Quantifying movable oil is essential for shale oil resource evaluation. This study applies continuous residual hydrocarbon extraction to assess movable oil in the first member of the Qingshankou Formation and investigate hydrocarbon migration and accumulation. Chromatographic results show that medium-to-low-carbon hydrocarbons extracted by n-hexane and dichloromethane represent free retained hydrocarbons, validating residual hydrocarbon analysis as an effective method for movable oil quantification. Molecular geochemical data reveal distinct oil-source relationships between upper and lower intervals, with strong biomarker consistency between lower shale residual hydrocarbons and sandstone crude oil, as indicated by key terpane and sterane ratios. Variations in free hydrocarbon content and aromatic migration parameters (4-/1-MDBT and 4,6-/1,4-DMDBT) indicate a short-distance migration from shale to adjacent thin sandstones. High movable oil contents in three interbedded sandstone layers confirm their reservoir potential.

The deep-buried dolomites of the Upper Ediacaran Qigebrak Formation in the Tarim Basin possess substantial hydrocarbon potential. Although diagenetic processes, particularly dolomitization, are recognized as critical for reservoir evolution in this formation, their specific impacts on reservoir quality remain poorly understood. This study utilizes newly acquired petrographic, stable isotope, and radiogenic isotope data, integrated with carbonate U-Pb ages and temperature data, to elucidate the relationship between dolomitization processes and reservoir evolution in the Qigebrak Formation. Seven distinct dolomite phases were identified: dolomicrite (D1), very fine crystalline dolomite (D2), fine crystalline dolomite (D3), fibrous dolomite cement (D4), bladed dolomite cement (D5), fine to medium crystalline dolomite cement (D6), and coarse crystalline saddle dolomite cement (D7). D1 and D2 exhibit delta 13C and 87Sr/86Sr ratios consistent with coeval Ediacaran seawater, indicating an initial syngenetic dolomitization event in restricted lagoon/tidal flat environments. This event formed via nearsurface dolomitization driven by the reflux of slightly evaporative seawater. D4 and D5 also precipitated in near-surface settings under seawater dolomitization conditions, but their depleted delta 13C and delta 18O values with elevated 87Sr/86Sr ratios suggest the involvement of meteoric water in the precipitation process. In contrast, most D3 and D6 were formed through burial dolomitization at elevated temperatures. D7 originated from hydrothermal dolomitization at 135-150 degrees C, characterized by progressively depleted delta 18O ratios with increasing burial depth and the mixing of 87Sr-enriched hydrothermal fluids. Notably, early syngenetic dolomitization preserved primary pores in the Qigebrak Formation despite long-term burial, whereas later burial and hydrothermal dolomitization primarily adjusted the preexisting pore systems. This study enhances our understanding of multistage dolomitization processes in the Qigebrak Formation and provides insights for future exploration of Precambrian successions. (c) 2026 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Sterane and terpane compounds are key biomarkers in petroleum geochemistry to identify organic matter sources,depositional environments,and thermal evolution.The reliability of their analysis directly affects the accuracy of oil-source correlation and maturity assessment.This study addresses the limitations of conventional GC-MS,including co-eluting peak interference and limited quantitative accuracy in complex matrices.Source rocks and crude oils from the Huagang Formation in the Xihu sag were analyzed to compare GC-MS and GC-MS/MS in characterizing sterane and terpane parameters and their diagnostic significance.Using single ion monitoring(SIM)and selected reaction monitoring(SRM)to obtain parameters,sterane triangular plots and ratio parameters were employed for comprehensive evaluation.Results showed consistent identification of organic matter sources,indicating mixed-source input dominated by lower aquatic organisms.The average differences in heterocyclic sterane parameters measurements including C29-20S/(20S+20R),C29-αββ/(ααα+αββ),and C27-αββ/(ααα+αββ)were all less than 0.05,with overall trends consistent across both methods.Notably,GC-MS/MS yielded C29-20S/(20S+20R)values closer to the evolutionary endpoint(～0.55),which aligned better with the measured vitrinite reflectance(Ro)values(0.84%-1.40%).For terpane parameters,GC-MS/MS showed an average Ts/Tm value approximately 0.18-0.28 higher than GC-MS,and a lower Ga/C30H value by approximately 0.11-0.12,yet both methods maintained consistency in depositional environment identification and maturity trends.In conclusion,while GC-MS remains valuable for routine sterane and terpane analysis,GC-MS/MS offers higher accuracy in maturity assessment for high maturity or complex matrix samples by enhancing selectivity and resolution,providing more reliable technical support for fine-scale geochemical analysis of sterane and terpane compounds.

