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Carbonate rocks exhibit significant heterogeneity as both a source rock and reservoir. Stylolite formation plays a crucial role in the enrichment of organic matter and the migration of geofluids within carbonate rocks. In order to study the enrichment mechanism of organic matter and the geofluid migration mode in the stylolites developed in carbonate rocks, stylolite-bearing core samples from the Dachigan structural belt in the eastern Sichuan Basin were collected. The stylolites and matrix were subjected to the total organic carbon (TOC) test and Rock-Eval pyrolysis, thin-section observation under fluorescent light, whole-rock X-ray diffraction, carbon and oxygen isotope analysis, and scanning electron microscopy. The organic matter occurring in the stylolites is mainly in the form of three types: soluble organic matter, pyrobitumen, and bitumen. This suggests that the organic matter within the stylolites mainly consists of secondary migrated organic matter. The stylolites also exhibit well-developed secondary dolomite and pyrite resulting from late-diagenetic recrystallization. These minerals contribute to the preservation of intercrystalline pore spaces and fractures, providing favorable conditions for oil and gas accumulation and migration. The strong cementation observed at the contact between the stylolites and matrix, along with the presence of secondary minerals nearby, may be attributed to the fractionation of light and heavy oil components during the migration of hydrocarbon fluids from the matrix to the stylolites. The thicknesses of the stylolites vary within the bulk, indicating severe diagenesis in thinner areas. Consequently, this leads to significant fractionation effects. The fractionation of crude oil components by stylolites poses challenges for the study of definitive oil–source correlations. To overcome these challenges, future research could investigate biomarker compounds to attempt oil–source correlations. Additionally, future efforts should take into consideration the spatial variation in the crude oil properties. Understanding the role of stylolites in organic matter enrichment and geofluid migration is crucial for optimizing exploration strategies in the Sichuan Basin, a region of growing importance in the energy industry. Moreover, our findings shed light on the complex interactions within stylolite-bearing rocks, which are not limited to this specific basin. These insights offer valuable contributions to the broader field of geology and reservoir characterization, enhancing our ability to predict and interpret similar geological formations globally. [ABSTRACT FROM AUTHOR]

Shales in the Carboniferous–Permian Fengcheng (FC) and Lucaogou (LCG) formations in Junggar Basin are important organic rich rocks containing significant oil resources. To evaluate the difference in sedimentary environment conditions and hydrocarbon-generating potential between the FC and LCG formations. Total organic carbon (TOC), Rock-Eval pyrolysis, solvent extraction, column fractionation, stable carbon isotope, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons and organic petrology from the source rocks of FC and LCG formations. were analyzed. The biomarker composition indicates that during the deposition of FC, LCG-1 to LCG-2, the sedimentary environment for the source rock formations changed with gradual decrease of salinity, from anoxic to dyoxic/suboxic in redox conditions, and from strong stratification to weakened stratification of water. The FC Formation source rock, with main telalginite (planktonic green algae), archaebacteria and minor terrestrial organic matter, deposited in the environment characterized by high salinity and strongly reducing condition. Its TOC content is relatively low with a high original hydrocarbongenerating potential of unit organic material. The LCG Formation source rock deposited in the environment with low salinity and large variations, the organic matter is mainly sourced from telalginite (planktonic green algae), lamalginite, bacteria and higher plants, resulting in strong heterogeneity of the source rock. The abundance of TOC is high, but the original hydrocarbon generation potential of unit organic matter is lower than that of FC Formation. The results provide a geochemical basis for further study of saline-brackish water sedimentary environment shales in the Junggar Basin. [ABSTRACT FROM AUTHOR]

Shallow‐delta sedimentary systems receive both terrestrial and marine organic matter. As oil and gas exploration activities determine that the source rocks of the deep‐water area of the Qiongdongnan Basin, northern South China Sea, are generally rich in and even dominated by terrestrial organic matter, this has led many researchers to examine the rules governing terrestrial organic matter enrichment in shallow‐delta sea sedimentary systems. However, the deep burial of source rocks in deep‐water areas and the relatively small amount of drilling undertaken have greatly restricted the study of these rules. In this study, the 'forward modeling' research method was used to observe and analyze the deposition and preservation of terrestrial organic matter through flume experiments, where the depositional results were carefully depicted and sampled. The total organic carbon content of selected samples was measured and when combined with qualitative observations and quantitative comparison results, the dominant enrichment areas of terrestrial organic matter were identified. The experimental results show that the overbank parts of the delta front, the dune countercurrent surface, the low‐lying parts, the delta front slope area and the shallow‐prodelta sea area are where terrestrial organic matter is predominantly enriched. This provides an important basis and guidance for the prediction of the development areas of marine source rocks with terrestrial input in the deep‐water areas of the Qiongdongnan Basin. [ABSTRACT FROM AUTHOR]

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]

