 67%. This approach highlights the most prospective exploration areas in superimposed basins, which, in general, is challenging and complex for traditional basin modeling tools. • Favorable hydrocarbon accumulation zones are quantitatively predicted. • Impact of tectonic movement on the petroleum accumulations is studied. • Petroleum favorable exploration zones in complex superimposed basin are predicted. • The coincidence rate of the application of the model is 87%. [ABSTRACT FROM AUTHOR]

Despite the enormous potential of continental shale oil resources, only recoverable shale oil is economically valuable. Shale oil development decisions can greatly benefit from an accurate assessment of recoverable shale oil resources. Three crucial factors for evaluating shale oil recoverable resources are shale oil content, movable oil ratio, and recovery factor. However, because of the significant variety of continental shale, it is challenging to define the aforementioned characteristics. The methodologies and technology for determining the shale oil recoverable resource evaluation parameters, particularly the shale oil content, movable oil ratio, and recovery factor, are thoroughly discussed in this paper. Taking the Tertiary Sha-3 continental shale in Dongpu and Nanpu sags in Bohai Bay Basin, eastern China as an example, the shale oil content was obtained by the pyrolysis method with light and heavy hydrocarbon calibration comprehensively considered, and the main controlling factors of shale oil content were explored. Four geological parameters, including the sedimentary facies, pore structure, surrounding rock quality, and burial depth, primarily regulate the variation features of continental shale oil. The formation mechanism and primary governing variables of movable oil were then revealed using a physical simulation experiment. As porosity and permeability rise, clay mineral content falls, oil viscosity falls, and kerogen concentration rises, the movable oil ratio rises as well. Through stepwise linear regression, a quantitative estimation model for movable oil ratio was developed. After that, a literature search was used to determine the average recovery factor for continental shale oil. Finally, using the volume method model and the Monte Carlo methodology, the recoverable shale oil resources in the Gaoliu area of Nanpu Sag were assessed. The retention, movability, and resource potential of continental shale oil are better understood as a result of this study. [ABSTRACT FROM AUTHOR]

The Lower Permian Taiyuan Formation is a lithic-abundant, tight gas sandstone reservoir in the Kangning field, northeastern Ordos Basin. To clarify the gas accumulation process in tight gas sands, the relationship between the densification of Taiyuan Formation sandstones and gas charging is analyzed. Core porosities of the Taiyuan Formation are mostly below 10%, and air permeabilities are below 1 mD. The sandstone reservoirs are characterized by strong heterogeneity. Dissolution pores and large amounts of micropores in clay minerals dominate the storage space. This porosity is the result of the impact of diagenesis. Porosity evolution modeling confirmed well the prediction of the period of sandstone densification (225-215 Ma) based on the basin analysis as well as diagenetic calibration. Strong compaction dominated the physical property (porosity and permeability) reduction with cementation as a subordinate factor. The integration of burial history and fluid inclusion data suggested that large quantities of gas migrated into the Taiyuan sandstone reservoirs from 176 Ma to 80 Ma when the reservoirs were already tight (porosity <10%). Gas expansion forces provided by source rocks were the main driving force for gas charging instead of buoyancy. Since the late Cretaceous, uplift activity had little influence on Taiyuan tight gas-sands due to self-generation and self-storage. Overlapping sandstone bodies interbedded with high-thickness and good-quality source rocks jointly determined development of tight gas sands. These study results may explain the gas accumulation process and provide insights into gas exploration and development when this method is applied elsewhere. • A new method to indicate the type of gas accumulation. • Three stages of gas accumulation process for Taiyuan Formation in Kangning area. [ABSTRACT FROM AUTHOR]

Decrease in TOC of the carbonate source rock with low total organic carbon (CSRLTOC) contents with hydrocarbon generation and expulsion (a, b, c) and the lower limit of TOC (LLTOC) decreases (d) with the increasing T max or VR. • The CSRLTOC (carbonate source rocks with low total organic carbon) can be effective to oil/gas accumulation in China. • The LLTOC (lower limit of total organic carbon) for effective carbonate source rocks at various mature stages is determined. • Some oil/gas fields have derived partly or mostly from the CSRLTOC. • The CSRLTOC should be conducted for more in-depth studies in the future. Carbonate rocks with high maturity and low total organic carbon content are widely distributed in China. Whether they can be effective source rocks remains controversial, but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks. Dataset reveals that as the depth or maturity increases, total organic carbon (TOC) content, atomic H/C ratio, and hydrocarbon generation potential of carbonate source rocks with low total organic carbon (CSRLTOC) all decrease because of hydrocarbon expulsion. Moreover, some pyrolysis experiments also indicate that the CSRLTOC are able to generate and expel a quantity of hydrocarbons. The above studies indicate that CSRLTOC can be effective source rocks for oil/gas pools. The unique features of carbonate rocks and CSRLTOC demonstrate that they are easier to expel hydrocarbons and contribute to oil/gas reservoirs than shale, and thus, the lower limit of TOC (LLTOC) of effective carbonate source rocks could be smaller than shale (0.5%). The LLTOCs of effective carbonate source rocks with type I-II 1 kerogen (mainly sapropelic group) at the levels of immaturity and low maturity, maturity, high maturity and over maturity are TOC = 0.5%, 0.5–0.3%, 0.3–0.2%, and 0.2–0.1%, respectively. If the carbonate source rocks are at the high-over mature stage, we could take 0.2% as the LLTOC. The present organic-poor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods, especially carbonate source rocks if type I-II 1 kerogen. Some typical oil and gas fields in China derive partly or mostly from the CSRLTOC, proving that CSRLTOC could be effective source rock in China. [ABSTRACT FROM AUTHOR]

Reliably and conveniently predicting oil and gas prospects and avoiding costly drilling mistakes are the goals pursued by oil explorers and companies. In 2013, the Anyue gas field was discovered in Sichuan Basin, and it was the first discovery of a giant natural gas field in Precambrian strata attracting extensive attention since then. However, the complexity of geology in the Sinian-Lower Paleozoic makes prediction of oil and gas prospects very challenging. Based on the comprehensive review of the Anyue gas field, we identify four functional elements, i.e., a source kitchen (S), paleo-uplift (U), reservoir depositional facies (D), and regional cap rock (C). The combination of these four functional elements in time and space (T- CDUS) controls the Sinian-Lower Paleozoic hydrocarbon accumulation. The probability for hydrocarbon accumulation is determined by a proposed index, Tcdus. As a tectonic movement can damage oil and gas reservoirs formed prior to the movement, a model for calculating the hydrocarbon reservoir preservation probability is also established. The oil and gas prospects in Sinian-Lower Paleozoic are predicted with the combination of the hydrocarbon accumulation and preservation model. 137 wells drilled in the Sichuan Basin show that 88.9% of the successful wells are distributed in the predicted zones. • Four functional elements control the formation of oil and gas reservoirs. • Damage of tectonic movement to petroleum accumulations is analyzed. • The oil and gas prospects of the Sinian-Lower Paleozoic are predicted. [ABSTRACT FROM AUTHOR]

Abstract Shale oil has gradually become the predominant target for unconventional hydrocarbon exploration in recent years. Unlike marine shales that have been extensively studied, lacustrine shales are not adequately characterized in the literature. In this study, we examined Shahejie Formation shales in the Nanpu Sag in the Bohai Bay Basin of China, and performed organic geochemistry, mineralogy, scanning electron microscopy (SEM), and N 2 and CO 2 gas adsorption analyses. The results show that the Shahejie shale can be divided into four lithofacies: siliceous shale, calcareous shale, argillaceous shale, and mixed shale. Calcareous shale was deposited in deep lacustrine environment, and usually has the highest total organic carbon (TOC) content with type I kerogen. Mixed shale, which was deposited in the transitional zone, has moderate TOC values. Argillaceous shale and siliceous shale were deposited in shallow lacustrine environment or near shore, and always have the lowest TOC values. Inorganic mineral pores are the dominant type of pore in all four shale groups. Argillaceous shale has a large amount of intraparticle pores with low average pore size and poor connectivity compared with calcareous shale. Limited organic matter pores are present in low matured lacustrine shales, whereas the porosity of some lacustrine shales may increase due to the occurrence of large number of organic pores leading to significant increase of total porosity. Micro-fissures within calcareous shale greatly improve reservoir quality. Overall, siliceous shale and argillaceous shale usually have low TOC, low residual hydrocarbon, relatively strong adsorption capacity, and poor connectivity of pores, which contain little free oil and are unfavorable for shale oil exploration. Calcareous shale has high TOC, high residual hydrocarbon, large pore size, and excellent fracturing nature, and should be the best target for lacustrine shale oil exploration. Highlights • Pore characterization in an argillaceous-siliceous-calcareous shale system was studied. • Inorganic mineral pores play the dominant role in the micro- and mesopores of lacustrine shales with low maturity. • Calcareous shale is the most favorable target for shale oil exploration. [ABSTRACT FROM AUTHOR]

