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Tight gas exploration plays an important part in China’s unconventional energy strategy. The tight gas reservoirs in the Jurassic Shaximiao Formation in the Qiulin and Jinhua Gas Fields of central Sichuan Basin are characterized by shallow burial depths and large reserves. The evolution of the fluid phases is a key element in understanding the accumulation of hydrocarbons in tight gas reservoirs. This study investigates the fluid accumulation mechanisms and the indicators of reservoir properties preservation and degradation in a tight gas reservoir. Based on petrographic observations and micro-Raman spectroscopy, pure CH4inclusions, pure CO2inclusions, hybrid CH4–CO2gas inclusions, and N2-rich gas inclusions were studied in quartz grains. The pressure–volume–temperature–composition properties (PVT-x) of the CH4and CO2bearing inclusions were determined using quantitative Raman analysis and thermodynamic models, while the density of pure CO2inclusions was calculated based on the separation of Fermi diad. Two stages of CO2fluid accumulation were observed: primary CO2inclusions, characterized by higher densities (0.874–1.020 g/cm3) and higher homogenization temperatures (> 210 °C) and secondary CO2inclusions, characterized by lower densities (0.514–0.715 g/cm3) and lower homogenization temperatures: ~ 180–200 °C). CO2inclusions with abnormally high homogenization temperatures are thought to be the result of deep hydrothermal fluid activity. The pore fluid pressure (44.0–58.5 MPa) calculated from the Raman shift of C–H symmetric stretching (v1) band of methane inclusions is key to understanding the development of overpressure. PT entrapment conditions and simulation of burial history can be used to constrain the timing of paleo-fluid emplacement. Methane accumulated in the late Cretaceous (~ 75–65 Ma), close to the maximum burial depth during the early stages of the Himalayan tectonic event while maximum overpressure occurred at ~ 70 Ma, just before uplift. Later, hydrocarbon gas migrated through the faults and gradually displaced the early emplaced CO2in the reservoirs accompanied by a continuous decrease in overpressure during and after the Himalayan event, which has led to a decrease in the reservoir sealing capabilities. The continuous release of overpressure to present-day conditions indicates that the tectonic movement after the Himalayan period has led to a decline in reservoir conditions and sealing properties.

Oil reservoirs have been discovered in the Mesoproterozoic strata in the Liaoxi Depression, NE China. In order to determine the source of oil shows of the Mesoproterozoic Gaoyuzhuang Formation and their organic geochemical characteristics, eight source rocks and reservoir cores from the Mesoproterozoic Gaoyuzhuang Formation and four source rocks from the overlying Middle Jurassic Haifanggou Formation were geochemically analysed. The distribution patterns of normal alkanes, acyclic isoprenoids, hopanes, steranes and triaromatic steroids of the Mesoproterozoic hydrocarbons from Well N-1 are consistent with those of source rock extracts from the Mesoproterozoic Gaoyuzhuang Formation in the Well L-1. The molecular marker compositions of source rock extracts from the overlying Middle Jurassic Haifanggou Formation are distinctively different from those of the Mesoproterozoic hydrocarbons. The results suggest that the Mesoproterozoic source rocks have significant petroleum generation potential. The Mesoproterozoic paleo-reservoir may be prospecting exploration targets in the Liaoxi Depression, NE China.

Oil-oil and oil-source rock correlations for nineteen crude oils and eight mudstone extracts from four sub-basins in the Muglad Basin were achieved by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Based on the characteristics of biomarker compositions and hierarchical cluster analysis, an outlier crude oil has been discovered in the Muglad Basin. The majority of discovered oils have similar geochemical characteristics and belong to an established oil family. This oil family has relatively higher (C19+C20)/C23TT and C24TeT/C26TT ratios, small amounts of long-chain tricyclic terpanes with a dominance of C21TT among C19 C23TT homologues, a lower abundance of C28 regular steranes with abundant C30 4α-methylsteranes, an unusually higher content of diasteranes and diahopanes. These biomarker ratios indicate the mixed inputs of aquatic plankton and terrigenous organic matter deposited under oxic to sub-oxic, and fresh to brackish water sedimentary environments. This oil family belongs to source rocks of the Abu Gabra Formation as proven by oil-source rock correlation. In contrast, the distribution characteristics of tricyclic and tetracyclic terpanes, C28 regular steranes, C30 4α-methylsteranes, diasteranes and diahopanes are all significantly different than those of the established oil family in the well Kela-1 oil (2563 m) within the Darfur Group reservoir. The geochemical signatures indicate that their related source rock is an algal dominated organic matter input, deposited in sub-oxic condition, and brackish to saline sedimentary environment. Moreover, the composition and characteristics of molecular markers of the well Kela-1 oil are highly consistent with those of the two mudstone samples from the Darfur Group. This clearly demonstrates that the Darfur Group mudstones are the potential source rocks for the outlier crude oil. Identification of the outlier crude oil shows that the Darfur Group mudstones have a certain potential regarding hydrocarbons resources that could provide petroleum companies an impetus to enhance their exploration efforts.

The thermodynamic stabilities of methyldibenzothiophene (MDBT) isomers and the mechanisms of 1,2-methyl shift, methylation and demethylation have been systematically investigated by molecular modelling. Combining with the geochemical data from a set of lacustrine mudstone samples in the Liaohe Basin (East China), this study reveals the possible reaction mechanisms governing the formation and occurrence of MDBT isomers in source rocks. The Boltzmann distribution of MDBTs in gas, water and cyclohexane was calculated to show the relative thermal stability with the increasing of temperature (burial depth). A total of eight reaction pathways involving in 1,2-methyl shift, methylation and demethylation chemical processes via ionic and free radical reactions, and the transition states and energy barriers are calculated. The results show that three main reaction pathways (P1, P3 and P6) are three possible pathways for the transformation reactions between DBT and MDBTs during the onset of the oil generation window to high maturity stages of sedimentary organic matter. The 1,2-methyl shift isomerization (P1) and low concentration of the possible precursor (2-methylbiphenyl) for 1-MDBT, cause the relative low abundance of 1-MDBT isomer during the oil generative window. Methylation of DBT with methyl radical attack (P3), the formation of MDBTs with TMAB as methyl donor (P6), the prevailing degradation reactions and much higher thermodynamic stability of 4-MDBT relative to 1-MDBT, makes the MDR maturity indicator regularly increase during the oil generative window to high maturity stages. Therefore, a previously proposed MDR maturity indicator was further confirmed by the theoretical calculations, molecular simulations and the geochemical data of the lacustrine mudstone samples in this study.

Study of the occurrence and formation of sedimentary organic matter in the Sichuan Basin, southern China, provides valuable insights into the biological sources and compositions of Neoproterozoic to Lower Cambrian shales. Rock samples from two sets of shales in the Sichuan Basin and adjacent areas, including the Ediacaran Doushantuo shale and the Lower Cambrian Qiongzhusi shale in the Gaoshiti-Moxi and Ziyang-Weiyuan areas were collected and geochemically analyzed in this study. The distribution of biomarkers, stable carbon isotope compositions and microfossil characteristics reveal that eukaryotic algae including chlorophytes, pelagophyceae, dinoflagellates, diatoms (or their ancestors) and sponges proliferated widely during the Late Neoproterozoic to Early Cambrian in the Sichuan Basin. Cyanobacteria, some α-Proteobacteria, rhizobiales and aerobic methanotrophs also made a contribution to organic matter in the studied shales. Aerobic methanotrophs that utilize 13C-depleted substrates such as CH4 as carbon sources may have been the cause of anomalous 13C-depleted kerogen in the study area.

