Lacustrine Source Rocks Research Articles

Tectonic evolution and source rocks development of the super oil-rich Bohai Bay Basin, East China

The Bohai Bay Basin, as a super oil-rich basin in the world, is characterized by cyclic evolution and complex regional tectonic stress field, and its lifecycle tectonic evolution controls the formation of regional source rocks. The main pre-Cenozoic stratigraphic system and lithological distribution are determined through geological mapping, and the dynamics of the pre-Cenozoic geotectonic evolution of the Bohai Bay Basin are investigated systematically using the newly acquired high-quality seismic data and the latest exploration results in the study area. The North China Craton where the Bohai Bay Basin is located in rests at the intersection of three tectonic domains: the Paleo-Asian Ocean, the Tethys Ocean, and the Pacific Ocean. It has experienced the alternation and superposition of tectonic cycles of different periods, directions and natures, and experienced five stages of the tectonic evolution and sedimentary building, i.e. Middle–Late Proterozoic continental rift trough, Early Paleozoic marginal-craton depression carbonate building, Late Paleozoic marine–continental transitional intracraton depression, Mesozoic intracontinental strike-slip–extensional tectonics, and Cenozoic intracontinental rifting. The cyclic evolution of the basin, especially the multi-stage compression, strike-slip and extensional tectonics processes in the Hercynian, Indosinian, Yanshan and Himalayan since the Late Paleozoic, controlled the development, reconstruction and preservation of several sets of high-quality source rocks, represented by the Late Paleozoic Carboniferous–Permian coal-measure source rocks and the Paleogene world-class extra-high-quality lacustrine source rocks, which provided an important guarantee for the hydrocarbon accumulation in the super oil-rich basin.

Evaluation of source rocks and prediction of oil and gas resources distribution in Baiyun Sag, Pearl River Mouth Basin, China

By conducting organic geochemical analysis of the samples taken from the drilled wells in Baiyun Sag of Pearl River Mouth Basin, China, the development characteristics of hydrocarbon source rocks in the sag are clarified. Reconstruct the current geothermal field of the sag and restore the tectonic-thermal evolution process to predict the type, scale, and distribution of resources in Baiyun Sag through thermal pressure simulation experiments and numerical simulation. The Baiyun Sag is characterized by the development of Paleogene shallow lacustrine source rocks, which are deposited in a slightly oxidizing environment. The source rocks are mainly composed of terrestrial higher plants, with algae making a certain contribution, and are oil and gas source rocks. Current geothermal field of the sag was reconstructed, in which the range of geothermal gradients is (3.5–5.2) °C/100 m, showing an overall increasing trend from northwest to southeast, with significant differences in geothermal gradients across different sub-sags. Baiyun Sag has undergone two distinct periods of extensional process, the Eocene and Miocene, since the Cenozoic era. These two periods of heating and warming events have been identified, accelerating the maturation and evolution of source rocks. The main body of ancient basal heat flow value reached its highest at 13.82 Ma. The basin modelling results show that the maturity of source rocks is significantly higher in Baiyun main sub-sag than that in other sub-sags. The Eocene Wenchang Formation is currently in the stage of high maturity to over maturity, while the Eocene Enping Formation has reached the stage of maturity to high maturity. The rock thermal simulation experiment shows that the shallow lacustrine mudstone of the Wenchang Formation has a good potential of generating gas from kerogen cracking with high gas yield and long period of gas window. Shallow lacustrine mudstone of the Enping Formation has a good ability to generate light oil, and has ability to generate kerogen cracking gas in the late stage. The gas yield of shallow lacustrine mudstone of the Enping Formation is less than that of shallow lacustrine mudstone of the Wenchang Formation and the delta coal-bearing mudstone of the Enping Formation. The numerical simulation results indicate that the source rocks of Baiyun main sub-sag generate hydrocarbons earlier and have significantly higher hydrocarbon generation intensity than other sub-sags, with an average of about 1 200×104 t/km2. Oil and gas resources were mainly distributed in Baiyun main sub-sag and the main source rocks are distributed in the 3rd and 4th members of Wenchang Formation. Four favorable zones are selected for the division and evaluation of migration and aggregation units: No. ▪ Panyu 30 nose-shaped structural belt, No. ▪ Liuhua 29 nose-shaped uplift belt and Liwan 3 nose-shaped uplift belt, No. ▪ gentle slope belt of Baiyun east sag, and No. ▪ Baiyun 1 low-uplift.

