Petroliferous Basins In China Research Articles

Helium resource in the petroliferous basins in China and its development prospects

Helium resource in the petroliferous basins in China and its development prospects

Environmental controls on organic matter enrichment of the lower Cambrian source rocks in the Tarim Basin, Northwest China

In the Tarim Basin, the largest petroliferous basin in China, a set of organic-rich deposition hosted in the lower Cambrian Yuertusi Formation is considered to be the main hydrocarbon source rocks. However, the limited and scattered distribution of the lower Cambrian constrains present studies mostly on some certain sections where samples are accessible, inhibiting a systematic understanding of characteristics of these source rocks and controls on organic matter enrichment. Here, we conduct a systematic study on a drillcore in northwestern margin of the Tarim Basin (Taxibei), integrated with previously published data from other available sections. These data show the Yuertusi Formation was mainly composed of mudstone/shale, carbonate and siliceous rocks, and distributed dominantly in the Taxibei, northern (Tabei) and eastern-northeastern areas of the Tarim Basin (Tadong-Tadongbei) with an increased thickness (mean 30 m to mean 60 m) towards the Tadong-Tadongbei areas. High total organic carbon (TOC) contents (up to 30%) were found in the mudstone/shale dominated intervals in both the Taxibei and Tabei areas, characterized by I-II kerogen and highly to over-matured thermal maturity. Paleo-redox reconstruction based on high V/(V + Ni) ratios (>0.6) and FeHR/FeT ratios (>0.38) suggests anoxic watermass conditions occurred during the deposition of the organic-rich intervals along with moderate sedimentation rate, providing consistently good preservation for organic matter accumulation. Based on a combined analysis, we infer primary productivity in surface waters exerted a key control on TOC contents of the organic-rich intervals, which was driven by nutrient supplies from hydrothermal activities with partial upwelling effects in the Taxibei area, and from upwelling waters in the Tabei area.

Relationship between fluorescence and IR parameters of oil inclusions and crude oils in the Dongpu Depression (Bohai Bay Basin, China)

The Dongpu Depression is an important petroliferous basin in China with abundance of hydrocarbons. The fluorescence characteristics of inclusion oils and their relationship with the present-day produced crude oils have never been systematically studied in the basin scale. This knowledge can further clarify ambiguities in petroleum system modeling and support exploration efforts in the area. To this end, core samples and crude oils were collected from the Shahejie Formation (Paleogene) in the Dongpu Depression to perform fluorescence and infrared spectroscopy from the source rock and the reservoir. Results show that the fluorescence characteristics of oil inclusions in the source rocks are consistent with oil inclusions in the reservoirs in the northern part of the depression, showing blue, green, yellow and orange fluorescence colors; whereas orange fluorescence is absent in the southern part in inclusions. Moreover, lack of a robust relationship between λmax and QF-535 suggests crude oils have gone through secondary alterations towards becoming heavy oil. Herein, comparison between the IR (CH2/CH3) and fluorescence parameters (λmax and QF-535) illustrates that CH2/CH3, although is suitable for organic matter thermal maturity assessment, cannot be directly applied to the expelled oils. Moreover, the occurrence of orange fluorescing oils in the horsts can be accounted for a higher hydrocarbon potential in these areas. Additionally, establishing a clear relationship between fluorescence and IR parameters failed in certain strata or structural unit, proving a complex tectonic setting of the depression. Overall, this study reveals the geologic nature of the Dongpu Depression is very complicated, which means additional analytical studies to resolve the oil-source correlation problem, in particular, would be necessary.

