Distribution Of Source Rocks Research Articles

Widespread coastal upwelling along the marginal Yangtze Platform (South China) during the early Cambrian: Implications for hyper-enrichment of organic matter

Early Cambrian Earth history witnessed significant changes in marine environments and biological evolution contemporaneous with extensive accumulation of organic-rich shale. However, major factors controlling hyper-enrichment of organic matter (OM) of lower Cambrian shale deposits remain controversial. Black shale of the lower Cambrian Niutitang Formation (NTT) of the Marginal Yangtze Platform (South China) deposited during the Cambrian middle Age 2 are especially organic-rich, with total organic carbon (TOC) concentrations of as much as 13.0 wt%. Geochemical evidence suggests that cool, dry paleo-climatic conditions that prevailed on the Yangtze Platform during the Cambrian Fortunian-late Age 2, induced vigorous coastal upwelling. Paleo-productivity level assessments indicate that the magnitude of primary productivity contemporaneous with deposition of the NTT shale deposits exceeded that of modern upwelling systems (e.g., Peruvian Margin). The widespread occurrence of phosphate nodules within these deposits and decreased Co-EF × Mn-EF values of associated lower Cambrian black shale successions deposited along the margin of the Yangtze carbonate platform suggest extensive coastal upwelling. Widespread and strong coastal upwelling in tandem with elevated surface water primary productivity and anoxic (even euxinic) bottom water conditions are manifested by deposition of OM hyper-enriched black shale during Cambrian Fortunian to middle Age 2 time. However, weakened seasonal upwelling that appears to have prevailed during the late Cambrian Age 2 was accompanied by accumulation of NTT shale deposits of diminished TOC content. In summary, this study provides robust evidence of extensive coastal upwelling along the Marginal Yangtze Platform during early Cambrian time that favored accumulation of OM hyper-enriched shale. These results help to elucidate the distribution of high-quality lower Cambrian natural gas source rocks in South China.

Impact of Tethyan domain evolution on the formation of petroleum systems in the Sichuan super basin, SW China

Based on the “unidirectional break-up and convergence” geodynamic model, this study investigates the impact of the evolution of the Tethyan domain on the formation of petroleum systems in the Sichuan super basin and explores the enrichment pattern of natural gas. The results show that, firstly, the Sichuan Basin and its surrounding areas have experienced two unidirectional rifting-aggregation cycles triggered by the Proto-Tethys Ocean and the Paleo-Tethys Ocean during the Neoproterozoic to Triassic. During Jurassic–Cenozoic, the Sichuan Basin is incorporated in the circum-Tibetan plateau basin-mountain coupled tectonic domain system. The episodic tectonic movements within the plate control the sedimentary infill styles. Second, the evolution of the Tethyan domain, paleoclimatic environment and major geological events controlled the formation and distribution of high-quality source rocks within the basin. The rift valley and intracratonic rift, passive continental margin slope, and intracratonic sags are favorable areas for the development of source rocks. Third, the evolution of the Tethyan domain, supercontinent cycles, global sea level changes, and tectono-climatic events controlled the distribution of carbonate platform and reservoir-caprock combinations. The cratonic platform margins and sub-platform internal high terrains are key areas for finding carbonate high-energy facies belts. Syndepositional paleo-uplifts and surrounding slopes, regional unconformities, and later faults zone are areas where large-scale carbonate reservoirs are distributed. The regional evaporite or shale caprock are beneficial for the large-scale preservation of oil and gas in the basin. Fourth, the spatio-temporal matching relationship of reservoir forming factors influenced by the early tectonic-sedimentary evolution pattern and the degree of later tectonic modification is the key to oil and gas enrichment. Future oil and gas exploration should focus on potential gas systems during the Sinian rift period, Cambrian pre-salt gas systems in the eastern and southern Sichuan, as well as whole oil and gas systems of Permian and Triassic.

Prototypical basin reconstruction and evolutionary models for the Precambrian of the Qinshui Basin, North China craton