Abstract In a petroleum accumulation system, the source rock plays a significant geological role in controlling hydrocarbon distribution, because it provides the material basis for hydrocarbon accumulation. Thus, quantitative evaluations of source rock control on hydrocarbon distribution are useful for predicting future hydrocarbon discoveries. Using the Huizhou Depression in the Pearl River Mouth Basin, China, as a case study, geostatistical methods and probability calculation were employed to analyze the controlling effect of source rocks. The Huizhou Depression can be divided into five petroleum accumulation systems of which the Huixi contains the largest known amount of hydrocarbons. A probability model of hydrocarbon accumulations is established for quantitative evaluation of potentially favourable hydrocarbon distribution. In each petroleum accumulation system, the larger the hydrocarbon expulsion intensity, the larger the amount of hydrocarbons that accumulates; the longer the distance from the hydrocarbon expulsion centre, the lower the amount of hydrocarbon accumulates. The hydrocarbon distribution limit controlled by the source rock is 3.5 times the radius of the source kitchen in the Huizhou Depression. In the current state of exploration, oil fields in the Huizhou Depression are mainly observed in the areas 1.5 to 2 times the source kitchen radius. Based on the prediction model, structural highs around hydrocarbon source kitchens are potentially favourable zones for structural hydrocarbon reservoirs, and the interiors of source kitchens are potentially targets for subtle hydrocarbon reservoirs and unconventional resources. Highlights • Quantitative evaluation of source rock control on hydrocarbon distribution is built. • Geochemical characteristic of source rocks is investigated. • Petroleum accumulation systems of Huizhou Depression are divided. [ABSTRACT FROM AUTHOR]

Abstract The Ordos Basin is a significant gas-producing region in China. Previous studies have shown that a large amount of the gas discovered in the upper Paleozoic strata originated from Carboniferous–lower Permian coal and its interbedded coal measure mudstone. To compare the hydrocarbon generation characteristics and process, two coal and coal measure mudstone samples from the Shanxi Formation in the Shenfu area were used in confined gold tube pyrolysis experiments. The analytical results revealed that both samples had poor oil generation potential, with maximum C 6+ yields of 171.65–175.67 mg/g TOC for the coal and 50.12–51.86 mg/g TOC for the coal measure mudstone at the heating rate of 2 °C/h and 20 °C/h. However, their gas generation potentials are good, and the coal exhibited higher values than the mudstone, with final laboratory C 1 –C 5 yields of 154.71–173.32 mg/g TOC for the coal and 82.52–98.96 mg/g TOC for the coal measure mudstone. Based on a comprehensive analysis of the indicators lnC 2 /C 3 , lnC 1 /C 2 and δ 13C 1 – δ 13C 2 , the hydrocarbon generation process can be divided into three stages. The results of extrapolation to geological conditions revealed that the main gas generation stages of both source rocks in the Shanxi Formation began in the Early Cretaceous. Because of the regional tectonic uplift and the decrease in the geothermal gradient in the Late Cretaceous, the C 1 –C 5 volume yields of the coal and the coal measure mudstone reached plateaus of 78.4 ml/g TOC and 32.7 ml/g TOC, respectively, in the Late Cretaceous, and the current transformation ratios of C 1 –C 5 for the coal and the coal measure mudstone are only 32.1% and 22.8%, respectively. Highlights • Geochemical characteristics of the lower Permian Shanxi Formation coal and coal measure mudstone were analysed. • Confined pyrolysis experiments were performed on the two selected sample types. • Hydrocarbon generation process were determined based on the indicators of yields and carbon isotopes. • Kinetic parameters were used to simulate the hydrocarbon generation history of the coal and coal measure mudstone. [ABSTRACT FROM AUTHOR]

Abstract The Dongpu Depression, especially the northern part, is an important natural gas-rich area in the Bohai Bay Basin, China. The gases found in Es3M, Es3L and Es4 are mainly distributed in the western part of Qianliyuan, in Weicheng, and in the middle of the Wenliu area, respectively. However, few studies have been conducted on the relationship between the development of source rock and the enrichment of gas in different hydrocarbon-generating sags. In this paper, gas genetic types, distribution characteristics and gas-source correlations were analyzed using data from molecular and stable carbon isotope components of natural gas, biomarkers and thin sections. The results indicate that Es3M gas is predominantly oil-type gas and that Es3L gas is mainly mixed gases dominated by coal-type gas, with average dryness coefficients (C 1 /C 1–5) of 0.877 and 0.920, respectively. By contrast, Es4 gas is dry with an average dryness coefficient of 0.970 and displays the typical characteristics of coal-type gas from Carboniferous-Permian coal measures. Carbon isotope reversals are common in the gas samples, while their causes are different. The mixing of coal-type gas and oil-type gas is the main reason for the gas carbon isotope reversals in Es3M and Es3L. By contrast, the widely existing carbon isotope reversal in Es4 gas in the central part of Wenliu is related to the long-term migration and enrichment of coal-type gas from Carboniferous-Permian rocks. Gas-source correlations indicate that Es3M gas is predominantly from the in situ cracking of Es3M oil and that coal-type gas mixed within Es3L gas is from the Carboniferous-Permian coal measures. Highlights • Genetic types of natural gas in the northern Dongpu Depression were summarized. • Gas carbon isotope reversals in different target layers were analyzed. • Es3M gas is predominantly from in situ cracking of Es3M oil. • Coal-type gas mixed within Es3L gas is from the Carboniferous-Permian coal measures. [ABSTRACT FROM AUTHOR]

Pore development and the location of retained hydrocarbons are key topics to understand the shale oil system. In an attempt to investigate pore network characteristics and oil retention in lacustrine shales, 24 samples from the Es 3 shales in the Dongpu Depression, Bohai Bay Basin were subjected to mineralogical, petrological, geochemistry and pore structure investigations using X-ray diffraction (XRD), thin section, field emission-scanning electronic microscope (FE-SEM), N 2 adsorption-desorption. After removing retained oil by solvent extraction, 12 selected samples were again analyzed by Rock-Eval pyrolysis and N 2 adsorption-desorption experiment. According to the mineral compositions and petrological characteristics, six types of lithofacies are identified in the Es 3 shales in the Dongpu Depression: laminated calcareous shales, lenticular calcareous shales, laminated argillaceous shales, massive argillaceous shales, laminated siliceous shales, and massive siliceous shales. Geochemical characteristics of these shales are quite different, as calcareous/laminated shales have higher TOC contents than massive argillaceous and siliceous shales. Interparticle pores, intraparticle pores, organic matter pores and fracture pores are observed in the studied samples, while bitumens were found retaining in well-connected pore network. The N 2 adsorption-desorption experiments on samples before and after solvent extraction reveal the different pore structure characteristics in six lithofacies and the occurrence of oil in lacustrine shales. In general, pore volumes within pore diameter range of 1.7–10nm and 10–200nm of samples show increase after solvent extraction, indicating that large and small pores in shales both play important roles in oil retention. Oil retained in shales can be divided as free and heavy fractions, as from the pyrolysis results. In lacustrine shales, larger pores are main storage space for hydrocarbon. Larger pores in different lithologies have similar capacity for free oil retention, while those in calcareous and argillaceous shales are more efficient for heavy oil retention. For both free and heavy oil, small pores in argillaceous shales are efficient for their storage. [ABSTRACT FROM AUTHOR]

