2023 Vol. 42, No. 3

2023, 42(3)
2023, 42(3)
Genesis of Coal-type Petroleum in China Offshore
Aliphatic biomarker signatures of early Oligocene−early Miocene source rocks in the central Qiongdongnan Basin: Source analyses of organic matter
Min Xu, Dujie Hou, Xiong Cheng, Jun Gan, Xinde Xu, Gang Liang, Wenjing Ding
2023, 42(3): 1-18. doi: 10.1007/s13131-022-2082-5
The geochemical signatures of fifty-four rock samples and three supplementary drill stem test (DST) oils from the Yacheng-Sanya formations in the central Qiongdongnan Basin (CQB) were analysed. Reconstruction of the early Oligocene−early Miocene (36–16 Ma) palaeovegetation and source analyses of organic matter (OM) were conducted using aliphatic biomarkers in ancient sediments and DST oils. Both the interpreted aquatic and terrigenous OM contributed to the CQB source rocks (SRs) but had varying relative proportions. The four distribution patterns derived from n-alkanes, terpanes, and steranes are representative of four OM composition models of the Yacheng-Sanya SRs, including model A, model B, model C, and model D, which were classified based on the increasing contribution from terrigenous OM relative to aquatic OM. Some terrigenous higher plant-derived biomarkers, including oleanane, des-A-oleanane, C29 ααα 20R sterane, bicadinanes, the C19/(C19 + C23) tricyclic terpane ratio, and other n-alkane-derived ratios suggest that angiosperms had increased proportions in the palaeoflora from early Oligocene to early Miocene, and the bloom of terrigenous higher plants was observed during deposition of upper Lingshui Formation to lower Sanya Formation. These findings are consistent with the incremental total organic carbon and free hydrocarbons + potential hydrocarbons (S1 + S2) in the lower Lingshui-lower Sanya strata with a significant enrichment of OM in the E3l1-N1s2 shales. The maturity- and environment-sensitive aliphatic parameters of the CQB SRs and DST oils suggest that all the samples have predominantly reached their early oil-generation windows but have not exceeded the peak oil windows, except for some immature Sanya Formation shales. In addition, most of the OM in the analysed samples was characterised by mixed OM contributions under anoxic to sub-anoxic conditions. Furthermore, terrestrial-dominant SRs were interpreted to have developed mainly in the Lingshui-Sanya formations and were deposited in sub-oxic to oxic environments, compared to the anoxic to sub-anoxic conditions of the Yacheng Formation.
Zircon U-Pb geochronology, Hf isotopes, and geochemistry constraints on the age and tectonic affinity of the basement granitoids from the Qiongdongnan Basin, northern South China Sea
Lijun Mi, Xiaoyin Tang, Haizhang Yang, Shuchun Yang, Shuai Guo
2023, 42(3): 19-30. doi: 10.1007/s13131-022-2078-1
Studies in the northern South China Sea (SCS) basement remain important for understanding the evolution of the Southeast Asian continental margin. Due to a thick cover of sediments and scarce borehole penetration, little is known about the age and tectonic affinity of this basement. In this study, an integrated study of zircon U-Pb geochronology, Hf isotopes, and whole-rock major and trace elements on seven basement granitoids from seven boreholes of Qiongdongnan Basin has been carried out. New zircon U-Pb results for these granitoids present middle-late Permian ((270.0±1.2) Ma; (253±3.4) Ma), middle to late Triassic ((246.2±3.4) Ma; (239.3±0.96) Ma; (237.9±0.99) Ma; (228.9±1.0) Ma) and Late Cretaceous ages ((120.6±0.6) Ma). New data from this study, in combination with the previous dataset, indicates that granitoid ages in northern SCS basement vary from 270 Ma to 70.5 Ma, with three age groups of 270–196 Ma, 162–142 Ma, and 137–71 Ma, respectively. Except for the late Paleozoic-Mesozoic rocks in the basement of the northern SCS, a few old zircon grains with the age of (2708.1±17) Ma to (2166.6±19) Ma provide clues to the existence of the pre-Proterozoic components. The geochemical signatures indicate that the middle Permian-early Cretaceous granitoids from the Qiongdongnan Basin are I-type granites formed in a volcanic arc environment, which were probably related to the subduction of the Paleo-Pacific Plate.