Solid bitumen in reservoir rocks is an allochthonous organic matter formed from bitumen or liquid hydrocarbons by various alteration processes. In this study, two types of altered solid bitumen have been identified in both the Ediacaran Dengying and Cambrian Longwangmiao formations in the Central Sichuan Basin using petrographic, microthermometric, chemical, and isotopic data. The hydrothermally altered bitumen (type 1) is associated with the pore-filling saddle dolomite and quartz in which homogenization temperatures of some fluid inclusions are more than 10 °C higher than the maximum burial temperature of the Dengying Formation. It has various anisotropic textures with significantly high bireflectance values (Rbmax − Rbmin, %) between 3.89% and 9.21%. Moreover, the hydrothermal alteration led to 13C-enrichment in the type 1 bitumen compared to the normally matured bitumen derived from the same source. On the other hand, the thermochemical sulfate reduction (TSR)-altered bitumen (type 2) has S/C atomic ratios of up to approximately 0.06. The δ34S values of type 2 bitumen are close to those of the carbonate-associated sulfate and similar to the TSR-derived H2S in the same reservoirs. A strong negative correlation exists between S/C atomic ratios and δ13C values in the type 2 bitumen in the Dengying Formation, which is interpreted as a result of an increasing incorporation of the 13C-depleted ethanethiol into the bitumen during TSR. This interpretation is consistent with the fact that the remaining ethane became isotopically heavier as TSR proceeded. In addition, the reactive sulfate for TSR was primarily derived from the hydrothermal sulfate, such as barite. This is evidenced by the replacement of hydrothermal barite by the 34S-enriched pyrite and the small differences in δ34S value between the barite and the type 2 bitumen (or the TSR-derived H2S) in the same strata. We suggest that TSR in the Ediacaran and Cambrian hydrocarbon reservoirs was probably initiated by hydrothermal activity.

The Muglad Basin is one of the largest petroliferous basins within the Central and Western African Rift System (WCARS). A total of 42 oil samples including light, normal and heavy crude oils from the Cretaceous reservoirs of the Muglad Basin were systematically analyzed to fully understand their geochemical characteristics and possible origins. The results of oil–oil correlation show that these Cretaceous crude oils are genetically identical and derive from the same source kitchen. Differences in physical properties and geochemical compositions may be attributed to secondary alteration, such as biodegradation. Lightly, moderately and severely biodegraded oils can be divided on the basis of the relative abundance of n–alkanes and acyclic isoprenoids in the oils and the presence of “humps” on gas chromatogram baselines, as well as of the relative abundance of 25–norhopanes. The biodegradation degree of Cretaceous oils in the Muglad Basin is closely related to the burial depth of oil reservoirs and the distance between the locations of oil reservoirs and adjacent faults. This study may have practical application for further petroleum exploration and development in the Muglad Basin.

The Paleogene lacustrine mudstone is one of the most important sets of source rocks in Termit Basin, Niger. However, studies on the Paleogene source kitchen are scarce. In this study, the source rock types, spatial distribution and their hydrocarbon generative potential within the Paleogene Sokor-1 and LV formations are systematically evaluated. A total of two third-order sequences (SSQ1 and SSQ2, from the bottom to top) and six systems tracts were identified in the Sokor-1 Formation, while the LV Formation mainly comprises pure shale and is regarded as a compositive stratigraphic sequence with no need of subdivision. Six types of source rocks could be distinguished within the three depositional environments of the sequence stratigraphic framework: (1) deep lake mudstones and shale deposited in the lacustrine deep-water facies, (2) shallow lake mudstones and carbonaceous mudstones occurring in the shallow lake environment, and (3) deltaic-front mudstones and prodeltaic mudstones developed in the deltaic facies. Deep lake mudstones/shale, which mainly occurred in the SSQ1-TST (transgressive systems tract), SSQ2-TST and LV Formation, are considered organic-rich source rocks with oil-prone generative potential. The shallow lake mudstones and deltaic mudstones were predominantly distributed in the tectonic slope and marginal areas of the Agadem Block within the lowstand systems tract and high stand systems tract of the SSQ1 and SSQ2. Nevertheless, geochemical results indicated that the deltaic mudstones are good gas-prone source rocks but the shallow lake mudstones were classified as poor potential source rocks. The variance in organic matter accumulation and preservation conditions within different depositional facies and the fluctuation of relative lake level are the controlling factors for the occurrence and distribution of high-quality source rocks. The effective Paleogene source kitchen is limitedly distributed around the depocenter of the Dinga Depression within a small scale, and therefore exploration for oil and gas resources from the Paleogene source kitchen should focus on the eastern Dinga fault-step zone adjacent to the depocenter of the Dinga Depression.

In order to select biomarker parameters that have potential to quantitatively distinguish between the Cambrian–Lower Ordovician and Upper Ordovician source rocks in the Tarim Basin with a total organic carbon content >0.5%, 42 Lower Paleozoic shale samples, were collected from a number of wells and outcrops in the Tarim Basin, northwestern China. The biomarkers in the extracts of the source rocks were analyzed by gas chromatography–mass spectrometry. The results show that C23 and C30 were dominant in the tricyclic terpanes and pentacyclic triterpanes, respectively, and C27 and C29 were both dominant in C27–29 steranes of the source rock samples. The distribution of n-alkanes, isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded, and they originated from a reducing depositional environment with considerable marine algal input in. Comparatively, Upper Ordovician source rock samples presented lower values of C28/C29 regular steranes and higher values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα). Abnormally lower values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα) were observed in the Cambrian–Lower Ordovician strata. The set of data in this study show that homohopane index including C31homohopane/C30hopane, C31-35homohopanes/C30hopane, and the ratios related to the triaromatic steroids including C26−20S/C28−20S TAS, C27−20R/C28−20R, (C26−20R + C27−20S)/C28−20S and C28/(C26+C27) TAS, could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the Tarim Basin. The values of those parameters (in above order) in Cambrian–Lower Ordovician source rocks were 0.3, >0.5, >0.85, and 0.8, >1.9, 1.2, respectively. The correlation between the source rock samples and 19 oils obtained from the Tadong, Tabei and Tazhong uplifts in the Tarim Basin show that most of the oils in Tabei uplift are probably from Upper Ordovician source rocks, the oils in Tadong uplift are more likely from Cambrian- Lower Ordovician source rocks, and the Tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks.

Two unusual tricyclic terpanes (compounds X and Y) and four tetracyclic terpanes (compounds X1, Y1, Z, and Z1) have been detected in crude oils from the Pearl River Mouth, Beibuwan, and Liaohe basi...

Upper Cretaceous marine mudstones are widely known as the most important source rocks in the Western African rift basins. However, geochemical studies on their occurrence and formation mechanism are scarce. In this study, a detailed geochemical investigation within a sequence stratigraphic framework was carried out to reconstruct the palaeoenvironmental conditions and to establish models on the depositional history of Upper Cretaceous marine mudstones in the Termit Basin (Niger), western African. A total of six third-order sequences (DSQ1, DSQ2, YSQ1, YSQ2, YSQ3 and MSQ1, from bottom to top) were identified in the Upper Cretaceous. The distribution of biomarkers from 33 mudstone samples in different sequences indicate that mudstones from YSQ3 were mainly deposited in a suboxic to oxic environment, with significant contributions from mixed terrigenous higher plants and lower aquatic organisms in fresh-brackish water column. In contrast, the other sequences (YSQ2, YSQ1, DSQ2 and DSQ1) mainly represent a suboxic to anoxic marine environment with a stratified water column (0.59

Pyrobitumen has been discovered in the dolostone reservoir of the Central Sichuan Basin, southwest China, accumulates coevally with hydrothermal minerals in pores. The pyrobitumen has strong anisotropy with an evolutionary sequence from mosaic to a fibrous texture. A similar sequence of anisotropy has also been observed in mesophase pitch during the process of coking coal. Reflectance of the pyrobitumen shows a positive correlation with anisotropic evolution and increasing depth. Hopane and sterane biomarkers are abundant in extracts of highly anisotropic pyrobitumen with smooth, rough, hyperplastic surface and honeycomb micropores. Methylpyrene and methylchrysene maturity indicators and the reflectance values of pyrobitumen show a positive linear correlation in Cambrian samples, but show random correlation in Sinian samples. Fluid inclusion microthermometry of hydrothermal minerals, together with the burial and heat flow history of the reservoir indicate an abrupt thermal event that may have impacted the generation and evolution of pyrobitumen. This appears to be confirmed by the reflectance values of pyrobitumen in the source rock and reservoir. The pyrobitumen in source rock was less heated by the hydrothermal fluid due to its poor porosity and have lower reflectance values. However, the pyrobitumen in weathering crust reservoir was more heated by the hydrothermal fluid and have higher reflectance values, because the reservoir rocks have higher porosity. The occurrence and characteristics of pyrobitumen indicate an in-situ pyrolysis of oil, which was triggered by an abrupt thermal event in the reservoir. The reconstruction of burial history and heat flow, as well as the hydrothermal fluid minerals of the dolomite reservoir all indicate the thermal event of the Sichuan Basin might correlate with the Emei mantle plume in Late Permian.