Abundant shale oil resources were recently found in the organic‐rich Permian Lucaogou (P2l) Formation shale, Jimusar Sag, Junggar Basin. Although some scholars have conducted some researches on the basic geochemical characteristics of the Lucaogou Formation shale, it is not enough to effectively guide the shale oil exploration in the area. This is because during the exploration process of shale oil, besides the basic geochemical characteristics of shale, its mineral composition, physical properties, and oil‐bearing characteristics are all very important evaluation parameters. To systematically evaluate the favourable shale oil exploration sections in the Lucaogou Formation, 265 pieces of shale cores were sampled from well Ji174 in this study. Besides the basic geochemical characteristics of the 265 pieces of cores, the mineralogical, petrophysical, and oil‐bearing characteristics were also analysed. Results show that the shale has abundant organic matter (average total organic carbon 3.51%, average petroleum generation potential 15.67 mg/g), dominated by Type II1 and Type II2 kerogen, and has entered the early‐mature to mature stage. The Upper Member of the Lucaogou Formation has relatively higher organic matter and organic matter type is relatively more oil prone compared to the Lower Member; however, organic matter thermal maturity is lower. The P2l Formation shale is characterized by high carbonate minerals and low clay minerals, and when considering mineralogical brittleness, both the Upper and Lower members are ideally fracturable in shale oil development. The P2l Formation shale has a wide porosity range (1.1–13.9%), and the Lower Member's porosity is slightly greater. The inversion of shale porosity in the P2l Formation is controlled by the differentiated mineral composition. The oil saturation index values of the Lower Member shale are significantly greater than the Upper Member shale. From the above, the Lower Member of the P2l Formation shale is ideally suited and has greater shale oil potential in the Jimusar Sag, Junggar Basin, especially in the depth intervals of 3,255–3,272, 3,282–3,298, and 3,317–3,334 m in the well Ji174. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies have postulated the contribution of present‐day low‐total organic carbon (TOC) marine carbonate source rocks to oil accumulations in the Tabei Uplift, Tarim Basin, China. However, not all present‐day low‐TOC carbonates have generated and expelled hydrocarbons; therefore, to distinguish the source rocks that have already expelled sufficient hydrocarbons from those not expelled hydrocarbons, is crucial in source rock evaluation and resource assessment in the Tabei Uplift. Mass balance can be used to identify modern low‐TOC carbonates resulting from hydrocarbon expulsion. However, the process is quite complicated, requiring many parameters and coefficients and thus also a massive data source. In this paper, we provide a quick and cost effective method for identifying carbonate source rock with present‐day low TOC, using widely available Rock‐Eval data. First, we identify present‐day low‐TOC carbonate source rocks in typical wells according to the mass balance approach. Second, we build an optimal model to evaluate source rocks from the analysis of the rocks' characteristics and their influencing factors, reported as positive or negative values of a dimensionless index of Rock‐Eval data (IR). Positive IR corresponds to those samples which have expelled hydrocarbons. The optimal model optimizes complicated calculations and simulation processes; thus it could be widely applicable and competitive in the evaluation of present‐day low TOC carbonates. By applying the model to the Rock‐Eval dataset of the Tabei Uplift, we identify present‐day low‐TOC carbonate source rocks and primarily evaluate the contribution equivalent of 11.87×109 t oil. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mesopores are the most developed in the over-mature Longmaxi shale. • The contribution of OM pores to pore area increases with increasing TOC content. • The clay mineral pores contribute the most pore area and volume in OM-poor shale. Pore types significantly affect the pore properties of shale reservoirs, but their contribution to shale pore space in over-mature marine shale remains controversial because organic and inorganic pores fail to be quantitatively characterized. In this study, mercury intrusion porosimetry, low-temperature N 2 and CO 2 adsorption, and image analysis were conducted on over-mature marine shale in the Lower Silurian Longmaxi Formation to quantify the contribution of pore types to pore volume (PV) and surface area (SA). The full-scale pore size distribution of over-mature shale indicates that mesopores account for the largest PV, while macropores are developed in organic matter (OM)-poor shale. By analyzing the influence of multiple factors and heterogeneity of different pore types, OM pores, pyrite framboid pores, clay mineral pores, and dissolution pores are identified as dominant types in OM-poor/moderate/rich shale. The PV and SA of these four pore types are quantitatively characterized by extracting parameters of shale components and pores using image analysis. In OM-poor shale, clay mineral pores contribute the most to the PV and SA, followed by OM pores and dissolution pores, and finally pyrite framboid pores. In OM-rich/moderate shale, the contribution of OM pores to the PV and SA is dominant and increases with the increasing total organic carbon content. The evolution of pore types throughout the maturation sequence is revealed based on previous studies. Especially at the over-mature stage, clay mineral pores contribute the most to pore space in OM-poor shale, while OM pores make a major contribution to pore space in OM-rich/moderate shale. [ABSTRACT FROM AUTHOR]

Large shale oil resources were recently been found in lacustrine organic-rich shale with strong heterogeneity, however, few studies have been conducted to examine their oil contents, resulting in considerable risks in lacustrine shale oil exploration. The Middle Permian Lucaogou Formation (P 2 l) shale in Jimusaer Sag is a typical lacustrine deposit with strong heterogeneity, and its shale oil resource shows great potential. Integrated geochemical characterization of 265 core samples were conducted and results show that the P 2 l shale developed in an anoxic lacustrine with stratified salty water and the organic matter in the upper sub-member shale of the P 2 l is more oil-prone than that in the lower sub-member shale of the P 2 l. The hydrocarbon generation potential of the P 2 l shale decreases with the kerogen types changing from Type I to Type III, however, the residual hydrocarbon contents of the P 2 l shale increases from Type I to II 1 kerogen, and then decreases from Type II 1 to III kerogen, this is mainly due to differentiated hydrocarbon expulsion efficiencies among different kerogen types. Based on S 1 and TOC values and the S 1 /TOC ratios, considering the oil enrichment degree, this study classified the shale oil resources in the P 2 l shale into four categories: enriched, moderately enriched, less efficient, and invalid resources. The enriched and moderately enriched resources are mainly shales with Type II 1 kerogen, followed by Type II 2 kerogen, and the middle interval of the Lower P 2 l shale is the most favourable target for further shale oil exploration. The improved evaluation criteria are applicable for evaluating shale oil plays with strong heterogeneity qualitatively and quantitatively in terrestrial lacustrine basins in other parts of the world. [ABSTRACT FROM AUTHOR]

Evaluation of carbonate heterogeneity, closely related to pore geometry and connectivity, is important in oil and gas field production and reservoir prediction. To profoundly understand the heterogeneity of Lower Carboniferous carbonate reservoirs in the Marsel area, including Visean (C 1 v) and Serpukhovian (C 1 sr), the pore structure and fractal characteristics are investigated using routine petrophysical measurements, X-ray diffraction (XRD), cast thin section analysis, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) tests. The origin of potentially prolific reservoir, the relationships between fractal dimension and reservoir physical properties, mineral composition and pore structure are also discussed. XRD results reveal that the mineral compositions are highly heterogeneous in C 1 v and C 1 sr. Routine petrophysical experiments indicate that C 1 v and C 1 sr are generally tight. However, some high porosity-permeability zones exist in C 1 sr, which is described as a potentially prolific reservoir. The cast thin section and SEM analyses identify three main pore types in the potentially prolific reservoir: interparticle dissolution pores, intercrystalline dissolution pores, and moldic pores. The MICP data reflect that the pore-throat radii in the prolific reservoir range from 0.29 to 4.6 μm, whereas those in tight section are typically <0.14 μm. The fractal dimensions are computed based on MICP data, and the fractal characteristics of the potentially prolific reservoir in C 1 sr are divided into two segments. In the tight section, relatively larger fractal dimensions are observed, indicating highly heterogeneous pore structure. The potentially prolific reservoir in C 1 sr is formed mainly because of penecontemporaneous dolomitization and leaching due to meteoric freshwater ingress. Different mineral compositions have differing effects on the fractal characteristics of the pore structure. Relatively good correlations of fractal dimension with porosity, permeability, sorting coefficient, and skewness indicate that fractal dimensions are helpful in reservoir quality evaluation and quantitative characterization of pore networks. [ABSTRACT FROM AUTHOR]