Rhenium-platinum group elements reveal seawater incursion induced massive lacustrine organic carbon burial

Organic carbon burial in ancient lacustrine settings is a crucial source of petroleum resources. Unlike the marine environment, the dynamics of organic carbon burial in the terrestrial realm are more complex due to the interplay of global and regional climate-tectonic factors. There appears to be a potential linkage between seawater incursion events (SWIEs) and the generation of lacustrine source rocks. However, reliable proxies to reconstruct the frequency and extent of SWIEs are currently lacking. Here, we explore the potential of rhenium-platinum group elements (Re-PGE) system as a novel proxy for determining SWIEs in the Nenjiang Formation of the lacustrine Songliao Basin in China that is noted for its high-quality source rock. By comparing marine and non-marine intervals, we validate the utility of Re-PGE fractionation patterns and osmium (Os) isotope compositions. Moreover, the Re/Ir ratios demonstrate two main episodes of quantitative seawater-lake water exchange. The comparison of variable indicators shows that the Re-PGE system is more sensitive to the changes in water sources, thus providing detailed information of frequency and exchange amount. The inverse variation between seawater contribution and total organic carbon content further implies that the massive sulfate influx from SWIEs facilitated bacterial sulfate reduction in the sediment pile, which had the effect of recycling nutrients (e.g., phosphorous) back into the water column. The SWIEs-triggered eutrophication induced a positive feedback loop between productivity and hypoxia, creating ideal conditions for the preservation of organic carbon. Our data reveals the detailed mechanism of SWIEs-triggered organic carbon burial and emphasizes the significant role of SWIEs in generating economically important hydrocarbon reservoirs.

Comprehensive evaluation of the Paleogene lacustrine source rocks within the Bodong Sag, Bohai Bay Basin using kinetics-based petroleum system modeling

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 (E2s3) and first member (E3s1) of the Shahejie Formation are mainly Type II1 organic matter, exhibiting high generation potential. The third member of the Dongying Formation (E3d3) contains mixed Type II1 and II2 organic matter, with moderate potential. The E2s3 and E3s1 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 E3d3 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 × 108 t of oil and 1.68 × 1011 m3 of gas. E2s3, E3s1, and E3d3 contribute 41.62%, 33.95%, and 24.43% respectively, with E2s3 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.

Detailed oil-source correlation within the sequence and sedimentary framework in the Fushan Depression, Beibuwan Basin, South China Sea

The Fushan Depression is one of the petroliferous depressions in the Beibuwan Basin, South China Sea. Previous studies have preliminarily explored the origin and source of crude oils in some areas of this depression. Nevertheless, no systematic investigations on the classification and origin of oils and hydrocarbon migration processes have been made for the entire petroleum system in this depression, which has significantly hindered the hydrocarbon exploration in the region. A total of 32 mudstone and 58 oil samples from the Fushan Depression were analyzed to definite the detailed oil-source correlation within the sequence and sedimentary framework. The organic matter of third member of Paleogene Liushagang Formation (Els3) source rocks, both deltaic and lacustrine mudstone, are algal-dominated with high abundance of C23 tricyclic terpane and C30 4-methylsteranes. The deltaic source rocks occurring in the first member (Els1) and second member (Els2) of the Paleogene Liushagang Formation are characterized by high abundance of C19+20 tricyclic terpane and oleanane, reflecting a more terrestrial plants contribution. While lacustrine source rocks of Els1 and Els2 display the reduced input of terrigenous organic matter with relatively low abundance of C19+20 tricyclic terpane and oleanane. Three types of oils were identified by their biomarker compositions in this study. Most of the oils discovered in the Huachang and Bailian Els1 reservoir belong to group A and were derived from lacustrine source rocks of Els1 and Els2. Group B oils are found within the Els1 and Els2 reservoirs, showing a close relation to the deltaic source rocks of Els1 and Els2, respectively. Group C oils, occurring in the Els3 reservoirs, have a good affinity with the Els3 source rocks. The spatial distribution and accumulation of different groups of oils are mainly controlled by the sedimentary facies and specific structural conditions. The Els2 reservoir in the Yong’an area belonging to Group B oil, are adjacent to the source kitchen and could be considered as the favorable exploration area in the future.