The Lower Jurassic Da'anzhai Member in the Sichuan Basin preserves a rare continental archive of the Toarcian Oceanic Anoxic Event (T-OAE). However, the mechanisms governing differential organic matter (OM) enrichment in its lacustrine shale successions remain debated. This study presents an integrated organic geochemical, biomarker, and paleoenvironmental investigation of the Da1 3 and Da1 submembers from the central Sichuan Basin, aiming to elucidate their contrasting OM accumulation processes and link them to global climatic forcing. The results demonstrate that the Da1 3 submember, deposited during the T-OAE climatic optimum, is dominated by aquatic (algal) OM, reflecting high primary productivity under warm-humid conditions and stable thermal stratification in an open lacustrine system. In contrast, the Da1 submember exhibits greater terrestrial input and elevated salinity under a hot-arid climate, where OM preservation was largely confined to deep, stratified settings due to frequent fluvial disruption in a closed basin. Integrating these findings with regional T-OAE records from the marine Qiangtang and lacustrine Ordos basins, we establish a unified open-lake (warm-humid) versus closed-lake (hot-arid) enrichment model that links global climatic perturbation, basin evolution, and sediment supply to differential lacustrine OM accumulation. Importantly, the interpreted lake evolution from a warm-humid, open system to a hot-arid, closed system finds a strong parallel in lacustrine records of the Early Cretaceous OAE1a from the Jiaolai Basin, eastern China. This consistency suggests that our proposed framework captures a recurring pattern of lacustrine response to extreme climate perturbations, thereby reinforcing its reliability and potential transferability to other basins and geological periods characterized by similar climatic forcing. This study advances the understanding of continental paleoenvironmental responses to global anoxic events and provides a predictive framework for shale oil exploration in analogous lacustrine settings worldwide.

Methyltrimethyltridecylchromans (MTTCs), molecular markers for paleosalinity reconstruction, have been widely found in sediments and crude oils from various origins and ages. However, the thermal stability of MTTCs remains ambiguous. In this study, the saturated and aromatic fractions were analyzed in a suite of oils exhibiting progressive thermal maturity from the Miaoxi area of the Bohai Bay Basin. The total concentrations of MTTCs in these oils decrease with increasing thermal maturity, reaching their lowest levels prior to the late oil window. The relative thermal stability of these alkylated compounds decreases with the degree of alkylation, which may result from their degradation via demethylation on the benzene ring. The position of the methyl group in dimethyl-MTTCs affects thermal stability, with an observed order of β-MTTC > ζ-MTTC > γ-MTTC. This corresponds to varying steric hindrance effects from different methyl positions on the benzene ring, leading to differential thermal stabilities among individual isomers. The β/γ-MTTC ratio can reflect changes in the thermal maturity of crude oils at the early mature stage. The α/δ-MTTC ratio and a cross-plot of MTTCI versus Pr/Ph ratios indicate that these oils, in which MTTCs survive, were derived from low-salinity source environments. Further sample analysis is required to determine whether thermal maturity influences the behavior of these proxies in paleosalinity assessments of oils from other sources. The findings of this study offer valuable insights into oil-oil and oil-source rock correlations, as well as paleoenvironmental diagnosis of early mature oils in which MTTCs are present.