Models of helium enrichment in helium-rich gas reservoirs of petroliferous basins in China

Owing to the characteristics of chemical inertness, low molecular weight, low boiling point, strong permeability and high heat conductivity, helium is extensively used in the fields of aerospace, low-temperature superconductivity, medicine and high tech, and it is a kind of irreplaceable strategic rare resource. So far, however, less research has been conducted on the enrichment of helium in natural gas reservoirs. To solve this problem, this paper discusses the classification standard of helium-rich gas reservoirs. Based on this, the origin, source and enrichment process of helium in typical helium-rich gas reservoirs are studied, highlighting the contribution of main helium sources to helium-rich gas reservoirs. In addition, helium enrichment models of helium-rich gas reservoir in petroliferous basins are established. And the following research results are obtained. First, 6 helium enrichment models of two categories (crust-source helium enrichment and crust-mantle mixing helium enrichment) are established for different petroliferous basins in China. Second, helium-rich gas reservoirs are characterized by using the multiple helium supply and diverse helium enrichment model. And no matter what enrichment model it is, the contribution of the main helium source to gas reservoirs is indispensable. Third, the helium in the gas reservoirs of central and western basins is mainly crust-source helium, and its enrichment models include ancient formation water moving upward along the fault to dilute helium, helium enrichment in the process of natural gas migration along ancient reservoirs, and helium enrichment of shale gas. Fourth, the helium in the helium-rich gas reservoirs of eastern basins is of crust-mantle mixing origin and its enrichment models are mainly hydrocarbon gas based, carbon dioxide based and nitrogen based. In conclusion, the establishment of the helium enrichment models of helium-rich gas reservoirs in petroliferous basins provides reference and guidance for studying helium enrichment laws and main controlling factors and is of great significance to the evaluation of favorable helium exploration areas.

A CO2 storage potential evaluation method for saline aquifers in a petroliferous basin

According to the requirements for large-scale project implementation, a four-scale and three-level CO2 storage potential evaluation method is proposed for saline aquifers in a petroliferous basin in China, considering geological, engineering and economic factors. The four scales include basin scale, depression scale, play scale and trap scale, and the three levels include theoretical storage capacity, engineering storage capacity, and economic storage capacity. The theoretical storage capacity can be divided into four trapping mechanisms, i.e. structural & stratigraphic trapping, residual trapping, solubility trapping and mineral trapping, depending upon the geological parameters, reservoir conditions and fluid properties in the basin. The engineering storage capacity is affected by the injectivity, storage security pressure, well number, and injection time. The economic storage capacity mainly considers the carbon pricing yield, drilling investment, and operation cost, based on the break-even principle. Application of the method for saline aquifer in the Gaoyou sag of the Subei Basin reveals that the structural & stratigraphic trapping occupies the largest proportion of the theoretical storage capacity, followed by the solubility trapping and the residual trapping, and the mineral trapping takes the lowest proportion. The engineering storage capacity and the economic storage capacity are significantly lower than the theoretical storage capacity when considering the constrains of injectivity, security and economy, respectively accounting for 21.0% and 17.6% of the latter.

Cretaceous opposite rotations of North China Block and southern Sikhote Alin, northeast China/Russia: Relation to rifting in the petroliferous Songliao Basin

Paleomagnetic data from northeast Asia confirm that the Korea/North China and southern Sikhote Alin blocks rotated in opposite directions from the Berriasian to the Campanian (145–72 Ma). The Songliao Basin evolved between these rotated blocks with synrift sequences dating from the Tithonian or the Berriasian/Hauterivian. Geologic maps and tomographic images demonstrate a curvilinear subduction zone with associated accretionary wedge/magmatic arc stretched from Sikhote Alin, to Japan and southeast Korea near the Cretaceous Tertiary boundary. Arc-related volcanism migrated over 1000 km southeastwards across northeast Asia to the Japan Sea and Sikhote Alin coast from ∼ 140 – 70 Ma. This suggests that opposite microplate rotations resulted from Pacificward retreat of a curved subduction zone from Early to Late Cretaceous. Toroidal or radial flows in the mantle wedge exerting basal drag on the over-riding microplates is a likely driving mechanism. Anisotropic tomography suggesting fossil curved mantle flows which match the forces required to produce opposite rotations and the distribution of crustal thickness and Vp/vs ratios under the Songliao Basin support this mechanism. A major petroliferous basin in China may therefore be the result of double saloon door tectonics occurring during the Cretaceous behind a contemporaneous continental arc.