Source Rock Prediction Using Well Log and Seismic Data: A Study of the Albian Stage in the Côte d’Ivoire Basin

The Côte d’Ivoire Basin is one of the most important oil production areas in Africa; and it is also the focus of global oil and gas exploration, but the degree of oil exploration and development in this basin are still in the early stages. Based on a comprehensive analysis of drilling, logging, and geochemical data in the Côte d’Ivoire Basin, this study comprehensively evaluates the Cretaceous lacustrine source rocks in the study area, establishes a total organic carbon (TOC) quantitative prediction model by using the total organic carbon method, determines the relationship between seismic reflections and the geochemical characteristics of the source rocks, and tracks the main source rocks horizontally. The results show that the TOC of the Lower Cretaceous Albian stage source rocks is 2.63 wt.%, the kerogen type II is dominant, and the vitrinite reflectance (RO) ranges from 0.3% to 0.9%. The source rocks are mainly in the low-mature to mature stage, and they are generally of good and premium quality. The distribution range of high-quality source rocks with TOC values of 4.00~5.00% is predicted using the total organic carbon method, and useing the seismic method, we determine that high-quality source rocks with larger thickness values and better continuity are mainly developed in the eastern depression center of the basin.

Petroleum systems analysis of the onshore Otway Basin, South Australia

An updated Petroleum Systems Model was built for the South Australian onshore Otway Basin, incorporating new seismic interpretation, seismic stratigraphy, core interpretations and a recent chemostratigraphy study. The Otway Basin is a divergent passive continental margin, with the Penola Trough the most significant hydrocarbon province discovered to date. Revision of the existing geochemistry database and seismic stratigraphy along with source rock model indicates potential good quality, liquids prone lacustrine source rocks are present in the Jurassic Casterton Formation (S0-P5) and the Early Cretaceous Basal Shale (McEachern Sand equivalent)/Lower Sawpit Shale (S1-P1/S1-P2). Gas prone source rocks are modelled in the Valanginian Upper Sawpit Shale (S1-P4/S1-P5) and Hauterivian Laira Formation (S1-P6–S1-P10). Modelling suggests liquid hydrocarbons were expelled from the Lower Sawpit Shale between 126 and 70 Ma, with the majority sourced from the Penola and Robe Troughs. Gas/condensate expulsion also took place up to 30 Ma from the Upper and Lower Sawpit Shale in the Penola and Robe Troughs. Gas is modelled to have been expelled from the Laira Formation in the Tantanoola Trough, between 100 Ma and present day.

Investigation on the thermal maturity, organic matter sources and depositional environment of lacustrine source rocks in the Dongying Formation of the Bozhong Sag, Bohai Bay Basin, China

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.

Paleoenvironmental Characteristics of Paleogene Lacustrine Source Rocks in the Western Bozhong Sag, Bohai Bay Basin, China: Evidence from Biomarkers, Major and Trace Elements

AbstractThe 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 (E3d3), the Shahejie Formation mbrs 1 and 2 (E2s1+2), and the Shahejie Mbr 3 (E2s3) 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 E2s1+2 and E2s3 are excellent source rocks, and that E3d3 is of the second good quality. E3d3 source rocks were formed under a warm and humid climate, mainly belong to fluvial/delta facies, the E3d3 sediments formed under weakly oxidizing and freshwater conditions. Comparatively, the depositional environments of E2s1+2 source rocks were arid and cold climate, representing saline or freshwater lacustrine facies, and the sediments of E2s1+2 belong to anoxic or suboxic settings with large evaporation and salinity. During the period of E2s3, the climate became warm and humid, indicating the freshwater lacustrine facies, and E2s3 was characterized by freshwater and abundant algae. Moreover, compared with other intervals, the OM origin of E3d3 source rocks has noticeable terrestrial input. The OM origin of the E2s1+2 and E2s3 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.

