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Abstract Source rocks are found in the Eocene Liushagang Formation (El 1: bottom , El 2 and El 3:top members), in the Weixi'nan sag, Beibuwan Basin. The source rocks are mainly mudrocks, except El 2 , which consists of organic-rich shales at the bottom and mudrocks in the middle-upper part. Total organic carbon (TOC) and Rock-Eval analyses of 146 samples indicate that El 2 rocks have the best hydrocarbon generating potential, especially for organic-rich shales that contain up to 10% TOC with HI > 800 mg/g. The vitrinite reflectance analysis suggests that El source rocks are marginally mature to mature. Geochemical analyses were conducted on 18 El rock extracts and 18 oil samples from the WZ12 field. El 2 and El 3 source rock have similar organic facies and are distinct from El 1. El 2 and El 3 have higher C 27 βα (20R + 20S) diasteranes/C 27 αββ (20R + 20S) steranes (C 27 DiaS/C 27 S), Gammacerane/Hopane (G/H) and (C 28 + C 29) tricyclic terpane/((C 28 + C 29) tricyclic terpane + Ts) (ETR), and low C 23 tricyclic terpane/hopane (C 23 /H) and C 24 tetracyclic terpanes/C 26 tricyclic terpane (C 24 /C 26). These biomarker distributions indicate that El 2 and El 3 rocks were deposited under more alkaline-saline environment and had less import of terrestrial organic matter relative to El 1. El 2 can be distinguished from El 3 as having higher 4-methylsteranes/C 29 ααα steranes (4MSI) and G/H. WZ12 oils all exhibit similar compositions and display biomarker characteristics that correlate with the El 2 and El 3 rock extracts. 1D thermal and burial history model shows that El 2 and El 3 began to generate a great deal of hydrocarbon at ∼11.5 Ma and 23 Ma, respectively. At this stage, traps formed and faults actively extended downwards into the El 2 and likely into El 3. Oils expelled from El 2 and El 3 migrated upward into shallow intervals along faults then laterally along the structural slopes and accumulated in areas with good fault sealing primarily in the Miocene age Nx and Nj strata. Meanwhile, under the effect of overpressure in the middle of organic-rich shales, oils from El 2 may have migrated downward into reservoirs in the upper part of El 3. Lithologic traps in El 1 reservoir may be charged with relatively immature oil derived from El 1. Highlights • Source rocks in El 2 , especially organic-rich shales in the bottom of El 2 are the most important source rocks. • Sedimentary environment of different intervals of El have been distinguished. • WZ12 oils generated from oil El 2 and El 3 source rocks based on biomarkers parameters of saturated hydrocarbon. • Faults and overpressure may be responsible for the distribution of WZ12 oils. [ABSTRACT FROM AUTHOR]

Measurements of TOC, Rock-Eval pyrolysis, carbon isotope composition, solvent extract, extract fractionation, and the gas chromatography-mass spectrum (GC-MS) of the saturated fraction were conducted on the Lower Permian Fengcheng Formation carbonate-bearing shales and argillaceous dolomites of Well FN7 in the northwestern Junggar Basin to understand the geochemistry and depositional environmental evolution of the carbonate-bearing lacustrine source rocks. The results indicate that the Fengcheng Formation carbonate-bearing shales are fair to good source rocks with Type II kerogen for generating oil and minor gas, and the argillaceous dolomites are poor source rocks. The thermal mature shales contain more residually generated hydrocarbon, whereas the argillaceous dolomites contain more migrated hydrocarbon. This property is favorable for shale oil or tight oil accumulation in the Fengcheng Formation. Biomarker analyses indicate that the Fengcheng Formation source rocks were formed in the anoxic reducing lacustrine depositional environment with a stratified water column and high salinity. The source rock organic matter has major aquatic phytoplankton and algae input and less terrestrial organic matter input. During the Fengcheng Formation period, oxidation had a weak increasing tendency and salinity had a weak decreasing tendency from the early to late periods under the background of a reducing environment. Except for poor source rocks in upper and lower parts, the good source rocks are primarily in the middle part of the Fengcheng Formation. [ABSTRACT FROM AUTHOR]