Biomarkers reveal the terrigenous organic matter enrichment in the late Oligocene−early Miocene marine shales in the Ying-Qiong Basin, South China Sea
Wenjing Ding, Youchuan Li, Lan Lei, Li Li, Shuchun Yang, Yongcai Yang, Dujie Hou
2023, 42(3): 31-53. doi: 10.1007/s13131-022-2081-6
The increase of total organic carbon content of the late Oligocene−early Miocene terrigenously-dominated marine shales in the shallower depth intervals was reported in the Ying-Qiong Basin, South China Sea. The organic enriched lower Sanya Formation shales (early Miocene) have biomarker characteristics of tropical/subtropical plants, with abundant high molecular weight n-alkanes, angiosperm-derived oleanane, rearranged oleananes I, II, II, tricyclic/tetracyclic terpanes including des-A-oleanane, X, *, Y, Z, Z1 and bicadinanes W, T, T1, R. The biomarker characteristics are suggestive of larger influx of the dominant tropical/subtropical angiosperms in flora under a warming and more humid climate during depositions of the lower Sanya Formation (early Miocene) than the older Lingshui Formation (late Oligocene). The tropical/subtropical angiosperm input was thought as the prime control of terrigenous organic matter enrichment relative to the redox condition, and the coeval sea level changes and seafloor spreading in the South China Sea. Enrichment of the terrigenous organic matter in the early Miocene shales is likely in association with the coeval peak East Asian summer monsoon intensity in the South China Sea.
The influence of organic sources and environments on source rock deposition during the periods of Cretaceous–Eocene and Oligocene–Miocene, northern Kalimantan
Lei Lan, Youchuan Li, Zhigang Zhao, Shuchun Yang, Qing Lin, Weilai Zhang
2023, 42(3): 54-64. doi: 10.1007/s13131-022-2080-7
The sedimentary system of Kalimantan has undergone significant development since the Oligocene. Previous research have largely ignored the capacity of the Cretaceous–Eocene sediments to produce hydrocarbons, focusing instead primarily on the Oligocene–Miocene coal as the principal source rocks. Shales and coals from the outcrops in the northern margin of Kalimantan were analyzed with palynological and geochemical methods to characterize the palaeoenvironmental and palaeoecological differences between the Cretaceous–Eocene and the Oligocene–Miocene samples. The high proportion of Cheirolepidoaceae, Schizaeoisporites and Ephedripites in the pollen assemblage from the Cretaceous–Eocene outcrops reflects an arid tropical/subtropical climate. The relatively low abundances of gymnosperm-derived biomarkers including isopimarane, β-phyllocladane, β-kaurane, suggest the gymnosperm features in flora. High C27/C29 ααα 20R sterane ratios, (C19–C29) tricyclic terpanes/C30 αβ hopane and extremely low oleanane/C30 αβ hopane, bicadinane T/C30 αβ hopane, and diterpenoid abundance indicate that there was a dominance of algae relative to higher plants in the organic matter. The gymnosperm-derived biomarkers, including isopimarane, β-phyllocladane, β-kaurane, suggest that palaeovegetation during this period was dominated by gymnosperms. The saline and reducing conditions in the bathyal and abysmal sea, evidenced by rather low Pr/Ph and high Gammarerane index, are beneficial for the preservation of hydrogen-rich organic matter. It is presumed that the Cretaceous–Eocene shales had great hydrocarbon generation potential in the southern South China Sea. During the period of Oligocene to Miocene in the Zengmu Basin and the Baram-Sabah Basin, the climate changed to a dominant humid and warm condition, which is corroborated by abundant pollen of Florschuetzia and Magnastriatites hawardi. Low C27/C29 ααα 20R sterane ratios, (C19–C29) tricyclic terpanes/C30 αβ hopane, and high oleanane/C30 αβ hopane, bicadinane T/C30 αβ hopane suggest that the palaeovegetation was dominated by angiosperms including the mangrove plants. The extremely abundant higher plants provide ample terrigenous organic matter for the formation of coal-measures in delta facies. The low gammacerane index and high Pr/Ph indicate the fresh and sub-oxic water in delta-neritic-abysmal faces, which is not beneficial for the accumulation of hydrogen-rich organic matter. Thus, the Oligocene–Miocene marine argillaceous rocks can be potential sources of natural gas.
