High-quality Hydrocarbon Source Rocks Research Articles

Retrogradation of carbonate platforms on a rifted margin: the late Ediacaran record of the northwestern Yangtze Craton (SW China)

The Late Ediacaran Dengying Formation, located in the Sichuan Basin of the northwestern Yangtze Craton, is of significant interest in oil and gas exploration due to its abundant pores and vugs within microbial mound-shoal complexes. However, there is still uncertainty regarding the spatiotemporal distribution and controlling factors of the platform margin. This study comprehensively analyzes the retrogradation pattern of the Dengying Formation platform margin using seismic data, well logs, field outcrops, and petrological characteristics. Our findings reveal that the Dengying Formation strata surrounding the rift basin at the northwestern of the Yangtze Craton can be divided into three main depositional facies: basin facies, slope facies, and platform margin facies. Additionally, based on the integration of lithological, log, and seismic characteristics, the Dengying Formation is subdivided into four third-order sequences, with five sequence boundaries and three seismic facies identified. Supported by sequence stratigraphy and geophysical data, we have reconstructed the tectono-sedimentary evolution of the multiple platform margins on the eastern side of the Deyang-Anyue rift in the Sichuan Basin during the late Ediacaran. Our findings indicate that the platform underwent two phases of retrogradation. The second-stage platform margin underwent retrogradation towards the interior, spanning a distance between 10 and 80 km, based on the initial configuration established by the first-stage platform margin. The main controls for progradation and retrogradation of carbonate platforms are eustatic sea-level changes and tectonic activity. Eustatic sea-level changes can be divided into constructive and destructive phases. Constructive phases are commonly observed in highstand systems tracts, while destructive phases are often associated with transgressive systems tracts and are related to platform retrogradation processes. However, sea-level changes alone cannot fully control the process of platform retrogradation. The thermal subsidence following mantle plume events likely played a significant role in the retrogradation of the platform in the study area. During this period, tectonic processes controlled the geometry of the platform and the deposition of carbonates in the platform margin-slope-basin environment. Additionally, karst-related mound-shoal complexes developed extensively along the platform margin of the Dengying Formation in the northwestern Yangtze Craton. The Lower Cambrian dark shales represent high-quality hydrocarbon source rocks, while the Dengying Formation exhibits an optimal source-reservoir configuration.

Sedimentary characteristics of mixed source fine-grained gravity-flow and its significance for shale oil exploration in a lacustrine depression basin: A case study of the Chang 73 Sub-member of the Triassic Yanchang Formation in Ordos Basin, NW China

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang73 Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 73 Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.

An Analysis of Silurian Paleo–Tethys Hydrocarbon Source Rock Characteristics in North Africa, the Middle East, and South China

In this study, we elucidate the genesis and distribution patterns of Silurian hot shale hydrocarbon source rocks by utilizing a comparative analysis of the evolutionary characteristics of plate tectonic activity in the Paleo–Tethys Ocean and the sedimentary filling characteristics of key basins in North Africa, the Middle East, and South China. We propose an explanation for the sedimentary genesis of world-class Silurian hydrocarbon source rocks in the Paleozoic craton basin of the ancient Tethys tectonic domain. This is achieved by scrutinizing the plate tectonic activity and evolution of the ancient Tethys Ocean and combining these findings with the paleotectonic sedimentation background of North Africa, the Middle East, and South China. Additionally, we compare Silurian hydrocarbon source rocks from these regions. The deep-water stagnant environment of the land shelf favors the preservation of organic matter, thereby forming high-quality hydrocarbon source rocks. Conversely, the shallow-water body of the land shelf is more turbulent, thus resulting in the poorer preservation of organic matter and, consequently, lower-quality hydrocarbon source rocks.

Newly discovered Botryococcus-rich source rocks in the western Qaidam Basin, NW China and their implications for shale oil exploration in saline lacustrine basins