The thick Precambrian strata in the Qinshui Basin (QB) have high potential for hydrocarbon exploration. Tectonic evolution restoration and prototype basin reconstruction are key to hydrocarbon exploration and evaluation for Precambrian strata. We studied the 2D/3D seismic data and Precambrian field outcrops of the QB to elucidate its geological attributes, major regional unconformities, tectonostratigraphic framework, tectonic evolution and prototype basin. The Precambrian strata of the QB are the sedimentary products of the continuous transgression from south to north and are more appropriately attributed to the Xiong'er Aulacogen in the Changchengian and Jixian periods. We identified six regional unconformities: the Archean or Palaeoproterozoic/Xiong'er Group, the Xiong'er Group/Chy, Chc/Chl, Chl/Jxl, the Jxl/Cambrian System, and the Ordovician/Carboniferous. Seven tectonostratigraphic units can be delineated: the Archean–Palaeoproterozoic, Xiong'er Group, Yunmengshan–Cuizhuang, Luoyukou, Jixian, Cambrian–Ordovician, and Carboniferous–Triassic tectonostratigraphic units. The tectonic evolution was divided into seven stages: the formation and evolution of the basement (∼1800 Ma), the synrift stage in the Xiong'er period (1800–1720 Ma), the postrift stage in the Yunmengshan–Luoyukou period (1720–1600 Ma), the passive continental margin basin stage in the Jixian period (1600–1400 Ma), the marine craton stage during the Cambrian–Ordovician (542–440 Ma), the marine–continental transitional–continental craton stage during the Carboniferous–Triassic (316–200 Ma), and the intense deformation stage (200–0 Ma). Three stages of prototype basin models of the Precambrian in the QB have been reconstructed: (1) In the synrift basin model, numerous normal faults have developed, and the section shows a symmetrical composite graben. The topography in the south is low, that in the north is high, and the northeast and northwest areas are palaeo-uplifted. A set of volcanic rocks with coarse-grained clasts was deposited. (2) In the postrift basin model, the fault activity weakened, and the depositional range became larger than that of the synrift stage, and a set of marine clastic–carbonate rocks was deposited showing a gradual coarsening–upwards trend. (3) In the passive continental margin basin model, the fault became almost inactive, the topography was low in the southeast and high in the northwest, and the depositional area was more widespread, resulting in a set of marine carbonates. These results contribute to a better understanding of the break-up process of the Columbia supercontinent and the Precambrian evolution of the North China Craton (NCC) and can be used to predict the distributions of source rocks and hydrocarbons.

Three-stages tectonic evolution controls the differential distribution of hydrocarbons in the West Java backarc basin

The West Java Basin is a typical back-arc basin in Southeast Asia and one of Indonesia's main petroleum provinces. In the past ten years, it has been found that the new hydrocarbon reserves are on the rise, and a amount of resources to be discovered is significant. The distribution of hydrocarbon resources and the tectonic evolution characteristics of the basin are systematically explored, and the differences in hydrocarbon distribution and its main controlling factors in the back-arc basin of West Java are analyzed. The uneven distribution pattern of hydrocarbon is mainly controlled by the differential distribution of source rocks in the subsidence period. The distribution of the main source rocks is influenced by the "three-stage" tectonic development process that governs the distribution of sedimentary facies in various sedimentary environments throughout the tectonic subsidence period. Second, the weakening of the crust and magmatism facilitates the production of high heat flow values in the shallow strata of the back-arc area, thereby accelerating the pyrolysis and hydrocarbon generation of source rocks. The study delineates the formation and distribution of various types of oil and gas across various blocks. Prospective basin exploration sites include the shallow-deep water zones in the basin's south-central sector, as well as underexplored terrestrial regions. The exploration history and hydrocarbon resources identified within the West Java back-arc basin support these findings.

Formation mechanism of the small-angle X-type strike-slip faults in deep basin and its controlling on hydrocarbon accumulation: a case study from the Tabei Uplift, Tarim Basin, NW China

In the central Tarim Basin, numerous hydrocarbon deposits were found along ultra-deep strike-slip faults, and its evolving progress and formation mechanism are research hotspots. The Paleozoic small-angle X-type strike-slip fault in the Tabei Uplift is the research subject in this article. Based on high-precision three-dimensional seismic data, three structural deformation layers were revealed: the rift system, weak strike-slip deformation and salt tectonics in the deep structural layer (Sinian–Middle Cambrian), the strong strike-slip deformation and karst-dissolution structure in the middle structural layer (Upper Cambrian–Middle Ordovician), and echelon normal faults in the shallow structural layer (Upper Ordovician–Carboniferous). The formation and evolution of strike-slip faults is jointly controlled by the distribution pattern of basement rift and the activities of surrounding orogenic belts, which can be divided into three stages. In the Middle to Late Cambrian, the initial subduction of the Paleo-Asian and Proto-Tethyan oceans precipitated the emergence of two sets of small-angle X-type strike-slip faults, striking NW and NE above the grooves of Precambrian rifts, influenced by local weak compressive stress. Affected by the closure of peripheral paleo-ocean, strike-slip faults deformed considerably in the Middle–Late Ordovician and were reactivated in the Silurian–Carboniferous, forming en-echelon normal faults in the shallow layer. The layered deformation structure of the strike-slip faults significantly affects the accumulation of hydrocarbons. The differential hydrocarbon enrichment of faults in the Tabei Uplift is collectively influenced by the distribution of source rocks and the migration of oil and gas. The topographical features of the Tabei Uplift, along with the distribution of strike-slip faults across tectonic units, have rendered the NE direction the preferential pathway for hydrocarbon migration. Additionally, impacted by the development of en echelon faults, the NE-trending faults offer superior conditions for hydrocarbon preservation and charging condition, compared to the NW-trending faults.