Hydrocarbon evaporative loss evaluation is important for shale oil resource assessment. We propose a mass balance model to evaluate the hydrocarbon evaporative loss (S 1-loss ) in this study. The proposed method is applied to a large Rock-Eval pyrolysis dataset on source rock samples from the Lucaogou Formation in the Jimusaer Depression of the Junggar Basin, NW China, as a case study to evaluate the S 1-loss . The results indicate that the S 1-loss can range from 0.12 to 7.25 mg/g Rock, corresponding to 11%–89% loss of the generated hydrocarbons (S 1-loss /(S 1 +S 1-loss )) due to evaporation before lab analysis. It appears that when Ro ≤ 1.3%, S 1-loss and S 1-loss /TOC decreases gradually with increasing Ro, likely corresponding to increasing oil density as results of heterogeneous source rock compositions and increased accommodation space via the creation of organic pores of large specific surface for heavy and large hydrocarbon molecules in oil window. When Ro > 1.3%, S 1-loss /TOC begins to increase with increasing Ro as more oil crack to gaseous and light hydrocarbons that are more susceptible to evaporative loss. In the case of similar Ro, the relative S 1-loss /TOC varies little among the samples but the absolute S 1-loss amount increases with increasing TOC. Restricted by regional geological conditions in this study, we focused on the S 1-loss for lacustrine type I kerogen only, and quantification of evaporative loss for other kerogen types remains a subject for further study. [ABSTRACT FROM AUTHOR]

The Tarim Basin in China is a complicated superimposed basin with four periods of hydrocarbon accumulations and three periods of modifications due to tectonic movements, forming complicated hydrocarbon reservoirs, many of which have gone through reservoir migration, reservoir scale reformation, reservoir damage and phase alterations a result of the tectonic events. Due to the complexity, the hydrocarbon exploration in this region has low success drilling rate. It is of great importance to study the destruction to the early hydrocarbon accumulations by the tectonic movements and quantitatively evaluate the remaining resource for understanding the current hydrocarbon distribution and guiding the exploration. This study analyzed six periods of large tectonic events in the Tarim Basin, and found the destroyed hydrocarbon volume was mainly controlled by four geological factors, including the ratio of the destroyed to the original hydrocarbon amount, tectonic event number and sequence, and the cap seal capacity. Larger ratio of destroyed hydrocarbon, less tectonic event number, later sequence and stronger cap seal capacity lead to more hydrocarbons preserved through the tectonic events. Based on the geological analysis, we established quantitative relationships among the destroyed hydrocarbon volume by the tectonic events (Q l ), the volume of primary hydrocarbon accumulation (Q 0 ), the ratio of destroyed hydrocarbon (K l ), the tectonic event number (n) and sequence (i), and cap seal capacity (fc). Using these relationships and the calibrated geological parameters, we delineated the hydrocarbon accumulation and destruction history and quantified the hydrocarbon loss due to the tectonic events for both the Tazhong area and the whole Tarim Basin. In the Tazhong area, four major accumulation events occurred at 510 Ma, 298 Ma, 227 Ma and 38 Ma with each accumulation underwent 68%, 59%, 28% and 0% of hydrocarbon loss due to later tectonic events. In total, over 60% of the original hydrocarbon reserves have been destroyed in the Tarim Basin, with an estimate of ca. 1.9 billion tons remaining hydrocarbons in the Tazhong area. At present, more than 95% oil and gas reservoirs and over 93% high quality exploration wells are located in the areas with high remaining resource potential predicted in this study, whereas 70% non-commercial production wells are located outside. Overall, this work shows that a careful evaluation of the damage to reservoirs from tectonic events can considerably enhance the success rate of hydrocarbon exploration. [ABSTRACT FROM AUTHOR]

Three rounds of exploration in Marsel, Kazakhstan show that the area has good petroleum geological conditions; however, the exploration degree is low and the resource potential is unclear. The total organic carbon ( TOC ) content is the criteria for effective source rocks and is also the key parameter to calculate the amount of resources; while previous evaluations of the effective source rocks and calculated amounts of resources were based on residual TOC , which gradually decreases with mass hydrocarbon expulsion; therefore, this will inevitably produce errors. For greater accuracy, the TOC recovery coefficient formula was used to recover the residual TOC in Marsel; then, the criteria of the effective source rocks were revised to calculate the Carboniferous and Devonian resources in Marsel. The results are as follows: for the source rocks in Marsel, the TOC recovery coefficient of the Carboniferous source rocks is approximately 1.6, while that for the Devonian is approximately 2.0; the revised lower limit of TOC for the effective source rocks is 0.3% and 0.25% for the Carboniferous and Devonian, respectively; after TOC recovery, the total amount of hydrocarbon expulsion is 26.7 × 10 12 m 3 , the total amount of recoverable resources is 2.62–5.10 × 10 12 m 3 , and the mean is 3.79 × 10 12 m 3 for the main source rocks. The effective source rocks evaluated based on the recovered TOC and the latter resources could reflect the quantity of source rocks and actual resource potential of Marsel more accurately. This has certain significance for the exploration of areas that have low levels of exploration with an unclear understanding of the resource potential as well as a high degree of thermal evolution and a low quantity of residual TOC . [ABSTRACT FROM AUTHOR]

Recent exploration has revealed the great potential of gas resources, complicated gas distributions and geochemical characteristics of natural gases in tight sandstones of the West Sichuan Basin, China (WSBC). The chemical, stable carbon and light hydrocarbon compositions of natural gases from WSBC were investigated to assess the genesis, sources, and migration pattern leading to variations in the geochemical and stable carbon isotope compositions. The analysis shows that the natural gas in the Jurassic tight sandstones occurs as dry gas or wet gas in some places; however, most of the gas in the deep Triassic tight sandstones is apparently dry gas. Carbon isotope distributions increased in the order δ13C1 < δ13C2 < δ13C3 < δ13C4. The light hydrocarbon content increased in the order of chain (i.e. non-cyclic) alkanes, cycloalkanes, and aromatic hydrocarbons. Relatively high ethane carbon isotope values and methylcyclohexane contents indicated the presence of lacustrine coal-formed gases. The Jurassic natural gas is in a lower thermally mature stage and possibly originated from a mixture of T3X3 and T3X5 black mudstones. The deep Triassic natural gas was possibly originated in the deep source rocks because the gas has a relatively higher thermal maturity. Positive correlations between burial depth and methane content, dryness coefficient, δ13C1, and δ13C1-δ13C2 for the deep Triassic gas suggest that the migration distance was short and the source was nearby or the gas accumulated in situ. The chemical compositions and geological studies revealed that the fractionation of the Jurassic natural gases in a tight reservoir was predominantly caused by vertical migration rather than the maturity of the local source rocks. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Shale oil exploration of the Permian Lucaogou Formation (P2l) has greatly advanced in the Jimusar Sag in the Junggar Basin. However, despite the strong exploration prospects of shale oil resources, the potential of shale oil resources in the Junggar Basin has been insufficiently assessed. This study performs systematical evaluation of P2l source rocks in the Jimusar Sag and proposes an improved hydrocarbon generation potential method to assess the hydrocarbon generation and expulsion characteristics and evaluates the shale oil resource potential of the P2l source rocks in the Jimusar Sag. The results show that the P2l source rocks are thick (up to 200 m) and are distributed over a wide area (up to 1200 km2) in the study region. It contains high TOC (2%-5%), characterized by dominantly type I and type II kerogen. The source rocks are in the low mature-mature stage, and possess good conditions for generation of hydrocarbons. The P2l source rocks reached the hydrocarbon expulsion threshold at a thermal maturity of 0.8% vitrinite reflectance. The efficiency of hydrocarbon expulsion of the source rock is 34%. The amounts of hydrocarbons generated and expelled were 65 × 108 t. Based on the above information, the shale oil resources potential is calculated to be 43 × 108 t, indicating very good shale oil resource prospect in the P2l source rocks of the Jimusar Sag. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Steptoean Positive Carbon Isotope Excursion Event (SPICE) represents one of the largest carbon cycle perturbations in the Cambrian, which coincided with climatic changes and dramatic extinction of shallow-shelf faunas. This carbon perturbation has been linked to a global expansion of marine euxinia (SPICE-OAE), but the precise timing, duration and extent of changing redox conditions across the SPICE event, as well as its influences on the coeval biotic evolution, remain unclear. Here, we report paired δ 98 Mo– δ 238 U data across this event, constrained by a radioisotopically anchored astrochronology, based on Alum Shale samples from the Albjära-1 and Billegrav-2 drill-cores in Scandinavia. The δ 98 Mo profiles are largely invariant around 1.08±0.21‰ with short-term fluctuations parallelling changes in pyritization of iron, possibly as a result of locally varying [H 2 S] aq in the water column and/or fluctuating reactive iron delivery. The bottom water is interpreted to have been primarily anoxic, but with frequent local variations between oxic and sulfidic conditions in a moderately restricted basin. A novel analytic method based on variation of Mo concentrations at millimeter scale stratigraphic resolution was applied to distinguish between local and global redox signatures, revealing that cyclic redox changes are recorded in the Alum Shale sea with periods of ∼0.3 Myr, ∼0.9 Myr and ∼0.15 Myr before, during and after the SPICE-OAE, respectively. Stratigraphic profiles of Mo, U, Mo/TOC ratios and δ 238 U in the two cores show comparable trends, facilitating recognition of three chemically distinct stages across the SPICE event: 1) global expansion of marine euxinia (SPICE-OAE; 497.9–496.6 Ma), 2) global contraction of euxinia accompanied by enhanced carbonate weathering inputs (496.6–495.8 Ma) and 3) general global oxygenation (495.8–494.4 Ma). The first in situ correlation between the onset of the SPICE-OAE and the end-Miaolingian extinctions in Baltica corroborates that expansive anoxic and sulfidic water masses likely contributed to the biotic crisis. The time gap between the start of the extinction and the appearance of the new fauna in Baltoscandia is estimated to be ∼600 kyr. • We present paired δ 98 Mo– δ 238 U data across the SPICE event from the Alum Shale constrained by an astrochronology. • The Alum Shale records two negative δ 238 U shifts of up to −0.46‰ and largely invariant δ 98 Mo at ∼1.08‰. • We suggest three chemically distinct stages across the SPICE event as supported by U biogeochemical model. • The first in situ correlation between the SPICE-OAE and the end-Miaolingian extinctions in Baltica confirms their connection. • The time gap between the start of the extinctions and the appearance of the new fauna in Baltoscandia is ∼600 kyr. [ABSTRACT FROM AUTHOR]