Owing to the high hydrocarbon content and photosynthetic efficiency of phytoplankton, planktonic microalgae account for the main source of petroleum hydrocarbons in marine and continental petroliferous basins. Therefore, the presence of algal residues in such basins is considered an important indicator of high-quality hydrocarbon source rocks. The southwestern region of the Qaidam Basin, characterized by lacustrine deposits from the Paleogene, has substantial petroleum resources. However, studies on the development of source rocks and the mechanisms involved in their organic matter enrichment are insufficient, thus hampering ongoing shale oil exploration efforts. Herein, abundant Botryococcus fossils were discovered in the main source rocks of the upper member of Xiaganchaigou Formation of Paleogene in Southwest Qaidam, accounting for >50 % of the total organic matter content. The palynofacies assemblages reflected relatively distal and oxygen-rich sedimentary environments. The algae-rich rocks usually exhibited distinctive laminated structures alternately composed of quartz-feldspathic, clay and carbonate laminae, indicating periodic climate fluctuations. The Botryococcus, which was mainly preserved in the coarse-grain quartz-feldspathic laminae, probably reflected heavy precipitation conditions and subsequently high nutrient inputs. Finally, the oxygen-rich, low salinity and eutrophic water was likely taking form which benefit the growth of these hydrobiontic algae. These algal-rich shales exhibited typically high hydrogen index (IH) and total organic carbon (TOC) values, indicating their high hydrocarbon-generation potential. Thus, they are important marker beds for high-quality source rocks in petroliferous basins. Concurrently, the coarse-grained detrital mineral laminae displayed excellent reservoir physical properties, probably providing sufficient reservoir space for planktonic algae-derived liquid hydrocarbon. Therefore, these Botryococcus-rich source rocks might represent important targets for ongoing shale oil exploration.

High-frequency sequences, geochemical characteristics, formations, and distribution predictions of the lower Cambrian Yuertusi Formation in the Tarim Basin

Deep–ultradeep carbonates have long been a hotspot in the field of oil and gas exploration in the Tarim Basin. The hydrocarbon source rock distribution is a key factor restricting deep–ultradeep oil and gas exploration, and in particular, there has been controversy over the sources of the deep oil and gas in the Tarim Basin. In addition, the black shale sediments of the Yuertusi Formation formed during the important transition period of the Precambrian/Cambrian and recorded globally significant climatic events. Research on this formation is of great scientific value for understanding the changes in the Earth's surface climate system during this major and critical transition period. Based on nine field sections, 36 wells, and 2D and 3D seismic data, this paper analyzes the sedimentary environment and hydrocarbon source rock distribution of the Yuertusi Formation in the Tarim Basin. The results show that the lower Cambrian Yuertusi Formation in the Tarim Basin can be divided into two third-order sequences (SQ1 and SQ2) and five fourth-order sequences (sq1-sq5), which experienced two primary transgressions during the deposition of sq2 and sq4. During the depositional period of sq2, hydrocarbon source rocks with the highest organic carbon content in the entire area formed under the influence of high paleoproductivity and upwelling currents. sq4 formed in the context of a second large-scale transgression, and the depositional center moved to the southwest. The organic matter content in sq4 is lower than that in sq2, but it features an extensive distribution and a large thickness. sq3 and sq5 are shallow-water deposits, primarily gray dolomite and algal dolomite with local argillaceous interlayers, that formed during marine regression phases. sq2 and sq4 of the Yuertusi Formation are primarily deep-water to shallow-water shelf sediments with stable thicknesses and are widely distributed in the slope area of the basin and on the shelf. These two sets of high-quality hydrocarbon source rocks indicate the huge hydrocarbon exploration potential of the deep Cambrian strata in the Tarim Basin. These research results can provide important scientific references for evaluating deep–ultradeep oil and gas resources in the Tarim Basin.

Characteristics of Cambrian tectonic-lithofacies paleogeography in China and the controls on hydrocarbons

In early Cambrian, the earth was in a state of active super-continental disintegration, ocean basin expansion, and seawater transgression. It was a critical period for hydrocarbons source rocks development on a global scale. In China for example, hydrocarbons reservoirs, including conventional and unconventional, have been discovered in the Cambrian rocks. By using regional tectonic analysis, stratigraphic correlation, and lithofacies comparison, we compiled a tectonic-lithofacies paleogeographic map and determined the lithofacies paleogeographic characteristics of the main plates of the early Cambrian period in China. Our results show that although the North China Plate was deposited in shallow-water shore facies, it was later uplifted and exposed to long-term denudation; and thus, lacked the depositional conditions necessary to form hydrocarbon source rocks. In contrast, both the South China and the Tarim Plates contain the complete platform-slope-basin facies sequence, which is the required depositional conditions to promote the formation of hydrocarbon source rocks. Furthermore, our results demonstrate that, during the period of Meishucun and Qiongzhusi formations, South China Plate was composed of deep-water sediments. Correspondingly, during the sedimentation of the Yuertusi Formation, several areas such as the Kalpin, Tabei, and Taxinan in the Tarim Basin, formed a gentle-sloping continental shelf sedimentary sequence of phosphorous siliceous rocks, muddy shales, marlstones, and doloarenite. In addition, we demonstrate that (1) abundant nutrients provided by the hydrothermal and coastal upwelling events, combined with the “periodic anoxic” bottom water conditions, resulted in the formation of high-quality hydrocarbon source rocks in the Yuertus Formation in the Tarim Basin; and (2) the intracratonic sag and continental shelf edge slope settings controlled the deposition of hydrocarbon source rocks in the Sichuan Basin.