Systematic geochemical analysis of 39 crude oils from the Termit Basin, Niger, all belonging to the same oil family, reveals distinct differences in bulk properties and molecular compositions, which were undoubtedly caused by differences in the extent of biodegradation. Various degrees of microbial alteration were identified in crude oils, which were classified as non-, slightly, moderately, or heavily degraded oils, according to their physical properties, bulk chemical compositions, and certain molecular biomarker characteristics. Examination of the possible origins and geological backgrounds of the biodegraded oils shows that reservoirs with deeper burial depths (greater than 1800 m), thicker caprocks (>100 m), relatively higher stratigraphic temperatures (>80 °C), and which have been subject to the influence of silent faults had not suffered significant biodegradation. The burial depth of reservoirs is therefore considered to be the main factor controlling the biodegradation process, and may offset other factors if the depth is sufficient. The Paleogene reservoirs in the Dinga Step-Fault Zone (DSFZ) that meet all of these conditions are therefore dominated by normal oils. Degraded oils of varying degrees are widely distributed in other tectonic units where at least one of these factors is absent. This study may have practical application for further petroleum exploration, prediction of petroleum properties, and the reduction of production risk.

Reservoir geochemistry has a practical application in petroleum exploration. A typical Paleozoic carbonate oilfield was selected from the Tabei Uplift of the Tarim Basin (northwestern China) to exhibit the method, application, and exploration implications of reservoir geochemistry. Oil–oil correlation indicates that all oils analyzed in this study belong to one single oil group. The overall oil migration direction traced by selected organic molecular markers is from the southern to the northern regions of the Halahatang region. The source kitchen for current oil accumulations in the carbonate reservoir is predicted to locate to the south of this oilfield, most likely between the Awati and Manjiaer depressions. Based on the characteristics of hydrocarbon-bearing inclusions and the histograms of the homogenization temperatures (Th) and ice-melting temperatures of associated aqueous inclusions, the oil charging temperatures were obtained. The stratigraphic-burial and geothermal histories for representative i...

Dibenzofurans, oxygen-containing heterocyclic aromatics, are ubiquitous in crude oils. Their geochemical implications in petroleum geochemistry have been studied in numerous previous publications. Here, we present the distribution of dibenzofuran and its methylated homologues, and calculate the polarity and solubility of methyldibenzofuran isomers based on the Connolly surfaces and molecular dynamics simulation. The results show that the geo-chromatographic fractionation effect is evident due to absorption onto the water/silica matrix carrier bed during the oil migration process. Two potential molecular indicators relative to dibenzofurans, i.e. the total content of dibenzofurans and the relative abundances of 1-methyldibenzofuran to 4-methyldibenzofuran, are proposed for tracing the oil migration orientation and filling pathways. These two indicators were applied successfully in Paleozoic marine carbonate and Tertiary lacustrine sandstone reservoirs. The variations of source input, depositional environment, and maturity for oils from same source bed/kitchen appear to have no significant effect on these two indicators. It suggested that these two dibenzofuran indicators are potentially effective molecular tracers for orientation and filling pathways in oil migration.

Organosulphur compounds (OSCs) play important roles in the formation, preservation, and thermal degradation of sedimentary organic matter associated with petroleum generation. However, with the ongoing develop of hydrocarbon prospecting in ultra-deep reservoirs, the thermal maturation and geochemical behaviours of OSCs in deep sedimentary basins during the metagenesis stage of thermal maturation has not been well understood. In this study, we modelled the thermal maturation of OSCs by a combination of gold-tube hydrous pyrolysis studies and quantum mechanics calculations based on density functional theory (DFT) to investigate the reaction pathways and kinetic properties. Dibenzothiophene (DBT) was selected as an over-mature model molecule due to its high degree of aromaticity and lack of side chains. The energy barrier of aryl-aryl bonding is estimated to be approximately 60 kcal/mol. Dehydrogenation of OSCs through aryl-aryl bonding mainly occurs in the Easy%Ro range of 2.3%–3.8%. We modeled the contribution of OSCs to organic-derived H2S at advanced maturity based on numerical simulations in MATLAB. The contribution of OSCs to H2S can be quantitatively predicted based on specific thermal histories and the kinetics of H release from biaryl formation. Taking the Sinian reservoirs in the central uplift of the Sichuan Basin as an example, the yield of organic-derived H2S contributed from S-rich solid bitumen (S/C atomic ratio ~0.06) was estimated to be approximately 1.58 mg/g. These results suggested that OSCs participated as carriers of organic S from the surface to deep strata. Desulphurisation of OSCs together with thermochemical sulphate reduction, could contribute to the subsurface accumulation of H2S, to the detriment of. the quality of hydrocarbon reservoirs and the durability of the drill.

Based on the thermodynamic stabilities and molecular simulation, two trimethyldibenzothiophene molecular indicators, that is, 2,4,6-/(1,4,6 + 1,4,8 + 3,4,6)-trimethyldibenzothiophene (TMDBT) and (2,4,7 + 2,4,8)-/(1,4,6 + 1,4,8 + 3,4,6)-TMDBT (the numbers indicate the position of methyl-substitute), were proposed to trace subsurface oil migration orientations and reservoir filling pathways. In this paper, we successfully applied the aforesaid parameters both in an Ordovician carbonate reservoir of the Tuoputai region of the Tarim Basin (NW China), and in an Eocene clastic lacustrine reservoir of the Fushan Depression in the Beibuwan Basin, South China Sea. Besides the migration fractionation is proven the main control factor for the relative contents of TMDBT isomers in corresponding oils, which indicates that the trimethyldibenzothiophene molecular parameters can serve as the effective oil migration tracer in an oil reservoir, especially in high to over mature and severely biodegraded oils.

Geochemical analysis of 64 oils sampled from an Ordovician carbonate reservoir in the Tuoputai region was undertaken to study the composition of molecular markers. All the oils have similar geochemical characteristics and belong to a single oil family. They are presumed to derive from the same source kitchen and have similar oil charging history. A histogram of homogenization temperatures (T h) of aqueous inclusions in reservoir rocks shows a bimodal distribution pattern, indicating that the Ordovician reservoir has been charged twice. Coupling the measured T h (°C) with the burial and geothermal histories reconstructed using 1D basin modeling, we relate the homogenization temperature to the relevant geological ages: i.e., 425–412 and 9–4 Ma, corresponding to the Middle to Late Silurian and the Miocene to Pliocene, respectively. The oil filling orientation and pathways are traced using molecular indicators related to alkyldibenzothiophenes and benzo[b]naphthothiophenes. The oil charging orientation is from south to north generally. It can be predicted that the Ordovician reservoirs were sourced from a kitchen located to the south of the Tuoputai region, most probably between the Awati and Manjiaer Depressions. Traps located in the southern side of the Tuoputai region, along the oil charging pathways, should therefore be preferred oil exploration targets.