The role of protolith composition in the formation of tin-enriched granitic melts: A modeling study using the example of the southwest China tin province

Important features of Sn mineralization are the heterogeneous geographic distribution and frequent regional separation from W mineralization in spite some similarities of Sn and W behavior during magmatic processes. Major Sn and W mineralization is often spatially associated with peraluminous granites, which are derived from partial melting of metasediments. Several concepts have been suggested to explain those features, such as a weathering-related Sn-enriched source, Sn redistribution between melts and restite during protolith melting, and extensive fractional crystallization. We demonstrate the importance of protolith composition for the formation of Sn (and W) granites by using a comprehensive bulk-rock composition dataset from Precambrian metasediments of the South China Sn-W province and employing a thermodynamic modeling approach. We used four compositional proxies for phase equilibria calculations, which are the metasediments of the Mengdong, Sibao, Pingbian, and Shuangqiaoshan Groups. It is well documented that those Precambrian metasediments are important protoliths of Sn granites in South China. We present quantitative evaluation of the control of protolith composition in the generation of Sn-enriched granitic melts using South China as example, but our conclusions may also be applicable to worldwide Sn–enriched granites. Our results indicate that the protolith major-element geochemistry controls the anatectic reactions and melt productivity at specific melting conditions, and consequently the partitioning behavior of Sn. Further, pre-enrichment of Sn is crucial to the fertility of granitic melt and may be a prerequisite, particularly for the formation of giant Sn deposits. We propose that the heterogeneous distribution of favorable source rocks is one of the important factors that control the spatial distribution of major Sn (and W) districts in South China and other regions worldwide.

Characteristics of Jurassic Source Rocks and Evaluation of Hydrocarbon Resources in Tarim Basin, western China

Jurassic strata in Tarim are significant for their hydrocarbon-rich content. However, the distribution of hydrocarbon source rocks in this region is uneven, and the conditions for hydrocarbon accumulation vary significantly among the different structural elements. Therefore, it is essential to investigate the distribution of Jurassic source rocks, the evolution history of hydrocarbon generation, and the mechanisms influencing hydrocarbon accumulation for effective hydrocarbon exploration and development. To achieve this, seismic, geologic, drilling, logging, and testing data were integrated to analyze the distribution and quality of hydrocarbon source rocks, the evolution of hydrocarbon generation, and the factors influencing hydrocarbon accumulation in Jurassic hydrocarbon source rocks in Tarim Basin. The study revealed that the Jurassic effective hydrocarbon source rock centers are mainly located in the piedmont areas of Kuqa depression, southwest depression, and southeast depression, as well as Yingjisu sag and Manjiaer sag in the eastern Tarim area. Based on a comprehensive evaluation and comparison of the potential of Jurassic hydrocarbon potential in each secondary structural unit, it is concluded that Kuqa Depression has the most favorable exploration zone due to its excellent hydrocarbon accumulation conditions and abundant hydrocarbon resources. Although the overall exploration potential of the southwest depression is lower than that of the Kuqa depression, the hydrocarbon resources potential of the piedmont thrust belt and the associated depression is very high. Similarly, the eastern part of the Manjiaer sag and the Altun piedmont zone of the southeastern depression still have promising exploration prospects, although the overall exploration potential of the eastern Tarim area and the southeastern depression is not as good as the former two.