Mechanism of carbonate cementation and its influence on reservoir in Pinghu Formation of Xihu Sag
Haiqiang Bai, Xiaojun Xie, Gongcheng Zhang, Ying Chen, Ziyu Liu, Lianqiao Xiong, Jianrong Hao, Xin Li
2023, 42(3): 65-75. doi: 10.1007/s13131-022-2079-0
Carbonate cements are the most abundant authigenic mineral and impact on physical properties greatly in sandstone reservoir. In this paper, Pinghu Formation of Xihu Sag was taken as a target. Characteristics, distribution and formation of carbonate cements were investigated via optical microscopy, cathodoluminescence (CL), electron probe and in-situ carbon-oxygen isotope. The results showed that carbonate cements varied in types and shapes. Calcite/dolomite mainly present as poikilotopic cements, while ferrocalcite/ferrodolomite/ankerite generally present as pore-filling cements. Carbon isotope (δ13C) values of carbonate cements were ranging from –7.77‰ to –2.67‰, with an average of –4.52‰, while oxygen isotope (δ18O) values were ranging from –18.94‰ to –12.04‰, with an average of –14.86‰. The δ13C/δ18O indicated that the paleo-fluid of carbonate cement was mainly freshwater. Organic carbon mainly came from organic matter within mature source rocks, and inorganic carbon came from dissolution of carbonate debris and early carbonate cements. Distinctive δ13C/δ18O values manifest that carbonate cements with different types formed in different periods, which make different contributions to the reservoir properties. Calcite/dolomite formed during eodiagenesis (70–90℃) and early mesodiagenesis stage (90–120℃), and were favorable to reservoir owing to their compacted resistance and selective dissolution. Ferrocalcite/ferrodolomite/ankerite formed during middle-late mesodiagenetic stage (above 120℃), and were unfavorable to reservoir due to cementing the residual intergranular pores. Hence, in order to evaluate the reservoir characteristics, it is of significantly important to distinguish different types of carbonate cements and explore their origins.
Origin of hydrocarbon fluids and discussion of abnormal carbon isotopic compositions in the Lishui-Jiaojiang Sag, East China Sea Shelf Basin
Jingqi Xu
2023, 42(3): 76-88. doi: 10.1007/s13131-022-2128-8
The hydrocarbon gases in the L1 gas field of the Lishui-Jiaojiang Sag have been commonly interpreted to be an accumulation of pure sapropelic-type thermogenic gas. In this study, chemical components, stable isotopic compositions, and light hydrocarbons were utilized to shed light on the origins of the hydrocarbon fluids in the L1 gas pool. The hydrocarbon fluids in the L1 gas pool are proposed to be a mixture of three unique components: mid-maturity oil from the middle Paleocene coastal marine Lingfeng source rock, oil-associated (late oil window) gas generated from the lower Paleocene lacustrine Yueguifeng source rock, and primary microbial gas from the paralic deposits of the upper Paleocene Mingyuefeng source rock. Here, for the first time, the hydrocarbon gases in the L1 gas pool are diagnosed as mixed oil-associated sapropelic-type gas and microbial gas via four pieces of principal evidence: (1) The abnormal carbon isotopic distributions of all methane homologues from C1 (CH4 or methane) to C5 (C5H12 or pentane) shown in the Chung plot; (2) the diagnostic 13C-depleted C1 compared with the thermogenic sapropelic-type gas model, while δ13C2 (C2H6 or ethane) and δ13C3 (C3H8 or propane) both fit perfectly; (3) the excellent agreement of the calculated carbon isotopic compositions of the pure thermogenic gas with the results of the thermal simulated gas from the type-II1 kerogen-rich Yueguifeng source rock; and (4) the oil-associated gas inferred from various binary genetic diagrams with an abnormally elevated gas oil ratio. Overall, the natural gases of the L1 gas pool were quantified in this study to comprise approximately 13% microbial gas, nearly 48% oil-associated sapropelic-type gas, and 39% of nonhydrocarbon gas. The microbial gas is interpreted to have been codeposited and entrained in the humic-kerogen-rich Mingyuefeng Formation under favorable low-temperature conditions during the late Paleocene-middle Eocene. The microbial gas subsequently leaked into the structurally and stratigraphically complex L1 trap with oil-associated sapropelic-type gas from the Yueguifeng source rock during the late Eocene−Oligocene uplifting event. A small amount of humic-kerogen-generated oil in the L1 gas pool is most likely to be derived from the underlying Lingfeng source rock. The detailed geological and geochemical considerations of source rocks are discussed to explain the accumulation history of hydrocarbon fluids in the L1 gas pool. This paper, therefore, represents an effort to increase the awareness of the pitfalls of various genetic diagrams, and an integrated geochemical and geological approach is required for hydrocarbon-source correlation.
