Salinity plays a critical role in lake ecosystems, organic matter enrichment, and source rock formation, and holds significant implications for oil and gas exploration. Therefore, the mechanisms underlying lacustrine salinization have attracted increasing attention. Previous studies of salinization mechanisms have often focused merely on correlations between various factors and salinity. This limited scope has created a gap in the study of quantitative thresholds and triggering conditions, which impedes a comprehensive understanding of heterogeneous salinization mechanisms across temporal and spatial scales. This study investigates the multifactorial controls on heterogeneous salinization of the Paleogene strata of the Bohai Bay Basin, quantitatively elucidating their influence and identifying distinct thresholds or triggering conditions for each factor. The results show that lacustrine salinization heterogeneity is governed by multiple factors, including evaporation, marine transgression, surface runoff and hydrothermal activity: 1) evaporation is the predominant driver of salinization when climatic aridity exceeds a critical threshold (Sr/Cu > 25); 2) mantle-derived hydrothermal fluids, not alone but in conjunction with formation brines, can lead to significant salinization; 3) marine transgression can cause substantial dissolved salt influx to lakes during prolonged and extensive transgression, and its salinizing effect is amplified during sustained arid climatic regimes; 4) surface runoff effectively reduces salinity, particularly near river mouths, and the dilution effect becomes significant only when the salinity exceeds 8 parts per thousand. Collectively, evaporation and large-scale marine transgressions primarily control temporal heterogeneity in lacustrine salinization, whereas hydrothermal activity and surface runoff govern spatial heterogeneity.

In recent years, significant progress has been made in the deep exploration of the Bozhong Sag within the Bohai Bay Basin, eastern China, leading to the discovery of metamorphic oil and gas reservoirs such as Bozhong 19-A, Bozhong 21-B and Bozhong 13-C. Globally, the oil and gas source and accumulation mechanisms of deep large reservoirs, due to strong subsurface heterogeneity and complex structural conditions, remain critical issues to be addressed in understanding the distribution patterns of deep hydrocarbon reservoirs. The study was based on a number of different analyses. These included geochemical analyses of hydrocarbons, the hydrocarbon generation history of sub-depressions, fluid inclusion identification and quantitative grain fluorescence measurements. The aim was to clarify the hydrocarbon sources and accumulation models across different structural belts in the southwestern Bozhong Sag. Results show that the hydrocarbon source rocks for the Bozhong 19-A, Bozhong 21-B and Bozhong 13-C structures all originate from the Shahejie Formation, though the primary hydrocarbon supply sub-sags vary. The study area has undergone three stages of hydrocarbon charging: an early stage of crude oil charging (12-5.1 Ma), a middle stage of highly mature oil and gas charging (5.1 Ma) and a late stage of natural gas charging (0 Ma). The dynamic hydrocarbon accumulation process in the deep layers of the southwestern Bozhong Sag is characterised by 'early-stage source control, mid-stage adjustment and late-stage formation'. The deep hydrocarbon accumulation model is governed by the maturity of the source area: oil reservoirs or highly volatile oil reservoirs form near source areas of moderate maturity, whereas condensate gas reservoirs or gas reservoirs develop near source areas of higher maturity. The findings enhance the understanding of deep hydrocarbon accumulation processes and provide valuable insights for the exploration and prediction of deep reservoirs under similar geological conditions worldwide.

Chlorite coats are believed to inhibit quartz cementation and preserve deeply-buried sandstone porosity. However, geologists face numerous challenges in evaluating the influences of chlorite coats in real cases. To tackle these challenges, this work reviewed a large number of case studies to discuss the proper way to evaluate their role using petrography. The following five main conclusions were drawn: (1) Compared to other coat parameters, coat coverage is more reliable in evaluating the influence of chlorite coats on quartz cements. (2) In addition to chlorite coats, quartz growth is influenced by multiple factors such as temperature, while sandstone porosity is affected by various factors including mechanical compaction; therefore, when evaluating the influence of chlorite coats, geologists should take these factors into account. (3) Even if no negative correlation exists between chlorite coats and quartz cements, and no positive correlation is observed between chlorite coats and sandstone porosity, one cannot simply conclude that chlorite coats do not inhibit quartz cements and protect sandstone porosity. (4) Chlorite coats can significantly occupy pore space, leading to a net porosity decrease. (5) Chlorite coats can undergo significantly dissolution, while whether this phenomenon is ubiquitous remains underexplored.