The Shuntuoguole region is situated at the center of the Tarim Basin, in NW China. It is bordered by the Tazhong Uplift to the west and the Manjiaer Depression to the east. Data obtained on samples collected from exploration wells highlight the large potential for gas discoveries in this region. Component and stable carbon isotopic analyses have been performed on gas samples from 5 wells in the Shuntuoguole region. The dryness coefficients of gas samples vary from 0.95 to 1.00, indicating that the gas samples belong to overmatured dry gas. The carbon isotopic values (δ13C) of all gas samples are heavier than −40‰, compared to the typical gas samples with different sources in the Tarim Basin, revealing that the gas in the Shuntuoguole region originated from source rocks in the Cambrian. The SN-1 exploitation well was selected to undertake fluid inclusion studies revealing the mechanisms of natural gas charging and fluid evolution. Data from the SN-1 well drill core indicate a 1,700- to 7,050-m-thick sedimentary sequence containing Ordovician to Tertiary strata. The Middle Ordovician Yijianfang Formation (Fm) constitutes one of the most important gas reservoirs. The Yijianfang Fm consists principally of limestone containing widely distributed sparry calcite. Numerous fluid inclusions are observed in calcite, and 4 types of fluid inclusions are recognized based on the observed phases and components. These are pure CH4 (P-type), CH4-bearing (C-type), aqueous (W-type), and solid-bearing (S-type) inclusions. The trapping temperatures and pressures, determined through the intersection of isochores from coeval fluid inclusions, reach 183 °C to 216 °C and 1,914 bars to 2,196 bars, respectively. When taking into account the regional burial history, we find the natural gas charged during the Neogene interval, from 20 Ma to 3 Ma.

A total of 11 core samples of Cambrian source rocks and 2 Cambrian-sourced crude oils (TD2, T904) were analyzed. High content of total organic matters indicates a good to excellent source rocks. However, high maturity was indicated by high Tmax values and very low content of HI index and S1+S2. The relative intact alkanes can be detected in most samples with no odd/even predominance. A series of relatively high abundance of terpanes and steranes can be detected on m/z 191 and m/z 217 chromatograms, respectively. Ratios of C21/C23 tricyclic terpanes are greater than 1.0. All samples of Yuli 1 and Tadong 2 show high abundance of C26∼C28 triaromatic steroids and triaromatic dinosteroids, with ratios of (C26+C27) TAS/C28TAS and TDSI greater than 1.0 and 0.62, respectively. Oil source rocks correlation between sampled rock extracts and typical Cambrian oils reveals that high-postmature Cambrian source rock extracts are well consistent with those two typical Cambrian oils that are usually used as end me...

The Moxi–Gaoshiti (MG) Bulge of the Sichuan Basin in southwestern China is unique because of its enormous gas field in old Cambrian and Proterozoic marine strata, where oils in carbonate reservoirs have been nearly completely cracked to solid bitumen and natural gas because of high to post-mature thermal maturity levels. Numerous solid bitumen-bearing dolomites obtained from boreholes in the MG Bulge provide an ideal opportunity to trace the major filling orientations for the Lower Cambrian paleo-oil reservoirs. Biomarker analysis was conducted on solid bitumens through quantitative detection with gas chromatography–mass spectrometry. Results show that all solid bitumen samples in the MG Bulge exhibit similar compositions of molecular markers, thereby implying that they belong to the same oil population and that paleo-oils should be derived from the same source kitchen/bed. Low pristane/phytane and dibenzothiophene (DBT)/phenanthrene ratios, as well as the predominance of DBT among DBT, dibenzofuran, and fluorene, are observed in the solid bitumen samples, which suggest that related paleo-oil reservoirs likely originated from a highly reducing marine shale environment. All isopleth maps of the DBT concentrations, 4-methylDBT/1-methylDBT, 4,6-dimethylDBT/(1,4 + 1,6)-dimethylDBT (DMDBT), and (2,6 + 3,6)-DMDBT/(1,4 + 1,6)-DMDBT ratios consistently indicate that the overall orientation of oil migration in the MG Bulge is primarily from the west-to-east direction, with the strongest filling point located at approximately the Mx12 well. The north-to-west direction is an additional filling orientation. These indicators show that those paleo-oils that had been cracked to pyrobitumens mainly originated from a source kitchen in the western part of the MG Bulge. A hydrocarbon source kitchen in the northern part of the MG Bulge may also have developed but further research on this is needed. The recognition of these solid bitumen molecular signatures, particularly the consistent results using DBT concentrations and alkyl DBT thermal maturity indicators, suggests a potential approach that can be applied to the study of paleo-oil reservoirs.

The distribution of phenyldibenzofurans (PhDBFs) and methyldibenzofurans (MDBFs) and their potential implications in petroleum organic geochemistry have been investigated for sediment extracts and crude oils derived from lacustrine shale, marine shale, marine carbonate, and terrestrial (fluvial/deltaic/fresh water) mudstone. PhDBF isomers in a set of lacustrine shales from the Liaohe Basin (East China) are identified in the m/z 244 mass chromatograms of the aromatic fraction by coinjection of internal synthetic standards on a high resolution capillary column (HP-5MS). The results show that the relative abundance of 4-PhDBF/2-PhDBF and 4-PhDBF/(2-PhDBF+3-PhDBF) increase gradually with increasing burial depth and maturity (R0 ≥ 0.6%), and have a good correlation with vitrinite reflectance in lacustrine shales from wells S202 and SG1 in the Liaohe Basin. These two ratios, defined as phenyldibenzofuran ratio-1 and -2 (PhFR-1 and PhFR-2), may be potential maturity indicators for mature sediments in this study....

Analysis of Mesoproterozoic sediment extracts from the North China Craton by gas chromatography–mass spectrometry metastable reaction monitoring revealed an extended series of C18–C33 13α(n-alkyl)-tricyclic terpanes. This discovery extends the previous known series to C33 with sixteen homologs. Because of the abundant presence of C22 and C27 homologs, and no isomeric peaks for the homologs > C24, it was confirmed that the side chain at the C-13 position is a long-chain n-alkyl group without a methyl substituent or an asymmetric carbon atom. The distribution of the extended tricyclic terpanes in the Mesoproterozoic sediments is mainly controlled by a primitive biological community, probably from certain specific bacteria and/or cyanobacteria. Their biological precursor was probably inhibited by high-salinity water conditions and not evolved during the deposition of the Gaoyuzhuang Formation.

A significant negative organic carbon isotope (δ13Corg) excursion occurred in the lower member of the lower Cambrian Qiongzhusi organic-rich shales in the Sichuan Basin, which is also reported in other contemporaneous strata throughout the Yangtze Platform. In this study, 47 lower Cambrian drill core samples were collected to investigate the possible formation mechanism of this negative δ13Corg excursion using multiple geochemical approaches. Primary paleoproductivity, redox conditions and organic carbon isotopic compositions were determined for these core samples of the Qiongzhusi Formation in the Sichuan Basin. The results suggest that the degree of microbially mediated carbon cycling and its contribution to the total sedimentary organic matter has an important influence on organic carbon isotopic compositions. An extensive transgression during the deposition of the lower member of the Qiongzhusi Formation brought abundant nutrients to primary producers and thus stimulated blooms, which is evidenced by the high primary productivity in the lower member of the Qiongzhusi Formation. In addition, marine transgression also provided sulfide to the seawater. Enhanced primary productivity accelerated the consumption of bottom water oxygen and thus promoted the establishment of euxinic bottom water conditions. The euxinic conditions promoted an anaerobic carbon cycle mediated by sulfate-reducing bacteria and chemoautotrophic bacteria, which resulted in the dissolved inorganic carbon reservoir becoming more depleted in 13C. Once these 13C-depleted carbons are utilized by primary producers, the 13C-depleted carbons will participate in the carbon cycle. This series of processes ultimately resulted in lower δ13Corg values within the lower Cambrian Qiongzhusi organic-rich black shales. A shift to positive and rather stable δ13Corg values in the relatively organic-poor sediments indicates a smaller contribution of 13C-depleted bacterial biomass (e.g., sulfate-reducing bacteria and chemoautotrophic bacteria), which is evidenced by more oxygenated ocean bottom conditions in the study area.