New interpretation on the provenance changes of the upper Pinghu–lower Huagang Formation within Xihu Depression, East China Sea Shelf Basin
Jinshui Liu, Shuai Li, Kaifei Liao, Yuchi Cui, Lei Shao, Peijun Qiao, Yi Lu, Yuanli Hou, Thian Lai Goh, Yongjian Yao
2023, 42(3): 89-100. doi: 10.1007/s13131-022-2127-9
Both Pinghu and Huagang formations are important hydrocarbon reservoirs of the Xihu Depression in the East China Sea Shelf Basin. Clarifying the source suppliers and restoring source-to-sink transport routes are of great significance to the future petroleum and gas undertakings. Previous researchers were largely confined by either limitation of geological records, highly dependence on a singular method or low-precision dating techniques. Our study integrated heavy mineral assemblages, geochemical analyses and detrital zircon U-Pb dating to reconstruct multiple source-to-sink pathways, and to provide a better understanding on the provenance evolution for the upper Pinghu–lower Huagang depositions of the Xihu Depression. At least three major provenances have been confirmed and systematically investigated for their separate compositional features. The Hupijiao Uplift (or even farther northern area) was dominated by a major Paleoproterozoic population peaked at ca. 1 830 Ma along with minor Mesozoic clusters. The Haijiao Uplift to the west and the Yushan Low Uplift to the southwest, on the other hand, generate opposite U-Pb age spectra with apparently larger peaks of Indosinian and Yanshanian-aged zircons. To be noted, both Indosinian and Paleoproterozoic peaks are almost identical in proportion for the Haijiao Uplift. The overall sedimentary pattern of late Eocene−early Oligocene was featured by both spatial and temporal distinction. The Hupijiao Uplift was likely to cast limited impact during the late Eocene, whereas the broad southern Xihu Depression was transported by a large abundance of materials from the nearby Haijiao and Yushan Low Uplifts. The northern source substantially extended its influence to the farther south during the early Oligocene by delivering plentiful sediments of higher-degree metamorphic parent rocks. Combined with the proximal western and southwestern suppliers, the overall Xihu Depression was under control from both distant and local provenances.
Cenozoic giant coal-bearing basin belt discovered in China’s sea area
Gongcheng Zhang, Ying Chen, Dongdong Wang, Zengxue Li
2023, 42(3): 101-112. doi: 10.1007/s13131-022-2134-x
Sea area is an important area of oil and gas exploration in China. It has been found that China’s sea area mainly consists of coal type oil and gas, and the exploration of coal-bearing series source rocks has become an important part of oil and gas exploration there. Through years of comprehensive geological research in China’s sea area, it has been revealed that it has undergone multiple occurrences of tectonic opening and closing movements in varying degrees in the Paleogene, forming 26 Cenozoic sedimentary basins of various types, such as active continental margin, passive continental margin, transitional continental margin and drift rift basins. In the present study, it is observed for the first time that coal type source rocks are mainly developed in 14 continental margin basins in China’s sea area, revealing that a very large C-shaped coal-bearing basin group developed there in the Cenozoic. Next, based on the coupling analysis of paleoclimate, paleobotany, paleogeography and paleostructure, it is observed that there are five coal-forming periods in China’s sea area, namely the Paleocene, Eocene, early Oligocene, late Oligocene and Miocene-Pliocene, and the coal-forming age is gradually new from north to south. It is also found that the coal seams in the sea area are mainly developed in three coal-forming environments in Cenozoic, namely delta, fan delta and tidal flat-lagoon. The coal seams developed in different environments are characterized by thin thickness, many layers and poor stability. However, the coal-bearing series source rocks in China’s sea area have a wide distribution range, very high thickness and large amount, thus forming a material basis for the formation of rich coal type oil and gas.
Early Cenozoic paleontological assemblages and provenance evolution of the Lishui Sag, East China Sea
Yingzhao Zhang, Yiming Jiang, Zhenghua Liu, Shuai Li, Ning Li, Jinshui Liu, Peijun Qiao, Kai Zhong, Shuhui Chen, Thian Lai Goh
2023, 42(3): 113-122. doi: 10.1007/s13131-022-2133-y
The East China Sea Shelf Basin generated a series of back-arc basins with thick successions of marine- and terrestrial-facies sediments during Cenozoic. It is enriched with abundant oil and gas resources and is of great significance to the petroleum exploration undertakings. Therein, the Lishui Sag formed fan delta, fluvial delta and littoral-to-neritic facies sediments during Paleocene–Eocene, and the research on its sedimentary environment and sediment source was controversial. This study analyzed the paleontological combination characteristics, and conducted a source-to-sink comparative analysis to restore the sedimentary environment and provenance evolution of the Lishui Sag during Paleocene–Eocene based on the integration of detrital zircon U-Pb age spectra patterns with paleontological assemblages. The results indicated that Lishui Sag was dominated by littoral and neritic-facies environment during time corroborated by large abundance of foraminifera, calcareous nannofossils and dinoflagellates. Chronological analysis of detrital zircon U-Pb revealed that there were significant differences in sediment sources between the east and west area of the Lishui Sag. The western area was featured by deeper water depths in the Paleocene–Eocene, and the sediment was characterized by a single Yanshanian peak of zircon U-Pb age spectra, and mainly influenced from Yanshanian magmatic rocks of South China Coast and the surrounding paleo-uplifts. However, its eastern area partly showed Indosinian populations. In particular, the upper Eocene Wenzhou sediments were featured by increasingly plentiful Precambrian zircons in addition to the large Indosinian-Yanshanian peaks, indicating a possible impact from the Yushan Low Uplift to the east. Therefore, it is likely that the eastern Lishui Sag generated large river systems as well as deltas during time. Due to the Yuquan Movement, the Lishui Sag experienced uplifting and exhumation in the late stage of the late Eocene and was not deposited with sediments until Miocene. Featured by transitional-facies depositions of Paleocene–Eocene, the Lishui Sag thus beared significant potential for source rock and oil-gas reservoir accumulation.