Petroleum geological characteristics and exploration targets of the oil-rich sags in the Central and West African Rift System

Based on seismic, drilling, and source rock analysis data, the petroleum geological characteristics and future exploration direction of the oil-rich sags in the Central and West African Rift System (CWARS) are discussed. The study shows that the Central African Rift System mainly develops high-quality lacustrine source rocks in the Lower Cretaceous, and the West African Rift System mainly develops high-quality terrigenous organic matter-rich marine source rocks in the Upper Cretaceous, and the two types of source rocks provide a material basis for the enrichment of oil and gas in the CWARS. Multiple sets of reservoir rocks including fractured basement and three sets of regional cap rocks in the Lower Cretaceous, the Upper Cretaceous, and the Paleogene are developed in the CWARS. Since the Late Mesozoic, due to the geodynamic factors including the dextral strike-slip movement of the Central African Shear Zone, the basins in different directions of the CWARS differ in terms of rifting stages, intervals of regional cap rocks, trap types and accumulation models. The NE–SW trending basins have mainly preserved one stage of rifting in the Early Cretaceous, with regional cap rocks developed in the Lower Cretaceous strata, forming traps of reverse anticlines, flower-shaped structures and basement buried hill, and two types of hydrocarbon accumulation models of “source and reservoir in the same formation, and accumulation inside source rocks” and “up-source and down-reservoir, and accumulation below source rocks”. The NW–SE basins are characterized by multiple rifting stages superimposition, with the development of regional cap rocks in the Upper Cretaceous and Paleogene, forming traps of draping anticlines, faulted anticlines, antithetic fault blocks and the accumulation model of “down-source and up-reservoir, and accumulation above source rocks”. The combination of reservoir and cap rocks inside source rocks of basins with multiple superimposed rifting stages, as well as the lithologic reservoirs and the shale oil inside source rocks of strong inversion basins are important fields for future exploration in basins of the CWARS.

Carbon Isotopic Behavior During Hydrocarbon Expulsion in Semiclosed Hydrous Pyrolysis of Type I and Type II Saline Lacustrine Source Rocks in the Jianghan Basin, Central China

Abstract Organic carbon isotopic analysis is a significant approach for oil-source correlation, yet organic carbon isotopic behavior during oil expulsion from saline lacustrine source rocks is not well constrained, and this hinders its wide application for fingerprinting oils generated by saline lacustrine source rock. To resolve this puzzle, semiclosed hydrous pyrolysis was conducted on typical saline lacustrine source rocks from the Qianjiang Formation (type I kerogen) and Xingouzui Formation (type II kerogen) sampled in the Jianghan Basin, China, under high-temperature high-pressure conditions (T = 275℃–400℃; P = 65–125 MPa). Experimental results show that there is minor carbon isotopic fractionation (<3‰) between pyrolyzed and nonpyrolyzed retained oil fractions during the main oil generation/expulsion stage of both type I and II source rocks. Carbon isotopic fractionations between expelled and retained oil fractions are also minor (<2‰) during this stage. The δ13C values of retained and expelled oil fractions generated by the type I saline lacustrine source rock correlate positively with the degree of oil expulsion, whereas the influence of oil expulsion on the δ13C values of oil fractions generated by the type II source rock was not consistent. In addition, carbon isotopic analysis also unravels the mixing of oil-associated gases with different maturity levels and/or generated via different processes. Outcomes of this study demonstrate that oil expulsion from type I and II saline lacustrine source rocks cannot be able to result in large-degree carbon isotopic fractionation, indicating that carbon isotopic analysis is a feasible approach for conducting oil-source correlation works in saline lacustrine petroleum systems.

Main controlling factors evolution on high-quality source rocks development in the Shanan Sag, Bohaibay Basin, NE China: Implication from structure, depositional environment, and organic matter

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.