Transition of seawater conditions favorable for development of microbial hydrocarbon source – Reservoir assemblage system in the Precambrian

Several commercial Precambrian petroleum systems have been discovered worldwide. The microorganisms found in the Precambrian successions play a significant role in the development of high-quality hydrocarbon source rocks and microbialites with a high reservoir porosity. Yet, it remains unclear regarding issue of environments in which microbial activities can lead to the development of organic-rich hydrocarbon source rocks or reservoirs, and how the microbial source rock and reservoir constitute an effective hydrocarbon source-reservoir assemblage system. During the Edicaran Doushantuo Formation period, high-quality black shale source rocks were widely developed, and the organic matter in the source rocks was mainly derived from microbes including bacteria and algae. The thickness of the source rocks is up to 200 m, and the total organic carbon (TOC) value is ∼ 17.9%. During the Dengying Formation period, thick-layered microbialites, including microbial stromatolite, thrombolite, and botryoidal dolomite were developed extensively, with the abundant primary framework and secondary dissolution pores. During the Precambrian evolution, the transition of the seawater environment controlled the development of microbial source rocks or microbialite reservoirs. In a deep, euxinic reducing seawater condition, microbial organic matter was well preserved and accumulated in fine-grain sediments, thus forming the high-quality microbial source rocks. Meanwhile, under such a reducing condition, a large amount of layered, nodular, granular, and strawberry pyrites were precipitated due to the effect of BSR reaction with extremely positive δ34S values (up to 39.0‰). Meanwhile, in a shallow turbulent high-energy oxidized seawater condition, especially in the intermittent “Dolomite Sea” environment, microbial dolomitization promoted the massive development of microbial dolomite mound/shoal frameworks, constituting the matrix for the development of large-scale microbialite reservoirs. During the later burial evolution processes, the oil and gas generated from the Doushantuo Formation source rocks migrated and accumulated into the overlying Dengying Formation microbialite reservoirs, which constituted an effective hydrocarbon source-reservoir assemblage system. Overall, the areas surrounding the Mianyang, west side of the Chengkou, and northwest side of the western Hubei troughs are considered to be the potentially favorable areas with microbial source rock-reservoir assemblage system. The microbial source-reservoir assemblages in these Precambrian strata worldwide are worth further exploring.

New discoveries on the first petroleum scientific drilling (QK-1) of the Qiangtang Basin, Tibetan Plateau