The gradual subduction-collision evolution model of Proto-South China Sea and its control on oil and gas
Xiaojun Xie, Wu Tang, Gongcheng Zhang, Zhigang Zhao, Shuang Song, Shixiang Liu, Yibo Wang, Jia Guo
2023, 42(3): 123-137. doi: 10.1007/s13131-022-2132-z
This study involved outcrop, drilling, seismic, gravity, and magnetic data to systematically document the geological records of the subduction process of Proto-South China Sea (PSCS) and establish its evolution model. The results indicate that a series of arc-shaped ophiolite belts and calcalkaline magmatic rocks are developed in northern Borneo, both of which have the characteristics of gradually changing younger from west to east, and are direct signs of subduction and collision of PSCS. At the same time, the subduction of PSCS led to the formation of three accretion zones from the south to the north in Borneo, the Kuching belt, Sibu belt, and Miri belt. The sedimentary formation of northern Borneo is characterized by a three-layer structure, with the oceanic basement at the bottom, overlying the deep-sea flysch deposits of the Rajang–Crocker group, and the molasse sedimentary sequence that is dominated by river-delta and shallow marine facies at the top, recording the whole subduction–collision–orogeny process of PSCS. Further, seismic reflection and tomography also confirmed the subduction and collision of PSCS. Based on the geological records of the subduction and collision of PSCS, combined with the comprehensive analysis of segmented expansion and key tectonic events in the South China Sea, we establish the “gradual” subduction-collision evolution model of PSCS. During the late Eocene to middle Miocene, the Zengmu, Nansha, and Liyue–Palawan blocks were separated by West Baram Line and Balabac Fault, which collided with the Borneo block and Kagayan Ridge successively from the west to the east, forming several foreland basin systems, and PSCS subducted and closed from the west to the east. The subduction and extinction of PSCS controlled the oil and gas distribution pattern of southern South China Sea (SSCS) mainly in three aspects. First, the “gradual” closure process of PSCS led to the continuous development of many large deltas in SSCS. Second, the deltas formed during the subduction–collision of PSCS controlled the development of source rocks in the basins of SSCS. Macroscopically, the distribution and scale of deltas controlled the distribution and scale of source rocks, forming two types of source rocks, namely, coal measures and terrestrial marine facies. Microscopically, the difference of terrestrial higher plants carried by the delta controlled the proportion of macerals of source rocks. Third, the difference of source rocks mainly controlled the distribution pattern of oil and gas in SSCS. Meanwhile, the difference in the scale of source rocks mainly controlled the difference in the amount of oil and gas discoveries, resulting in a huge amount of oil and gas discoveries in the basin of SSCS. Meanwhile, the difference of macerals of source rocks mainly controlled the difference of oil and gas generation, forming the oil and gas distribution pattern of “nearshore oil and far-shore gas”.
Supply of terrigenous organic matter from tidal flat to the marine environment: An example of neritic source rocks in the Eocene Pinghu Formation, Xihu Depression, East China Sea Shelf Basin
Shanshan Zhou, Youchuan Li, Jianping Li, Wenjing Ding, Xin Li, Weilai Zhang
2023, 42(3): 138-150. doi: 10.1007/s13131-022-2141-y
The terrigenously-dominated marine shales which were deposited in the lower Eocene Pinghu Formation were thought to be a potential source rock in the Xihu Depression of the East China Sea Shelf Basin. However, the exceptionally high total organic carbon content (TOC, >6% on average) of the tidal sand ridge samples was not compatible with their sedimentary environment, indicating coal-bearing sedimentary debris may have been transferred from the coast to the ocean. In this study, new sights into the origins and supply of organic materials in the coastal environment were proposed in the neritic organic matter of the Eocene Pinghu Formation. A discriminant model was developed using plynofacies analysis data to pinpoint the source of organic materials in marine source rocks. The discrimination results suggested that marine mudstones were associated with tidal flat mudstones rather than deltaic ones. The biomarker characteristics of mudstones deposited in various environments support this assertion, indicating that the supply of plant materials in tidal flats is the primary organic matter source for the marine environment. The organic matter abundance was elevated in tidal flats due to their superior preservation conditions. Additionally, the lithological assemblage of tidal flats suggests that tidal currents can scour marshes and then transport dispersed terrigenous organic materials to neritic areas. These findings indicate that coal-bearing sedimentary debris was likely transferred from the coast to the ocean, and tidal currents are thought to be the dominant mechanism driving organic matter from the tidal to the marine environment.