Determining the place of paleo-oil cracking is crucial to understanding the formation mechanism of the gas reservoirs in the Precambrian–Cambrian strata. For the old strata the attainable information is limited because most paleo-oil have been cracked to gas at high thermal maturity, such as the Anyue giant gas field in the Moxi–Gaoshiti Bulge of Sichuan Basin, Southwestern China. In this study, the place of paleo-oil cracking was discussed based on solid bitumen found in the reservoirs. Numerous solid bitumen from the deep boreholes in the Precambrian Dengying (Z2dn2 and Z2dn4) and Lower Cambrian Longwangmiao (Є1l) formations of the Moxi–Gaoshiti Bulge were analyzed. Elemental and carbon isotope analyses and optical microscopic observations, such as Raman spectroscopy, scanning electron microscopy, and electronic probe, were performed on these solid bitumen samples. The obtained parameters, including the reflectance of the solid bitumen, parameters I(D)/I(G) and S(D)/S(G) from the Raman spectra, the compositions and carbon isotope of gas, burial history, and the predicted “equivalent vitrinite reflectance Ro” (EqVRo) for reservoirs, jointly support that solid bitumen are epigenetic bitumen in reservoirs other than primary bitumen in source rocks. This study provides a relatively solid evidence of in situ cracking of paleo-oil and the thermochemical sulfate reduction reaction between paleo-oil and dolomite reservoirs. Pyrite were found both in carbonate rocks and argillaceous rock, and the sulfur in pyrite is perceived as a pathway of H2S formation. The results provide a basis for understanding the formation mechanism of the deep huge gas field that was derived from paleo-oil cracking, and may contribute to future petroleum exploration in deep Precambrian–Lower Cambrian systems.

Mango proposed a significant invariance of four isoheptanes, in which a ratio of sums (K1) remains at approximately 1, regardless of the absolute concentrations. Gas chromatographic analyses of 70 oils from the Tarim Basin demonstrated that most of these oils share invariant K1 values close to 1.0, even if they are derived from different sources and have varying degrees of maturity and biodegradation. However, a few unexpectedly high K1 values occurred in certain oils from the Tazhong-4 Oilfield in the Tazhong Uplift, and these oils are characterized by abnormally high contents of dibenzothiophenes (DBTs). An excellent positive correlation between the elevated K1 values and the abnormally high DBT contents indicated that the thermochemical sulfate reduction (TSR) alteration, which contributed to the increased DBT contents, likely resulted in the elevated K1 values. Methylhexanes (MHs) are assumed to be more susceptible to TSR than dimethylpentanes (DMPs), and the terminal methyl groups of C7 compounds are...

The geochemistry, origin and charge history of oils from the Yuqi area of Tarim Basin have been investigated, through GC, GC-MS and fluid inclusion microthermometry analysis. The Yuqi oils accumulated mainly in three intervals: (1) in the Lower-Middle Ordovician Yingshan Formation (O 1-2 y ) carbonate reservoirs; (2) in the overlying Upper Triassic Halahatang Formation (T 3 h ); and (3) in the Lower Cretaceous Yageliemu Formation (K 1 y ) sandstones. Oils from different reservoirs have distinct physical properties, varying from extra-heavy (O 1-2 y), heavy (T 3 h ), to light oils (T 3 h and K 1 y ). However, their geochemical compositions show a high degree of similarity, which indicates that they derive from the same source rock. Abundant tricyclic terpanes, gammacerane, dibenzothiophene and C 21 C 22 steranes, together with a low level of diasteranes, indicate an anoxic marine source rock for oils in the Yuqi area. Oil-oil correlation shows that Yuqi oils derive from the same source bed as Tahe oils. The co-occurrence of intact n -alkanes and 25-norhopanes in all the samples supports the proposition that there is a mixture of an early filled severely biodegraded oil and a late filled fresh oil. In this study, charge history is examined on the basis of integration of fluid inclusion homogenization temperature data with 1D burial-thermal history models. Two episodes of oil charging are identified in the O 1-2 y reservoir (well YQX1-1) at around 436-420 Ma (Middle-Late Silurian) and 10-3 Ma (Miocene to Pliocene), respectively. For the samples from the T 3 h and K 1 y intervals, only one episode of oil charge is indicated by the homogenization temperatures of coexisting aqueous inclusions with an inferred timing around 10-3 Ma. The T 3 h heavy oil reservoir is assumed to be a secondary hydrocarbon pool, which accumulated by re-migration and re-distribution of hydrocarbons from O 1-2 y hydrocarbon pools. The few early biodegraded oils in the K 1 y light oils were probably picked up along the migration pathway during the late fresh oil charging.

Dibenzothiophenes (DBTs) and benzo[ b ]naphthothiophenes (BNTs) are important sulfur heterocyclic aromatic compounds in oils and sedimentary rock extracts. Based on both migration fractionation effects and differences in thermal stability of different isomers, DBTs and BNTs can be used to trace oil migration orientations and filling pathways. They are present in significant abundances in oils, including light oils and condensates, in which biomarkers are commonly absent or present only in extremely low concentrations. Molecular biomarker compositions indicate that all oils and condensates in the Eocene clastic reservoir of the Fushan Depression (Beibuwan Basin, South China Sea) belong to a single oil population. In this study, three geochemical indicators relating to DBTs and BNTs, namely; (1), 4-/1-methyldibenzothiophene (4-/1-MDBT); (2), total content of DBTs; and (3), benzo[ b ]naphtha[2,1- d ]thiophene/(benzo[ b ]naphtha[2,1- d ]thiophene+benzo[ b ]naphtho[1,2- d ]thiophene) ([1,2]BNT/([1,2]BNT+[1,2]BNT)) were applied to trace oil migration orientations and filling pathways. The results show that these parameter values gradually decrease from northeast to southwest, towards the Huachang Uplift, and from north to south towards the Bailian Faulted-Belts. Therefore, the source kitchen for identified oil accumulations in the Huachang Uplift is likely to be located in the Bailian Sag, which lies to the northeast of the Huachang Uplift. Zones in the Bailian Sag, situated upstream of the preferred oil filling pathways are likely to be the most favorable prospecting regions. It is concluded that relative parameters of DBTs and BNTs are practical molecular indicators for tracing oil migration orientations and filling pathways.

The well RP3–1 lies in the Tarim Basin, which is located in the southern Xinjiang Uygur Autonomous Region in northwest China. This region is one of the world's largest frontier basins. Drilling data that we collected on the well RP3–1 reveal a 2398–7066 m thick sedimentary sequence, containing Ordovician to Tertiary strata. The Yijianfang (O2yj) formation of the Middle Ordovician sequence is one of the most important reservoir stratums. The O2yj formation consists mainly of bioclast and intraclast limestones. Two different fluid types are identified by the associations they have with veinlets, which are colors caused by petroleum inclusions under UV light. Early-stage petroleum fluids are characterized by a blue-white color under UV-epifluorescence microscopy, and early-stage aqueous fluid inclusions are mainly homogenized between 54 and 102 °C, with salinities ranging from 12.0 to 16.7 wt% NaCl equiv. Late-stage petroleum fluids are characterized by a yellow color under UV-epifluorescence microscopy, and late-stage, aqueous-fluid inclusions are mainly homogenized between 85 and 135 °C, with salinities ranging from 2.0 to 8.0 wt% NaCl equiv. In order to obtain accurate measurements of both the trapped temperature (Ttrap) and pressure (Ptrap) of these fluids, we use 10 pairs of petroleum inclusions and coeval aqueous fluid inclusions to measure the intersection of a number of isochors. The Ttrap of early-stage fluids we surveyed were between 82 and 97 °C, with the Ptrap ranging from 97 to 259 bars. In contrast, the Ttrap of late-stage fluids were between 141 and 170 °C, with Ptrap values ranging from 430 to 608 bars. When combined with the regional buried history, the early-stage fluids charged from 422 Ma to 412 Ma BP, and the late-stage fluids charged from 20 Ma to 6 Ma BP.

A total of 32 oils and condensates were collected from four gas/oil fields (Dalaoba, Luntai, Kuqa, and Yakela) in the Yakela Faulted Uplift (Tarim Basin, northwest China). The composition of biomarkers and a previous study show that oils and condensates in the Dalaoba, Luntai, and Kuqa gas/oil fields originated from lacustrine mudstone/shale source rocks deposited in an oxic environment. Oils and condensates from the Yakela gas/oil field are of typical marine carbonate origin. Some phenyl-substituted polycyclic aromatic hydrocarbons, including phenylnaphthalenes (PhNs) and terphenyls (TrPs), were unequivocally detected in most of these oils and condensates. They are present in significant abundance in oils and condensates from the Dalaoba, Luntai, and Kuqa gas/oil fields. However, the concentrations of PhNs and TrPs are relatively lower or below the detection limit in oils and condensates from the Yakela gas/oil field. The ratios of 2-PhN/2-MN and m-TrP/3-MBP are used in this study to describe the abundan...