The Bonan area, characterized by complex fault systems and multistage tectonic evolution, represents a critical hydrocarbon exploration target in the southern Bohai Bay Basin. Understanding the fault systems, depositional systems, and source rock distributions is crucial to hydrocarbon prospecting in this area. Here, we present a comprehensive characterization of the Cenozoic fault systems using high-precision 3D contiguous seismic data for the Bonan area. Analysis reveals, for the first time, a "three groups, three types" fault system organization that fundamentally controls the spatial-temporal distribution of depositional systems and source rocks. Specifically, three directional fault groups (NNE, NE, and EW/NWW), three fault types based on kinematic properties (strike-slip, extensional, and strike-slip-extensional), and three distinct tectonic evolution stages are identified. It is demonstrated that the early Cenozoic depositional systems were governed by the coupled interaction of EW-trending basin-controlling faults and NNE-striking-slip faults. In contrast, late Cenozoic deposition was controlled by a unified basin subsidence. These findings provide a new conceptual framework for understanding fault-controlled hydrocarbon systems in complex rift basins and offer practical guidance for exploration targeting in the Bonan area and analogous basins 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.

Deep layer, deep water, and unconventional areas are the inevitable trend of oil and gas exploration and development in the world. Hydrocarbon generation and accumulation are significant and challenging topics in deep oil and gas exploration, particularly in the context of the geological setting characterized by high temperature and pressure in deep layers. This study introduces an innovative approach by utilizing the findings from hydrogenation thermal simulation experiments to constrain the hydrocarbon generation kinetics parameters within the basin simulation process. By integrating these results with geochemical characteristics, key factors such as paleo-lake salinity, high temperature, excess pressure, and deep hydrogen-rich fluids were analyzed, revealing the unique characteristics of hydrocarbon generation evolution in the deep Paleogene source rocks of Bozhong Sag. It is found that salinization of lacustrine basin is beneficial to the enrichment of organic matter and the formation of high-quality source rocks. The high temperature is favorable to the thermal evolution of source rocks and hydrocarbon generation of organic matter, and it also leads to the cracking of hydrocarbons and affects the phase state of hydrocarbons. Excess pressure is conducive to the expulsion and migration of hydrocarbons and is the driving force of hydrocarbon migration and accumulation. Hydrogenation effect of deep hydrogen-rich fluids, especially near deep-seated faults, significantly improves the hydrocarbon generation potential of source rocks. The hydrogenation thermal simulation experiment has strongly confirmed this viewpoint, and with sufficient external hydrogen, the total hydrocarbon production can be increased by 2.5–3.2 times.

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 Paleocene-Eocene thermal maximum (PETM, similar to 56 Ma) was driven by global carbon release and temperature increases. Studies of marine strata have indicated that volcanic activity during the PETM led to significant carbon emissions. However, the impact of volcanic activity on terrestrial strata and whether this activity was regional remain unclear. Here, we conducted astrochronological analysis, polycyclic aromatic hydrocarbons (PAHs) analysis, organic petrological identification, and paleo-environmental proxies to identify lacustrine records of PETM from the Miaoxi Depression in the Bohai Bay Basin, eastern China. The analysis of natural gamma-ray (GR) series indicates that PETM occurred at 56.06 Ma and lasted approximately 0.58 Ma within the formation. Significant GR series changes, carbon isotope excursions, and red bed sedimentation were observed during the PETM. Paleo-environmental proxy changes, including decreased paleo-productivity, arid paleo-climate, increased paleo-salinity that promoted strongly reducing conditions, and decreased lake level, were correlated with the GR series and delta C-13(VPDB) excursions. The duration of the PETM event suggests that terrigenous sediments may record climate events in great detail. The PETM corresponds to a peak in 405 kyr eccentricity, but other changes in the astronomical cycle do not align with the paleo-environment data, indicating that astronomical forcing may not be the primary driver of the PETM. Increased PAHs and volcanic proxies suggest enhanced volcanic activity during this period, indicating that the PETM event was associated with regional volcanic activity.