Effective reservoirs in petroliferous basins are generally defined as rocks that have relatively high porosity and permeability, which have the capacity for accumulating and storing hydrocarbons under the conditions of sufficient hydrocarbon supply. Previous studies show that the lower limit of porosity of effective reservoirs decreases with increasing burial depth. However, the existing assessment methods for effective reservoirs mainly use well test data or petrophysical properties of a reservoir, without considering the influence of depth. These methods are not practical for petroleum exploration. In this study, the dynamic mechanism of hydrocarbon accumulation was studied using physical simulation experiments, which revealed that capillary force is a basic control for hydrocarbon accumulation. The relationship between the capillary force ratio of surrounding rock to reservoir rock and their oiliness was statistically analyzed. Finally, a novel discriminating standard and quantitative evaluation for determining effective reservoirs was established and its reliability was confirmed. This study shows that the capillary force ratio of surrounding rock to reservoir rock determines the reservoir effectiveness. Moreover, a reservoir is more effective when the ratio is larger, thus more favorable for hydrocarbon accumulation. The reservoir potential index (RPI) ranging from 0 to 1 is introduced to characterize the hydrocarbon-bearing property of a reservoir. Rservoirs are classified as effective reservoirs when RPI <1 and ineffective reservoirs when RPI >1. The effective reservoirs are further divided into four grades based on the RPI value: Grade I: 0 ≤ RPI ≤0.25; Grade II: 0.25 < RPI ≤0.5; Grade III: 0.5 < RPI ≤0.75; Grade IV: 0.75 < RPI <1, in which the grade I refers to the most favorable reservoir. It is conformed as a practical and reliable method. • Dynamic mechanism of hydrocarbon accumulation is simulated. • Discriminating standard of effective reservoirs is established. • Capillary force difference inside and outside of rock is intrinsic for effectiveness. [ABSTRACT FROM AUTHOR]

Significant progress has recently been made in tight oil exploration within the Permian Lucaogou (P2l) Formation of the Jimusar Sag. However, current tight oil exploration deployment of the P2l Formation is mainly based on reservoir prediction, which is high risk for tight oil exploration. In this study, the geological and geochemical characteristics of the P2l Formation source rocks, including the distribution, sedimentary environment, organic matter abundance, kerogen types and thermal maturity were investigated. Hydrocarbon generation and expulsion intensity were evaluated through an improved hydrocarbon generation potential methodology, and the significance of source rocks in tight oil source and occurrence was systematically investigated. Results indicate that P2l Formation source rocks with total organic carbon >1.0 wt% occur widely (an area up to 1500 km2), are thick (up to 160 m), were deposited in a lacustrine weakly reducing sedimentary environment with relatively low salinity, have a high total organic content with a mean value of 3.12 wt%, are dominated by type II kerogen and have reached the early mature to mature stage. Modelling results indicate that the source rocks reached the hydrocarbon generation threshold and hydrocarbon expulsion threshold at 0.48% and 0.86% vitrinite reflectance, respectively. The comprehensive hydrocarbon expulsion efficiency was approximately 30%, and the maxima of hydrocarbon generation and expulsion intensities for P2l Formation source rocks are 1200 × 104 and 425 × 104 t/km2. The tight oil is sourced from adjacent source rocks that are interbedded with, or are close to, the reservoirs. The migration of oil generated from the source rocks occurs over very short distances. The oil filling degree index (oil bearing thickness/P2l Formation thickness) is higher at a closer proximity to the source rocks, and where it is higher the hydrocarbon generation intensity of the source rocks is also elevated. In addition, the greater the hydrocarbon expulsion intensity of the source rocks, the higher the daily oil production values (ton/day) from prospect wells. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

There are two sets of carbonate source rocks in the Lower Carboniferous layers in Marsel: the Visean (C 1 v) and Serpukhovian (C 1 sr). However, their geochemical and geological characteristics have not been studied systematically. To assess the source rocks and reveal the hydrocarbon generation potential, the depositional paleoenvironment and distribution of C 1 v and C 1 sr source rocks were studied using total organic carbon (TOC) content, Rock-Eval pyrolysis and vitrinite reflectance (Ro) data, stable carbon isotope data, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis data. The data were then compared with well logging data to understand the distribution of high-quality source rocks. The data were also incorporated into basin models to reveal the burial and thermal histories and timing of hydrocarbon generation. The results illustrated that the average residual TOC contents of C 1 v and C 1 sr were 0.79% and 0.5%, respectively, which were higher than the threshold of effective carbonate source rocks. Dominated by type-III kerogen, the C 1 v and C 1 sr source rocks tended to be gas-bearing. The two source rocks were generally mature to highly mature; the average Ro was 1.51% and 1.23% in C 1 v and C 1 sr, respectively. The source rocks were deposited in strongly reducing to weakly oxidizing marine–terrigenous environments, with most organic material originating from higher terrigenous plants and a few aquatic organisms. During the Permian, the deep burial depth and high heat flow caused a quick and high maturation of the source rocks, which were subsequently uplifted and eroded, stopping the generation and expulsion of hydrocarbons in the C 1 v and C 1 sr source rocks. The initial TOC fitted by the △logR method was recovered, and it suggests that high-quality source rocks (TOC ≥ 1%) are mainly distributed in the northern and central local structural belt. [ABSTRACT FROM AUTHOR]