Objective The Chang 7 and Chang 6 Members of the Yanchang Formation in the Jingbian area of the Ordos Basin are important source rock intervals. However, systematic studies on their hydrocarbon generation potential and contributions to hydrocarbon accumulation remain limited, which has hindered further exploration efforts in this region. Therefore, this study applies geochemical analytical methods to systematically characterize the distribution and the geochemical properties of the Chang 7 and Chang 6 source rocks in the Jingbian area. In addition, the geochemical signatures of the crude oils from the Yan 9 and Chang 2 reservoirs in the Jingbian Oilfield were investigated. Methods By comparing the geochemical parameters of the Chang 7 and Chang 6 source rocks in the study area with those of the Zhangjiatan Shale in the basin center, the potential sources of the Yan 9 and Chang 2 reservoir oils were identified through oil–source correlation. Results The Chang 7 source rocks are moderate to excellent-quality source rocks, characterized by wide distribution, large thickness, Type Ⅰ–Ⅱ2 kerogen, and a mature thermal evolution stage. Biomarker characteristics indicate mixed organic matter inputs and deposition under reducing conditions with relatively low salinity. Low Pr/Ph ratios and high C27/C29 regular sterane ratios suggest that the organic matter was predominantly derived from aquatic algae and other lower organisms and accumulated in a reducing freshwater to brackish-water depositional environment. In contrast, the Chang 6 source rocks are moderate to good-quality source rocks, dominated by Type Ⅱ2–Ⅲ kerogen, and generally at the low-maturity to mature stage. Moderate Pr/Ph ratios and relatively low C27/C29 regular sterane ratios indicate a predomint innput of terrigenous higher plantsand deposition under weakly reducing conditions. Oil-source correlation results reveals that the Yan 9 and Chang 2 crude oils have the same originand were primarily sourced from the Chang 7 source rocks, with a minor contribution from the Chang 6 source rocks. The distinct geochemical characteristics of the Zhangjiatan Shale compared with the Yan 9 and Chang 2 crude oils further suggest that the Zhangjiatan Shale was not the major hydrocarbon source for these oils. Conclusions (1) The Chang 7 Member is the primary and high-quality source rock in the Jingbian area, exhibiting advantages over the Chang 6 Member in terms of thickness, lateral distribution, organic matter abundance, and hydrocarbon generation potential. (2) Distinct biomarker signatures differentiate the Chang 7 (aquatic/algal-derived organic matter under reducing conditions), Chang 6 (terrigenous organic matter under weakly reducing conditions), and Zhangjiatan Shale (highly aquatic organic matter under strongly reducing conditions) source rocks. (3) The Yan 9 and Chang 2 crude oils share a common origin and were primarily sourced from local Chang 7 source rocks, with possible minor contributions from the Chang 6 source rocks. These oils show no direct genetic relationship with the Zhangjiatan Shale in the basin center, supporting a predominantly local hydrocarbon charging model in the Jingbian area. [ Significance ] This study provides critical geochemical evidence for understanding the local hydrocarbon system, resolving uncertainties regarding oil sources, and evaluating exploration potential in the Jingbian area of the Ordos Basin.

Methyltrimethyltridecylchromans (MTTCs), a class of oxygen-containing aromatic derivatives, have been used as indicators of paleosalinity in source rocks and crude oils. However, the reliability of these compounds as indicators in mature organic matter remains unclear, hindering a definitive assessment of their significance for oil–oil or oil–source rock correlation. In this study, a suite of mature oils and associated source rocks from the Paleogene Shahejie (E2s) Formation in the Machang area, Dongpu Depression, Bohai Bay Basin, were analyzed. The distribution of bulk compositions and biomarkers in the oils and source rock extracts suggests a genetic relationship, indicating that the oils were derived from similar organic matter (predominantly algae and aquatic macrophytes) and depositional environments (low salinity), with comparable maturity levels (within the middle oil window). The β/γ-MTTC ratio, a proposed maturity indicator, appears unreliable in mature organic matter, as evidenced by its poor correlation with established maturity proxies (e.g., C29 24-ethylcholestanes αββ/(ααα + αββ)) in the studied samples. In contrast, MTTC-based salinity paraments (α/δ, α/γ, MTTCI, and the cross-plot of MTTCI versus Pr/Ph) consistently reflect a low-salinity depositional environment for these crude oils and source rocks, except in the ternary diagram of relative alkylation abundances. These findings suggest that MTTC-derived paleosalinity indicators may serve as effective tools for oil–oil or oil–source rock correlation within the middle oil window. This study provides evidence supporting the broader applicability of MTTC-based proxies for paleosalinity reconstruction and correlation studies, particularly in mature organic matter under geological conditions. The results also offer insights for regional petroleum exploration in saline lacustrine basins.

Aryl isoprenoids (AIs), a series of aromatic carotenoid derivatives, have served as significant biomarkers for paleoecology reconstructions. However, the effect of biodegradation on AIs remains unclear to date. Here, a confirmed family of progressively biodegraded oils from the Miaoxi Depression, Bohai Bay Basin was analyzed to investigate the fate of these compounds during severe biodegradation under natural conditions. Two pseudohomologous series with carbon numbers ranging from 13 to 31 and 40 were identified as 2,3,6- and 2,3,4-AIs in the reference oil. The concentrations of total AIs showed a gradual decline with increasing biodegradation but AIs survive in the most severely biodegraded oil (with a PM rank of 8 and an M-MN2 of 983) in this study. The differential proximity of adjacent methyl groups on the benzene ring may result in 2,3,4-AIs being more resistant to biodegradation compared to 2,3,6-AIs. No preferential biodegradation of individual AIs is proceeded systematically by carbon number, which may result from competition of two biodegradation pathways and favor of specific microbial communities. Moreover, the AIs-bearing proxies, including aryl isoprenoid ratio (AIR) and concentrations of total AIs, 2,3,6-AIs and 2,3,4-AIs, exhibit a significant decrease with increasing microbial alteration. Therefore, these proxies must be employed with caution to reconstruct photic-zone euxinia (PZE) for crude oils with potential microbial degradation.