Dissolution mechanism of a deep-buried sandstone reservoir in a deep water area: A case study from Baiyun Sag, Zhujiang River (Pearl River) Mouth Basin
Jihua Liao, Keqiang Wu, Lianqiao Xiong, Jingzhou Zhao, Xin Li, Chunyu Zhang
2023, 42(3): 151-166. doi: 10.1007/s13131-022-2142-x
Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers. In this paper, the Enping Formation and Zhuhai Formation in Baiyun Sag of South China Sea was taken as a target. Based on the thin section, scanning electron microscopy, X-ray diffraction, porosity/permeability measurement, and mercury injection, influencing factors of dissolution were examined, and a dissolution model was established. Further, high-quality reservoirs were predicted temporally and spatially. The results show that dissolved pores constituted the main space of the Paleogene sandstone reservoir. Dissolution primarily occurred in the coarse- and medium-grained sandstones in the subaerial and subaqueous distributary channels, while dissolution was limited in fine-grained sandstones and inequigranular sandstones. The main dissolved minerals were feldspar, tuffaceous matrix, and diagenetic cement. Kaolinization of feldspar and illitization of kaolinite are the main dissolution pathways, but they occur at various depths and temperatures with different geothermal gradients. Dissolution is controlled by four factors, in terms of depositional facies, source rock evolution, overpressure, and fault activities, which co-acted at the period of 23.8–13.8 Ma, and resulted into strong dissolution. Additionally, based on these factors, high-quality reservoirs of the Enping and Zhuhai formations are predicted in the northern slope, southwestern step zone, and Liuhua uplift in the Baiyun Sag.
Influence of the Moho surface distribution on the oil and gas basins in China seas and adjacent areas
Yimi Zhang, Wanyin Wang, Linzhi Li, Xingang Luo, Dingding Wang, Tao He, Feifei Zhang, Jing Ma
2023, 42(3): 167-188. doi: 10.1007/s13131-022-2136-8
Owing to the strategic significance of national oil and gas resources, their exploration and production must be prioritized in China. Oil and gas resources are closely related to deep crustal structures, and Moho characteristics influence oil and gas distribution. Therefore, it is important to study the relationship between the variation of the Moho surface depth undulation and hydrocarbon basins for the future prediction of their locations. The Moho depth in the study area can be inverted using the Moho depth control information, the Moho gravity anomaly, and the variable density distribution calculated by the infinite plate. Based on these results, the influences of Moho characteristics on petroleum basins were studied. We found that the Moho surface depth undulation deviation and crustal thickness undulation deviation in the hydrocarbon-rich basins are large, and the horizontal gradient deviation of the Moho surface shows a positive linear relationship with oil and gas resources in the basin. The oil-bearing mechanism of the Moho basin is further discussed herein. The Moho uplift area and the slope zone correspond to the distribution of oil and gas fields. The tensile stress produced by the Moho uplift can form tensile fractures or cause tensile fractures on the surface, further developing into a fault or depression basin that receives deposits. The organic matter can become oil and natural gas under suitable chemical and structural conditions. Under the action of groundwater or other dynamic forces, oil and natural gas are gradually transported to the uplift or the buried hill in the depression zone, and oil and gas fields are formed under the condition of good caprock. The research results can provide new insights into the relationship between deep structures and oil and gas basins as well as assist in the strategic planning of oil and gas exploration activities.