<p indent=0mm>QK-1 is the first scientific drilling designed to solve the key problems in petroleum geology in the Qiangtang Basin on the northern Tibetan Plateau. Its altitude is <sc>5030 m,</sc> and the well depth is <sc>4696.2 m.</sc> It is also the highest-altitude scientific drilling on the globe, and is located in the Lake Bandaohu area in the northern Qiangtang Basin. By performing the QK-1 and its affiliated shallow drillings, a series of new discoveries has been made in regard to high-quality regional seals, source rocks, petroleum displays and the Jurassic strata succession. (1) Two sets of caprocks, which could be regional high-quality seals, were discovered through drilling, i.e., an anhydrite layer in the Quemoco Formation (J_1-2<italic>q</italic>) <sc>(354–365 m</sc> thick) and a gypsum-bearing mudstone in the Xiali Formation (J₂<italic>x</italic>) <sc>(230–252 m</sc> thick). Comprehensive analyses of drilling and seismic data show that the distribution of these caprocks is regionally continuous and stable. Moreover, petroleum displays and well logging data reveal that these two caprocks are characterized by low porosity, good permeability (the mean permeability is <sc>0.265×10⁻³ μm²,</sc> and the mean breakthrough capillary pressure is <sc>9.0 MPa),</sc> and good sealing. (2) The northern Qiangtang Basin contains high-quality hydrocarbon source rocks in Upper Triassic deltaic facies. The affiliated shallow drillings reveal a good Upper Triassic hydrocarbon source rock in a coastal swamp-deltaic plain facies on the northern Qiangtang Basin margin. The two mudstone source rock members have a thickness of <sc>36.0 m</sc> and total organic carbon (TOC) values of 2.0%–3.6%, and a thickness of <sc>61.0 m</sc> and TOC values of > 1.0%, respectively. The organic matter type is II₁-, II₂- and III-type kerogen with high to over-thermal maturation. Well logging, seismic data and facies analyses indicate that the pro-deltaic to shelf mudstones in the covered area in the central northern Qiangtang Basin are better source rocks than those in the deltaic plain on the basin margin. (3) Well logging petroleum displays provide important insights for petroleum reservoir formation and destruction. Based on petroleum displays and evidences of gas release in 250 outcrops, the QK-1 scientific drilling has also discovered 14 petroleum anomalies in the Eocene Suonahu Formation, Middle Jurassic Buqu Formation and the Upper Triassic Nadigangri Formation. One anomaly is oil-bearing, while thirteen anomalies are gas-bearing. The total hydrocarbon value of gas logging is 10%, and is mainly methane in composition. The petroleum displays of well logging occurred after drilling through the Xiali or Quemoco formations which are caprocks, while outcrop petroleum displays are mainly distributed on the basin margin or central uplift which are strong tectonic deformation zones. The widespread petroleum displays indicate good hydrocarbon-generation conditions and petroleum reservoir formation/destruction processes, while the distribution characteristics of petroleum displays indicate that the high-quality regional-seals-covered area in central basin could be favorable plays of petroleum reservoirs. (4) The QK-1 drilling cut through a sedimentary succession from the Upper Jurassic to Upper Triassic, i.e., from the upper to the bottom, the Upper Jurassic Suowa Formation (J₃<italic>s</italic>), Middle Jurassic Xiali (J₂<italic>x</italic>) and Buqu Formations (J₂<italic>b</italic>), Middle-Lower Jurassic Quemoco Formation (J_1-2<italic>q</italic>) and Upper Triassic Nadigangri Formation (T₃<italic>nd</italic>), in which the whole Jurassic strata is completely continuous. The Jurassic sedimentary succession shows a transition from alluvial facies to shore, shallow shelf and carbonate platform facies up section. This finding breaks the previous view that suggests a lack of Lower Jurassic strata, and provides new basics for Qiangtang Basin analysis. Overall, the discovery of two regional high-quality seals in the Jurassic Quemoco and Xiali formations in the Qiangtang Basin refutes the previous traditional perception, which suggests poor petroleum preservation conditions. The northern Qiangtang Basin contains high-quality source rocks with great hydrocarbon generation potential in the Upper Triassic deltaic facies. Petroleum display anomalies and distribution characteristics reveal that the Qiangtang Basin not only underwent large-scale hydrocarbon generation, reservoir formation/destruction processes, but also developed favorable display of petroleum preservation. The establishment of a continuous Jurassic strata succession provides a new evidence and valid framework for petroliferous basin analysis of the Qiangtang Basin.

Controlling factors of high-quality hydrocarbon source rocks developed in lacustrine shallow-water zone of the Junggar Basin, northwestern China

Deposits in the lake center, transitional steep slope, and marginal slope of the early Permian alkaline lacustrine system of the Mahu sag, Junggar Basin, have been studied via organic petrology and organic geochemistry to identify high-quality source rocks and investigate potential controlling factors. Organic geochemical data demonstrate that source rocks deposited on the steep slope and on the gentle slope show higher total organic carbon, higher Rock-Eval S2 and hydrogen index values, and lighter carbon isotopes of kerogen than those deposited in the lake center. Organic petrology results show: 1) enrichment of lamalginite in dolomitic laminae of the steep slope, 2) abundant alginite and sporinite in tuffaceous laminae of the gentle slope and, 3) only minor alginite relics and bituminite in evaporite-rich sediments of the lake center. Therefore, the lake slope sediments of the Fengcheng Formation have better source rock quality than the central sediments, which differs from the predicted lacustrine source rock models that place the organic-richest sediments in lake centers. The Mahu sag lake center deposits show strong microbial reworking and contain a high proportion of evaporite minerals, which dilute the organic matter. On its two slopes, shrinkage cracks and erosive surfaces indicate a shallow water setting for the deposition of alginite-rich sediments. These shallow-water organic-rich sediments could be formed and preserved through: 1) local hydrothermal input linked to normal faults, 2) a strong stratified water column related to lateral salinity variations, 3) early silicification of microbial mats induced by evaporation, and 4) high water alkalinity.