Characteristics and origin of low-organic-matter carbonate source rocks in the Middle–Upper Ordovician, Tarim Basin, northwestern China

The continually-discovered large volumes of marine hydrocarbons indicate a huge exploration potential within the carbonate rocks of Ordovician in the Tarim Basin, northwestern China. The Middle–Upper Ordovician (O2+3) carbonate source rocks are at a highly mature stage. These rocks presently contain a low amount of total organic carbon (TOCpd ≤0.5%) (TOCpd means the present-day TOC). A mass balance approach is used to identify the source rocks that have expelled hydrocarbons. The characteristics and origin of hydrocarbon expulsion from low-TOCpd source rocks are studied, which are significant for oil exploration in the basin. The results showed that a low-TOCpd O2+3 source rock having expelled hydrocarbons was type I and had a very narrow oil window. With a high rock maturity and a high original TOC (TOCo) value exceeding 1%, nearly 80% of the generated hydrocarbons were expelled. The content of gammacerane and C28 steranes in the oil and source rock extracts were relatively lean, with a V-shaped distribution of regular steranes, suggesting a possible genetic relation between the low-TOCpd source rocks and the marine oil in the Tabei area. Hydrogen-rich planktonic algae or acritarchs were the main hydrocarbon parent materials, distributed in the subsiding platform-slope facies. Organic matter was preserved under reducing conditions, and source rocks were formed with a favorable kerogen type and a moderate hydrocarbon generation potential. This study illustrates the hydrocarbon expulsion of low TOCpd source rocks in northern Tarim Basin, which is greatly significant for analyzing the genesis of marine crude oil in Tarim Basin, and evaluating the distribution of marine source rocks. This research method is theoretically significant for oil and gas exploration in the same type of highly-mature carbonate paleobasins.

Current Status of Helium Resource Research and Prediction of Favorable Areas for Helium Reservoir in China

As an unconventional oil and gas reservoir, helium gas reservoirs have gradually become a focus of attention. In recent years, with the continuous increase in demand for helium gas, the uneven distribution of global helium resources has attracted China’s attention to helium resources. In this study, a method for predicting favorable areas of helium gas was proposed based on the natural gas exploration theory and the idea of “finding gas in enrichment areas”. We conducted an in-depth study and analysis of the types of helium gas formations in China by comprehensively using geochemical and isotope-testing data, identifying the distribution of helium source rocks in China. Based on this, we conducted directed analyses of the transport channels and caprock conditions for helium gas, and summarized the enrichment modes of helium gas. Using this method, we predicted five favorable areas for the enrichment of helium gas in China, providing an important basis for the future exploration and development of helium resources in China.

Research on fault-controlled hydrocarbon accumulation in rifted lacustrine basins based on structural geological modeling: a case study of the Banqiao area in Bohai Bay Basin, China

The Banqiao area in the Bohai Bay Basin has experienced three stages of extensional deformation, leading to the formation of numerous fault-bound traps. Faults, acting as boundary conditions for these traps, play a crucial role in hydrocarbon accumulation. In this study, we conducted a 3D structural modeling of the area using high-resolution 3D seismic data and established a fault-reservoir database based on previous research. Our findings reveal four levels of faults in the Banqiao area: basin-controlling faults, boundary faults, derivative master faults, and secondary adjusting faults. The structural units can be categorized into subsag areas, slope areas, stress tran-sition zones, bifurcation and main incised fault zones, and southern block areas. The segmented growth of the main boundary faults controls the evolution of the subsags, with the subsidence center gradually shifting eastward from Rift Phase I to Rift Phase II, aligning with the distribution of source rocks. Fault-bound traps in the Banqiao area include single faults, intersecting faults, and side faults. Faults primarily act as barriers to lateral hydrocarbon migration during the process of hydrocarbon accumulation, while also providing pathways to a lesser extent. By integrating the fault-reservoir database with the fault system classification, we identified four types of fault-controlled hydro-carbon accumulation models: like-dipping fault barrier model, oppositely-dipping fault barrier model, intersecting fault barrier model, and reactivation-controlled secondary hydrocarbon ac-cumulation model. This structural geological model effectively demonstrates the spatial configura-tion of faults and their role in hydrocarbon accumulation in the Banqiao area. The fault control mechanisms presented in the model can also be applied to other blocks in the Bohai Bay Basin, laying a foundation for future petroleum exploration in continental rifted basins and facilitating the ap-plication of big data algorithms in various geoscientific research fields.

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.

