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The pore structure of a carbonate reservoir determines the fluidity and storage characteristics of hydrocarbon, and it is vital for reservoir evaluation and oil and gas production. The Visean and Serpukhovian carbonate reservoirs in the Marsel exploration area of the Chu-Sarysu Basin possess a complex lithology and pore structure, which significantly affect production efficiency. The pore structure of the reservoirs was investigated in this study via X-ray diffraction, conventional petrophysical measurements, cast thin section analysis, scanning electron microscopy (SEM), imaging logging, and high-pressure mercury injection (HPMI). The findings show that the lithology in this area mainly comprises fine-grained limestone, bioclastic limestone, and silty limestone; moreover, the mineral assemblage in this region is markedly different. The types of carbonate reservoir pore spaces are complex and diverse. Primary pores are mainly biological cavity pores with small pore size, mostly isolated individuals, poor connectivity, and weak percolation capacity. Secondary dissolution pores, which are dominated by intergranular dissolution pores, intragranular dissolution pores, and microfractures, are the dominant storage spaces in the Carboniferous carbonate reservoirs. Through an innovative approach of combining imaging logging and cast thin section analysis, it was concluded that mainly three types of fractures exist, namely conductivity fractures, fissures, and resistive fractures. Moreover, structural fractures in this area are relatively developed, which substantially increases the percolation capacity of the reservoirs and it is conducive to oil and gas flow. However, some structural fractures are filled with calcite, destroying the effectiveness of the fractures and reducing the percolation capacity. Based on capillary pressure curves, the samples were divided into three types, of which the physical properties decrease in the order of type I > type III > type II. The fractal dimensions obtained from HPMI and SEM data characterize the complexity of the reservoir microstructure in the study area. The formation of high-porosity and high-permeability zones was caused by quasi-syngenetic dolomitization and syngenetic–quasi-syngenetic dissolution. In this study, the pore structure of carbonate reservoirs in the area was evaluated quantitatively, which is highly significant to improving oil and gas recovery. [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]

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]

Hydrocarbon migration mechanism in deep Ordovician reservoir of the Halahatang area, Tarim Basin, is investigated based on the analysis of the geological setting and geochemical parameters. The parameters successfully analyzed include oil–gas properties, crude oil maturity [Ts/(Ts + Tm) ratio], gas carbon isotopic ratios, and trace elements (B, V, Ni, and Cu) in crude oil. The results show that hydrocarbons migrate into the Ordovician of the Halahatang area from the Manjiaer Depression and the Halahatang area itself through unconformities and faults. The overall hydrocarbon migration orientation in the area is generally from south to north. The oil and gas migration process determine parameters vary regularly from south to north as follows: oil density, V/Ni ratio, Ni/Cu ratio, and concentrations of V, Ni and (Ni + B) in crude oil regularly increase, whereas Ts/(Ts + Tm), gas/oil ratio, and dryness coefficient gradually decrease, and methane show a trend toward depletion of heavy carbon isotopes. Additionally, there exist obvious anomalies of geochemical parameter values in the Repu3 well block, which indicates that a local fault intersection could provide a favorable hydrocarbon filling point. Based on these findings, further exploration activities in the deep Ordovician reservoir of the Halahatang area should be conducted in the southern part of the Halahatang area along the oil charging pathways, and fault intersection zones should be emphasized. [ABSTRACT FROM AUTHOR]

The hydrocarbon accumulation model of Ordovician reservoirs in the Tazhong area, Tarim Basin, was investigated based on the analysis of the key factors controlling hydrocarbon generation, migration, and enrichment process and history. The results show that the hydrocarbon generation processes are controlled by Cambrian-Lower Ordovician and Upper Ordovician source rocks distributed in the Tazhong area and the Manjiaer Sag. Vertical hydrocarbon migration distance is controlled by the vertical source-reservoir distribution, faults, and caprocks. Structure played a major role in the lateral hydrocarbon migration along NW-SE direction. Lateral migration pathways were governed by unconformities, transport faults, and reef-beach body reservoirs. A part from the sedimentary control, hydrocarbon enrichment is controlled by recent tectonic events. The hydrocarbon charging points refer to the intersection of northeast- and northwest-trending faults. Two types of hydrocarbon accumulation models could be retained, (a) mixed-sourced area model, in which the hydrocarbons are sourced from both the Tazhong area and the Majiaer Sag, and (b) single-sourced area model, in which the hydrocarbons are only sourced from the Tazhong area itself. [ABSTRACT FROM AUTHOR]