Application of back-propagation neural networks in the distribution of oil sand reservoirs\u2014a case study from the western slope of the Songliao Basin, China

High-quality hydrocarbon source rocks are present in the upper Cretaceous layer in the western slope of the Songliao Basin. Oil and gas have accumulated in these rocks at the shallow edge of the basin, which has led to the formation of oil sand resources. This study uses the back-propagation (BP) neural network method to predict the distribution of oil sand reservoirs and is the first study of its kind in China. First, based on the basic data collected by core sample, well log and geochemical analyses, and the reasonable selection of samples, the cores are divided into mudstone, siltstone, fine sandstone, medium sandstone, and sand, according to lithology. Second, a three-layer BP neural network model is constructed with two hidden S-type layers and one linear output layer. Third, through a comparison of the effect of different numbers of training sessions of the sample data, we demonstrate that the accuracy of the model can be increased to 90% after training the network 100,000 times. Then, the log-derived data of rocks with unknown lithologies are input into the neural network to predict whether they contain oil sands. We show that the BP neural network method can predict the distribution of oil sand reservoirs in the target horizon of the study area, and the results are consistent with research results on the corresponding sand reservoirs and sedimentary facies. Thus, we conclude that it is feasible to use the BP neural network method to predict the distribution of oil sand reservoirs.

Pressure–temperature–time–composition (P–T–t–x) of paleo–fluid in Permian organic–rich shale of Lower Yangtze Platform, China: Insights from fluid inclusions in fracture cements

Recently, much attention has been paid to Permian shale from the Yangtze Platform because of the high–quality hydrocarbon source rock. This research investigates fracture veins derived from conventional cores of the Permian Leping Formation in the Pingle Depression, Lower Yangtze Platform. Based on petrographic observations, micro–Raman spectroscopy, and fluorescent spectrometry, hybrid CH4–C2H6 gas inclusions, pure CH4 inclusions, dissolved CH4 aqueous inclusions, and different abundant blue-green fluorescence petroleum inclusions were discovered in quartz and calcite. The pressure–temperature–time–composition properties of the CH4–bearing inclusions were obtained by quantitative Raman analysis and thermodynamic models. The pore fluid pressure (59.05–87.78 MPa) calculated from Raman shift of C–H symmetric stretching (v1) band of methane indicates inclusions trapped at close to lithostatic pressures, which can provide a key evidence for understanding the formation and evolution of overpressure. PT entrapment conditions, reconstruction of hydrocarbon generation and simulation of burial history permit to constrain accumulation time sequence of paleo-fluid in shale veins. Blue-green fluorescence with high-mature petroleum fluid trapped in late Cretaceous (~81–71 Ma); hybrid CH4–C2H6 fluid trapped in early Paleogene (~68–61 Ma); and CH4 fluid trapped in Paleogene (~62–51 Ma). Dry gas generation period close to the age of maximum burial depth at early Himalayan uplift period.

Characteristics of lacustrine deepwater fine-grained lithofacies and source-reservoir combination of tight oil in the triassic chang 7 member in Ordos Basin, China

The lithofacies characteristics of fine sedimentary rocks affect the hydrocarbon generation capacity and reservoir performance of tight oil and are closely related to the sedimentary environment. On the basis of field outcrops and core observations, combined with logging data, thin section identification, grain size analysis, organic carbon analysis and physical property tests, the fine sedimentary facies of the Chang 7 Member in the Ordos Basin are studied. Nine lithofacies types are identified in the study area, including planar and trough cross-bedding sandstone facies (Spt), massive bedding sandstone facies (Sm), slump deformation fine sandstone facies (Ssd), sand-mud interbedded facies (Sb), rippled siltstones facies (Fr), horizontal bedding mudstone facies (Mh), massive bedding mudstone facies (Mm), horizontal bedding shale facies (S) and tuff facies (T). Lithofacies types Spt, Ssd, Fr are developed in the gentle slope belt in the northeastern study area, which is a delta front-semi-deep lake sedimentary environment and mainly contains underwater distributary channel and estuary dam sedimentary microfacies types. Lithofacies types such as Sm, Sb, Mh and T are developed in the southwestern steep slope zone of the study area, which is a channel-type gravity flow sedimentary environment, and sedimentary microfacies types such as near-source channels, far-source channels and leading-edge dolomites are developed. In the vertical direction, during the Chang 73 sedimentation period, the lake surface area and water depth reached their maxima, and S and Mh lithofacies developed, which were conducive to the formation of high-quality hydrocarbon source rocks for tight oil. During the Chang 72-Chang 71 sedimentation period, the lake basin shrank, and the reducibility of the water body weakened. The delta front sand body composed of Spt lithofacies in the northeast and the sandy clastic flow sand body composed of Sm in the southwest constitute the main tight oil reservoirs. High-quality hydrocarbon-generating lithofacies and favourable reservoir lithofacies are spatially superposed and developed, forming two source-reservoir combinations in the Chang 7 tight oil source-reservoir formation and near source-reservoir formation.