The Member 4 of the Paleogene Shahejie Formation(Es4) in the Bohai Bay Basin is interspersed with a set of high-quality source rocks typified as intercalation of red and black mudstones. A large amount of petroleum(crude oil) originates from these source rocks. The Paleocene-Eocene Thermal Maximum(PETM)event occurred during the deposition of Es4 in the Bohai Bay Basin, and the organic matter enrichment model under this event is worth further investigation due to its relationship with and influence on petroleum accumulations. Well LK25-A, as the first oil exploration well drilled into the Es4 in Miaoxi area, serves as a valuable case to study organic matter accumulation. In this study, we integrate total organic carbon(TOC),Rock-Eval pyrolysis, microscopic observation, vitrinite reflectance(VRo), elemental analysis, and gas chromatography-mass spectrometry(GC-MS) to evaluate the hydrocarbon generation potential, organic matter types, thermal maturity, and sedimentary environment of the Es4 in Miaoxi area. The analysis of maceral and rock pyrolysis data reveals that Type I and Type II organic matter make up the majority of Paleogene mudstones in this region. The TOC and rock pyrolysis data show that mudstones in this area have high organic matter abundance and oil-generation potential. The measured vitrinite reflectance distribution of mudstone samples, which ranges from 0.3% to 0.74%, demonstrates that the Paleogene strata are at the immature to mature stage, and the samples from this area contain a sizable amount of bituminite and mineralbituminous groundmass. The analysis of biomarkers in the mudstone samples indicates that most of the mudstones in this area are in lacustrine and brackish-hypersaline lacustrine environment under a reducing condition, and some of the red mudstones in the Es4 are formed under a suboxic condition. Based on the size and morphology of the pyrite framboid, the redox conditions of the water mass during deposition or diagenesis are further analyzed. The ratio of the size of framboid pyrite(D) to the size of its micrograins(d) suggests that the Es4 black mudstone developed in an anoxic sulfuretted water environment. Both the inorganic and organic geochemical indexes show that the Paleogene paleoclimate has a great influence on the source rocks of Es4 in this area. The distribution of red-black strata in the area is the result of the combined action of the water redox state and the climatic variation during the PETM event. The development models of organic-rich source rocks from the Es3 and Es4 in this area have been created in light of potential connections between rapid redox variation and the PETM event during the Paleogene. These models may offer a theoretical guidance for petroleum exploration in Miaoxi area of Bohai Bay Basin and other contemporaneous continental basins around the world.

The crude oil types in the Miaoxi area of the Bohai Sea area are highly complex, and the source rocks and genetic types remain unclear. Using multivariate statistical analysis, researchers can comprehensively examine the interrelationships among multiple correlated variables, which is particularly suitable for large-scale data mining and regional oil-oil and oil-source analysis. In this study, based on the biomarker parameter index system, hierarchical cluster analysis (HCA) and principal component analysis (PCA), common methods in multivariate statistical analysis, were applied for oil-oil and oil-source correlation of crude oil from multiple layers in the Miaoxi area. Three types of crude oil were detected. Type Ⅰ crude oil are characterized by a low C23TT/C30H ratio with relatively low maturity. It may be derived from a freshwater lacustrine reducing environment with abundant input of terrigenous organic matter. This type of crude oil shows a strong correlation with the source rocks in the first and second members of the Shahejie Formation in the eastern sag of the Huanghekou Depression. Type Ⅱ crude oil is at a mature stage and features lower C23TT/C30H and G/C30H ratios compared to Type Ⅰ, but with slightly higher Pr/Ph, sterane/hopane, and C19TT/C23TT ratios. These features also indicate a freshwater lacustrine environment with terrestrial organic matter input. It is inferred that Type Ⅱ oil is mainly sourced from the third member of the Shahejie Formation, with contributions from source rocks in both the eastern sag of the Huanghekou Depression and the southern sag of the Miaoxi Depression. Type Ⅲ crude oil is at a mature stage, exhibiting a wide distribution range in multiple biomarker parameters, including ETR[(C28TT+C29TT)/(C28TT+C29TT+ Ts)], G/C30H, C23TT/C21TT, Pr/Ph, C23TT/C30H C24Te/C26TT, C27/C29 regular sterane, and 4-methyl sterane/ C29 regular sterane. These variations reflect heterogeneity in the depositional environment of oil source rocks and organic matter types. It can be concluded that type Ⅲ crude oil is mixed-source oil, likely derived from the third and fourth members of the Shahejie Formation. Alternating least squares analysis results indicated that Type Ⅲ crude oil is mainly derived from the source rocks in the fourth member of the Shahejie Formation, with a contribution rate of 85% to 93%, while the contribution from the third member is only 7% to 15%.