The coupling control of biological precursors and environmental factors on β-carotane enrichment in alkaline lacustrine source rocks: A case study from the Fengcheng formation in the western Junggar Basin, NW China

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation (Fm.) are typically alkaline lacustrine source rocks, which are typified by impressively abundant β-carotane. Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments. However, the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm. remains obscure. Based on a comprehensive investigation of the bulk, molecular geochemistry, and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes, we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm. The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm. The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane, which is suggested by a good positive correlation between the 2-methylhopane index, 7- + 8- methyl heptadecanes/Cmax, C29%, and β-carotane/Cmax in sedimentary rocks and the predominance of cyanobacteria with abundant β-carotene in modern alkaline lakes. The enrichment of β-carotane requires the reducing condition, and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold. The paleoclimate conditions do not considerably impact the enrichment of β-carotane, but they have some influence on the water’s paleosalinity by affecting evaporation and precipitation. While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm., paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.

PALYNOFACIES AND ORGANIC GEOCHEMISTRY OF LACUSTRINE SOURCE ROCKS: THE POTRERILLOS – CACHEUTA SOURCE ROCK SYSTEM IN THE TRIASSIC CUYO BASIN, WEST‐CENTRAL ARGENTINA

This study presents an integrated investigation of the Upper Triassic Potrerillos – Cacheuta lacustrine source rock in the Cuyo Basin of western Argentina. Data came from palynofacies analyses, organic petrography, Rock‐Eval pyrolysis and mineralogical studies based on X‐ray diffraction analyses. An 80 m thick outcrop section was studied and is interpreted to represent the transition from shallow‐lacustrine sediments influenced by fluvial discharges (uppermost Potrerillos Formation) to the deposits of a deep, permanent lake (Cacheuta Formation). Three palynofacies were defined. Palynofacies I is characterized by shallowing‐upward cycles with abundant woody material, and was deposited under an oxic, disturbed water column. Palynofacies II and III occur in laminated shales rich in amorphous organic matter (AOM) and freshwater algal material (Botryococcus) respectively, which were deposited under oxygen‐depleted conditions. In general, the detrital material present suggests an input derived from fluvial discharges; however, interbedded tuffs altered to analcime and smectite suggest the transformation of vitric material in pyroclastic ash under saline to alkaline water conditions. Kerogen Types II/III and III with high total organic carbon values indicate a moderate oil‐ and gas‐prone source rock whose thermal maturity varies from immature to the early oil window (Tmax: 430‐438 °C; vitrinite reflectance: 0.59‐0.67 % VRo; and thermal alteration index: 2‐2+).This study demonstrates the importance of palynofacies analyses for the interpretation of depositional changes and associated controls in lacustrine shale successions. When integrated with data from organic geochemistry, palynofacies analysis is an important tool in the evaluation of a source rock's thermal maturity and hydrocarbon generation potential.

The implication of kinetics characteristics of modern organisms for the hydrocarbon generation evolution of organic matter components in the lacustrine source rocks: A case study of the Dongying Formation from the Bozhong Sag, Bohai Bay Basin

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.

Paleoenvironmental Evolution and Organic Matter Accumulation in a Hydrocarbon-Bearing Depression in the East China Sea

Investigating the paleoenvironment and characteristics of source rocks in sedimentary basins is crucial for understanding organic matter accumulation and guiding hydrocarbon exploration. The Lishui Sag, a significant hydrocarbon-bearing depression in the East China Sea, has experienced extensive marine transgression and increasing salinity in the Paleocene, but the changes in accumulation factors of organic matter during this evolution process remain unclear. Through a comprehensive analysis of total organic carbon (TOC), major and trace elements, and biomarker data, this study investigates the characteristics of source rocks from two lithostratigraphic units, namely the Paleocene Yueguifeng and Lingfeng formations, to gain deep insight into the effects of paleoenvironment on organic matter accumulation and hydrocarbon distribution. Our results indicate that the Lishui Sag transitioned from a closed lake to an open-marine environment in the Paleocene, with a shift from warm-humid to arid climate conditions. The biomarker distribution suggests a change in the origin of organic matter, with a higher input of terrestrial organic matter in the Lingfeng Formation. During the early stage, the lacustrine source rocks in the lower Yueguifeng Formation were formed in a relatively humid and anoxic environment within brackish water, resulting in a substantial influx of terrestrial and lacustrine algae organic matter. In contrast, in the late stage, the marine source rocks in the overlying Lingfeng Formation were developed in an arid and oxidizing environment. The lacustrine source rocks in the Yueguifeng Formation were notably more favorable to developing good-quality source rocks. Compared with the other regions, the western and northeastern parts of the study area have greater hydrocarbon generation potential due to the wider distribution of high maturity and organic-rich source rocks, with higher terrestrial and algal organic matter input. Moreover, considering the practical circumstances in the exploration, the northeastern part of the Lishui Sag is recommended as the next exploration target zone.