Bio-environmental interactions and associated hydrocarbon generation in a saline lake basin: A case study of the Palaeogene interval in the Dongpu Sag, eastern China

• Different hydrocarbon-generating organisms occur in different saline palaeoenvironments. • The palaeoenvironment affects the hydrocarbon-generating organisms and source rock potential. • Source rocks deposited in saline environments have a low oil generation threshold and are characterised by multi-stage oil generation. • Saline environments form source rocks with a good organic matter type and high hydrocarbon generation potential. Saline lake basins are important Cainozoic petroliferous basins in China. There are significant differences in the palaeoenvironments of these basins. Different biological assemblages resulted in varying hydrocarbon generation characteristics and potential. Therefore, the bio-environmental interactions are key to understanding the oil generation from terrestrial organic matter. In this study, we investigated the organic petrology and geochemistry of the Palaeogene Shahejie Formation in the Dongpu Sag, Bohai Bay Basin, China. The depositional palaeoenvironment varied from saline to freshwater. Cyanobacteria and coccolithophores were deposited in the saline environment, and green algae and higher plant debris were deposited in the freshwater to brackish water environments. The algal biomass in the sediments increased with salinity. Thermal and pyrolysis experiments revealed differences in the hydrocarbon generation potential of the source rocks deposited in different sedimentary environments. The source rocks deposited in the saline water environment have a high hydrocarbon generation potential. Two immature to low-mature oil generation peaks were observed, which are mainly due to the pyrolysis of coccolithophores, cyanobacteria, and dinoflagellates. Only one oil generation peak was observed for source rocks deposited in the freshwater environment, which is mainly due to the pyrolysis of dinoflagellates and green algae. The hydrocarbon generation potential and two stages of oil generation of the source rocks deposited in the saline environment may be due to the type of hydrocarbon-generating organisms and the catalytic effect of the high-salinity diagenetic environment. The results show that the sedimentary environment of saline lake basins affects the hydrocarbon-generating organisms and hydrocarbon generation process.

Architectural Element Analysis of Nonmarine Oil and Gas Reservoir in China, the Research History, Progress and Future Trend: A Review

Since Miall’s architectural element analysis (AEA) was introduced to China in 1986, the method has been widely applied to studying the heterogeneity of oil and gas reservoirs in China. The development history, research field, research progress, and potential limitations of AEA in China have been reviewed based on 601 Chinese documents retrieved from the China National Knowledge Infrastructure (CNKI) database and some English documents retrieved from various databases. The development of sedimentary AEA in China experienced four stages: (1) the conceptual and introduction stage (1985–1995), (2) the test and trial stage (1995–2005), (3) the popularization and application stage (2005–2015), and (4) the innovative stage (2015 to present). The research field covers all reservoir types of continental petroliferous basins in China. Papers published on AEA were distributed in the fields of (1) alluvial fan reservoirs, (2) braided channel reservoirs, (3) meandering channel reservoirs, (4) fan delta reservoirs, (5) braided river delta reservoirs, (6) shallow-water delta reservoirs, (7) large-scale delta reservoirs, (8) beach and bar reservoirs, and (9) lacustrine deepwater gravity flow reservoirs. Among all these fields, the architecture of alluvial fans, braided channels, meandering channels, braided river delta, and fan delta reservoirs was mainly researched. AEAs in China have made outstanding innovations in hierarchical sedimentary boundary surface classification, depositional architectural models, and quantitative reservoir architectural modeling. Combining the hierarchical AEA with geostatistical modeling, big data, and artificial intelligence technology and comprehensively using geology, geophysics, and production information to realize the hierarchical integrated reservoir architectural characterization are the development directions of the AEA in China and around the world.