Basin-range coupling relationship and prediction of favorable targets for ultra-deep subsalt exploration in the Tarim Basin, Northwest China

The Tarim Basin holds significant geological interest due to its complex basin-range coupling relationship and promising prospects for hydrocarbon exploration. The basal Cambrian in this basin records all tectonic movements of surrounding plates and influences the distribution of source rocks, reservoirs and caprocks. Its morphological evolution not only reflects changes of the uplift-depression framework but also indicates the migration pathways of hydrocarbons. Based on seismic profile interpretations, the structural morphologies of the basal Cambrian and active faults in various geological periods are obtained and related to regional plate tectonics. Furthermore, favorable targets for ultra-deep hydrocarbon exploration are predicted by identifying structural hinge zones. The results show that, (1) the initial amalgamation of the Tarim Plate with South Kunlun Terrane in the southwest at the end of the Middle Ordovician led to uplift of the Southwestern Depression, Bachu uplift, and Tazhong uplift, with the formation of NW-SE-trending boundary faults, (2) the amalgamations of the Tarim Plate with South Kunlun Terrane in the southwest, and Altun-Qilian Terrane in the southeast, as well as the bidirectional subduction of the South Tianshan Ocean in the north at the end of the Ordovician led to uplift of the Southwestern Depression, Bachu uplift, Tazhong uplift, southeastern basin, and Tabei Uplift, with the development of the Bachu-Tazhong fault system trended NW-SE, Madong back-thrust fault system trended NE-SW, and Tabei fault system trended E-W, (3) the amalgamation of the Qaidam Block and Tarim Plate in the southeast and the continued subduction of the South Tianshan Ocean in the north at the end of the Silurian led to uplift in the Madong structural belt, Gucheng low uplift, Tadong low uplift, Shuntuoguole low uplift, and Tabei Uplift, with a large-scale reactivation of the Madong back-thrust fault system and small-scale activities of the Bachu-Tazhong and Tabei fault systems, (4) the closure of both the North and South Tianshan Oceans in the Carboniferous and the intense uplift of the Tianshan orogenic belt in the Permian in the north, as well as the amalgamation of the Tarim Plate with Tianshuihai Terrane in the southwest, led to the development of the Tabei Uplift, Shuntuoguole low uplift, Awati sag, northwestern Bachu uplift, Tazhong uplift, and Southwestern Depression, with the strong activities of the Tabei and Bachu-Tazhong fault systems, and weaker activity of the Madong back-thrust fault system, (5) the amalgamations of the Tarim Plate with North Qiangtang Block, South Qiangtang Block, and Lhasa Block during the Mesozoic in the southwest led to uplift of the Southwestern Depression, Bachu uplift, Tazhong uplift, and Tadong low uplift, with the strong activity of the Bachu-Tazhong fault system, (6) the remote effect resulting from the collision between the Indian and Eurasian plates in the Cenozoic led to significant uplift of the Bachu uplift and varying degrees of subsidence in other basin areas, with intense fault activities in the southwestern, northwestern, and northern regions. Based on the distribution of Middle-Lower Cambrian source rock, reservoir, and caprock, the favorable targets for ultra-deep subsalt exploration include the central-eastern Tazhong uplift, central Lunnan low uplift, central Shuntuoguole low uplift, southwestern Markit slope, northwestern Yecheng sag, and northwestern Bachu uplift.

Depositional environment and source rock distribution of the Late Cretaceous Rakopi formation in the deepwater Taranaki Basin, New Zealand

The interpretation of depositional environments and evaluation of potential source rocks in deepwater frontier basins are challenging because of the limited data availability. The Deepwater Taranaki Basin, the northwest extensional area of the Taranaki Basin in New Zealand, hosts the Late Cretaceous Rakopi Formation, which is one of the most important source rocks and potential reservoirs in the Taranaki Basin. This study utilized seismic data and well ties from the Romney-1 well to interpret three seismic units in the study area, representing different depositional environments and evolutions. These units were correlated with the regional Megasequences B2–B4 in the Deepwater Taranaki Basin. Sediment transport and progradation mainly occurred from the northeast to the southwest direction. Fluctuations in relative sea levels influenced the shifting of the shelf and marginal environments, potentially creating coal-bearing strata in the study area. The key source rock strata were identified within units 2 and 3 characterized by kerogen type III, coaly source rocks in a marginal environment. A minor source rock interval was found in unit 2, characterized by kerogen type II, marine organic matter in a bathyal environment in the depocenter of the study area. Coal seams were identified using seismic and well log data. However, thin coal seams were challenging to distinguish clearly, sometimes showing a similar response to sandstone beds, because of the limited seismic resolution. Thus, within limited data areas, source rock predictions can be made in the context of depositional interpretation.