The Rongchang area, boasting abundant shale gas resources, has emerged as a pivotal region for reserve augmentation and production increase. Influenced by the Huayingshan Fault Zone, a multitude of faults have developed within this area. During the shale gas development process, the injection of high - pressure fluids and hydraulic fracturing operations are likely to trigger alterations in the stress field adjacent to the faults, thereby exerting an impact on the fault stability. Through the comprehensive utilization of geological and geophysical data, this research delves deeply into the contemporary in - situ stress environment of the Wufeng - Longmaxi Formation in the study area. In combination with the Coulomb criterion, it assesses the stability of the faults in the study area and probes into the factors influencing fault stability. The findings reveal that the contemporary in - situ stress relationship in the study area is characterized by SHmax > Sv > Shmin, placing it in a strike - slip stress environment. The evaluation of fault stability demonstrates that the faults in the western and southern parts of the study area exhibit poor stability, while those in the northeastern part possess relatively better stability. Moreover, it has been discovered that there exist disparities in stability along the same fault. There is a necessity to establish a fault model to further explore the stability factors. The results indicate that in the study area, faults with a dip angle of occurrence less than 10°, as well as those with an azimuth angle less than 30° and a dip angle greater than 60°, enjoy good stability. Conversely, for faults with an azimuth angle exceeding 50°, their stability diminishes as the dip angle increases. When the azimuth angle reaches 70° and the dip angle is greater than 70°, the stability is rather poor. This clearly indicates that the occurrence of faults stands as a crucial factor influencing fault stability. As the pore pressure rises, the range of azimuth angles and dip angles of potentially unstable faults gradually expands, signifying that pore pressure is also an essential factor affecting fault stability. This research furnishes a theoretical foundation for circumventing potential risks and attaining efficient development during the shale gas development process.

A significant climatic shift and marine incursion have been reported in the Paleogene sediments from the LishuiJiaojiang Sag, leading to the deposition of two distinct source beds: the lacustrine mudstones of the Yueguifeng Formation (E1y, ca. 66-64.6 Ma) and the marine mudstones of the Lingfeng Formation (E1l, ca. 61.6-59.2 Ma). However, the impact of this transition on source rock development remains unclear. We present a comprehensive geochemical and paleontological analysis of the E1y and E1l source rocks. Both total organic carbon (TOC) and Rock-Eval pyrolysis data show that the abundance and type of organic matter in the Jiaojiang Sag are superior to those in the Lishui Sag, and the hydrocarbon potential of the E1y is better than that of E1l. These differences are associated with paleoecological transition driven by climate change, marine incursion, and tectonic subsidence. During the E1y deposition, low values of strontium/copper, boron/gallium and C21/C23 tricyclic terpanes indicate a more fresher water column under the humid climate. In the Jiaojiang Sag, abundant freshwater algal fossils (Pediastrum and Botryococcus) and notable algal biomarkers (C30 tetracyclic polyprenoids, C30 TPP) suggest that lower aquatic organisms were the primary biological source. Moderate subsidence rate created the isolated lacustrine environments with a stable, anoxic bottom water (low pristane/phytane and high vanadium/ chromium), further enhancing the development of high-quality source rocks. In contrast, the good source rock developed in the Lishui Sag is attributed to open lake hydrology, more oxic conditions (higher pristane/phytane and lower vanadium/chromium), and a reduced algae contribution (lower C30 TPP levels). During the E1l deposition, as the climate transitioned to arid conditions and continued marine incursion, the appearance of Spiniferites ramosus fossils and the further decline in C30 TPP from the E1l source rocks indicate the replacement of freshwater algae by marine algae. Intense fault activity increased the influx of terrestrial organic matter (higher oleanane, T-bicadinane, and isopimarane levels) into the basin via the expansion of braided river deltas. Redox indicators reveal the oxygen-rich conditions hindered organic matter preservation, resulting in fair to good shallow marine source rocks. Our findings demonstrate the sensitivity of paleoecology to the late Paleocene climate fluctuations and marine incursion, and further highlight the diversity of Paleocene paleoecology resulted in unique geochemical characteristics and different hydrocarbon potential of source rocks.