The thermal history of sedimentary basins is a key factor for hydrocarbon accumulation and resource assessment, and is critical in the exploration of lithospheric tectono-thermal evolution. In this paper, the Cenozoic thermal histories of nearly 200 wells and the Mesozoic thermal histories of 15 wells are modeled based on the vitrinite reflectance and apatite fission track data in Bohai Bay Basin, North China. The results show that the basin experienced Early Cretaceous and Paleogene heat flow peaks, which reveals two strong rift tectonic movements that occurred in the Cretaceous and the Paleogene in the basin, respectively. The thermal evolution history in Bohai Bay Basin can be divided into five stages including (1) the low and stable heat flow stage from the Triassic to the Jurassic, with the heat flow of 53 to 58 mW/m; (2) the first heat flow peak from the Early Cretaceous to the middle of the Late Cretaceous, with a maximum heat flow of 81 to 87 mW/m; (3) the first post-rift thermal subsidence stage from the middle of the Late Cretaceous to the Paleocene, with the heat flow of 65 to 74 mW/m at the end of the Cretaceous; (4) the second heat flow peak from the Eocene to the Oligocene, with a maximum heat flow of 81 to 88 mW/m; and (5) the second thermal subsidence stage from the Neogene to present, with an average heat flow of 64 mW/m. [ABSTRACT FROM AUTHOR]

The upper limit of maturity for hydrocarbon generation (ULMHG) is represented by vitrinite reflectance (VR) that corresponds to the point at which the amount of further hydrocarbon generation is less than 1 % of the total hydrocarbon generation. Six methods were employed to study the ULMHG of the Cambrian and Ordovician carbonate source rocks in the Tarim Basin Platform (TBP), based on the mechanism of hydrocarbon generation, retention, and expulsion. These methods are reduction in atomic ratio for organic elements, variation in hydrocarbon generation potential, optimum simulation of organic materials based on material balance principle, thermal simulation experiment, variation in hydrocarbon retention amount, and variation in hydrocarbon expulsion amount. Studies showed that, although there are some differences between the results obtained using different methods, these differences are small. The ULMHG in the TBP varies from 4.10 to 4.84 % with an average value of 4.55 %, which is in good agreement with the characteristics of hydrocarbon generation under actual geological conditions. The VR of the Cambrian and Ordovician carbonate source rocks in the Tazhong uplift, the Bachu uplift, and the Tabei uplift ranges from 0.64 to 2.34 %, indicating that these areas have not reached the ULMHG. The VR in the Tadong area is very close to the ULMHG, and the VR of most source rocks in the Manjiaer sag has exceeded the ULMHG. In summary, the carbonate source rocks in the TBP have generated large amounts of hydrocarbons and still have high hydrocarbon generation potential to form large-scale oil and gas reservoirs, indicating good hydrocarbon exploration prospects in the deep parts of the TBP. Therefore, the study of ULMHG of carbonate source rocks is of great significance for evaluating the oil and gas resource potential and predicting favorable exploration areas in deep or carbonate rock formations. [ABSTRACT FROM AUTHOR]