Geochemical properties, mechanism of formation, and source of solid bitumen in the Ediacaran Dengying Formation from the central to northern Sichuan Basin, China

Several large gas reservoirs, including the Weiyuan and Anyue gas fields, have been discovered in the Ediacaran Dengying Formation in the Sichuan Basin, indicating vast opportunities for deep-petroleum exploration. Understanding the origin and evolution process of these gas reservoirs is crucial for further petroleum exploration and development. Although the burial history of the Sichuan Basin and its surrounding areas is basically dominated by Paleozoic burial and preservation, and Meso-Cenozoic uplift and erosion. However, the source of the Dengying Formation natural gas in the Sichuan Basin is still a subject of controversy between the Ediacaran and Cambrian source rocks due to high thermal maturity, secondary alteration, and multistage tectonic activities. This study aims to examine the investigation on the properties and genetics of Dengying solid bitumens that existed in the central to northern Sichuan Basin, in order to provide insights into the source and evolution history of the Dengying paleo-oil reservoirs. For this purpose, petrologic, elemental, and organic geochemical analyses were used to investigate the properties, origin, and formation mechanism of solid bitumens. The results indicate that these solid bitumens have suffered intense thermal alteration (EqVRo 2.1%–5.3%), and are classified as pyrobitumens. Solid bitumens in the Gaoshiti bulge and the north slope area were subjected to tectonic thermal events (e.g., Emeishan igneous province) and relatively strong thermochemical sulfate reduction (TSR) alteration. These solid bitumens were assumed to be pyrolysis products of light (condensate) oil. In contrast, solid bitumens in the Weiyuan bulge and the Micangshan uplift area were not influenced by tectonic thermal events but experienced weak TSR alteration, and originated from either normal or heavy crude oil. Moreover, a systematic bitumen-source correlation was carried out using δ13Corg values, bound steranes, and trace element distributions. Results reveal that the solid bitumens in the central Sichuan paleo-uplift were mainly derived from the lower Cambrian Maidiping Formation, those in the north slope area originated from the lower Cambrian Qiongzhusi Formation, while those in the Micangshan area were sourced from the Ediacaran Doushantuo Formation. Structural and burial history analysis implies that the differences in the source of the Dengying paleo-oil reservoir (i.e., solid bitumen) between these study areas are attributed to the distribution of source rocks and their contact relationship with reservoirs. This research successfully obtained insights into the source and genetic mechanism of paleo-oil reservoirs by analyzing the properties of high-maturity solid bitumens, which can guide further exploration and development of deep petroleum resources.

Sedimentary Characteristics and Hydrocarbon-Generation Potential of the Permian Pingdiquan Formation in Dongdaohaizi Sag, Junggar Basin, Northwest China.

After several years of extensive exploration, a series of oil and gas reservoirs or structures have been discovered in the Dongdaohaizi sag, situated in the hinterland of the Junggar Basin. However, in recent years, some wells around the sag have failed. Herein, sedimentary facies, the plane distribution of dark mudstone, geochemical characteristics of source rocks, and the control of sedimentary facies on the distribution and development of source rocks in the Permian Pingdiquan Formation in the Dongdaohaizi sag were systematically investigated. A combined geological and geochemical method was used to fully understand the distribution law and resource potential of the source rocks in this area. Investigation results indicate that the Pingdiquan Formation primarily inherited continuous deposition, and the sedimentary facies of the sag, such as semideep lake, shore-shallow lake, prefan delta, fan delta front, and fan delta plain, were developed from the center sag to the outside. The fan delta facies, predominantly distributed in the northern margin of the sag, are characterized by lithology mainly composed of sandy conglomerate and coarse sandstone. The sandstone has a coarse particle size and poor sorting ability, while lacustrine mudstone is widely developed in the sag with a large sedimentary thickness, which is the primary source rock of the Dongdaohaizi sag. The source rocks in the Pingdiquan Formation exhibit medium-to-good quality. The organic matter is categorized as types II1-III, and the thermal evolution maturity is in the mature-high mature stage, indicating a high resource potential. The development of the source rock in the Dongdaohaizi sag is closely related to the sedimentary environment. The sedimentation rate of fan delta facies is high and burial is fast, bringing a large amount of terrestrial organic matter to the lake, thereby increasing the nutrition and organic matter quantity of the lake. Due to the stable water of the lake, low energy at the bottom, and medium conditions of strong reduction, a large amount of organic matter accumulated in the deep to semideep lake facies of the study area, resulting in the development of thick dark mudstone with a thickness of over 400 m. The abundance of organic matter has reached a good level, and the maturity and content of organic matter are high, signifying a high hydrocarbon-generation potential.