Volcanic reservoirs lacking a paleoweathering crust in Carboniferous deposits of the Junggar Basin, western China: Characteristics, controlling factors and hydrocarbon potential

The Junggar Basin is the basin with the richest exploration results of volcanic reservoirs among petroliferous basins in China. The oil test data and production data show there also exhibit good hydrocarbon displays in the Kebai Fault Zone where the paleo-weathering crust is not developed. In this study, we utilized core data and thin sections to investigate volcanic reservoir rocks lacking paleo-crusts of weathering in Carboniferous deposits in the western Junggar Basin, Northwestern China. The studied interval is dominated by seven types of volcanic rocks, two types of metamorphic rocks and three types of sedimentary rocks, which are grouped into six lithofacies. From northeast (NE) to southwest (SW) of study area, the dominant rock type changes from sedimentary rock, to pyroclastic rocks, tuffites, and then to welded pyroclastic rock, lavas, respectively volcanic rocks. And the corresponding dominant lithofacies changes from delta facies, to volcanic-sedimentary facies, explosive facies, and then to sub-volcanic facies, and effusive facies. Fault-induced fractures and dissolved pores are the dominant reservoir space types. Faults and lithology jointly played a significant role in controlling the macro-distribution, physical properties, and hydrocarbon accumulation in volcanic reservoirs. The geological setting controlled the influences of primary and secondary processes. This study also reveals that the volcanic lavas with high-angle fractures of effusive facies located in SW of the study area are favorable lithofacies for hydrocarbon accumulation. This study highlights the differences in volcanic reservoir rocks with and without paleo-crusts of weathering in terms of reservoir spaces, controlling factors and hydrocarbon accumulation. This study also reveals that volcanic reservoir lacking paleo-crust of weathering can have a high hydrocarbon potential.

Investigation on hydrocarbon generation and expulsion potential by deep learning and comprehensive evaluation method: A case study of Hangjinqi area, Ordos Basin

The Hangjinqi area is a significant gas exploration area from the northern Ordos Basin, an important petroliferous basin in China. Although a considerable amount of achievements has been made in the natural gas exploration field, the controversy about the hydrocarbon generation characteristics of source rocks has raged unabated for ten years. In addition, few studies have systematically investigated hydrocarbon expulsion potential in the Hangjinqi area.In this paper, we adopted a series of new approaches to assess the hydrocarbon generation and expulsion potential of source rocks in Hangjinqi. Specifically, we analyzed the rock pyrolysis, kerogen composition, and biomarker data and applied Convolution Neural Network (CNN) in the field of deep learning to determine organic matter abundance, type, maturity and depositional environment. Then, two techniques, the hydrocarbon generation kinetics method and the hydrocarbon generation potential method, were implemented to obtain both the hydrocarbon generation and expulsion quantity. The results reveal that the new technique CNN is suitable for TOC prediction, and the performance is improved largely compared to traditional methods, e.g., Back Propagation neural network (BP) and △log R. Rock pyrolysis data, kerogen macerals and biomarker data reveal that the source rocks are gas-prone and dominated by type Ⅲ. By the integrating hydrocarbon generation potential method and hydrocarbon generation kinetics method, the hydrocarbon generation quantity of the study area is 20.35 × 1012m3. The results of the hydrocarbon potential method indicate that about 23.7 × 108t and 0.74 × 108t hydrocarbons were expulsed from coal and mudstone, respectively. According to comprehensive analysis, the hydrocarbon generation and expulsion quantities in the Hangjinqi area are extensive, indicating good prospects for natural gas exploration. These findings have significant implications for the understanding of natural gas resources potential and the distribution of favorable natural gas areas.

Status-quo of proved oil/gas initially-in-place in China as of 2020 and its variation trend in the past decade

This study analyzes the status-quo of the proved oil/gas initially-in-place and its variation trend, the proved undeveloped oil/gas initially-in-place, and the remaining proved technically recoverable reserves (TRR) of oil/gas in China as of 2020 based on statistics. As shown by the results, the proved oil initially-in-place (OIIP), the proved undeveloped OIIP, and the remaining proved TRR of oil in China are mainly distributed in the Bohai Bay, Ordos and Songliao Basins, and those of free gas are mainly in the Ordos, Sichuan, and Tarim Basins. From 2011 to 2020, the largest increment in the proved OIIP, the proved undeveloped OIIP and the remaining proved TRR of oil occurred in the Ordos Basin, followed by the Bohai Bay Basin, while that in the proved gas initially-in-place (GIIP), the proved undeveloped GIIP, and the remaining proved TRR of gas occurred in the Ordos Basin, followed by the Sichuan Basin. In addition, a comprehensive analysis reveals that the petroliferous basins in China with the potential of reserve addition and production growth include the Ordos Basin, the Bohai Bay Basin, the Sichuan Basin, and the Tarim Basin.