The Dongpu Depression is a rift lake in the Bohai Bay Basin of eastern China, where abundant oil and gas resources were discovered. Previous geochemical investigations of Dongpu Depression oils have revealed a notable lack of data regarding the isotopic compositions of individual polycyclic aromatic hydrocarbons (PAHs) from the discovered Cenozoic crude oils. Using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), a novel approach combining molecular parameters and stable carbon isotope compositions of individual PAH was applied to study the depositional environment conditions, the different source inputs, the maturation stage and to identify the potential family of five crude oils selected from Dongpu Depression. A decrease of Ga/C30H together with low Pr/Ph ratios and changes in isotopic compositions indicate that the five Dongpu Depression oils were derived from OM deposited in an anoxic saline environment with a stratified water column where mixing of saline water and freshwater occurred. The low isoprenoid, tricyclic and tetracyclic terpane ratios in combination with the delta C-13 values of 1,6-DMN, 1,2,5-TMN, 1,3,6,7-TeMN, 9-MP and 1-MP attest that their sources have a mixed origin with high aquatic organism (planktonic) input and a lower land plant (C-3 plant) contribution. The delta C-13 values of phenanthrene between-30.0 %o and-20.0 %o indicate that the studied Dongpu Depression oils belong to a single family generated from the peak to the late oil generation stage (RcTNR-2 between 0.85 and 1.15 %). A correlation with various basins indicates that oil samples featured by delta C-13 values of Phen between-30.0 %o and-20.0 %o correspond to crude oils derived from mixing of aquatic and high plant contributions with different degrees of mixtures, which have not yet reached their cracking stage (Rc < 2.0 %). Crude oils from the Carboniferous and Mesozoic-Cenozoic Tarim rocks, Australian petroleum systems, Termit Basin, Bongor Basin, Fushan Depression and the studied Dongpu Depression oils belong to that group of oils. Crude oils characterized by lighter phenanthrene isotopic compositions (delta C-13 values of Phen <-30.0 %o) are mainly derived from marine/aquatic input and have already entered the cracking window (Rc > 2.0 %). This group of oils is represented in this study by the Cambrian-Ordovician Tarim Basin oil samples. The research shows the importance of the aromatic isotopic compositions in petroleum system characterization and could be used as a reference for a practical exploration campaign of petroleum.

当前中国已经进入常规-非常规油气并举勘探时代.四川盆地是我国最重要的天然气产区之一,常规、非常规气资源有序聚集.前人对相同烃源来源的石炭系黄龙组常规天然气(以下简称黄龙组天然气)和龙马溪组非常规页岩气(以下简称龙马溪组页岩气)地球化学组成差异及控制因素的研究较薄弱,为此通过大量的常规、非常规天然气组分、同位素和稀有气体组成分析,结合实际地质条件,对黄龙组天然气和龙马溪组页岩气地球化学组成差异进行了详细分析,并探讨了形成差异的控制因素,取得以下认识:①受成熟度、储层岩性、参与裂解物质等因素影响,同源的黄龙组天然气和龙马溪组页岩气组分和同位素组成差异明显;②黄龙组天然气CH4含量一般低于龙马溪组,而前者重烃气含量稍高,主要是由于成熟度差异导致;③龙马溪组页岩气中有机质热降解生成的H2S易与金属反应而被消耗掉,导致页岩气中不含H2S,黄龙组天然气中H2S应来源于有机质热降解;④黄龙组天然气CO2含量明显高于龙马溪组页岩气,与碳酸盐岩矿物高温分解关系密切,页岩中的N2含量低,是由于N2快速的运移扩散速率所导致;⑤成熟度越高,同源常规气与非常规气CH4碳同位素分馏程度越大,重烃气碳同位素也有类似特征,但是分馏程度没有CH4明显;⑥封闭体系内不同母质来源(干酪根、滞留油和湿气)裂解生成天然气的混合导致高-过成熟页岩气出现负碳同位素系列(δ13C1＞δ13C2＞δ13C3),而先期形成的伴生气与后期形成的原油裂解气混合是造成黄龙组天然气倒转(δ13C1＞δ13C2＜δ13C3)的主要原因.