The unclear understanding of the source rock characteristics for the Paleogene Dongying Formation in the Bozhong Sag leads to the lack of cognition in oil and gas exploration. This research systematically evaluated the discrepancy in geochemical characteristics of source rock intervals in the Dongying Formation based on detailed analysis using Rock‐Eval pyrolysis, total organic carbon (TOC), vitrinite reflectance (Ro), maceral components, stable carbon isotopes and biomarker parameters. It discusses the organic matter sources, depositional environment, thermal maturity and hydrocarbon generation potential in the source rock intervals of the Dongying Formation. The organic matter abundance in the third member (E3d3) and lower sub‐member of the second member (E3d2L) of the Dongying Formation is higher, with an average TOC of 2.08% and 1.03%, respectively, indicating that these source rock intervals could have good and excellent quality. The source rocks of the Dongying Formation predominately contain exinite maceral group and type II1 and II2 kerogen. The thermal evolution is mainly in the low‐mature and mature stages. The organic matter sources in the Dongying Formation are mainly dominated by mixed origin (the carbon isotope reversal of aromatics also provides a potential explanation). However, the values of 1,2,5−/1,3,6‐trimethylnaphthalene, (1,2,5,6 + 1,2,3,5)‐tetramethylnaphthalene/tetramethylnaphthalenes, methyldibenzofuran/methylphenanthrene (MDBF/MP) and dibenzofuran/phenanthrene (DBF/PHEN) parameters in the E3d3 and some samples in E3d2L decipher relatively smaller values than other source rock intervals, proving the enhancement of the contribution of lower organisms. Moreover, the terrestrial input in the bottom‐up deposition process of the Dongying Formation gradually increases. The depositional environment reveals the Dongying Formation source rocks mainly developed freshwater sulphur‐poor lacustrine facies and shallow lake‐fluvial delta facies in an open clay‐rich sedimentary environment with poor water column stratification, belonging to the typical weak oxidation–reduction depositional conditions. The comprehensive geochemical evaluation concludes that the favourable supply of lower organisms and stable clay‐rich depositional environment in the E3d3 interval of the Dongying Formation are potential explanations for the formation of lacustrine source rocks with high organic matter abundance, intermediate thermal evolution and excellent hydrocarbon generation potential, providing a good foundation for the exploration and development prospects of hydrocarbons.

Based on the data from 3D seismic surveys, drilling, sidewall coring, thin sections, and tests, this paper analyzes Meso-Cenozoic geotectonic dynamics, buried-hill reservoir characteristics, and differential enrichment patterns of oil and gas in the buried hills, as well as case studies of typical reservoirs, to systematically discuss the conditions required for the formation of buried-hills and reservoirs and accumulations in the large oil and gas fields in deep to ultra-deep composite buried hills in the Bohai Sea.. The key findings are as follows. First, deep to ultra-deep composite buried hills developed in the offshore Bohai Bay Basin primarily due to the double-episode destruction of the North China Craton in the Yanshanian and Himalayan.. The Tanlu Fault’s activity and the destruction of the North China Craton worked together to create the destruction center, which moved and converged episodically from the Bohai Bay Basin’s margins towards the Bozhong Depression. This led to the formation of two development zones for composite buried hills and an orderly process of mountain-building within the offshore Bohai Bay Basin. e. the middle and inner rim zones within the Bozhong Depression. Second, under the coupling of favorable lithologies and multi-stage structures, the middle and inner rim zones are favorable for the formation of reservoirs in fluid dissolution–pore/fracture zones underlying the weathering crust. Third, Massive hydrocarbons were produced along the middle and inner rim zones during the Episode II craton destruction, which caused overpressure. These hydrocarbons then moved to and accumulated in the composite buried hills. Excellent conditions for the accumulation of hydrocarbons are still present in the interior and lower portions of these buried hills. . These results encourage a change in buried hill research to investigate composite buried hills in three dimensions. It should be noted that the multi-stage volcanic structures in the inner rim zone of the depression and the deep to ultra-deep composite buried hill interiors in the middle rim zone are significant successor areas for further Bohai Sea exploration.