Development prospect of natural gas industry in the Sichuan Basin in the next decade

The Sichuan Basin is the earliest natural gas discovery and utilization area in the world, where China's modern natural gas industry started. Through nearly 7 decades of development, a natural gas industrial system with complete upstream, midstream and downstream industry chain has been established, and its natural gas production has been among the top in China for half a century. And it has played an important role in the process of improving the primary energy consumption structure and constructing a “beautiful China”. The evaluation result of national oil and gas resources in the 13th Five-Year Plan shows that the natural gas resource extent in the Sichuan Basin is 39.94 × 1012 m3, ranking the first in petroliferous basins of China. In addition, the proven rate of natural gas resource is only 15% and it is still in the early and middles stages of exploration. Therefore, the Sichuan Basin has great potential in natural gas exploration and development. The natural gas production in the Sichuan Basin in 2020 is 565 × 108 m3 (including all production enterprises of PetroChina and Sinopec in the Sichuan Basin), taking the first place in China major gas producing areas. Over the next decade, the China puts forward new development concepts from the perspective of development circumstances. The construction of Chengdu–Chongqing economic circle will be promoted and a national natural gas strategic production base with yearly gas production of 1000 × 108 m3 will be built up in the Sichuan Basin, which marks a new golden development age of natural gas industry in the Sichuan Basin. From the perspective of development potential, deep and ultra-deep marine carbonate gas reservoirs, deep shale gas and continental tight sandstone gas will be the key exploration and development fields to search for giant gas fields in the Sichuan Basin. It is recommended to strengthen core technology researches according to the thoughts of “coordinated development of marine and continental, conventional and unconventional, structural and lithologic” and relying on theoretical and technological progress. In the future, it will enter a booming development period when the quality of natural gas resources will be continuously improved and the production rate will be increased continuously. In the meantime, take advantage of the opportunities of prosperity development to break through the development bottleneck of increasingly complex work objects, incomplete pipeline network system and insufficient high end and high additional value industries. Under the situation of normalized low oil price, construct a new-type industrial system with the coordinative development of natural gas production, transportation, storage and marketing and the innovation and efficiency improvement in the whole industrial chain. It is predictable that the natural gas industry in the Sichuan Basin will start in a new peak growth period and a high-quality development period in the next decade, and its yearly natural gas production will exceed 1000 × 108 m3, and the yearly natural gas production of PetroChina Southwest Oil & Gasfield Company will reach 800 × 108 m3 in 2030. Obviously, the Sichuan Basin will be a world-class natural gas production and supply base.

Genetic Types, Distribution Patterns and Enrichment Mechanisms of Helium in China’s Petroliferous Basins

Helium is a strategic resource with many scientific, technical and industrial applications. However, despite its importance and scarcity, there is a lack of understanding on its origin, migration and accumulation processes. In view of this, the distribution patterns and enrichment mechanisms of helium are studied based on systematic analysis of the genetic types of helium in petroliferous basins in China. The helium in the petroliferous basins in China is mainly radiogenic, which involves the decay of U and Th. As expected, we found that, in one gas field, the helium content in natural gas increases with the age of reservoir rock, and in the same layer of the same structure within one reservoir, the helium content in natural gas in high altitude part of the structure is higher than that in the lower part. Enrichment of helium is closely related to old deep groundwater. The helium produced by radioactive decay in old strata is dissolved in water and transported and enriched through water migration. Due to large uplift of the structure during the Himalayan period, the helium dissolved in water was exsolved and accumulated with natural gas in the trap above the water. And thus, the helium-rich natural gas reservoirs formed. So, the helium contents in natural gas fields increase gradually along the flow direction of groundwater. The area with large range of uplift often shows higher helium content. This study not only provides a basis for the study of helium enrichment research, but also provides a new idea for industrial helium resource exploration.