Factors Controlling Organic Matter Enrichment in Alkaline Lacustrine Source Rocks: A Case Study of the Late Paleozoic Fengcheng Formation in the Junggar Basin, NW China

AbstractThe late Paleozoic Fengcheng Formation shale (LPF shale) in the Junggar Basin, NW China, is the oldest alkaline source rock discovered in the world, providing a unique perspective with which to explore organic matter (OM) enrichment in alkaline lake environments. Combined with the organic carbon isotope profile and paleoenvironmental proxies, this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment, accompanied by frequent volcanic activity. High TOC values are concentrated in two intervals with frequent fluctuations in OM types. A negative excursion due to changes in sedimentary OM source is found in the δ13Corg profile. The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu. This implies that the extreme arid climate has led to high salinity, resulting in strong reducibility and changes in paleontological assemblages, which in turn controlled the differential enrichment of OM. The Fengcheng Fm. high‐quality source rocks are the result of the combined action of climatic events, volcanism, high‐salinity water environment and superior hydrocarbon‐generating organisms. The results provide new insights into the formation conditions of terrestrial alkaline high‐quality source rocks and the factors controlling alkaline OM enrichment.

Gas source of the Middle Jurassic Shaximiao Formation in the Zhongjiang large gas field of Western Sichuan Depression: Constraints from geochemical characteristics of light hydrocarbons

The Zhongjiang gas field is a typical large gas field in terrigenous strata of the Western Sichuan Depression. It remains debatable which member of the Upper Triassic Xujiahe Formation served as the source rocks and how significant the member contributed to the gas accumulations in the Zhongjiang gas field. In this study, we analyzed the essential characteristics of the Lower Jurassic source rocks and the geochemical features of light hydrocarbons in natural gas from the 2nd (T3x2) and 4th members (T3x4) of the Upper Triassic Xujiahe Formation (T3x), as well as the Middle Jurassic Shaximiao (J2s) and Qianfoya (J2q) formations. Based on this, we explored the sources of the natural gas in the Zhongjiang gas field and determined the natural gas migration patterns and their effects on the properties of light hydrocarbons in the natural gas. The results indicate that the Lower Jurassic lacustrine source rocks of the Zhongjiang gas field contain humic organic matter, with vitrinite reflectance (Ro) values ranging from 0.86% to 0.98%. Samples meeting the criterion for effective source rocks [total organic carbon (TOC) content ≥0.75%] exhibited an average TOC content of merely 1.02%, suggesting significantly lower hydrocarbon generation potential than source rocks in the underlying T3x, which show higher thermal maturity and TOC contents. For natural gas samples from T3x2, T3x4, J2s, and J2q reservoirs, their C5–7 iso-alkane content was significantly higher than their n-alkane content, and their methylcyclohexane (MCH) index ranged from 59.0% to 77.3%, indicating the predominance of methylcyclohexane in C7 light hydrocarbons. As indicated by the origin identification and gas-source correlation based on the geochemical features of light hydrocarbons, the natural gas in the Zhongjiang gas field is typical coal-derived gas. The gas from the primary pay zone of the Shaximiao Formation, with significantly high K1, (P2 + N2)/C7, and P3/C7 values, predominantly originated from the 5th member of the T3x and migrated in the free phase, with a small amount possibly sourced from the Lower Jurassic source rocks. The dissolution and adsorption during gas migration led to a decrease in the aromatic content in C6–7 light hydrocarbons and an increase in the iso-heptane values. Therefore, their effects must be considered when determining the gas origin and thermal maturity based on the aromatic content in C6–7 light hydrocarbons and iso-heptane values.