There are a mass of transitional types of rock including proportionally-predominated shales, secondary siliciclastic sandstones, and carbonate rocks which are in small proportion (major dolostone and minor limestone) in Lucaogou Formation of the Jimusaer Sag of Junggar Basin. These rocks were deposited in brackish lacustrine environment under alkaline to weak alkaline conditions. The various rock types were all formed in reduction environment, but there is the broadest oxidation–reduction varying condition for the shale, and environmental reductivity where dolostones deposited is stronger than that of sandstones. The organic matter in source rocks comes from either aquatic organism or terrestrial organism. As reservoir, parts of sandstones and dolostones contain more primitive solid organic matters which might act as the hydrocarbon-generating material base. Parent material types of source rocks in different lithologies are mainly mixed type of type II 2 -I and have oil-prone characteristic. In the study area, the Ro value corresponding to oil generation peak for source rock is Ro=0.9%, lower than the classic model oil peak Ro=1.0% and thermal maturity of source rocks in multifarious types fall in the stage of oil generation. Determined by relationships of TOC content vs. Bitumen/TOC and TOC content vs. HCI, the lower TOC limit for active rocks is around 1.0%, hereby, about 90% of the shales, all of the limestone, and a third of the dolostone and sandstone are active source rocks. Therefore, there are preferable oil-generating potential of lithologically different source rocks in Lucaogou Formation of the Jimusaer Sag. The shales are chiefly oil-prone source rock and parts of dolostone, sandstone and limestone also have a contribution to the oil reservoirs. [ABSTRACT FROM AUTHOR]

In the brackish water sedimentary environment of the Junggar Basin in Xinjiang, the organic-rich shale and shale oil formation with relatively low maturity, is characterized by relatively high abundance of C 28 regular steranes. However, the biogenic source of organic matter and favorable sedimentary environment for the type of source rock has been thoroughly classified. In this paper, organic geochemical and petrological, analyses of the Lucaogou Formation shale in the Jimusaer Sag, Junggar Basin in the north of Xinjiang Uygur Autonomy Region were carried out. The source rocks in the Lucaogou Formation are mainly composed of two types of maceral including lamalginite (cyanobacteria) and telalginite (green algae), with the former predominantly fueling the high C 28 regular sterane content. Petrological, organic and inorganic geochemical analyses suggest that the lamalginite was developed in low-salinity water while the telalginite was developed in high-salinity water. Based on geochemical and organic petrological characteristics, both the lamalginite and telalginite have good hydrocarbon-generating potential, but in the lower thermal evolution stage of the Lucaogou Formation source rock, telalginite have higher hydrocarbon-generating rate. • Lamalginite (cyanobacteria) is the primary source of C 28 regular steranes. • The lamalginite (cyanobacteria) is more developed when the salinity is low and the telalginite (green algae) is more developed when the salinity is high. • In the lower thermal evolution stage of the Lucaogou Formation source rock, the lamalginite (cyanobacteria) has lower hydrocarbon-generating rate than the telalginite (green algae). [ABSTRACT FROM AUTHOR]

According to the geochemistry, organic petrography, stable carbon isotopic compositions and biomarker analyses of the Late Miocene Zeit Formation shale samples in the Tokar area, Red Sea Basin, the paleoenvironment and hydrocarbon-generating potential were analyzed. The Zeit Formation shales from the Wells S. Suakin-1 and Tokar-1 contain mostly Type III kerogens with minor Type II organic matter (OM), indicating more favorable gas-generating potential. OM of the shale samples from the Late Miocene Zeit Formation are predominantly of terrigenous higher plants. Kerogens from the Well Tokar-1 show abundant brown liptinites, indicating the mixed origin of planktonic/bacterial and terrigenous OM. The Late Miocene Zeit Formation was deposited in dysoxic/suboxic, reducing and salt water environments. According to the results of Tmax, Ro and biomarker maturity parameters, the most shale samples from the Zeit Formation are in early mature to mature stages. The shales from the Late Miocene Zeit Formation are generally fair to good source rock. The shales from wells Suakin-2, Talla-2 and Tokar-1 have high hydrocarbon-generating potential. • OMs from the Zeit Fm. Are predominantly of terrigenous higher plants. • Shales were deposited in dysoxic/suboxic, reducing and salt water environments. • Shales from the Zeit Formation are in early mature to mature stages. • Shales from the Zeit Formation are generally fair to good source rock. [ABSTRACT FROM AUTHOR]