Paleoproductivity of the Chang 7 unit in the Ordos Basin (North China) and its controlling factors

Primary paleoproductivity provides basic organic matter for the formation of high-quality hydrocarbon source rock. A progressive method to quantitatively calculate paleoproductivity on the basis of total organic carbon (TOC) has been proposed to evaluate the contribution of primary paleoproductivity to a hydrocarbon source rock with high organic matter abundance. In this study, pyrite contents, organic maceral compositions and high-resolution sedimentation rates (SRs) were used to recover the amount of organic matter oxidation and to exclude the effect of terrestrial clastic input during the deposition of the Chang 7 sediment. Multiple calculation methods were used to quantitatively reconstruct paleoproductivity, and the results suggest that the Ordos Basin was a eutrophic lacustrine basin (approximately 2 × 104 mg C cm−2 kyr−1 to 10 × 104 mg C cm−2 kyr−1) during the Chang 7 stage compared with modern lake productivity. The paleoclimatic analysis indicates that a warm humid paleoclimate was beneficial for increasing the weathering intensity, which could have provided more nutrient inputs to the depositional environment with incoming terrestrial debris. In addition, the plate collision between the North China Block and Yangtze Block generated moderate volcanic ash that spread to the study area and provided large amounts of essential nutrients for algae blooms. Moreover, a brackish water condition promoted upward mixing of deep water, which inhibited water mass stratification and then facilitated primary paleoproductivity. In general, the Chang 7 high-quality hydrocarbon source rock is a result of increased paleoproductivity resulting from suitable paleoclimate conditions, brackish water masses and abundant essential nutrient inputs.

Shallow Gas Accumulation Discipline in Heidimiao Layers of Daqing Changyuan in Northern Songliao Basin

Shallow gas has been a hotspot in present natural gas exploitation and development due to its shallow depth, low cost of exploitation and development, high production and good benefits. Combined data of core, log, 3D seismic and indoor analysis experiments have been studied. Results indicated: In northern region of Daqing Changyuan, shallow gas can be defined as oil type associated gas from underlying reservoir while in the southern region as mainly degradation gas of crude oil mixed with oil type gas of heavy hydrocarbon component from bacteria degradation in early stage. Main accumulation-affected factors of shallow gas in Daqing Changyuan contain: (1) accumulation of shallow gas was based on early large reservoir as source of gas, which was formed by supply of many high-quality hydrocarbon source rocks; (2) many reservoir cap combinations in prograded delta front sedimentary environment controlled accumulation location; (3) quaternary structural trap related with source reservoir fracture controlled horizontal distribution of shallow gasaccumulation. Gas reservoir was defined as structural gas reservoir.Consequently, for degradation gas reservoir and associated gas reservoir of southern and northern Daqing Changyuan respectively, formation mechanism and accumulation model have been founded.

Petrologic and geochemical evidence for the formation of organic-rich siliceous rocks of the Late Permian Dalong Formation, Lower Yangtze region, southern China

Abstract Black shales are economically and scientifically important because they are commonly associated with high-quality hydrocarbon source rocks and metal accumulations formed during periods of abrupt environmental change. Organic-rich siliceous rocks of the Late Permian Dalong Formation developed extensively within Middle–Upper Yangtze region (western Yangtze platform); however, the formation mechanism of its formation in the Lower Yangtze region (eastern Yangtze platform) remains a subject of debate due to a lack of outcrop exposure. Petrologic, mineralogic, and geochemical data are presented for three sections of the Dalong Formation within the Lower Yangtze region. Geochemical ratios including V/(V + Ni), V/Cr, Ni/Co, U/Th, and V/Sc suggest that the Dalong Formation was deposited in an oxygen-poor environment. The elevated concentration of Mo (>100 ppm) further suggests that free H2S existed in the bottom water, which was favorable for the accumulation of large amounts of organic matter. Mo/TOC ratios reveal that water flow in the Lower Yangtze Basin was moderately to strongly restricted during deposition of the Dalong Formation. Flow within the upper–middle parts of the Yangtze Basin was also restricted, suggesting that northern marginal basins on the Yangtze Platform formed part of a single ocean during the Late Permian (i.e., Changhsingian). Cherts of the Dalong Formation reflect high primary productivity, similar to that of modern-day upwelling zones, which increased the amount of organic matter. These results, combined with previous work, suggest that the long-term oxygen-poor conditions during deposition of the Dalong Formation were caused by restricted water circulation in a unique paleogeographic environment characterized by low latitude and submarine paleo-highlands. Such settings of low latitude characterized by tropical climate favored the growth of siliceous zooplankton in the zones with submarine paleo-highlands having created topographic barriers to water circulation. Restricted bottom-water flow and oceanic anoxia during the Late Permian were likely the main controls on the accumulation of organic-rich rocks. Elevated nutrient levels caused by frequent oceanic upwelling along the western margin of the South China Craton and global volcanism at this time facilitated siliceous organism productivity.