Controlling factors on the charging process of oil and gas in the eastern main sub-sag of the Baiyun Sag, Zhujiang River (Pearl River) Mouth Basin
Cong Chen, Xiangtao Zhang, Guangrong Peng, Zulie Long, Baojun Liu, Xudong Wang, Puqiang Zhai, Bo Zhang
2023, 42(3): 189-200. doi: 10.1007/s13131-022-2140-z
The eastern main sub-sag (E-MSS) of the Baiyun Sag was the main zone for gas exploration in the deep-water area of the Zhujiang River (Pearl River) Mouth Basin at its early exploration stage, but the main goal of searching gas in this area was broken through by the successful exploration of the W3-2 and H34B volatile oil reservoirs, which provides a new insight for exploration of the Paleogene oil reservoirs in the E-MSS. Nevertheless, it is not clear on the distribution of “gas accumulated in the upper layer, oil accumulated in the lower layer” (Gasupper-Oillower) under the high heat flow, different source-rock beds, multi-stages of oil and gas charge, and multi-fluid phases, and not yet a definite understanding of the genetic relationship and formation mechanism among volatile oil, light oil and condensate gas reservoirs, and the migration and sequential charge model of oil and gas. These puzzles directly lead to the lack of a clear direction for oil exploration and drilling zone in this area. In this work, the PVT fluid phase, the origin of crude oil and condensate, the secondary alteration of oil and gas reservoirs, the evolution sequence of oil and gas formation, the phase state of oil and gas migration, and the configuration of fault activity were analyzed, which established the migration and accumulation model of Gasupper-Oillower co-controlled by source and heat, and fractionation controlled by facies in the E-MSS. Meanwhile, the fractionation evolution model among common black reservoirs, volatile reservoirs, condensate reservoirs and gas reservoirs is discussed, which proposed that the distribution pattern of Gasupper-Oillower in the E-MSS is controlled by the generation attribute of oil and gas from source rocks, the difference of thermal evolution, and the fractionation controlled by phases after mixing the oil and gas. Overall, we suggest that residual oil reservoirs should be found in the lower strata of the discovered gas reservoirs in the oil-source fault and diapir-developed areas, while volatile oil reservoirs should be found in the deeper strata near the sag with no oil-source fault area.
Integrated gravity and magnetic study on patterns of petroleum basin occurrence in the China seas and adjacent areas
Tao He, Wanyin Wang, Zhizhao Bai, Xingang Luo, Jing Ma, Yimi Zhang
2023, 42(3): 201-214. doi: 10.1007/s13131-022-2139-5
The China seas and adjacent areas contain numerous petroleum basins. One of the main challenges for future oil and gas exploration is to identify the inherent patterns of petroleum basin distribution. The formation and evolution of petroleum basins along with the migration and accumulation of oil and gas are often closely related to the tectonic environment. The gravity and magnetic fields with high lateral resolution and wide coverage provide important data for regional tectonic research. Based on the gravity data in the Global Satellite Gravity Anomaly Database (V31.1) and magnetic data from the Earth Magnetic Anomaly Grid (2-arc-minute resolution) (V2), this study uses integrated gravity and magnetic field technique to obtain integrated gravity and magnetic field result for the China seas and adjacent areas, and then adopts the normalized vertical derivative of the total horizontal derivative technique to conduct partition. Finally, it identifies the relationship between the partition characteristics and tectonics as well as the patterns of petroleum basin occurrence. The research shows that the partition of gravity and magnetic field integrated result has a good correlation with the Neo-Cathaysian tectonic system and tectonic units. The petroleum basins are characterized according to three blocks arranged from north to south and four zones arranged from east to west. The north−south block structure causes the uneven distribution of oil and gas resources in the mainland area and the differences in the hydrocarbon-bearing strata. Petroleum basins are more abundant in the north than in the south. The ages of the main oil- and gas-bearing strata are “Paleozoic–Mesozoic, Paleozoic–Mesozoic–Cenozoic, and Paleozoic–Mesozoic”, in order from north to south. The difference in the overall type of oil and gas resources in all basins is controlled by the east–west zonation. From east to west, the oil and gas resource type exhibits a wave-like pattern of “oil and gas, gas, oil and gas, gas”. The vertical distribution is characterized by an upper oil (Mesozoic–Cenozoic) and lower gas (Mesozoic–Paleozoic) structure. Within the study area, the Paleozoic marine strata should be the main strata of future natural gas exploration.
Late Eocene–early Miocene provenance evolution of the Crocker Fan in the southern South China Sea
Yuchi Cui, Lei Shao, Wu Tang, Peijun Qiao, Goh Thian Lai, Yongjian Yao
2023, 42(3): 215-226. doi: 10.1007/s13131-023-2148-z
There are many large-scale Cenozoic sedimentary basins with plentiful river deltas, deep-water fans and carbonate platforms in the southern South China Sea. The Crocker Fan was deposited as a typical submarine fan during the late Eocene–early Miocene, and stretches extensively across the entire Sarawak–Sabah of the northern Borneo area. However, systematic analyses are still lacking regarding its sediment composition and potential source suppliers. No consensus has been reached yet on the provenance evolution and sedimentary infilling processes, which seriously impeded the oil-and-gas exploration undertakings. By combining with sedimentary-facies identification, heavy mineral assemblages, elemental geochemistry and detrital zircon U-Pb dating, this paper aims to generalize an integrated analysis on the potential provenance terranes and restore source-to-sink pathways of the Crocker Fan. In general, the Crocker Fan was initially formed over the Cretaceous–lower/middle Eocene Rajang Group by an angular Rajang unconformity. The continual southward subduction of the proto-South China Sea resulted in magmatic activities and subsequent regional deformation and thrusting along the Lupar Line in the northern Borneo. The lowermost Crocker sequence is featured by a thick conglomerate layer sourced from in-situ or adjacent paleo-uplifts. From the late Eocene to the early Miocene, the Crocker Fan was constantly delivered with voluminous detritus from the Malay Peninsula of the western Sundaland. The Zengmu Basin was widely deposited with delta plain and neritic facies sediments, while the Brunei-Sabah Basin, to the farther east, was ubiquitously characterized by turbiditic sequences. The Crocker Fan successions are overall thick layers of modest-grained sandstones, which formed high-quality reservoirs in the southern South China Sea region.