In order to study the organic matter source in stylolite and its effect on hydrocarbon migration, carbonate rock core samples containing stylolites in the eastern Sichuan Basin were collected. The stylolite and matrix in the cores were analyzed by total organic carbon (TOC), Rock-Eval pyrolysis and gas chromatography-mass spectrometer (GC–MS) for the saturated fractions. The organic matter is more enriched in the stylolite than the matrix, and the stylolite have higher hydrocarbon expulsion efficiency than the matrix, which can be used as a supplement to the later generation of the source rock. The hydrocarbons formed by the matrix aggregated near the stylolites and cause the Hydrocarbon Index of the matrix which adjacent to the stylolites to become higher, this indicates that the stylolite is an important channel for hydrocarbon discharge of carbonate rocks. The ratios of close molecular biomarker in stylolite indicate that the deposition environment, biological origin and maturity of organic matter in the stylolites are consistent with the matrix, therefore, the organic matter in the stylolite should be inherited from the matrix, but the light to heavy molecular biomarker ratios of the stylolite indicate that component fractionation occurred when the hydrocarbons generated in the matrix migrated to the stylolite. Especially, a series of light to heavy molecule ratios of C 19 to C 23 tricyclic terpanes and C 14 to C 19 normal alkane of stylolite are lower than that of matrix, this indicates that the hydrocarbon fractionation in the course of hydrocarbon migration along the stylolite is more obvious than that in the matrix. • The organic-rich stylolite in low-abundance carbonate source rocks is an important supplement to the parent material for hydrocarbon generation. • The stylolite is important pathway for fluid migration in carbonate rocks. • The hydrocarbons in the matrix underwent a fractionation effect as they migrated from the matrix to the stylolite. • The hydrocarbon fractionation in the course of hydrocarbon migration along the stylolite is more obvious than that in the matrix. [ABSTRACT FROM AUTHOR]

This work systematically studied the significant geochemical heterogeneity of natural gases inside and outside the buried hills of the Xinglongtai area, West Sag, Liaohe Depression, Bohai Bay Basin. The gases inside the buried hills are thermogenic, whereas those outside the buried hills are dominated by secondary microbial gases, with a biodegraded thermogenic gas contribution. The humic- and sapropelic-type gases are primarily derived from the Es 3 and Es 4 source rocks, respectively. The gases inside the buried hills are primarily derived from the Es 3 source rocks, and decreasing δ13C 1 and C 1 /C 1-5 ratio towards the northeast are a result of the decreasing maturity of the adjacent Es 3 source rocks. The different geochemical characteristics of the gases outside the buried hills results from the relative contribution of secondary microbial and thermogenic gases, the maturity and organic type of the thermogenic gas, and biodegradation. In the Xinglongtai area, secondary microbial gas is an important shallow Paleogene exploration target, and the natural gas inside the buried hills is primarily enriched near the Es 3 source rocks. The findings and methods of this work are significant for studies on the origin and accumulation of natural gas inside and outside the buried hills in other petroliferous sags of the Bohai Bay Basin. Considering the geological and geochemical significance of the spatial geochemical heterogeneity of natural gas is essential for researching the origin and accumulation mechanism of gas and oil in complex exploration areas with multiple source rocks. • Sapropelic- and humic-type gases are from Es 4 and Es 3 source rocks, respectively. • Decreasing δ13C 1 and C 1 /C 1-5 inside buried hills results from varied maturities. • iC 4 /C 2 , 13C 1 and 13C 2 reveal mixing of secondary microbial and thermogenic gas. [ABSTRACT FROM AUTHOR]