Discovery and geological significance of high quality hydrocarbon source rocks in interglacial of Neoproterozoic in the eastern part of the southern margin of North China

A conglomerate of glacial deposits from Neoproterozoic Sinian is generally developed in the southern margin of the North China Plate. However, whether the hydrocarbon source rock exists in a conglomerate of glacial deposits is still undeterminable as it has not been studied previously. The developmental characteristics of the conglomerate were systematically observed and measured. The conglomerate was subsequently divided into four sets, and the gray-black shale was discovered in between them. The said shale has the thickness of 36 m, 46 m, and 7 m, respectively from top to bottom. The study presents the highly-abundant organic matter in the shale, which belong to the high-quality hydrocarbon source rock. The organic matters are derived from lower organisms. The discovery of thick and high-quality hydrocarbon source rocks in conglomerates opens up new exploration areas for conventional natural gas, as well as provides a powerful basis for shale gas exploration. The dropstones that appeared in hydrocarbon source rocks and conglomerates are cogent evidence of glaciation. The findings provide an important guiding significance for the genesis of the conglomerate.

Hydrocarbon accumulation and formation of composite reservoirs in KL16-1 of Laizhou Bay Sag, Bohai Sea, China

The Laizhou Bay Sag, one of the oil-bearing sags with large exploration potential in the South Bohai Sea of China, contains two sets of high-quality hydrocarbon source rocks, i.e. Member 3 and Member 4 of Shahejie Fm (E2s3 and E2s4). As an important hydrocarbon accumulation zone in the sag, the KL16-1 low bulge is found to have oil reservoirs in the Neogene Guantao Fm (N1g), the Paleogene Dongying Fm (E3d), the Paleogene Shahejie Fm (E2s), and the Mesozoic buried hill formation. It is characterized by distinct features of composite reservoirs. Nonetheless, its hydrocarbon accumulation process and mechanism are unknown yet. In this paper, the hydrocarbon generation modelling of source rocks, combined with the characteristics of oil-source rock biomarker compounds and fluid inclusions, is used to restore the hydrocarbon accumulation process in the KL16-1 low bulge. Crude oil in all three KL16-1 plays is characterized by low Pr/Ph, low gammacerane, high 4-methyl sterane and high dinosterane. In other words, it is oil of low maturity to maturity. As the biomarker index of the oil is very similar to that of the E2s3 source rocks, it is thought that E2s3 is the primary source rock layer, while E2s4 is the secondary source rock layer. In addition, episodic oil and gas charging happened in two phases under high pressure in the late period (from 5.0 Ma till now).

Characteristics of source rocks, resource potential and exploration direction of Sinian-Cambrian in Sichuan Basin, China