Integrated well-log data and seismic inversion results for prediction of hydrocarbon source rock distribution in W segment, Pearl River Mouth Basin, China

Hydrocarbon source rocks possess a high concentration of organic matter and can generate significant quantities of hydrocarbons, which are integral components within hydrocarbon systems. It is critical for well deployment and hydrocarbon production enhancement to accurately anticipate the geographical dispersion of hydrocarbon source rocks in hydrocarbon-bearing basins. Total Organic Carbon (TOC) serves as a pivotal metric for assessing hydrocarbon source rocks, enabling the estimation of their spatial distribution through TOC prediction. In this study, we present a Machine Learning-based process for predicting the distribution of hydrocarbon source rocks, utilizing well-log data and seismic inversion results. Based on the TOC value of the well curve, the hydrocarbon source rock quality is divided into four categories: normal, medium, good, and excellent. We identify the sensitive parameters governing hydrocarbon source rock quality through cross-plot and correlation analysis. Machine Learning will predict the spatial distribution of hydrocarbon source rock by establishing a relationship between hydrocarbon source rock quality and sensitive parameters. The prediction results are evaluated by comparing the performance of Random Forest, Gradient Boosting Decision Tree (GBDT), eXtreme Gradient Boosting tree (XGBoost), and Histogram-Based Gradient Boosting Decision Tree (HGBDT) using confusion matrix and ROC curve. The findings consistently demonstrate that HGBDT exhibits superior prediction capability, accurately anticipating the spatial distribution of hydrocarbon source rocks. The prediction outcomes align well with the depositional properties of the research area and demonstrate excellent agreement with the well data.

Dynamic evaluation on sealing capacity of caprocks of the Meso-Neoproterozoic reservoirs in Ordos Basin, China

The Meso-Neoproterozoic is a new play in the Ordos Basin. A deeper understanding about the dynamic relationship between the caprocks and the source rocks is needed. Based on the comprehensive analysis of hydrocarbon source development characteristics of the Meso-Neoproterozoic and its overlying strata, as well as the formation contact relationships, lithology characteristics and exploratory drilling data, it is recognized that the Meso-Neoproterozoic contains two types of petroleum accumulation assemblage, that is, the “self-sourced indigenous” and “upper source rock-lower reservoir” assemblages. The former is mainly controlled by the development and distribution of source rocks of the Changcheng System, with the Lower Cambrian shale sequence as its caprock. The later is controlled by the superposition between the Meso-Neoproterozoic and its overlying source rocks and this assemblage is mainly distributed in Hangjinqi and Pingliang areas with the Carboniferous-Permian shale sequence as its caprock. The dynamic evaluation on the displacement pressure serves to reconstruct the displacement pressure history of the caprock. The results show that the shale sequence of the Cambrian Maozhuang Formation in well XY 1 in the southern Ordos Basin has possibly acquired the ability of sealing natural gas since the early of Late Triassic. Its displacement pressure increased rapidly up to 20 MPa during the Late Triassic-Jurassic and keeps at 9.2 MPa at present, indicating fair sealing ability. The Carboniferous-Permian caprocks in Hangjinqi area could have acquired the ability to seal natural gas in the Late Jurassic-Early Cretaceous, and the present-day displacement pressure is 9–12 MPa, indicating good sealing ability. The upper Paleozoic caprock in Pingliang area has been able to seal natural gas since the Early Jurassic, with a maximum displacement pressure of 23 MPa during the Cretaceous period and a current value of 17–20 MPa, indicative of strong ability to seal natural gas. The sealing ability of caprocks of both the “self-sourced indigenous” and “upper source rock - lower reservoir” assemblages has come into being earlier than or at least no later than the peak gas generation of the source rocks and therefore the caprocks are dynamically effective in geohistory. The Meso-Neoproterozoic reservoirs in the Ordos Basin are well preserved and probabally of better potential for exploration in terms of the caprock-source rock combination.

A method for predicting the TOC in source rocks using a machine learning-based joint analysis of seismic multi-attributes

Total organic carbon (TOC) is an important geological feature of source rocks, which is typically estimated based on expensive core or logging data. However, as oil and gas exploration move into low exploration areas (deep layers, deep water, or new exploration areas), it is becoming increasing significant to predict the distribution of source rocks in these areas to reduce drilling costs. Traditional seismic inversion methods based on well logging and seismic data are difficult to implement in low exploration areas due to a lack of evaluation parameters and well logging data of hydrocarbon source rocks. Seismic data contains rich information about underground media. There are more diverse ways to extract information from seismic data as seismic attribute analysis and machine learning techniques advance. Seismic attributes can be classified into two categories: attributes calculated by mathematical methods, such as instantaneous amplitude, Energy Half Time (EHT), and instantaneous frequency; and physically meaningful attributes obtained through geophysical calculations, such as Elastic Impedance (EI), λ, Young's modulus and μ, etc. In the case of the L Depression in the South China Sea, the EHT, EI, density, and λ are sensitive attributes of source rocks. Using Gradient Boosted Decision Trees (GBDT), eXtreme Gradient Boosting (XGBoost), Random Forests (RF), Hist-Gradient Boosted Decision Trees (HGBDT), and Local Cascade Ensemble (LCE) methods, we predicted the TOC of source rocks. The prediction results show that it is feasible to predict TOC of source rocks based on seismic attributes, and the prediction accuracy rate is above 80%, which can provide good data support for further drilling deployment in areas with low exploration areas.