The Pearl River Mouth Basin in the South China Sea has accumulated >2 km of Eocene sediments in its deep basin, and has become the exploration focus due to the recent discoveries of the HZ25-7 oil field in the Eocene Wenchang (E 2 w) Formation. In this study, the geochemical characteristics of potential source rocks and petroleum in the HZ25-7 oil field are investigated and the possible origins and accumulation models developed. The analytical results reveal two sets of potential source rocks, E 2 w and Enping (E 2 e) formations developed in the study area. The semi-deep-to-deep lacustrine E 2 w source rocks are characterized by relatively low C 29 steranes, low C 19 /C 23 tricyclic terpane (<0.6), low C 24 tetracyclic terpane/C 30 hopane (<0.1), low trans-trans-trans -bicadinane (T)/C 30 hopane (most <2.0), and high C 30 4-methyl sterane/ΣC 29 sterane (>0.2) ratios. In contrast, the shallow lacustrine and deltaic swamp-plain E 2 e source rocks are characterized by relatively high C 29 steranes, high C 19 /C 23 tricyclic terpane (>0.6), high C 24 tetracyclic terpane/C 30 hopane (>0.1), variable yet overall high T/C 30 hopane, and low C 30 4-methyl sterane/ΣC 29 sterane (<0.2) ratios. The relatively low C 19 /C 23 tricyclic terpane ratios (mean value: 0.39), low C 24 tetracyclic terpane/C 30 hopane ratios (mean value: 0.07), high C 30 4-methyl sterane/ΣC 29 sterane ratios (mean value: 1.14), and relatively high C 27 regular sterane content of petroleum in the HZ25-7 oil field indicate that the petroleum most likely originated from the E 2 w Formation mudstone in the Huizhou Depression. One stage of continuous charging is identified in the HZ25-7 oil field; oil injection is from 16 Ma to present and peak filling occurs after 12 Ma. Thin sandstone beds with relatively good connectivity and physical properties (porosity and permeability) in the E 2 w Formation are favorable conduits for the lateral migration of petroleum. This petroleum accumulation pattern implies that the E 2 w Formation on the western and southern margins of the Huizhou Depression are favorable for petroleum accumulation because they are located in a migration pathway. Thus exploration should focus in these areas in the future. [ABSTRACT FROM AUTHOR]

Shales from the Lower Silurian Longmaxi Formation in the Sichuan Basin are among the most important shale gas reservoirs in China, and have been investigated because of their great shale gas potential. To understand the pore structure and fractal characteristics of the shales, a series of experiments was conducted on core samples from the Lower Silurian Longmaxi Formation in the Sichuan Basin of China, including X-ray diffraction (XRD), total organic carbon (TOC) content and vitrinite reflectance (Ro) analysis, field emission-environmental scanning electron microscope (FE-ESEM) observation, and low-pressure N 2 adsorption-desorption experiments. Frenkel-Halsey-Hill (FHH) method was applied to calculate fractal dimensions. In addition, the pore genesis, the relationships between composition and thermal maturity, the pore structure parameters, and the fractal dimensions are discussed. FE-ESEM observation results show that the Longmaxi Formation shales are dominated by organic-matter (OM) pores along with interparticle (interP) pores, intraparticle (intraP) pores and fracture pores. This study identified the fractal dimensions at relative pressures of 0–0.45 and 0.45–1 as D 1 and D 2 respectively. D 1 ranged from 2.60 to 2.71 and D 2 ranged from 2.71 to 2.82. D 1 was typically smaller than D 2 , indicating that the smaller pores in shales were more homogeneous than the larger ones. The formation of these OM pores is owing to kerogen deformation during the thermal maturation, which results in a large number of nanopores. The pore structure of the Longmaxi Formation shales is primarily controlled by TOC content and thermal maturity. TOC content is a controlling factor on the fractal dimensions as it exhibited positive correlations with D 1 and D 2 . Fractal dimensions are useful for the characterization of the pore structures complexity of the Longmaxi Formation shales because D 1 and D 2 correlate well with pore structure parameters as they both increase with the increase of surface area and the decrease of average pore diameter. [ABSTRACT FROM AUTHOR]

Tight sandstone gas is regarded as a significant unconventional resource. One intriguing problem for tight sandstone gas accumulation is how gas was trapped in the tight-sandstone reservoirs. In this paper, we attempted to look for a reasonable explanation for this, which has attached great theoretical and practical significances. In this study, a dynamic equilibrium equation characterizing the critical buoyancy threshold (CBT) was established based on physical experiments, and the formation mechanism of tight gas and CBT were discussed. The explanations were further testified through the tight-gas reservoirs of the Upper Paleozoic Ordos Basin. A dynamical CBT occurs during the charge process in tight-gas reservoirs, and water- gas inversion with separating phases is dominated before natural gas reaches the CBT. At this stage, the driving force for gas movement is abnormal gas pressure, and almost no buoyancy exists. The pore throats, smaller than those corresponding to the CBT, enable the formation water to resist the gas to move upward. As a result, with no water-gas displacements, the gas migrates upward in a piston-like style forming continuous gas reservoir. The corresponding pore throat and depth of the CBT are geologically significant, which indicates a critical condition. However, the CBT condition is not constant and varies significantly according to the geologic setting. The case study of the tight-gas reservoirs of the Upper Paleozoic Ordos Basin indicates that the critical pore-throat radius was 1.77 µm by the end of the Early Cretaceous, with the corresponding critical permeability and porosity value of 0.686 mD and 10.6%, respectively. In contrast, its current counterparts are 1.33 µm, 0.39 mD, and 8.6%, respectively, which indicates that the original tight sandstone gas reservoir has shrunk during the geologic history. [ABSTRACT FROM AUTHOR]

The petroliferous Permian system of the Junggar Basin in northwest China is predominantly a conventional oil exploration region. To assist in the unconventional tight oil reservoir exploration, the hydrocarbon expulsion potential of source rocks and its influence on the tight oil distribution of the Middle Permian Lucaogou Formation (P 2 l) in the Jimsar Sag are discussed. The researches are based on geological and geochemical characteristics such as the distribution, abundance, type, thermal maturity of source rocks, and “sweet spots”. Meanwhile, hydrocarbon expulsion intensity and quantity were evaluated with hydrocarbon generation potential method. The Middle Permian Lucaogou source rocks have wide distribution and high thickness (mostly thicker than 100 m). The source rocks exhibit low to high TOC (mainly ranges from 1.0 wt% to 5.0 wt%) and primarily contain type-II kerogen, resulting in significant hydrocarbon expulsion potential under moderate mature thermal evolution (R o mainly ranges from 0.55% to 1.05%). According to the hydrocarbon expulsion modelling, the source rocks reached the hydrocarbon expulsion threshold at 0.73% R o and the hydrocarbon expulsion rate became greatest at 0.9% R o . The comprehensive hydrocarbon expulsion efficiency was approximately 48%. About 18.7×10 8 t of hydrocarbons were expulsed from the Lucaogou source rocks in the Jimsar Sag. The hydrocarbon expulsion intensities are relatively large and rang from 150×10 4 t/km 2 to 400×10 4 t/km 2 in the center and west area of the Sag, where is the superimposed area of source rock thicker than 200 m, TOC greater than 3.5% and R o higher than 0.85% in the sag. There are 17 exploratory wells obtaining commercial oil flow in the Sag. 16 of the 17 exploratory wells are located in the area with hydrocarbon expulsion intensity greater than 50×10 4 t/km 2 . By contrast, other exploratory wells outside the hydrocarbon expulsion coverage area have slight show of oil, even dry. In addition, there is positive correlation between the hydrocarbon expulsion intensity and the daily output of well testing. It is concluded that the tight oil exploration should focus on the area with hydrocarbon expulsion intensity greater than 100×10 4 t/km 2 , which is a favorable area for commercial hydrocarbon accumulation and exploration. [ABSTRACT FROM AUTHOR]