The Tabei Uplift is one of the largest petroleum accumulation region in China. After decades of exploration and development a number of commercial oil reservoirs have been discovered in the Mesozoic formations suggesting a great potential for hydrocarbons. The Mesozoic oils were sampled from 18 wells in the Tabei Uplift. A total of 6 Ordovician oil samples were also collected for contrast analysis. On the basis of oil–oil correlation, all of the 24 crude oils belong to the same oil population and probably derived from the same source kitchen. Despite the coexistence of “UCM”,25-norhopane and intact n -alkanes which is generally interpreted as twice oil filling history, the homogenization temperature ( T h ) of brine inclusion indicates a single oil filling event which happened at 4–3 Ma, with the major frequency ranging from 110 °C to 120 °C. By means of T s /( T s + T m ), MPI−1, 4-/1-MDBT and total concentration of carbazole compounds, the oil filling orientation in Yakela and Yuqi areas were from NEE to SWW and NW to SE, respectively. The Luntai fault is believed to be an oil-migrating fault connecting the Ordovician reservoirs with Mesozoic reservoirs. Based on the physical properties, the geochemical characteristics and the oil filling histories, it can be concluded that the Mesozoic reservoirs in the Tabei Uplift were of secondary oil accumulations derived from the deeper Ordovician reservoirs.

The Ordovician reservoir in the Halahatang Depression is one of the most commercially prolific oil reservoirs in the Tabei Uplift of the Tarim Basin, NW China. For this study, a total of 94 oil samples were collected from 88 exploration/production wells in this oilfield. All of the oils possess similar organic molecular compositions and belong to the same oil population. Oil-to-source correlation results indicate that oil accumulations discovered in Halahatang Oilfield were derived from the Upper Ordovician source rocks. The homogenization temperatures of fluid inclusion in the Ordovician reservoir in the Halahatang Oilfield range from 90 °C to 100 °C and from 140 °C to 160 °C. The bimodal distribution pattern shown in a histogram of the homogenization temperatures indicates that the Ordovician oil reservoir in the Halahatang Oilfield has been charged twice. Coupling this information with the reconstructed burial and geothermal histories by 1-D basin modeling, the homogenization temperatures can be related to the relative geological ages of 419 Ma to 410 Ma and 16 Ma to 8 Ma, namely, the Middle to Late Silurian and the Middle Miocene respectively, representing the two phases of oil charging and entrapment. The oil migration orientation and charging pathways can be traced using molecular indicators, such as Ts/(Ts + Tm) and TMNr parameters. The overall oil migration orientation in the oilfield is generally from south to north. Therefore, the source kitchen for the Halahatang Oilfield is predicted to be to the south side of the depression. The southern part of the Halahatang Depression, along the oil charging pathways, is identified as the most promising region for oil exploration.

Phenylnaphthalenes (PhNs), terphenyls (TrPs), quaterphenyls (QtPs) and triphenylbenzenes were identified by the use of authentic standards in GC–MS analysis. Phenyl derivatives of fused ring polycyclic aromatic hydrocarbons (PhPAHs) and polyphenyls (PPhs) were observed in selected lacustrine shales and related oils from the Tarim, Liaohe and Bebuwan Basins in China. All isomers of phenylnaphthalene and terphenyl were observed in these oils. The more stable 2-PhN and m-Trp, p-TrP isomers are predominant among phenylnaphthalenes and terphenyls, respectively. The effect of maturity on the distribution patterns of phenylnaphthalenes and terphenyls in a set of lacustrine shales from the Liaohe Basin, East China was investigated. The depth trends of three currently used maturity indicators were investigated. Analysis was based on the relative abundance of phenylnaphthalene and terphenyl isomers, namely PhNR (2-PhN/1-PhN), TrP1 (p-TrP/o-TrP) and TrP2 ((m-TrP+p-TrP)/o-TrP) in lacustrine shales from well S202 in the Liaohe Basin, East China. The indicators increase slightly with relatively great deviations within low to intermediate maturation stages and then increase abruptly in high maturity stages. Therefore, they are not suitable maturity indicators for lacustrine source rocks in this study. An alternative thermal maturity indicator, defined as p-TrP/m-TrP was proposed. It exhibits a uniform increase with increasing maturity and is probably a more suitable indicator for use in maturity assessment in lacustrine source rocks and related oils. This study broadens the current understanding of the occurrence and distribution of PhPAHs and PPhs in sedimentary organic matter of various origins.

Twenty-two oil samples and eight source rock samples collected from the Tarim Basin, NW China were geochemically analyzed to investigate the occurrence and distribution of phenylphenanthrene (PhP), phenylanthracene (PhA), and binaphthyl (BiN) isomers and methylphenanthrene (MP) isomers in oils and rock extracts with different depositional environments. Phenylphenanthrenes are present in significant abundance in Mesozoic lacustrine mudstones and related oils. The relative concentrations of PhPs are quite low or below detection limit by routine gas chromatography–mass spectrometry (GC–MS) in Ordovician oils derived from marine carbonates. The ratio of 3-PhP/3-MP was used in this study to describe the relative abundance of phenylphenanthrenes to their alkylated counterparts—methylphenanthrenes. The Ordovician oils in the Tabei Uplift have quite low 3-PhP/3-MP ratios ( 0.10), associating with high Pr/Ph, low ADBT/ADBF, high Ts/(Ts + Tm), and C30 diahopane/C30 hopane ratios. Therefore, the occurrence of significant amounts of phenylphenanthrenes in oils typically indicates that the organic matter of the source rocks was deposited in a suboxic environment with mudstone deposition. The phenylphenanthrenes may be effective molecular markers, indicating depositional environment and lithology of source rocks.

Organic-rich Mesoproterozoic rocks from the Xiamaling and Hongshuizhuang formations, with a moderate thermal maturity within the oil window, are well preserved in the Yanliao Rift, Northern China. Their organic geochemical compositions, especially biomarkers, organic matter origin, and depositional environment, are discussed in detail using GC, GC–MS and GC-MS-MS techniques. The results indicate that the Xiamaling and Hongshuizhuang samples share similar molecular compositions, which are distinct from those of Phanerozoic bitumens, suggesting a unique microflora in the North China sea during the Middle Mesoproterozoic (1.45–1.30 Gyr). The biomarkers in the saturated fraction were dominated by 13α(n-alkyl)-tricyclic terpanes (13αNATTs) and drimanes. The 13αNATTs appeared to be diagnostic biomarkers for special, ancient microbes in the Proterozoic, as they were only detected in Meso–Neoproterozoic bitumens and were absent in Phanerozoic sequences. Abundant drimane series, including extended drimanes, may be sourced from the ring C cleavage of 13αNATTs or may directly originate from the same precursor of 13αNATTs, based on the observation that the concentrations of drimanes and 13αNATTs exhibited a good correlation. Elevated C19 and C20 regular tricyclic terpanes together with abundant rearranged hopanes, which are usually typical fingerprints for terrestrial rocks, are distinct features of the Xiamaling and Hongshuizhuang marine rock samples. These compounds are considered to be derived from unique prokaryotes that were present in the Mesoproterozoic sea, which could synthesize abundant precursors of rearranged hopanes and low molecular tricyclic terpanes. Pristane (Pr) and phytane (Ph), mainly originated from the phytol side-chain of chlorophyll, were only present in very low abundances, even in the low maturity and organic-rich Xiamaling oil shales (total organic carbon >20%). This may indicate a contribution of phototrophs with other molecular proxies, rather than phytanic lipids in their chlorophylls. An absence or trace amounts of steranes in the samples may reveal a low contribution of eukaryotes, although previous studies have found a wide occurrence of eukaryotes in the Xiamaling and Hongshuizhuang formations, as well as in even more ancient rocks in the Yanliao Rift. Low Pr/Ph ratio values (