Sedimentary Environment and Organic Matter Enrichment Model of Saline Lake Source Rock in the Linhe Depression, Hetao Basin, China

The well-developed mudstone and gypsum mudstone in the Oligocene Linhe Formation (E3l) in the Hetao Basin are the main source rocks for gypsum. However, the sedimentary environment and organic matter (OM) enrichment factors of E3l are not clear, and this inhibits the prediction of hydrocarbon source rock distribution and resource calculation. Major and trace elements, total organic carbon (TOC), pyrolysis using the Rock-Eval II, and saturated hydrocarbon gas chromatography–mass spectrometry (GC–MS) analyses were performed in this study. The results show that E3l was deposited in brackish water and saline–ultrasaline water, with weak oxidation reduction in an arid and hot environment. Terrestrial input inhibits OM enrichment, while the redox, paleosalinity, paleoclimate, and paleoproductivity play a catalytic role. The main controlling factors of the same lithologic source rocks are different: terrestrial input and paleoclimate have a greater impact on mudstone, and the redox and paleosalinity were more favorable to gypsum mudstone. Although the main controlling factors are different for different lithologies, their OM enrichment characteristics are still consistent. The E3l water body was deep, and the contribution of nutrients from terrigenous debris to OM enrichment was less. In addition, the water retention environment changed significantly during the E3l sedimentary period, resulting in fewer nutrients, which limited the improvement of surface water paleoproductivity. The arid climate increased water evaporation and salinity, which to some extent prevented consumption and decomposition. Weak oxidation-reduction fluctuations and the stratification of the water body were obvious, and this was not only conducive to the enrichment of OM but also to its preservation.

How to distinguish between marine and lacustrine sedimentary environments? —A case study of Lishui Sag, East China Sea Shelf Basin

The identification and characterization of marine versus lacustrine sedimentary environments can aid in the assessment of source and reservoir rock distribution, which is important for hydrocarbon exploration. Analysis of sedimentary facies, trace fossils, micropaleontology, geochemical characteristics, and relative water-levels was conducted in this study. Analysis of the core revealed typical sedimentary structures, including fourteen lithofacies and eleven lithofacies associations, which are indicative of wave-influenced origins and tidal processes. The sediments of the Yueguifeng Formation and upper Mingyuefeng Formation were deposited in shallow water settings, which later transitioned into the semi-deep water environments of the Lingfeng Formation and lower Mingyuefeng Formation. Sediments of the lower Mingyuefeng Formation and Lingfeng Formation were deposited in semi-brackish water or brackish water environments, whilst from the lower Mingyuefeng Formation to upper Mingyuefeng Formation and lower Lingfeng Formation to Yueguifeng Formation, the environment was semi-brackish to freshwater. The Lishui Sag experienced a large-scale and rapid relative water-level increase from the Yueguifeng Formation to the lower Mingyuefeng Formation and a decrease during the upper Mingyuefeng Formation. The relative water-level of the Lingfeng Formation and the lower Mingyuefeng Formation was consistent with the inferred eustatic global sea-level, and the relative water-level of the Yueguifeng Formation and the upper Mingyuefeng Formation deviated from the inferred global eustatic sea-level. The typical lithofacies associations, micropaleontology, and geochemical characteristics are identified as the key indicators to determine the sedimentary environments. The sedimentary environment analysis results show that the Lingfeng Formation to the lower Mingyuefeng Formation developed in a marine setting, whereas the Yueguifeng Formation and the upper Mingyuefeng Formation developed in a lacustrine sedimentary environment. The sediments deposited within shallow marine settings form good source rocks, the coastal sediments form good reservoirs, sediments of shallow lacustrine can generate source rock, and the deep-lacustrine sediments form good source rocks that produce waxy crude. Therefore, accurately distinguishing the difference between marine and lacustrine basins is important in terms of the overall understanding of the basin and the hydrocarbon play elements contained within it.