The Liaohe (辽河) depression is an important part of the Bohai (渤海) Bay Basin, and the Bohai Bay Basin located in the center of lithospheric destruction and thinning in the eastern North China Craton. The North China lithospheric thinning activities have been verified from evidences of structural geology, petrology, geochemistry and geophysics, but there are still some controversies on their timing, mechanism and controlling factors. The sedimentary basin is a thin-bedded geologic unit with a limited distribution in the upper lithosphere, and its formation and evolution represent the shallow response of the deep geodynamic process. Therefore, its thermal evolution is closely related to the deep dynamic conditions. In this article, the Mesozoic-Cenozoic thermal history of the Liaohe depression is reconstructed using the vitrinite reflectance and apatite fission track data. Meanwhile 'thermal' lithospheric thicknesses in the Mesozoic and Cenozoic are calculated using the geothermic method on the basis of the above thermal history results. The results show that the Liaohe depression has under- peaks, 81 and 83 mW/m, respectively, corresponding to two strong rift movements. Accordingly, the depression has experienced two different levels of thinning processes in the Cretaceous and the Paleogene since the Mesozoic, and the 'thermal' lithospheric thicknesses were 60 and 50 km, respectively. This may reveal that the depression has experienced two large-scale destructions. The work may provide valuable geothermal evidence for initial revealing the time, process and stage of the lithospheric thinning in the Liaohe depression. [ABSTRACT FROM AUTHOR]

An unconventional, continuous petroleum system consists of an accumulation of hydrocarbons that is found in low-matrix-permeability rocks and contain large amounts of hydrocarbons. Tight-sand gas in the Jurassic and shale gas within the fifth member of Xujiahe Formation (Tx) are currently regarded as the most prolific emerging unconventional gas plays in China. The conventional and systematical evaluation of Tx source rocks was carried out for the first time in the western Sichuan basin (WSD). Hydrocarbon generation and expulsion characteristics (including intensity, efficiency, and amount) of Tx source rocks were investigated. Results show that Tx source rocks are thick (generally >200 m), have high total organic content (TOC, ranging from 2.5 to 4.5 wt%), and dominated by III-type kerogen. These favorable characteristics result in a great hydrocarbon generating potential under the high thermal evolution history ( R > 1.2%) of the area. An improved hydrocarbon generation potential methodology was applied to well data from the area to unravel the hydrocarbon generation and expulsion characteristics of Tx source rocks in the WSD. Results indicate that the source rocks reached hydrocarbon expulsion threshold at 1.06% R and the comprehensive hydrocarbon expulsion efficiency was about 60%. The amount of generation and expulsion from Tx source rocks was 3.14 × 10 and 1.86 × 10 t, respectively, with a residual amount of 1.28 × 10 t within the source rocks. Continuous-type tight-sand gas was predicted to develop in the Jurassic in the Chengdu Sag of the WSD because of the good source-reservoir configuration (i.e., the hydrocarbon generation and expulsion center was located in Chengdu Sag), the Jurassic sandstone reservoirs were tight, and the gas expelled from the Tx source rocks migrated for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from Tx source rocks is up to 9.3 × 10 t. The Tx gas shale has good accumulation potential compared with several active US shale-gas plays. Volumetrically, the geological resource of shale gas is up to 1.05 × 10 t. Small differences between the amounts calculated by volumetric method compared with that by hydrocarbon generation potential methodology may be due to other gas accumulations present within interbedded sands associated with the gas shales. [ABSTRACT FROM AUTHOR]

The process of organic matter transformation into oil and gas is also a balance process of hydrocarbon transformation. This article probes to distinguish the oil expulsion history from gas expulsion history based on the hydrocarbon generation, hydrocarbon residual, and hydrocarbon expulsion processes of the source rocks. In this method, the first step is to study the hydrocarbon expulsion rate by means of hydrocarbon generation potential method; the second step is to study the oil generation rate by means of the heating-pressuring experiment method; the third step is to study the oil residual rate by means of the mathematical method. The difference between the values of oil generation rate and oil residual rate is defined as the oil expulsion rate, while that between the values of hydrocarbon expulsion rate and the gas expulsion rate is defined as the gas expulsion rate. Then, combined with the geological parameters of source rocks, the oil and gas expulsion history can be obtained. This study on Es Source rocks, Nanpu Sag, Bohai Bay Basin, China shows that the primary expulsion period of Es source rocks is Guantao-Minghuazhen period. [ABSTRACT FROM AUTHOR]