Exploration breakthrough and its significance of Gulong lacustrine shale oil in the Songliao Basin, Northeastern China

Lacustrine shale oil resources are abundant in many petroliferous basins in China. The shale oil formations are characterized by low API gravity, high viscosity, poor mobility, high clay content, low brittleness and etc. Exploration of lacustrine shale oil started relatively late in China, but its progress is very rapid and breakthroughs have been made successively. In this short communication, we introduced the most significant shale oil discovery which is made recently in the Qingshankou Formation of the Gulong sag in the Songliao Basin. Key exploratory wells including Guye 1H (GY1H), Yingye 1H (YY1H) and Guye 2HC (GY2HC) tested stable and high oil flow in shale reservoirs, revealing the relatively stable and high oil production capacity of shale in the Gulong sag of Daqing Oilfield. It marks a leap of petroleum theoretical recognition from lacustrine shale generating oil to producing oil and from the theory of traditional oil migration and accumulation to the theory of indigenous oil accumulation in organic-rich shale. Although lacustrine shale oil exploration and development still faces many challenges in China, its prospect is quite promising.

Emeishan mantle plume and its potential impact on the Sichuan Basin: Insights from numerical modeling

The Sichuan Basin, adjacent to the Emeishan large igneous province (ELIP), is one of the large petroliferous basins in China. The effects of the ELIP on the basin thermal history and hydrocarbon generation have attracted much attentions. A 2D viscoplastic model is applied to simulate the evolution of the Emeishan mantle plume, and then the effects of the plume on the Sichuan Basin are discussed. Constrained by the pre-eruption uplift and volcanic activity, the modeling infers that the initial hemispherical Emeishan plume had a radius of 85–150 km and an excess temperature of 150–300 K. The plume head extended laterally after reaching the bottom of the lithosphere. The plume head-lithosphere interaction led to lithosphere erosion and partial melting atop the plume head. Near the junction between the inner and intermediate ELIP zones, the lithosphere erosion was the most serious and the mantle melting degree was the highest. There the basalt might erupt from the Xiaojing-Qiaojia fault, and flow to the intermediate ELIP zone due to terrain. That can explain the concentration of Emeishan basalts in the intermedia ELIP zone and the abnormal high paleo-heat flow in the southwestern Sichuan Basin. Thermal convection induced in the upper mantle prevented the plume head from spreading beneath the lithosphere, and the lateral extent of the plume head was 1200–1300 km. Only a few unmelted plume materials could reach the central Sichuan Basin, which should be the boundary of the outer ELIP zone. The Permian basalts and high paleo-heat flow anomalies in the central Sichuan basin might be derived from the subsequent lithospheric extension, which led to the upwelling and decompression melting of both the plume and asthenosphere materials. Since the plume head couldn't migrate to the northeastern Sichuan Basin, the surface observations there should be independent of the Emeishan plume.

Organic matter inputs and depositional palaeoenvironment recorded by biomarkers of marine‐terrestrial transitional shale in the Southern North China Basin

In recent years, significant breakthroughs have been achieved in shale gas exploration and exploitation of marine and continental shales in petroliferous basins of China. As an important petroliferous basin in China, marine–terrestrial transitional shale was widely distributed among the Southern North China Basin (SNCB). However, biomarkers distribution characteristics of marine–terrestrial transitional shale in the SNCB, which can reveal the organic matter inputs and paleoenvironmental characteristics, is not widely concerned. In general, organic matter in a transitional environment can be affected by the amounts of organic matters input from fresh water and aquatic organisms from the sea. To systematically investigate the organic geochemical characteristics of biomarkers in marine–terrestrial transitional shale from SNCB, core samples from the Upper Shihezi, Shanxi and Taiyuan formations were obtained from a single borehole in the SNCB. Gas chromatography–mass spectrometry (GC–MS) analysis was conducted on the saturated hydrocarbons. n‐Alkanes, acyclic isoprenoid alkanes, terpanes, and steranes were detected to study the change in organic matter inputs and sedimentary paleoenvironment of the marine–terrestrial transitional shale. The results reveal that organic matter inputs obviously changed during the sedimentary period in the study area of the SNCB. In addition, the terrestrial higher plants contributed more organic matter to shale from the Upper Shihezi Formation compared to the Taiyuan Formation and the Shanxi Formation. The shift of organic matter inputs is consistent with marine regression in the Early Permian period, which may significantly influence the organic matter input characteristics. Meanwhile, the marine–terrestrial transitional shale, including samples from the Taiyuan, Shanxi and Upper Shihezi formations, was deposited in a relatively oxic condition during the depositional process.