The Bozhong Sag is the largest petroliferous sag in the Bohai Bay Basin, and the source rocks of Paleogene Dongying and Shahejie Formations were buried deeply. Most of the drillings were located at the structural high, and there were few wells that met good quality source rocks, so it is difficult to evaluate the source rocks in the study area precisely by geochemical analysis only. Based on the Rock-Eval pyrolysis, total organic carbon (TOC) testing, the organic matter (OM) abundance of Paleogene source rocks in the southwestern Bozhong Sag were evaluated, including the lower of second member of Dongying Formation (E3d2L), the third member of Dongying Formation (E3d3), the first and second members of Shahejie Formation (E2s1+2), the third member of Shahejie Formation (E2s3). The results indicate that the E2s1+2 and E2s3 have better hydrocarbon generative potentials with the highest OM abundance, the E3d3 are of the second good quality, and the E3d2L have poor to fair hydrocarbon generative potential. Furthermore, the well logs were applied to predict TOC and residual hydrocarbon generation potential (S2) based on the sedimentary facies classification, using ΔlogR, generalized ΔlogR, logging multiple linear regression and BP neural network methods. The various methods were compared, and the BP neural network method have relatively better prediction accuracy. Based on the pre-stack simultaneous inversion (P-wave impedance, P-wave velocity and density inversion results) and the post-stack seismic attributes, the three-dimensional (3D) seismic prediction of TOC and S2 was carried out. The results show that the seismic near well prediction results of TOC and S2 based on seismic multi-attributes analysis correspond well with the results of well logging methods, and the plane prediction results are identical with the sedimentary facies map in the study area. The TOC and S2 values of E2s1+2 and E2s3 are higher than those in E3d3 and E3d2L, basically consistent with the geochemical analysis results. This method makes up the deficiency of geochemical methods, establishing the connection between geophysical information and geochemical data, and it is helpful to the 3D quantitative prediction and the evaluation of high-quality source rocks in the areas where the drillings are limited.

The organic matter （OM） enrichment mechanisms and depositional environment characteristics of lacustrine source rocks in the western Bozhong Sag,Bohai Bay Basin in Northeast China remain controversial.To address these issues,based on Rock-Eval pyrolysis,kerogen macerals,H/C and O/C ratios,GC-MS,major and trace elements,the Dongying Formation Member （Mbr） 3 （E 3 d 3 ）,the Shahejie Formation mbrs 1 and 2 (E 2 s 1+2 ),and the Shahejie Mbr 3 （E 2 s 3 ）source rocks in the western Bozhong Sag were studied.The above methods were used to reveal their geochemical properties,OM origins and depositional environments,all of which indicate that E 2 s 1+2 and E 2 s 3 are excellent source rocks,and that E 3 d 3 is of the second good quality.E 3 d 3 source rocks were formed under a warm and humid climate,mainly belong to fluvial/delta facies,the E 3 d 3 sediments formed under weakly oxidizing and freshwater conditions.Comparatively,the depositional environments of E 2 s 1+2 source rocks were arid and cold climate,representing saline or freshwater lacustrine facies,and the sediments of E 2 s 1+2 belong to anoxic or suboxic settings with large evaporation and salinity.During the period of E 2 s 3 ,the climate became warm and humid,indicating the freshwater lacustrine facies,and E 2 s 3 was characterized by freshwater and abundant algae.Moreover,compared with other intervals,the OM origin of E 3 d 3 source rocks has noticeable terrestrial input.The OM origin of the E 2 s 1+2 and E 2 s 3 are mainly plankton and bacteria.Tectonic subsidence and climate change have affected the changes of the depositional environment in the western Bozhong Sag,thus controlling the distribution of the source rocks,the geochemical characteristics in the three intervals of lacustrine source rocks have distinct differences.Overall,these factors are effective to evaluate the paleoenvironmental characteristics of source rocks by biomarkers,major and trace elements.The established models may have positive implications for research of lacustrine source rocks in offshore areas with few drillings.