The Paleogene Xingouzui Formation is one of the exploration targets in the Jiangling Depression from Jianghan Basin, Central China. However, the origin and source of crude oils remains controversial. In the present work, a total of 16 fresh source rocks and 23 oil samples were collected and geochemically investigated to clarify the source of organic matters, sedimentary environment and evolution degree. The Xingouzui Formation source rock samples are mainly of fair−good source rocks, with the TOC content, S1 and S2 of 0.36%–1.36% (mean of 0.74%), 0.14–0.38 (mean of 0.27 mg HC/g rock) and (mean of 1.12 mg HC/g rock), respectively. And these rocks are dominated by type Ⅱ2 kerogen within “oil window” maturity stage, with the Ro value of 0.6%–1.2% (mean of 0.9%). The biomarker results indicate that the source rocks of the Xingouzui Formation are mainly deposited in anoxic reducing environment, which main source materials are the mixture of terrestrial higher plants and lower marine organisms. The Pr/Ph values of the oils and source rocks range of 0.47–0.64 and 0.23–1.07, respectively. And the δ13C values of the oils and source rocks range from −34.93‰ to −26.41‰ and from −28.3‰ to −23.8‰, respectively. Family-I and family-Ⅱ oils were separated based on the results of biomarkers and stable carbon isotope, indicating that family-I oils, mainly occurring in the Wancheng fault zone and Gonggan slope zone, are characterized by relatively lower homohopane series, higher mature, and less input of terrestrial higher plants. Comparatively, family-Ⅱ oils mainly occur in the Jingzhou anticlinal zone, with the characterization of small amount of homohopane series, less mature and more terrigenous organic matter input. Family-I oils mainly originate from of the Ⅲ and Ⅱ groups of the lower Xingouzui Formation (Ex1-Ⅲ and Ex1-Ⅱ) source rocks in the Wancheng fault zone and less contribution from Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone; while family-Ⅱ oils are mainly derived from the Ex1-Ⅲ source rocks in the north of Jingzhou anticlinal, and a small amount comes from the Ex1-Ⅱ source rocks of the Shiqiao monoclinal zone. In addition, Ex1-Ⅱ and Ex1-Ⅲ source rocks in the Wancheng fault zone show some contribution to family-Ⅱ2 oils. This work could provide scientific theoretical guidance for petroleum exploration and exploitation in the Jiangling depression.

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (Є1m), the paleoclimate was arid and sea level was relatively low in the study area. The depositional environment indexes (UEF-MoEF, gammacerane index, β-carotane and VEF) suggested that the upper Є1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (Є1q). Furthermore, the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Є1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the Є1q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.

Carbazole (CA) and alkylated carbazoles are common nitrogen-containing heterocyclic aromatic compounds widely found in crude oils and sedimentary rocks extracts. Their geochemical significances in petroleum exploration have been widely investigated in previous studies. Using density functional theory (DFT) and molecular dynamics (MD) simulation, we calculated the polarity of alkylated carbazole isomers/homologues and their adsorption energy with water and α-quartz, respectively. The calculated results show that there are significant differences in migration fractionation effects for alkylated carbazole isomers/homologues. This study confirmed the validity of previously proposed carbazole migration indicators, i.e. 4-/1-MCA (the relative abundance of 4-methylcarbazole to 1-methylcarbazole), 2,5-/1,8-DMCA (2,5-dimethylcarbazole to 1,8-dimethylcarbazole) and 2,4-/1,8-DMCA (2,4-dimethylcarbazole to 1,8-dimethylcarbazole) on the basis of quantum chemistry and molecular simulation. All these three parameters have been successfully applied in a lacustrine sandstone reservoir in the Fushan Depression, Beibuwan Basin (South China Sea). The calculations of Gibbs free energy show that the differences in thermal stability between the isomers of alkylcarbazoles can be negligible. The source input and depositional environment have no significant effect on these indicators for oils from a same oil family. Hence, the values of these parameters are mainly controlled by subsurface petroleum migration processes. Therefore, the molecular parameters relative to alkylated carbazoles can be used as practically effective geochemical markers to trace the oil filling orientation and pathways for subsurface oil migration.

Dibenzofuran and its methyl substituted homologues are the important oxygen-containing polycyclic aromatic compounds in crude oils. The ratio of 1-/4-methyldibenzofuran has been successfully used in sandstone reservoir tracing oil migration orientation. Here we focus on the geo-chromatographic fractionation effect of methyldibenzofuran (MDBF) in dolomite reservoirs. We calculated the charge density differences between 1-methyldibenzofuran (1-MDBF) and 4-methyldibenzofuran (4-MDBF) based on density functional theory. The E I n between MDBF isomers and dolomite (104) surface was also calculated according to molecular dynamics simulation. The result show that comparing with the 4-MDBF, there is strong interaction between 1-MDBF and dolomite (104) surface leading the ratio of 1-MDBF and 4-MDBF increase with the migration distance. There is no significant correlation between bitumen reflectance and the ratio of 1-MDBF and 4-MDBF in our samples. In addition, molecular simulation results show that the MDBF isomers are thermodynamically stable and dissociation of Ph-CH3 is difficult to occur under geological condition. All these demonstrate that 1-MDBF and 4-MDBF can be survived after oil-cracking to gas and pyrobitumen. Therefore, in theory, the ratio of 1-/4-methyldibenzofuran is also regarded as an index for oil migration orientation in dolomite reservoir. Gas chromatography-mass spectrometry results show that MDBFs occur at relatively higher abundance in the bitumens of the Ediacaran Dengying Formation (Z2dn4) and Cambrian Longwangmiao Formation (Є1l) in the Sichuan Basin. The ratio of 1-/4-methyldibenzofuran has been successfully used to trace the crude oil filling pathway in the Z2dn4 and Є1l dolomite reservoir in the Sichuan Basin in this study. Two preferential filling orientations (i.e. from west to east and from north to south) for paleo-oil have been identified in the contour maps of 1-/4-methyldibenzofuran in the Z2dn4 and Є1l, which is consistent with the geological background.

Light hydrocarbons (LHs) are one of the main petroleum fractions in crude oils, and carry much information regarding the genetic origin and alteration of crude oils. But secondary alterations—especially biodegradation—have a significant effect on the composition of LHs in crude oils. Because most of the LHs affected in oils underwent only slight biodegradation (rank 1 on the biodegradation scale), the variation of LHs can be used to describe more the refined features of biodegradation. Here, 23 crude oils from the Dawanqi Oilfield in the Tarim Basin, NW China, eleven of which have been biodegraded to different extents, were analyzed in order to investigate the effect of slight to minor biodegradation on C6–C7 LHs. The study results showed that biodegradation resulted in the prior depletion of straight-chained alkanes, followed by branched alkanes. In slight and minor biodegraded oils, such biodegradation scale could not sufficiently affect C6–C7 cycloalkanes. For branched C6–C7 alkanes, generally, monomethylalkanes are biodegraded earlier than dimethylalkanes and trimethylalkanes, which indicates that branched alkanes are more resistant to biodegradation, with the increase of substituted methyl groups on parent rings. The degree of alkylation is one of the primary controlling factors on the biodegradation of C6–C7 LHs. There is a particular case: although 2,2,3-trimethylbutane has a relative higher alkylation degree, 2,2-dimethylpentane is more resistant to biodegradation than 2,2,3-trimethylbutane. 2,2-Dimethylpentane is the most resistant to biodegradation in branched C6–C7 alkanes. Furthermore, the 2-methylpentane/3-methylpentane and 2-methylhexane/3-methylhexane ratios decreased steadily with increasing biodegradation, which implies that isomers of bilateral methyl groups are more prone to bacterial attack relative to mid-chain isomers. The position of the alkyls on the carbon skeleton is also one of the critical factors controlling the rate of biodegradation. With increasing biodegradation, Mango’s LH parameters K1 values decrease and K2 values increase, the values of n-heptane and isoheptane decrease, and the indices of methylcyclohexane and cyclohexane increase. LH parameters should be applied cautiously for the biodegraded oils. Because biodegraded samples belong to slight or minor biodegraded oils, the values of n-heptane and isoheptane from Dawanqi Oilfield can better reflect and determine the “Biodegraded” zone. When the heptane value is 0–21 and the isoheptane value is 0–2.6, the crude oil in Dawanqi Oilfield is defined as the “Biodegraded” zone.