Inorganic and organic fluids are widely found in sedimentary basins during oil and gas migration and accumulation. This research investigates two types of sandstone reservoirs with different pressure states and fluid evolution of the Shahejie Formation in the western slope belt of Dongpu Sag. Based on petrographic observations, micro-Raman spectroscopy, and fluorescent spectrometry, petroleum inclusions, CH 4 -bearing inclusions, and CO 2 -bearing inclusions were discovered in quartz-hosted minerals. The pressure–temperature–time–composition ( P – T – t – X ) properties of the CH 4 and CO 2 inclusions were obtained by quantitative Raman analysis and thermodynamic models. Minimum trapped pressure was predicted by the thermodynamic modeling of petroleum inclusions. Saffron yellow petroleum inclusions represent an early Oligocene oil-charging event (∼32–30 Ma), and the two kinds of reservoirs were all kept in moderate-overpressure conditions (average pressure coefficient: 1.26). Due to rapid uplift and denudation (Dongying movement) in the middle Oligocene in step II, crude oil degrades into CH 4 and CO 2 (density of CH 4 gas inclusions: 0.1010–0.1339 g/cm 3 ; density of CO 2 gas inclusions: 0.450–0.612 g/cm 3 ) and retains low-maturity (0.67–1.04%) bitumen in situ . Tectonic destruction causes the reservoir to be released to normal pressure. With the reburial of the Es 3 Formation, step II entered the middle stage of oil generation again (average pressure coefficient: 0.90). However, in step I, with secondary hydrocarbons occurring, second-stage oil (performance for pale blue fluorescence in petroleum inclusions) accumulated at ∼12–0 Ma (late Miocene–Pliocene). The second oil charging event in step I remains a reservoir under high-overpressure conditions (average pressure coefficient: 1.67).

The Fukang Sag in the Junggar Basin is the main exploration block. However, the origin and source of crude oil are still controversial, which seriously affects the well locating and exploration in this area. In the present work, 30 source rocks and 21 crude oils were collected for geochemical analysis to clarify the source of the organic matter, the sedimentary environment, and the evolution degree. Among them, the source rocks of the Pingdiquan Formation are type II1 organic matter with good quality, the source rocks of the Badaowan Formation are type II2-III organic matter with fair–good quality, and the source rocks of the Xishanyao Formation are type II2 organic matter with fair quality. All source rocks are in the mature stage. The results of the biomarker compounds show that the lacustrine mudstone of the Xishanyao Formation and the coal-measure mudstone of the Badaowan Formation were deposited in reducing environments. The former was mainly from lower aquatic organisms, and the latter was from terrestrial higher plants. The mudstone of the Pingdiquan Formation was formed in a weakly oxidizing–weakly reducing depositional environment, and its parent material was of mixed origin. Based on the results of the biomarker compounds and carbon isotopes, the crude oils were divided into three categories. The Family I crude oil has the characteristics of low maturity, low salinity, and more input of low-level aquatic organisms, and the carbon isotope has a good affinity with the lacustrine mudstone of the Xishanyao Formation. The Family II crude oil shows medium maturity, low salinity, mainly higher plant input, and heavy carbon isotope, mainly derived from the Badaowan Formation coal-measure mudstone. The Family III crude oil is characterized by high maturity, high salinity, mixed parent materials, and light carbon isotope and originates from the mudstone of the Pingdiquan Formation. The results provide a reference for oil and gas exploration and development in the eastern area of the Junggar Basin; the future research will focus on well areas with high maturity near the Fukang fault zone.

The distribution of benzo[b]naphthofurans (BNFs) and benzo[b]naphthothiophenes (BNTs) and their geochemical significance have been investigated for a set of delta-lacustrine mudstone samples from the Beibuwan Basin, South China Sea. BNF and BNT isomers in source rock extracts are identified in the m/z 218 and m/z 234 mass chromatograms of the aromatic fractions, respectively. The relative content of BN12F and BN21T increases as the increasing of burial depth and maturity, which is corresponding to relative higher calculated thermostability among the BNF and BNT isomers. Two maturity parameters, BNFR (abundance of BN12F relative to BN21F) and BNTR (abundance of BN21T relative to BN12T), both correlate well with vitrinite reflectance (Ro%), suggesting that these parameters are potential indicators for assessment of source rock maturity (Ro>0.5%). BNTR is more suitable for the mature stage (Ro<1.0%), as errors in measurement increase in overmature stage due to the low abundance of BN12T. The significant correlation between BNFs and DBF indicates their similar origin, likely an oxic environment with higher plant input. The thermal instability of BNFs at the overmature stage suggests that they may be converted to other compounds at high thermal maturity. The absolute concentrations of BNTs show a growing trend as maturity increases, indicating a thermal origin. BNTs may be generated from phenylnaphthalene isomers by incorporating a sulfur atom into biphenyl during catagenesis. This study achieves a better understanding of the occurrence and significance of BNFs and BNTs in sedimentary organic matter.