The hydrocarbon potential of the Bodong Sag remains unclear. Investigation and comprehensive evaluation of Paleogene lacustrine source rocks is essential to determine the exploration potential and direction. This study examined the Paleogene source rocks using organic geochemical methods, including total organic carbon (TOC), Rock-Eval pyrolysis, maceral composition, vitrinite reflectance (Ro), and kinetic analysis. For the first time in this study area, a three-dimensional (3D) kinetics-based petroleum system modeling approach was employed to reconstruct the burial history, thermal maturity, and hydrocarbon generation history of the source rocks. The resource mass was evaluated based on the calculated generation mass. Results indicate that the third member (E 2 s 3 ) and first member (E 3 s 1 ) of the Shahejie Formation are mainly Type II 1 organic matter, exhibiting high generation potential. The third member of the Dongying Formation (E 3 d 3 ) contains mixed Type II 1 and II 2 organic matter, with moderate potential. The E 2 s 3 and E 3 s 1 source rocks matured early, entering the oil window in the Early Oligocene (∼30.3 Ma) and Middle Oligocene (∼28 Ma), respectively, and are currently in the wet gas to dry gas stage. The E 3 d 3 source rocks matured later, entering the oil window at the end of the Oligocene (∼24.6 Ma) and are currently in the late oil to the wet gas stage. Subsidence and burial have resulted in higher maturity in the southern subsag compared to the northern subsag, with the margins remaining in low maturity to immature stages. Resource estimates for the Bodong Sag source rocks are quantified at 8.33 × 10 8 t of oil and 1.68 × 10 11 m 3 of gas. E 2 s 3 , E 3 s 1 , and E 3 d 3 contribute 41.62%, 33.95%, and 24.43% respectively, with E 2 s 3 source rocks being the major contributor. The southern subsag, accounting for 76.92%, is the primary hydrocarbon generation kitchen. The significant increase in natural gas resources highlights the prospects for natural gas exploration in the Bodong area.

The hydrocarbon generation evolution characteristics of lacustrine source rocks are crucial for understanding hydrocarbon resources. In this research, we focus on the mudstone found in the third member of the Paleogene Dongying Formation (E3d3) within the Bozhong Sag of the Bohai Bay Basin as a case study. An analysis of the discrepancy in geochemical characteristics and kinetic parameters was carried out by utilizing the modern organisms (pine pollen and benthic algae) analogies to the organic matter (OM) components (Pinaceae sporopollen and benthic algae amorphous). Furthermore, the bulk hydrocarbon evolution of the Dongying Formation mudstones was further discussed in combination with a water-added sealing thermal simulation experiment and basin simulation. Our findings indicate that the pine pollen and benthic algae have higher OM abundance compared to the E3d3 mudstones, and their OM types are both Ⅱ1-Ⅱ2, which possess excellent potential for oil generation. In contrast, the E3d3 mudstones are predominantly in the low mature to mature thermal evolution stage. When we compare hydrocarbon generation rates and transformation ratios using Rock-Eval pyrolysis, it becomes evident that modern organisms are more prone to generate large amounts of hydrocarbons even at a low mature stage. The kinetic parameters of pine pollen are notably higher compared to those of benthic algae and the E3d3 mudstone samples. In parallel first-order reactions, the activation energy obtained by the single-frequency factor model (SFFP model) displays a narrow distribution range, while the activation energy distribution range of the multi-frequency factor model (MFFP model) is broader. The MFFP model focuses more on the variations of hydrocarbon kinetic parameters throughout the entire process, which is more in conformity with the actual geological process. The enclosed thermal simulation experiment and basin simulation analysis unveil the presence of two distinct phases of hydrocarbon generation within the Dongying Formation. Furthermore, it becomes evident that abundant benthic algae and Pinaceae sporopollen components play vital roles in both of these phases. In conclusion, the method of applying modern organisms to analogize the hydrocarbon generation evolution characteristics of the parent components of lacustrine source rocks seems feasible. Meanwhile, the Dongying Formation in the Bozhong Sag also possesses broad exploration prospects for hydrocarbon resources.