This paper utilizes multi-methods, such as core observation, slice identification, isotope analysis, trace element analysis, fluid inclusion technique and so on, to study the causes of the dolomite in the Nanpu Sag and the origins of the dolomite reservoir. The study results show that the forming environment of dolomite in the Nanpu Sag is a fresh-water lake environment, and the dolomite is the product of dolomitization which is caused by volcanic thermal fluids in the Early Dongying period. With the development of intergranular pores in the dolomitization process, a lot of dissolved pores/vugs and fractures were formed by denudation in the later periods because of the influence of thermal fluids including the associated fluids of volcanic activities and the expelled hydrocarbon fluids of the source rocks. On the whole, these secondary dissolution spaces greatly enhance the reservoir ability of the dolomite, and there are enough reservoir spaces in the dolomite in the Nanpu Sag. [ABSTRACT FROM AUTHOR]

The concept of hydrocarbon accumulation in network was presented on basis of the overall analysis of the formation and evolution characteristics of the continental faulted basin and of the systemic research on the major controlling factors on the hydrocarbon accumulation. The hydrocarbon accumulation in network can be defined as hydrocarbon accumulation in a three-dimensional network system which is constituted by the hydrocarbon migration passages under multiple dynamics, following the hydrocarbon generation from source rocks. The research shows that the hydrocarbon accumulation in network is composed of four elements, i.e., hydrocarbon source (source rock kitchen), hydrocarbon accumulation terminal (trap), network pathway connecting source and terminal (transporting system), and network potential driving hydrocarbon migration in the network pathway (migration dynamics). Compared with other networks, hydrocarbon accumulation in network has three basic characteristics: the irreversible geological process of material and information flow in the network; the loss of material and information in the flow process in the network; the multiple dynamics in the flow process. Interaction of all the elements in the geological process can be called hydrocarbon accumulation in network. There are three basic models for hydrocarbon accumulation in network, that is, hydrocarbon accumulation in the network source area, hydrocarbon accumulation in the network pathway, and hydrocarbon accumulation in the network terminal. The key in the application of the hydrocarbon accumulation models in network in practice is to confirm the major accumulation stage and the function range of the four elements controlling the hydrocarbon firstly, to predict the profitable accumulation region by superposition of the favorable areas confirmed by four elements consequently, and to evaluate the oil-bearing property of the trap as well as confirm drilling targets. This paper takes the Dongying Depression in the Bohai Bay Basin as an application example. [ABSTRACT FROM AUTHOR]

Combined evidence from the outcrop and the fluid inclusion assemblage (FIA) analysis indicates that there exist two episodes of fluid flow controlled by the tectonic activity. The first episode was recorded mainly in the reservoir rock of the Honghuayuan Formation, representing the fluid flow of hydrocarbon charging. The second episode occurred mainly along the fault systems, representing the fluid movement when the ancient oil reservoir was destroyed. The host mineral morphology, homogeneous temperature, and salinity of the FIAs record an episodic fluid movement. Characters of high homogenous temperature, low salinity and a quick temperature variation of the first episode fluid flow may indicate an early-stage fluid eruption, and correspondingly, fine-grained calcite was formed. Temperature of the erupted fluid tended to decrease during its mixing with the upper formation fluid and finally had the same temperature as the upper formation. From then on, the temperature was rather steady and favored the growth of the coarse calcite. Due to this character of the temperature variation of the episodic fluid flow, we can use the homogenous temperature of the FIA of the coarse calcite to date the formation and the destruction time of the Majiang ancient oil reservoir. Episodic fluid flow was known for its inhomogeneous trapping, which resulted in the failure of dating according to the burial history. But taking a close look at its temperature variation, we think that the latest stage of fluid flow, characterized by steady state temperature and grow of the coarser crystals, can be used for dating. It will be of great value if this method is proved to be effective. The formation and the destruction time of the Majiang ancient oil reservoir were dated to be in the Indosinian Period and the late Yanshan-early Hymalayan Period respectively. This conclusion is in great discrepancy with the common accepted idea that the Majiang ancient oil reservoir was formed and destroyed during the Caledonian tectonic movement. Even so, this paper further discussed its reliability from the view of the source rock evolution and also the local tectonic evolution. [ABSTRACT FROM AUTHOR]