Dibenzothiophenes (DBTs) and benzo[ b ]naphthothiophenes (BNTs) are the main sulfur-containing heterocyclic aromatic compounds in crude oils and sedimentary rock extracts. Because of the relatively higher electronegativity and an unshared pair of electrons in the outer shell orbital of the sulfur atom, hydrogen bonds can be formed between the sulfur and hydrogen atoms of the surrounding medium in carrier beds. Therefore, DBTs and BNTs are potential geochemical markers for tracing oil filling pathways. A correlation of oils in the Halahatang Oilfield of the Tabei Uplift in the Tarim Basin (NW China) shows that the molecular marker compositions of all oils are quite similar, implying that these oils have been sourced from the same source kitchen and belong to a same oil family. Using molecular indicators relative to DBTs and BNTs, namely, 4-/1-MDBT (4-/1-methyldibenzothiophene), 4,6-/(1,4 + 1,6)-DMDBT (4,6-/(1,4 + 1,6)-dimethyldibenzothiophene), (2,6 + 3,6)-/(1,4 + 1,6)-DMDBT ((2,6 + 3,6)-/(1,4 + 1,6)-dimethyldibenzothiophene), and [2,1]BNT/([2,1]BNT + [1,2]BNT) (benzo[ b ]naphtho[2,1- d ]thiophene/(benzo[ b ]naphtho[2,1- d ]thiophene + benzo[ b ]naphtho[1,2- d ]thiophene)), this paper traces the oil migration orientation and filling pathways in this oilfield. The isopleth maps of these molecular indicators unequivocally reveal an overall oil migration, filling points, and preferred filling pathways orientation from south to north. Based on this, a potential source kitchen is predicted in the southern part of the Halahatang Depression, most probably at the Manxi (Shuntuoguole) Low Uplift, just between the Awati and Manjiaer depressions. The southern part of the Halahatang Depression, along the oil charging pathways, is predicted to be the most promising region for oil exploration.

In 2013, a great breakthrough of deep petroleum exploration was achieved in the Cambrian pre-salt intervals of Wells Zhongshen1 (ZS1) and Zhongshen1C (ZS1C), Tazhong Uplift. However, the hydrocarbon discovery in the Cambrian pre-salt intervals has triggered extensive controversy regarding the source of marine oils in the Tarim Basin. The geochemistry and origin of the Cambrian pre-salt hydrocarbons in Wells ZS1 and ZS1C were investigated using GC, GC-MS and stable carbon isotope technique. These hydrocarbons can be easily distinguished into two genetic families based on their geochemical and carbon isotopic compositions. The oil and natural gases from the Awatage Formation of Well ZS1 are derived from Middle- Upper Ordovician source rocks. In contrast, the condensate and gases from the Xiaoerbulake Formation of Wells ZS1 and ZS1C probably originate from Cambrian source rocks. The recent discovery of these hydrocarbons with two different sources in Wells ZS1 and ZS1C suggests that both Middle-Upper Ordovician-sourced hydrocarbons and Cambrian-sourced petroleums are accumulated in the Tazhong Uplift, presenting a great exploration potential.

Four rearranged hopane series were simultaneously detected in oils from the Muglad Basin, one of several rift basins on the Central African Shear Zone (CASZ). The series are identified as having the structure of 17α(H)-diahopane (D series), 18α(H)-neohopane (Ts series), early eluting rearranged hopane (E series) and 21-methyl-28-norhopanes (Nsp series). The four rearranged hopane series exhibit strong positive correlations. The slope and intercept of the linear equation in plots of C29D/C29H vs C30D/C30H are 1.10 and 0.003 respectively, possibly implying identical biological origins and formation mechanisms. The relatively low slope ( Δ G ) calculated by density functional theory (DFT), the thermodynamic stabilities of C30 isomers follow the order C30D > C30H > C30E.

A total of 30 core samples of organic matter-rich Lower Toarcian shale (Posidonia Shale) were geochemically analyzed to study systematic changes in the quantitative and qualitative composition of specific biomarkers at different thermal maturities ranging from 0.48 to 1.45% vitrinite reflectance (VRr). The samples are derived from 5 wells (Hils Syncline, NW Germany) each representing a different level of maturation (0.48, 0.68, 0.73, 0.88, 1.45 %VRr). In NW Germany, Posidonia Shale was deposited under marine, oxygen-deficient bottom water containing initially high amounts of TOC (above 10%) and marine type I-II kerogen. Upon maturation, organic carbon has been lost and hydrogen index values have decreased. Biomarker ratios reflecting this maturation very well include ratios of short over long-chain n-alkanes, ratios of pristane and phytane over nC17 and nC18, Ts/(Ts + Tm), C29Ts/(C29Ts + C29H), ratios based on DMDBTs, and TA(I)/TA(I + II) ratios (triaromatic steroids). The evolution of different steroid-, hopane-, naphthalene-, phenanthrene- and dibenzothiophene-related parameters is documented and compared to literature. Concentrations of individual hydrocarbons were measured in order to obtain quantitative data on biomarker abundance and their evolution upon maturation. Whereas there is a general trend of increasing biomarker content from immature (0.48 %VRr) to early mature (0.68 %VRr) conditions and of decreasing biomarker content with maturation from 0.68 to 0.88 %VRr, there are some exceptions from this trend, especially at low levels of maturity. Almost no polycyclic biomarkers were detected at the highest maturity level of 1.45 %VRr in well Haddessen, where n-alkanes and aromatic hydrocarbons are predominant. The overall pattern of polycyclic biomarker abundance is not only determined by isomerization and aromatization processes, but mainly by preferential degradation of thermally less stable biomarkers leading to the observed modifications of biomarker maturity parameters. Furthermore, outer and inner sections from the 10 cm wide cores were compared in order to find out, whether 40 years of dry storage in a core repository affected the biomarker parameters. No significant and systematic differences have been observed, with exception of the most mature Haddessen well, where low molecular weight n-alkanes (

All four phenyldibenzothiophene (PhDBT) isomers were unequivocally identified in coals for the first time by the co-injection of authentic standards and comparison of the retention indices with those reported in previous studies. Based on density functional theory (DFT) calculations, the relative thermodynamic stability order of the phenyldibenzothiophene isomers (PhDBTs) was determined to be 3-PhDBT > 2-PhDBT > 4-PhDBT > 1-PhDBT. All PhDBT isomers were observed in a suite of coals from the Junggar and Ordos basins, and the effects of thermal maturity on their distributions were systematically investigated. Based on geochemical data and theoretical calculations, two phenyldibenzothiophene maturity indicators, i.e., PhDR1 (2-PhDBT/4-PhDBT) and PhDR2 ((2-PhDBT + 3-PhDBT)/4-PhDBT), are proposed. For high maturity coals (≥0.96 %Ro), PhDR1 and PhDR2 increase gradually with increasing thermal maturity. Two preliminary calibrations of these parameters against measured %Ro were made: %Rc = 0.60 × PhDR1 + 1.00 (≥1.00 %Ro) and %Rc = 0.30 × PhDR2 + 1.00 (≥1.00 %Ro). Compared to their corresponding methylated counterparts, phenyldibenzothiophenes are particularly useful as molecular markers of maturity at higher levels of thermal stress and provide a molecular assessment for sedimentary rocks containing types II and III kerogens where few biomarker parameters are available. PhDBTs in coals are likely to be generated during diagenesis/catagenesis as intermediate products in the formation of more condensed heterocyclic polycyclic aromatic compounds, such as triphenyleno[1,12-bcd]thiophene and benzobisbenzothiophenes, which were detected in the highly mature samples. This study broadens the current understanding of the occurrence, distribution, geochemical significance, and origin of phenyldibenzothiophenes in sedimentary organic matter.