The sources and enrichment of organic matter in a sediment core in the first member of the Xiagou Formation (Kg) from the Chang 2-2 borehole of the Jiuquan Basin, NW China, have been examined using Rock-Eval, maceral, carbon isotopes and biomarker data. This data indicates that highly variable organic matter sources and preservation conditions in response to climate change. TOC content, HI, and δC value were strongly correlated with the abundance of gammacerane, woody organic matter content, steranes/hopanes ratio, and C sterane content. This correlation demonstrates the importance that the control of the salinity of the depositional environment and organic matter sources can have upon the enrichment, type, and carbon isotopic composition of organic matter. In the Jiuquan Basin's relatively high temperature and arid climate, high salinity lakes with high primary productivity of algae, planktons, and bacteria, and good organic matter preservation conditions (anoxic bottom water) resulted in the enrichment of isotopically-light algae-bacterial organic matter. In the Jiuquan Basin's regions with a relatively low temperature and wet climate, fresh lakes with low primary productivity of algae, planktons, and bacteria received significant terrigenous high plants input, resulting in the deposition of a low abundance of isotopically heavier terrestrial organic matter. [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 research reveals the depositional environment can affect the organic matter enrichment. The major elements and trace elements are important indexes to reconstruct the depositional environment of Ordovician subsalt marine formations in the central-east Ordos Basin, combining organic parameters with inorganic parameters. Based on the microfacies, the study area is divided into gypsum-bearing dolomite flat, dolomite flat, gypsum-salt depression and limy dolomite flat. The higher Ti/Al ratios and the characteristics of trace elements show that there were detrital inputs in the sedimentation process. When the amounts of detrital inputs are large enough, they can obviously adsorb the elements and dilute the organic matter. Therefore, gypsum-bearing dolomite flat and dolomite flat have lower organic matter accumulation with higher detrital input. Lower P/Ti and Ba excess indicate the deficiency of paleoproductivity. Sr/Ba is higher than 0.6, indicating marine-continental transitional and marine environment. Even the paleosalinity is higher, previous researches have demonstrated the descending sea levels and strong hydrodynamic conditions are unfavorable for organic matter preservation. Lower paleoclimate index "C" and higher Sr/Cu illustrate arid and hot conditions. The value of V/(V + Ni) suggests that the limy dolomite flat is under the oxidizing conditions, and the three other environments are reductive to a certain extent. The Mo-U covariation shows that the water body is under strongly restricted conditions. These conditions are not conducive to biological growth and the accumulation of organic matter, causing low ancient productivity. The partial correlation analysis shows: the organic matter enrichment is affected by multiple factors; paleoproductivity, paleosalinity and paleoclimate have greater influence on the results of organic matter enrichment, and detrital input and oxidation conditions can decrease the paleoproductivity, indicating multiple factors are interrelated and interact with each other. • Major and trace elements are used to reconstructs the depositional environment of four types microfacies. • Paleoproductivity, paleosalinity and paleoclimate have greater influence on the organic matter enrichment. • Depositional environment has direction relationship with low paleoproductivity, causing low organic matter enrichment. [ABSTRACT FROM AUTHOR]

• Sedimentary conditions in the Lucaogou Formation source rocks are described. • The telalginite generated hydrocarbons earlier than the lamalginite. • New parameter index to identify effective source rocks is proposed. The Middle Permian Lucaogou Formation in the Jimusaer Sag is a hotspot for exploring and developing lacustrine tight oil in the Junggar Basin, NW China. In this study, we evaluated the tight oil potential from the perspective of source rock hydrocarbon generation and expulsion. Based on a detailed organic, petrological, and geochemical characterization of the target interval within the key well JHBE, kinetic experiments were performed on representative shale samples from the well JHBE and the threshold of hydrocarbon expulsion of the source rock was established. The findings of this study revealed that the Lucaogou Formation shales were deposited in a dysoxic to anoxic and clay-poor lacustrine environment with variable salinity and their organic matter (OM) was contributed from both lamalginite and telalginite with minor vitrinite and inertinite, resulting in good to excellent source rock potential. The lamalginite was deposited in water with low salinity, whereas the telalginite developed in water with relatively higher salinity. Although the greater contribution of lamalginite resulted in a higher OM content than that of telalginite, the former generated a lower amount of hydrocarbons than the latter, because telalginite is capable of generating hydrocarbons earlier than lamalginite, which is indicated by the higher HCI (Hydrocarbon index, S 1 × 100/TOC), EOM/TOC (extraction of organic matter/total organic carbon), C 29 ββ/(αα + ββ), and C 29 ααα20S/(20S + 20R) sterane values for telalginite than for lamalginite and is also evident from the difference in activation energy distributions between lamalginite and telalginite source rocks. As determined from the relationships of sterane maturity parameters C 29 ββ/(αα + ββ) and C 29 ααα20S/(20S + 20R) vs EOM/TOC ratio and HCI values, the lower C 29 ββ/(αα + ββ) and C 29 ααα20S/(20S + 20R) limits for active source rocks were approximately 0.28 and 0.44, respectively. [ABSTRACT FROM AUTHOR]