Hydrocarbon generation and migration in the Fuxin Basin during the Cretaceous evolution of the North China Craton, NE China

The extensive thinning and destruction of North China Craton (NCC) during the Early Cretaceous led to the development of numerous rift basins and petroleum systems. However, the specific relationship between NCC evolution and hydrocarbon generation and accumulation in these sedimentary basins is still unclear. In this study organic geochemistry and oil-source correlation analyses were conducted to investigate the hydrocarbons generation potential and migration pathway in the representative Fuxin Basin. Results demonstrate that the semi-deep lacustrine source rocks were developed in the Jiufotang Formation (K1jf) in the west and the upper Shahai Formation (K1sh) in the east, both of which are enriched with organic matter content and hydrocarbon generation potential of kerogen Types II to III. Based on the hierarchical cluster analysis of crude oils biomarker fingerprints, two crude oil types (A and B) were deduced. The Type A crude oil is characterized by high gammacerane, low pristane/phytane, relatively high C29 regular sterane and methylphenanthrene index, which is consistent with the K1jf biomarker characteristics in the west. Therefore, the Type A crude oil is likely generated from the K1jf source rock that accumulated in turbidite sand bodies of the K1jf and/or migrated to the K1sh reservoir through strike-slip faults. Type B crude oil is dominated by low gammacerane, moderate pristane/phytane, high C27 regular sterane and 1,2,5-trimethylnaphthalenes, which is in good agreement with the K1sh4 features. The basin simulation revealed that hydrocarbon generation of source rocks in the Jiufotang and Shahai formations was linked to a rapid subsidence of the basin, which was induced by intense extension with the NCC destruction during the Early Cretaceous. Subsequently, the rapid subduction of the Western Pacific plate during the earliest Late Cretaceous led to the development of extensive epigenetic fractures, enabling extensive hydrocarbon migration in the Fuxin Basin. This study sheds light on oil sources in sedimentary basins in an extensional setting and provides insights into the dynamic process of hydrocarbon generation and migration associated with NCC evolution.

Periods and Processes of Oil and Gas Accumulation in the HZ-A Structure Double Paleogene Field, Pearl River Mouth Basin

The source of oil and gas and the stages of oil and gas accumulation in the “double-Paleo” field of the HZ-A structure in the Pearl River Mouth Basin are analyzed, and the spatiotemporal coupling relationship of the key conditions of oil and gas accumulation are discussed to reconstruct the process of oil and gas accumulation. Based on previous research results, which are based the characteristics of biomarker compounds, the oil and gas in the HZ-A structure double Paleogene field came from the Paleogene Wenchang Formation hydrocarbon source rocks in the HZ26 sub-sag. By means of the casting thin section identification and inclusion homogenization temperature measurement, this paper reveals the three major hydrocarbon accumulation periods and corresponding fluid charging types in the “double-Paleo” field of the HZ-A structure in the Pearl River Mouth Basin. The results show that 13.8–10 Ma is the charging period of low mature crude oil, 10–5.3 Ma is the charging period of mature crude oil, and from 5.3 Ma is the natural gas charging period. Based on actual geological, drilling, logging, and seismic data, the key conditions for hydrocarbon accumulation in the HZ-A structure “double-Paleo” field are sorted out; that is, the source conditions are characterized by high-quality lacustrine source rocks generating early oil and late gas and a near-source continuous hydrocarbon supply. The reservoir conditions are characterized by weathering and superposition of a fracture zone that transforms into a reservoir, and a large-scale sandstone rock mass that transforms into a reservoir. The caprock conditions are characterized by the stacking of several thin mudstones that form a seal and the combination of multiple lithologies that block hydrocarbon migration. The trap conditions are characterized by multistage uplift structure traps and fracture-lithology combination control traps. The transport conditions are characterized by multi-stage cross-bed transport of source-connected faults and lateral differential transport of shallow sand in deep fractures. Finally, oil and gas accumulation models of the HZ-A structure double Paleogene field were established, and the accumulation process was reconstructed. The overall process involved three stages, with the first stage being the localized oil-displacing-water mode, the second being the large-scale oil-displacing-water mode, and the third being the late progressive gas-displacing-oil mode.