Methyltrimethyltridecylchromans (MTTCs) are biomarkers that are commonly used to identify immature-low mature source rocks or crude oil. MTTCs are abundant in the mature crude oils found in the Machang area of the southern Dongpu Depression in the Bohai Bay Basin. To explore the mechanism behind the enrichment of MTTCs in mature crude oils, a detailed study was conducted to investigate their distribution characteristics in crude oil samples. The study integrated tectonic evolution history, distribution characteristics, and thermal history of source rocks, molecular fingerprints characteristics of crude oils, and catalytic characteristics and combination features of clay minerals in the wall rock of crude oils. Based on this analysis, two enrichment models were proposed: (1) The first model suggests that crude oils were mixed with a small amount of immature-low mature soluble bitumen containing MTTCs; (2) The second model proposes that the evolutionary mechanism of clay minerals in the reservoir could reduce the decomposition rate and degree of MTTCs, resulting in their relative enrichment. Therefore, this study provides insights into the enrichment mechanism of MTTCs in mature crude oils and highlights the importance of considering various factors, such as tectonic evolution history, source rock characteristics, and catalytic properties of clay minerals, to understand the distribution and enrichment of biomarkers in crude oils.

以鄂尔多斯盆地杭锦旗地区山西组-太原组煤系烃源岩为研究对象,通过总有机碳测定、岩石热解分析、碳同位素测定及GC-MS分析等多种技术手段对两套烃源岩的地球化学特征进行了系统研究.结果表明,研究区山西组-太原组烃源岩有机质含量均较高,有机质类型以Ⅲ型为主,生烃潜力好.其中山西组烃源岩整体生烃潜力较太原组强,而太原组烃源岩成熟度则较山西组高.通过将两套烃源岩生物标志化合物特征进行对比分析发现,山西组烃源岩具有高姥植比,高(C19三环萜烷+C20三环萜烷)/C21三环萜烷值,高C24四环萜烷、C29规则甾烷、氧芴相对含量和低伽马蜡烷相对丰度的特征,反映其可能形成于氧化-偏氧化的淡,水环境,有机质来源以高等陆生植物为主;太原组烃源岩则具有低姥植比,低(C19三环萜烷+C20三环萜烷)/C21三环萜烷值,低C24四环萜烷、C29规则甾烷相对含量,低伽马蜡烷相对丰度和高硫芴相对含量的特征,说明其形成环境可能为相对偏还原的淡水环境,有机质来源中低等水生生物的贡献较多.基于两套烃源岩生物标志化合物组成特征上的差异,选取合适的生标参数建立了相应的有机地化判识指标将两套烃源岩区分开,这对研究区后续的油源对比及油气勘探工作有着较为重要的意义.

烃源岩有机相是有机质质量分数、有机质类型、有机地球化学特征和沉积环境的综合体现,利用有机相评价烃源岩特征是现今石油勘探中的有效方法.渤海海域庙西地区(黄河口东洼、庙西南洼)烃源岩有机相的研究相对薄弱,制约了该地区优质烃源岩的探寻,因此需要对该地区有机相纵向分布特征、平面展布特征及烃源岩形成机理展开研究.在对庙西地区东营组、沙河街组和孔店组烃源岩评价基础上,结合干酪根显微组分及生物标志物组成特征,综合考虑烃源岩沉积环境、沉积相及有机质生源特征等因素,进行有机相综合评价并预测优质烃源岩发育区.研究结果表明,庙西南洼、黄河口东洼中部沙三段及庙西南洼中南部沙一二段为优质烃源岩,其有机相类型为半深湖混源母质相(AB相)到滨浅湖陆源母质相(B相).古水深和古气候是控制庙西地区沙河街组湖相烃源岩发育的主要环境因素,古水深影响输入湖泊的有机质生源母质类型和有机质的保存条件,古气候变化控制输入湖泊的沉积物含量和浮游生物的繁衍程度.