The buried-hill trend is a typical style of the petroliferous structure in the faulted basin of eastern China. The Xinglongtai structure trend in the Liaohe subbasin, Bohai Bay Basin is a buried faulted-hill trend resulting from the periodic faulting of the Tai’an-Dawa faults. The structure was cut by various faults, which are interlinked and constitute the conduits for petroleum migration. Petroleum accumulated in the Tertiary sandstones and pre-Tertiary basement of metamorphosed rocks through the migration pathways of the fault system. Petroleum in the structure was derived from different hydrocarbon kitchens to form a hybrid field complex. Petroleum charged the structure from different directions and accumulated in various reservoirs through different pathways in different times. The accumulations in the buried hill trend of Xingongtai are composed of two types of traps in the two stratigraphic systems: traps in the Tertiary sandstones were formed by the fault blocks and by the pinchout of the sandstone, and traps in the pre-Tertiary basement of metamorphosed rocks were formed by fissures in the inner part of the buried hills and by the unconformity near the surface of the buried hills. [ABSTRACT FROM AUTHOR]

The evolution of Tarim Basin experienced several tectonic movements, of which Caledonian movement caused a serious damage to reservoirs of Silurian. The earlier reservoirs size and the later hydrocarbon loss are two key factors to exploration prospect of Silurian. On the basis of net thickness of bituminous sandstone and recovery coefficient of crude oil converting to bitumen, the scale of the hydrocarbon loss of palaeo-accumulation in bitumen sandstone of Silurian was calculated (86.30×108t). The determination of net thickness of bitumen sandstone was completed mainly by several methods, such as physical property demarcation, observing lithology, grains with oil inclusion (GOI) technology, and quantitative grains fluorescence (QGF) technology. The samples of black sandstone and white sandstone were collected and compared by means of the methods mentioned above. The results showed that there was little hydrocarbon, or none, in the white sandstone. The recovery coefficient was obtained by the analyses of plentiful physical parameters of crude oil and bitumen sandstone, based on the hypothesis that nonhydrocarbon compounds and bitumen were invariable in the process of bitumen forming. The calculated result, which is more scientific and actual, indicates the exploration prospect is more promising in Silurian of the Tarim Basin. [ABSTRACT FROM AUTHOR]

The Liaohe Oilfield in the Liaohe Western Depression of the Bohai Gulf Basin is the third-largest oil producing province and the largest heavy oil producing oilfield in China. A total of 65 oil samples, 35 rock samples and 36 reservoir sandstone samples were collected and analyzed utilizing conventional geochemical and biogeochemical approaches to unravel the mechanisms of the formation of the heavy oils. Investigation of the oils with the lowest maturity compared with the oils in the Gaosheng and Niuxintuo oilfields indicates no apparent relation between the maturity and physical properties of the heavy oils. It is suggested that the heavy oil with primary origin is not likely the main mechanism responsible for the majority of the heavy oils in the Liaohe Western Slope. The absence and/or depletion of n-alkanes etc., with relatively low molecular weight and the occurrence of 25-norhopane series in the heavy oils as well as the relatively high acidity of the oils all suggest that the majority of the heavy oils once experienced secondary alteration. The fingerprints of the total scanning fluorescence (TSF) of the inner adsorbed hydrocarbons on the reservoir grains and the included hydrocarbons in fluid inclusions are similar to that of the normal oils in the area but are different from the outer adsorbed and reservoired free oils at present, further indicating that most of the heavy oils are secondary in origin. Analyses of bacteria (microbes) in 7 oil samples indicate that anaerobic and hyperthermophilic Archaeoglobus sp. are the dominant microbes relevant to oil biodegradation, which coincides with the shallow commercial gas reservoirs containing anaerobic bacteria derived gas in the Gaosheng and Leijia teotonic belts. The biodegradation most likely occurs at the water/oil interface, where the formation water is essential for microbe removal and nutrient transportation. We think that biodegradation, water washing and oxidization are interrelated and are the main mechanisms for the formation of the heavy oils. Biodegradation was the predominant process with water washing being a prerequisite, and oxidization acting as a metabolic manifestation. This study provides unique approaches for further investigation of the formation mechanisms of heavy oils in general, and may provide some important insight for the exploration of shallow biogas in the area. [ABSTRACT FROM AUTHOR]