The lower limit of hydrocarbon accumulation in the Kuqa Depression is investigated. The Kuqa Depression in the Tarim Basin is one of the largest gas-producing regions in China. High yields of hydrocarbons have been found in depths below 7000 m, which suggests that the deep strata have great potential for hydrocarbon exploration. However, for effective deep exploration, the effective limits such as maximum depth or minimum physical properties of reservoirs have not yet been established. The lower limit of hydrocarbon accumulation, i.e. the conditions at which hydrocarbon accumulation is no longer possible, is determined by analysing the characteristics of effective reservoir space, reservoir permeability, fluid potential differences inside and outside the reservoir, and drilling results of exploratory wells in actual geological conditions. Research shows that the lower limit of hydrocarbon accumulation is reached when irreducible water saturation reaches 100%, reservoir permeability reaches as low as 0.01 × 10−3 µm2, fluid potential ratio of outside to inside the reservoir (fluid potential ratio of the surrounding rock to sandstone reservoir) is less than 5.5, and exploratory wells reach reservoirs that are all dry layers. From these four aspects, the lower limit of hydrocarbon accumulation corresponds with reservoir porosity of approximately 2.5%, permeability of approximately 0.01 × 10−3 µm2, pore throat radius of approximately 0.003-0.004 µm, and burial depth of approximately 8200 m. Determining the lower limit of hydrocarbon accumulation in the Kuqa Depression will contribute to predicting favourable exploration areas and evaluating effective resources. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Deep marine carbonate oil and gas resources in the Sichuan Basin are currently key exploration areas. As an important tectonic thermal event, the Emeishan mantle plume has an important influence on the source rocks under the Middle-Lower Permian in the Sichuan Basin. Therefore, the better understanding of the effect of Emeishan basalt eruption on source rock is vital for the exploration of marine oil and gas resources. On the basis of the thermal history study of the Sichuan Basin, this study restored the thermal maturity of the algal dolomite of the Sinian (Ediacaran) Dengying Formation in the Sichuan Basin in normal evolution. Using the hydrocarbon generation potential variation method, the hydrocarbon generation and expulsion characteristics of normally evolved algal dolomite and algal dolomite affected by basalt eruption were calculated. Through comparison, the influence of the Emeishan mantle plume on the algal dolomite of the Sinian Dengying Formation is analyzed. The research indicates that the thermal effect of the Emeishan mantle plume promotes the maturation of algal dolomite, which leads to the shortening of the oil generation period and the early arrival of the gas generation peak. The main hydrocarbon expulsion period of the normally evolved algal dolomite should be from the end of the Triassic to the end of the Jurassic, while the main hydrocarbon expulsion period of the algal dolomite affected by basalt eruption was advanced to the end of Silurian to the end of Triassic. In the current period, due to the high maturity of algal dolomite, the Emeishan mantle plume has little impact on the total resources. • Influence of Emeishan mantle plume on hydrocarbon generation and expulsion characteristics of algal dolomite of Sinian Dengying Formation. [ABSTRACT FROM AUTHOR]

In total, 2.37 million tons of marine crude oil originating from mixed source rocks has been discovered in the Tarim basin. Geological and geochemical analyses have confirmed that these mixed hydrocarbons are mainly from two sets of source rocks, including the Cambrian – Lower Ordovician and Middle-Upper Ordovician hydrocarbon source rocks. In this study, we determined the set of source rocks primarily responsible for the mixed hydrocarbons and the next location to be explored. Differences in n-alkane carbon isotopes in end-member oils from Cambrian–Lower Ordovician and Middle-Upper Ordovician source rocks were examined. A material balance model and simulation methods were used to evaluate the relative amounts contributed by each source. The results from known reserves in the Tazhong area show that the mixing ratio or contribution is up to 65% from Cambrian–Lower Ordovician source rocks and is generally higher than that from Middle-Upper Ordovician source rocks. The discovery of deep hydrocarbons has caused the total oil contribution from the Cambrian–Lower Ordovician to increase. The mixing ratio of Cambrian–Lower Ordovician oil varies depending on the well, formation, and block. It increases from west to east horizontally and from top to bottom vertically. Hydrocarbons from Cambrian–Lower Ordovician source rocks migrate upward along faults, and the mixing ratio decreases as the distance from the oil source fault increases. Favorable areas for Cambrian–Lower Ordovician hydrocarbon exploration are deep layers and areas near the fault zone that are connected to deep layers. The material balance model for carbon isotopes and evaluation methods for relative contributions considered differences in relative concentration and carbon isotope structure of n-alkanes. Herein, new methods for the identification and evaluation of hydrocarbons in the petroleum system of this superimposed basin are presented. [ABSTRACT FROM AUTHOR]

Combined with the actual geological settings, tight oil is the oil that occurs in shale or tight reservoirs, which has permeability less than 1 mD and is interbedded with or close to shale, including tight dolomitic oil and shale oil. The Fengcheng area (FA), at the northwest margin of the Junggar Basin, northwest China, has made significant progress in the tight oil exploration of the Fengcheng (P1f) Formation recently, which indicates that the tight oil resources have good exploration prospects. Whereas the lack of recognition of hydrocarbon generation and expulsion characteristics of Permian P1f source rocks results in the misunderstanding of tight oil resource potential. Based on the comprehensive analysis of geological and geochemical characteristics of wells, seismic inversion, sedimentary facies, tectonic burial depth, etc., the characteristics of P1f source rocks were investigated, and the horizontal distributions of the following aspects were predicted: the thickness of source rocks, abundance and type of organic matter. And on this basis, an improved hydrocarbon generation potential methodology together with basin simulation techniques was applied to unravel the petroleum generation and expulsion characteristics of P1f source rocks in FA. Results show that the P1f source rocks distribute widely (up to 2039 km2), are thick (up to 260 m), have high total organic content (TOC, ranging from 0.15 to 4 wt%), are dominated by type II kerogen and have entered into low mature-mature stage. The modeling results indicate that the source rocks reached hydrocarbon generation threshold and hydrocarbon expulsion threshold at 0.5% Ro and 0.85% Ro and the comprehensive hydrocarbon expulsion efficiency was about 46%. The amount of generation and expulsion from the P1f source rocks was 31.85 × 108 and 15.31 × 108 t, respectively, with a residual amount of 16.54 × 108 t within the source rocks. Volumetrically, the geological resource of shale oil is up to 15.65 × 108 t. Small differences between the amounts calculated by the volumetric method compared with that by hydrocarbon generation potential methodology may be due to other oil accumulations present within interbedded sands associated with the oil shales. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Replacing coal with cleaner hydrocarbon resources is a viable solution to reach carbon neutrality goals over time, especially given the current lack of growth in green energy resources (e.g., hydro, solar, wind, geothermal) globally. Hydrocarbon resources offer a low-carbon, low-waste, large-scale, rapid solution. Dynamic continuous hydrocarbon accumulation (DCHA) exists widely in sandstone and shale reservoirs and has enormous resource potential. However, understanding the accumulation mechanisms and models of DCHAs in different structural locations is highly challenging. In this study, we first extensively review hydrocarbon resources derived from hydrocarbon accumulation and identify key scientific issues and theories. We then take the well-known typical DCHA in the Bohai Bay Basin in eastern China as a case, and combine multiple analytical methods, such as biomarker analysis, petroleum and aqueous inclusion analysis, quantitative fluorescence analysis, microscopic thin section observations, and basin modeling, to investigate petroleum sources, charging times, and charging forces. The results of our study show that the DCHA is a superimposed accumulation of multiple petroleum reservoir types with multiple petroleum sources, charging times, charging forces, and modes. Petroleum reservoirs at high structural locations are conventional trap reservoirs accumulated by buoyancy in the early stage, with long migration distances. Petroleum reservoirs at the sag location are deep unconventional petroleum reservoirs accumulated by the petroleum generation pressurization force in the middle stage, with short migration distances. Petroleum reservoirs at the slope location are superpositions of conventional traps and deep unconventional petroleum reservoirs in the middle and late stages, with moderate migration distances. [ABSTRACT FROM AUTHOR]