The Anyue giant gas field was discovered in the Sinian-Cambrian Central Sichuan region of the Sichuan Basin in 2013, with geological reserves up to 1 × 1012m3, which is the first time for the exploration of natural gas paleo-reservoirs in the world. The gas source studies suggest that the Sinian natural gas is originated from the Sinian and Cambrian hydrocarbon source rocks, and the systematical study on the Sinian and Cambrian ancient source rocks has important scientific and practical significance for the global oil and gas geologic domain of the ancient stratum. Based on the drilling data and field profile observation of Sinian and Cambrian in Sichuan Basin, with adoption of the interpretation data of 28000 km-seismic and new drilling data, combined with geochemical analysis of source rocks of 2315 samples, this paper systematically studied the high quality hydrocarbon source rock center, where the source rocks are mainly distributed along the Mianzhu-Changning craton inner rift, with a accumulative thickness reaching 200–450 m, and 50–100 m for the thickness of source rocks in other areas. The hydrocarbon source rocks of the Sinian-Cambrian contributed about 56 %–63% of natural gas resources of the whole basin. Systematical evaluations have been conducted to the mudstone source rocks and their distribution in the III-section of Sinian Dengying Formation, where TOC value is ranging from 0.04% to 4.73%, with an average of 0.65%. The thickness of the source rocks in Central Sichuan region is ranging from 10 to 30 m. The oldest Sinian source rocks that can form large gas fields in China were systematically studied for the first time, and the total gas production intensity of the Sinian source rocks in the great Central Sichuan region is (15–28) × 108 m3/km2, where the gas source conditions for the formation of large gas fields are available. By using the genetic method and analogy method, the amount of natural gas resources of the Sinian-Cambrian in the basin are re-evaluated as (4.65–5.58) × 1012 m3, and the resources potential of natural gas is huge. The amount of natural gas resources in the Central Sichuan block accounts for about 66% of the total basin resources, which is the preferred selection for current exploration.

Discovery and basic characteristics of high-quality source rocks found in the Yuertusi Formation of the Cambrian in Tarim Basin, China

The Upper Paleozoic strata of the Tarim Basin have abundant resources of marine oil and gas. In the Tahe area, Halahatang area, and Tazhong area of the basin, many large-scale oilfields have been found. These oilfields have a confirmed oil and gas reserves worth more than 2.5 billion tons and have completed the annual output of more than 14 million tons of marine oil and gas equivalent. The belief that the only main hydrocarbon source rocks are of the Cambrian or Ordovician is still controversial. Chemists have made significant progress and have effectively lead the oil and gas exploration in Tarim Basin. Due to the complexity of the basin and the limitation of samples, the research work, and fine contrast is restricted. In this article, we investigated the Cambrian strata outcrop of Tarim Basin in detail. By analyzing a lot of outcrops, high-quality hydrocarbon source rocks of Yuertusi Formation have been found in more than 10 outcrop points in Aksu region. The source rocks' lithology is black shale with total organic carbon (TOC) content that ranges between 2% and 16%. Total organic carbon (TOC) of the black shale layer could be as much as 4%–16%, especially in the outcrops of the Yutixi and Shiairike. This by far is the best marine hydrocarbon source rock that was found in China. The source rocks were distributed consistently in the Aksu region, the thickness of which is about 10–15 m. It was formed in a sedimentary environment of a middle gentle slope to a low gentle slope. Organic matter enrichment is controlled by the upwelling currents. The thick strata of dolostone that developed in the Xiaoerblak Formation are considered to be good reservoirs of the beach and microbial reef in the upper strata of Yuertusi Formation. No hydrocarbon source rocks have been found in the outcrop of Xiaoerblak Formation. The thick strata of gyprock and mudstone development are a set of satisfactory cap layer in the Lower Cambrian. This hydrocarbon accumulation combination has great exploration potential.

Geological characteristics and exploration potential of diamictite tight oil in the second Member of the Permian Lucaogou Formation, Santanghu Basin, NW China

Abstract Based on the latest exploration data of tight oil, the evaluation of the “seven properties” relation of diamictite tight oil in the second Member of the Permian Lucaogou Formation in Santanghu Basin was carried out. The second Member of the Lucaogou Formation developed a new suit of high-quality hydrocarbon source rocks, which are characterized by wedging distribution, big thickness, high organic content and good organic type, and the overall being in the oil-generating window. Complex in lithology, the second Member of the Lucaogou Formation is a set of laminated structure diamictite combination composed of detrital material of terrigenous origin, dust fall, carbonate rock in variable proportions. With low natural gamma and high electrical resistivity, strong heterogeneity, micropores as main matrix porosity, it has features of ultra-low porosity and extra-low permeability; the fractures in it provide some reservoir space, and make great contribution to permeability. The Member has good oiliness (oil saturation from conventional core analysis is 15%−80%), high content of brittle minerals (80%−90%), clay mineral content of less than 10% in general; core sensitivity test shows that it varies widely in water sensitivity and velocity sensitivity, and is weak in acid sensitivity. Through evaluating the “seven properties” relation, it is concluded that the main control factors of the tight oil layer are the maturity of hydrocarbon source rock and reservoir physical property. The “sweet spots” of diamictite tight oil in the second Member of the Lucaogou Formation were selected, that is, the areas with favorable reservoir physical properties in the mature hydrocarbon source rock of the sag.