This paper analyzes the quantitative grain fluorescence (QGF) and quantitative grain fluorescence on extract (QGF-E) properties of 101 rock samples by using quantitative grain fluorescence techniques. The samples are collected from five wells in tight sandstone and thin siltstone in the third sector of the Shahejie Formation in the Niuzhuang sag of the Dongying depression. It was observed that both the tight sandstone and thin siltstone show relatively high fluorescence intensity of hydrocarbon, which suggests that they are possibly good subtle oil-migration pathways in the present or geological time. These thin subtle oil-migration pathways afford important clues for the researches on hydrocarbon accumulation in lithological reservoirs in the middle and lower of E s 3 in deep sag zone, which has the hydrocarbon source from the upper of E s 4 when there is no apparent fault playing oil migration conduits to connect lithologic traps and deep source rocks. This study shows good prospect of QGF techniques in discriminating oil migration pathways and paleo-oil zones. The results of this study may be of great significance to the researches on hydrocarbon accumulation mechanism of subtle reservoirs in the Dongying depression and other areas. [ABSTRACT FROM AUTHOR]

Apatite Fission Track (AFT) data from the Songliao Basin indicates that the late stage tectonic movements in the Songliao Basin have zoning in space and episodes in time. The late stage tectonic movements started from the east part of the basin and migrated westward. AFT ages in the east part of the basin are older than those in the west part of the basin, suggesting that the uplift occurred earlier in the east than in the west. The denudation thickness in the east part of the basin is significantly greater than that in the centre and west. The thermal history evolved two episodes of rapid cooling and subsequent slow cooling processes. Age-depth relationship derived from the AFT data indicates a four-episode denudation history. Further Monte Carlo random simulation of the AFT data reveals the four changing points of the thermal evolution at 65 Ma, 43.5 Ma, 28 Ma and 15 Ma, respectively. The uplifting and denudation rates from different episodes of evolution are proportional to the plate convergence rate. Based on the above analyses and the regional geologic background, it is concluded that the late stage thermal events in the Songliao Basin are the far field response to the subduction of the Pacific Plate under the Eurasian Plate. The first episode of the rapid cooling probably started at the end of the Nenjiang Formation, climaxed at the end of the Cretaceous and ceased at the Late Eocene. The subsequent slow cooling lasts another 15 Ma. The first episode of the evolution is the far field response to the major episode of the Yanshan Movement and subsequent series of the tectonic reorganization, especially the directional change of the Pacific Movement and also the subduction of the Indian Plate underneath the Eurasian Plate. While the second episode of the evolution is the far field response to the extension and closure of the Sea of Japan. Extension led to the migration and converging of the mantle heat flow to the Sea of Japan and resulted in the rapid cooling of the Songliao Basin. [ABSTRACT FROM AUTHOR]

This study presents an overview on the geological setting and geochemical characteristics of Pleistocene shallow gas accumulations in eastern Qaidam Basin, NW China. Five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km² and 7.9 trillion cubic feet (tcf) of proven plus controlled gas reserves. The dominance of methane (>99.9%) and the δ 13C and δD values of methane (-68.51‰ to - 65.00‰ and - 227.55‰ to - 221.94‰, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (< 75°C). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments is made possible by the lake basin's high altitude (2600 -3000 m) , high water salinity (> 15%) and strong stratification. The deposition and extensive lateral occurrence of shore and shallow lake sands/silts in beach sand sheets and small sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals were provided by such factors as intermittent vertical variations in the sediment lithologies, hydraulic trapping due to mudstone water saturation, the hydrocarbon gradient created as a result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400 - 800-m-thick mudstones and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication. [ABSTRACT FROM AUTHOR]