Sequence stratigraphy and sedimentary characteristics of the shelf-edge delta and slope fan systems in the Late Oligocene, Baiyun Sag, Pearl River Mouth Basin, China

The Pearl River Mouth Basin in the South China Sea is one of the most important offshore petroliferous basins in China. Specifically, the shelf-edge delta and slope fan systems in the deepwater area of the Baiyun Sag have been regarded as the most important objects for hydrocarbon exploration, and the sequence stratigraphic structure, sedimentary evolution, and “source to sink” relationship between them is an important basis for predicting favorable areas for hydrocarbon exploration. In this study, the sequence stratigraphic structure and sedimentary system of the Late Oligocene Zhuhai Formation in the Baiyun Sag of the Pearl River Mouth Basin were studied based on a comprehensive dataset that integrated seismic, logging, and core data. A total of four third-order sequences were determined in the second-order composite sequence (CS3) according to nature of unconformities. Within each third-order sequence, the characteristics of different sedimentary systems were successfully identified. Within CS3-1 and CS3-2 the transgressive systems tracts (TST) are relatively thick, and the highstand systems tracts (HST) encompass mainly the outer-shelf deltaic deposits with thin foresets. As for CS3-3 and CS3-4, the lowstand systems tract (LST) and falling-stage systems tracts (FSST) are dominated by shelf-edge deltas and slope fans deposits, whereas the TST are very thin. The depositional evolution and “source to sink” relationship between the shelf-edge deltas and associated slope fans are revealed as follows: there are mainly three stages of shelf-edge deltas, and their sedimentary paleo-water depths were 794.9 m, 822.6 m, and 556.8 m, with progradation angles of 8.5°, 7.7°, and 5.7°, respectively; the thick delta front (200–500 m) and slump deposit are the main identification features of the shelf-edge delta. During the period of LST, although the progradation angle of the shelf-edge delta was the largest, there were few slope fans or only mud-rich slope fan deposits in the front of the delta due to the influence of relative sea-level rise, and during the period of FSST, the slump and redeposition of the delta front sand bodies provided the sand source for the formation of the slope fans, thus forming the sand-rich fans. Moreover, according to the root mean square amplitude slices and 3D seismic profile, it was found that the slope fans in this area are directly related to the development of the channels between the deltas and slope fans; generally, the larger and denser the channel, the larger the slope fan area.

Hydrocarbon accumulation depth limit and implications for potential resources prediction

The horizontal distribution and periodic accumulation of hydrocarbon in reservoirs are well understood. However, our understanding on the vertical distribution of hydrocarbon accumulation in reservoirs is not clear and remains mysterious to some extent. We proposed a concept of hydrocarbon accumulation depth limit (HADL) to characterize the hydrocarbon’s vertical distribution in petroliferous basins, which is determined by statistical analyses of the variation trends of reservoir layers’ essential properties, including hydrocarbon saturations (So), movable hydrocarbon ratios (Mo) and dry layer ratio (Ko), when the depth is increased. A total of 80,762 drilling results from 12,237 exploration wells in six representative petroliferous basins in China were collected and analyzed. The reservoir layers’ essential properties So, Mo and Ko and their correlations with porosity, permeability, pore throat radius and thermal evolution degree were investigated. The critical values of So, Mo and Ko that define HADL were quantified to be So = 0, Mo = 0 and Ko = 100. Our study indicates that the HADL in petroliferous basins varies from less than 3,000 m to more than 13,000 m deep, depending on the hydrocarbon composition, reservoir lithology, reservoir age, geothermal gradient, tectonic movement, etc. Two factors play an essential role in the formation and variation of HADL: 1) the depletion of hydrocarbon generation potential of source rocks which cuts off hydrocarbon contribution for reservoirs formation and 2) the termination of differential compaction which eliminates capillary pressure difference between the outer surrounding rocks and inner reservoir layers, ending the dominant driven force for hydrocarbon migration and accumulation in deep and tight reservoir layers. All proven oil reserves of 33.95 billion tons equivalent in China, as well as world’s discovered 52,926 oil and gas reservoirs and their unproved potential resources, are distributed above the HADL we defined.