Lacustrine source rocks have a significant oil potential and contribute to 95 % of oil reserves in China's offshore areas, Nature of these rocks is related to climate and structure and they are commonly heterogeneous. Mechanism of organic matter accumulation in these rocks has been a matter of debate. Based on Rock-Eval pyrolysis, organic petrographic examinations, gas chromatography-mass spectrometry (GC-MS) analysis, and trace elements analysis, this paper illustrates the paleoclimate, tectonic activity, paleo-productivity, and preservation conditions recorded in high-quality source rocks of the Shanan Sag. The aim is to understand the controlling factors on the deposition of these source rocks. The results indicated that the organic matter of source rocks in middle segment of the third member of the Shahejie Formation (Es3m) was mainly from phytoplankton (average content = 52.2 %), the development of source rocks was controlled by high paleo-productivity. The primary productivity was low during the period of the first and second members of the Shahejie Formation (Es1-2), and the high gammacerane index, and low Pr/Ph ratios indicated that the source rocks were mostly controlled by a good reduction environment with high salinity. The source rocks of the third member of the Dongying Formation (Ed3) were characterized by high terrestrial materials (average exinite content = 62 %) and low to moderate Pr/ Ph ratios, which indicated the source rocks were affected by both paleo-productivity and preservation environment. A productivity-controlling model in Es3m was established along with a co-controlling model of productivity and preservation in Ed3, which was largely developed under the background of a warm-humid climate and strong depression-controlled fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity.

A large gas field with reserves of nearly 200 billion m3-BZ19-6-was discovered in the Bozhong Depression in the Bohai Bay Basin in 2018. There is a considerable difference between the amount of natural gas that would traditionally be expected to be generated by the thermal degradation of low-mature kerogens and the resources that have been confirmed by exploration. Therefore, the geochemical characteristics and the genesis of gas have become crucial aspects of investigating deep natural gas in the Bozhong Depression. The deep gas in the depression is predominantly methane. Its dry coefficient (C1/C1-5) ranges from 0.73 to 0.94, which is generally characterized as wet gas. The main non- hydrocarbon gases are CO2 (1.26 %-52.00 %) and N2 (0.1 %-0.74 %), with traces of H2S (10.44 x 10-6-36.63 x 10-6 ppm). The natural gases are thermogenic oil-type gases from the Shahejie and Dongying Formations. The deep natural gas in the Bozhong Depression is mainly derived from kerogen degradation, with contributions from oil cracking gas in the BZ1/19 and BZ2/3 structures. Complex carbon isotopic reversals are caused by the filling and mixing of natural gas with different maturities from the same source, evaporative fractionation due to the filling of late-stage high-mature natural gas, and Rayleigh fractionation under deep exogenous temperatures in the presence of transition metals. Combining the analysis of the fluid properties of natural gas, the evaluation of the performance of the migration system, and the understanding of the accumulation background indicates a high possibility that the gas was supplied from multiple hydrocarbon sources over long distances in the late stage. Thus, advantageous traps with high temperatures, close proximity to source kitchens, and favorable migration conditions are the preferred targets for future natural gas exploration in the Bozhong Depression. (c) 2024 Sichuan Petroleum Administration. Publishing services by Elsevier B.V. on behalf of KeAi Communication Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Chlorite coats are widely recognized as a key element in preserving sandstone porosity because it can inhibit the growth of quartz cements. However, the alteration of chlorite coats and its potential influences on sandstone porosity are rarely discussed. Therefore, this work used reactive transport models under different petrographic and geochemical conditions to investigate the influence of chlorite coats on sandstone porosity in a major dissolution window (100 degrees C). The HCO3-rich (CO2-charged) and HCO3-depleted (organic acids-charged) waters were injected to induce mineral dissolution and precipitation. The results indicate that the alteration of chlorite coats may result in sandstone porosity reduction. The HCO3-rich water leads to a porosity decrease mainly through the precipitation of magnesite and siderite resulting from chlorite dissolution. In contrast, the HCO3- depleted water causes a porosity decrease mainly through the redistribution of kaolinite and quartz cements. Factors, including pCO2, organic acid concentration, coat coverage, coat thickness, and grain size, have secondary influences on net porosity change. In comparison, factors, including chlorite mineralogy, detrital lithology, and the reduction of K-feldspar dissolution rate caused by chlorite coat, have negligible influences. The alteration of chlorite coats may introduce significant mis-interpretation to the analysis of the relationship be-tween chlorite coats and sedimentary facies. Moreover, the actual impact of pore-filling chlorite on porosity reduction may be either underestimated or overestimated. Therefore, the alteration of chlorite coats should be taken into consideration in future studies.