In recent years, new oil reservoirs have been discovered in the Eocene tight sandstone of the Huilu area, northern part of the Pearl River Mouth basin, South China Sea, indicating good prospects for tight oil exploration in the area. Exploration has shown that the Huilu area contains two main sets of source rocks: the Eocene Wenchang (E 2 w) and Enping (E 2 e) formations. To satisfy the requirements for further exploration in the Huilu area, particularly for tight oil in Eocene sand reservoirs, it is necessary to re-examine and analyze the hydrocarbon generation and expulsion characteristics. Based on mass balance, this study investigated the hydrocarbon generation and expulsion characteristics as well as the tight oil resource potential using geological and geochemical data and a modified conceptual model for generation and expulsion. The results show that the threshold and peak expulsion of the E 2 w source rocks are at 0.6% vitrinite reflectance and 0.9% vitrinite reflectance, respectively. There were five hydrocarbon expulsion centers, located in the western, eastern, and northern Huizhou Sag and the southern and northern Lufeng Sag. The hydrocarbon yields attributed to E 2 w source rocks are 2.4 × 10 11 tons and 1.6 × 10 11 tons, respectively, with an expulsion efficiency of 65%. The E 2 e source rock threshold and peak expulsion are at 0.65% vitrinite reflectance and 0.93% vitrinite reflectance, respectively, with hydrocarbon expulsion centers located in the centers of the Huizhou and Lufeng sags. The yields attributed to E 2 e source rocks are 1.1 × 10 11 tons and 0.2 × 10 11 tons, respectively, with an expulsion efficiency of 20%. Using an accumulation coefficient of 7%–13%, the Eocene tight reservoirs could contain approximately 1.3 × 10 10 tons to 2.3 × 10 10 tons, with an average of 1.8 × 10 10 tons, of in-place tight oil resources (highest recoverable coefficient can reach 17–18%), indicating that there is significant tight oil potential in the Eocene strata of the Huilu area. [ABSTRACT FROM AUTHOR]

The process and mechanisms of secondary hydrocarbon migration in the Huixi half-graben, Pearl River Mouth Basin, were investigated on the basis of geological analysis of the strata and study of the porosity and permeability of the reservoir rocks, fluid potential, oil properties, and geochemistry of oil-source correlation. The results suggest that the hydrocarbons of the Zhujiang Formation in the Huixi half-graben were derived from source rocks of the Eocene Wenchang Formation and the Eocene-Oligocene Enping Formation in the Huizhou Sag. The hydrocarbons migrated laterally from northeast to southwest. The sandstone in the upper member of the Zhujiang Formation exhibited superior physical properties (porosity and permeability) and connectivity than the lower member. Thin sandstone beds with good physical properties and stable distribution in the upper member of the Zhujiang Formation were the main carrier beds for lateral hydrocarbon migration. [ABSTRACT FROM AUTHOR]

Although it has been shown that the potential of tight-sand gas resources is large, the research into the mechanisms of hydrocarbon charging of tight sandstone reservoirs has been relatively sparse. Researchers have found that there is a force balance during hydrocarbon charging, but discriminant models still have not been established. Based on the force balance conditions observed during gas migration from source rocks to tight sandstone reservoirs, a calculation formula was established. A formula for identifying effective source rocks was developed with the gas expulsion intensity as the discrimination parameter. The critical gas expulsion intensity under conditions of various burial depths, temperatures, and pressures can be obtained using the calculation formula. This method was applied in the Jurassic tight sandstone reservoirs of the eastern Kuqa Depression, Tarim Basin, and it was calculated that the critical expulsion intensity range from 6.05 × 108 m3/km2 to 10.07 × 108 m3/km2. The critical gas charging force first increases with depth and later decreases with greater depths. The distribution range of effective gas source rocks and total expelled gas volume can be determined based on this threshold. This method provides new insight into and method for predicting favourable tight-sand gas-bearing regions and estimating their resource potentials. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Although a large amount of petroleum has been discovered in the Tarim Basin, the origin of the oils is still disputed. Yingmaili Oilfield was selected as a case study to further examine hydrocarbon generation and accumulation in the basin. Detailed geochemical studies were conducted to unravel the formation mechanisms. The oils in the field correlate well with Middle–Upper Ordovician (O 2+3 ) rocks (or similar rocks). Oil–oil and oil–source rock correlation by maturity parameters of saturate and aromatic hydrocarbons suggest that the Yingmaili and adjacent oils are primarily comparable to source rocks deeper than 6000 m. The results also showed that carbonates might be important potential source rocks as indicated by a relatively high abundance of aromatic sulfur compounds and relatively high C 29 /C 30 -hopane and C 35 /C 34 -hopane ratios of the oils, which are typical features of carbonate source rocks. We suggest the oil from wells YG2 and YG2-1C are more suitable to be end members from the O 2+3 source rocks (or similar rocks) than the oil from well YM2 as previously suggested. We also suggest that the previous model of hydrocarbon migration along the Ordovician unconformity should be revised. [ABSTRACT FROM AUTHOR]

The Middle Eocene Wenchang (WC) formation in the Huizhou depression of the Pearl River Mouth basin is an important geological element for petroleum exploration in the South China Sea. In this study, the geological and geochemical characteristics of the WC source rocks, including the distribution, sedimentary environment, organic matter type, hydrocarbon generation potential and thermal maturity were investigated. Hydrocarbon generation history and hydrocarbon expulsion intensity and quantities were evaluated through one dimensional basin modelling and an improved hydrocarbon generation potential methodology. The WC source rocks are widely distributed and have an elevated thickness (mostly thicker than 200 m, with a maximum thickness greater than 1000 m); they were deposited in a lacustrine weak oxidation–weak reduction sedimentary environment with low salinity. The source rocks have low to high organic matter abundance (TOC mainly ranges from 0.27 to 3.59 wt% with an average of 1.39 wt%) dominated by II 1 –type and II 2 –type kerogen, resulting in significant hydrocarbon generating potential under moderate–high thermal evolution (R o mainly ranges from 0.65% to 1.2% on the margins of sags and greater than 1.2% in the centre of sags). The source rocks became mature (R o = 0.5%) in the early Miocene and began to generate hydrocarbons. They reached the hydrocarbon expulsion threshold at 0.8% R o and the hydrocarbon expulsion rate became greatest at 1.1% R o . The expulsion intensities in the H26 sag, H21 sag, X24 sag and X23 sag are relatively large, with their greatest expulsion intensities being 104 × 10 6 , 71 × 10 6 , 62 × 10 6 and 53 × 10 6 t/km 2 , respectively. The comprehensive hydrocarbon expulsion efficiency was approximately 75%. About 690.1 × 10 8 t of hydrocarbons were expulsed from the WC source rocks in the Huizhou depression. [ABSTRACT FROM AUTHOR]

The Qibei sag (QS), at the western region of the Bohai Bay Basin (BBB), eastern China, has made significant progress in the deep gas exploration recently, indicating that the deep gas reservoirs have good exploration prospects. The deep gas reservoirs within the second member of the Shahejie Formation of the Paleogene (E 2 s 2 ) are buried at approximately 5000 m. The sources and hydrocarbon accumulation mechanisms were investigated by analyzing the regional structure, integrating geologic and geochemical parameters of source rocks and natural gas. Analyzing results show that, in the QS and adjacent areas (QSAA), three sets of source rocks (E 2 s 1 , E 2 s 2 , E 2 s 3 ) were developed, which differ in organic chemical characters, but have abundant organic matters in fair-good standard. Affected by burial history, E 2 s 3 source rocks have entered into the high mature-over mature stage and E 2 s 1 source rocks are mainly in mature stage. The methane content of natural gas (78%∼91%), the methane carbon isotope value (−41.3‰∼−34.6‰) and the positive carbon isotope sequence (δ 13 C 1 <δ 13 C 2 <δ 13 C 3 <δ 13 C 4 ) indicate that the deep gas is organic origin and mainly from thermogenic gas. The relationship between light hydrocarbon character parameters and thermal evolution degree indicates that the deep gas in the QS is mainly from E 2 s 2 and E 2 s 3 source rocks of the sag. Since the different hydrocarbon generation history, there exist two hydrocarbon accumulation periods in QS, and the early period is characterized by oil accumulation, which occurs at the shallows, such as the Binshen6 block; the late period is characterized by gas accumulation, which happens in the depth mostly, such as the Binshen22 block. The different hydrocarbon generation history results in the relative enrichment of gas in the depth and the phenomenon of oil–gas inversion. [ABSTRACT FROM AUTHOR]