Geochemistry and Mineralogy of Lower Paleozoic Heituao Shale from Tadong Low Uplift of Tarim Basin, China: Implication for Shale Gas Development

Tarim Basin is the largest Petroliferous basin in China, while its shale gas development potential has not been fully revealed. The organic-rich black shale in middle Ordovician Heituao Formation from Tadong low uplift of Tarim Basin has been considered as an important source rock and has the characteristic of large thickness, high organic matter content and high thermal maturity degree. To obtain its development potential, geochemical, mineralogical and mechanics research is conducted based on Rock-Eval pyrolysis, total organic carbon (TOC), X-ray diffraction (XRD) and uniaxial compression experiments. The results show that: (1) the TOC content ranges between 0.63 and 2.51 wt% with an average value of 1.22 wt%, the Tmax values are 382–523 °C (average = 468.9 °C), and the S2 value is relatively low which ranges from 0.08 to 1.37 mg HC/g rock (averaging of 0.42 mg HC/g rock); (2) the organic matter of Heituao shale in Tadong low uplift show poor abundance as indicated by low S2 value, gas-prone property, and post mature stage (stage of dry gas). (3) Quartz is the main mineral component in Heituao shale samples, accounting for 26–94 wt% with an average of 72 wt%. Additionally, its Young’s modulus ranges from 20.0 to 23.1 GPa with an average of 21.2 GPa, Poisson’s ratio ranges between 0.11 and 0.21 (average = 0.15); (4) the fracability parameter of brittleness index (BI) ranges between 0.28 and 0.99 (averaging of 0.85), indicating good fracability potential of Heituao shale of Tadong low uplift and has the potential for shale gas development. This study reveals the shale gas accumulation potential in middle Ordovician of the Tarim Basin, and beneficial for future exploration and production practice.

Silicon isotopic constraints on the genesis of cherts in the Ordovician sedimentary succession in Tarim Basin, Western China

• Hydrothermal fluids play an important role in the formation of cherts . • Si-O-Sr isotopes of the cherts to constrain the source and migration of the fluids. • Fault activity links the channels of hydrothermal migration during the late Hercynian. The Tarim Craton is the largest petroliferous basin in China, and thick chert have been found in the Ordovician carbonate reservoir during drilling. Owing to the tight chert, the storage performance may decrease and lead to drilling failure. In this study, silicon, oxygen, and strontium isotope compositions of chert were tested to constrain the source and migration of Si-rich fluids, which is helpful for understanding the reservoir and avoiding drilling failure. The Ordovician silicon dioxide in cherts have δ 30 Si values from +1.29‰ to +3.50‰, δ 18 O values from 20.21‰ to 27.50‰, and 87 Sr/ 86 Sr values from 0.709 to 0.714. The correlation between δ 30 Si- and δ 18 O reveals that the formation of cherts is related to the activity of low-temperature hydrothermal fluids, and the value of 87 Sr/ 86 Sr indicates the contribution of the deep crust source to the silicon-rich fluid. Comprehensive analyses showed that the deep thermal fluid was active during the formation of cherts. The late Hercynian tectonic movement led to the development and activity of deep and large faults, and the deep-source silica hydrothermal fluid entered the Ordovician reservoir along the fault from bottom to top, filling the pores, densifying the rocks, and causing deterioration of the storage performance. Therefore, through multi-geochemistry (Si-O-Sr isotopes), the spatial distribution characteristics of silica can be determined to avoid the distribution interval and member of silica during drilling. Moreover, the identification and distribution prediction of the genesis of cherts is of great significance for oil and gas exploration.