Study on the distribution characteristics of faults and their control over petroliferous basins in the China seas and its adjacent areas
Xin’gang Luo, Wanyin Wang, Ying Chen, Zhizhao Bai, Dingding Wang, Tao He, Yimi Zhang, Ruiyun Ma
2023, 42(3): 227-242. doi: 10.1007/s13131-022-2138-6
As one of the main controlling factors of oil and gas accumulation, faults are closely related to the distribution of oil and gas reservoirs. Studying how faults control petroliferous basins is particularly important. In this work, we investigated the plane positions of major faults in the China seas and its adjacent areas using the normalized vertical derivative of the total horizontal derivative (NVDR-THDR) of the Bouguer gravity anomaly, the fusion results of gravity and magnetic anomalies, and the residual Bouguer gravity anomaly. The apparent depths of major faults in the China seas and its adjacent areas were inverted using the Tilt-Euler method based on the Bouguer gravity anomaly. The results show that the strikes of the faults in the China seas and its adjacent areas are mainly NE and NW, followed by EW, and near-SN. Among them, the lengths of most ultra-crustal faults are in the range of 1 000–3 000 km, and their apparent depths lie between 10 km and 40 km. The lengths of crustal faults lie between 300 km and 1 000 km, and their apparent depths are between 0 km and 20 km. According to the plane positions and apparent depths of the faults, we put forward the concept of fault influence factor for the first time. Based on this factor, the key areas for oil and gas exploration were found as follows: the east of South North China Basin in the intracontinental rift basins; the southeast region of East China Sea Shelf Basin, the Taixinan and Qiongdongnan basins in the continental margin rift basins; Zhongjiannan Basin in the strike-slip pull-apart basins; the Liyue, Beikang, and the Nanweixi basins in the rifted continental basins. This work provides valuable insights into oil and gas exploration, mineral resource exploration, and deep geological structure research in the China seas and its adjacent areas.
Impact of microorganism degradation on hydrocarbon generation of source rocks: A case study of the Bozhong Sag, Bohai Bay Basin
Wei Li, Yufei Gao, Youchuan Li
2023, 42(3): 243-253. doi: 10.1007/s13131-023-2177-7
The discovery of the Bozhong 19-6 gas field, the largest integrated condensate gas field in the eastern China in 2018, opened up a new field for the natural gas exploration deep strata in the Bohai Bay Basin, demonstrating there is a great potential for natural gas exploration in oil-type basins. The ethane isotope of the Bozhong 19-6 condensate gas is heavy, showing the characteristics of partial humic gas. In this paper, aimed at the source rocks of the Bozhong 19-6 gas field in the Bohai Bay Basin, the characteristics of the source rocks in the Bozhong 19-6 structural belt were clarified and the reason are explained from impact of microorganism degradation on hydrocarbon generation of source rocks why the condensate oil and gas had heavy carbon isotope and why it showed partial humic characteristics was explored based on the research of parent materials. The following conclusions were obtained: The paleontology of the Bozhong 19-6 structural belt and its surrounding sub-sags is dominated by higher plants, such as angiosperm and gymnosperm. During the formation of source rocks, under the intensive transformation of microorganism, the original sedimentary organic matter such as higher plants was degraded and transformed by defunctionalization. Especially, the transformation of anaerobic microorganisms on source rocks causes the degradation and defunctionalization of a large number of humic products such as higher plants and the increase of hydrogen content. The degradation and transformation of microorganism don’t transform the terrestrial humic organic matter into newly formed “sapropel” hydrocarbons, the source rocks are mixed partial humic source rocks. As a result, hydrogen content incrased and the quality of source rocks was improved, forming the partial humic source rocks dominated by humic amorphous bodies. The partial humic source rocks are the main source rocks in the Bozhong 19-6 gas field, and it is also the internal reason why the isotope of natural gas is heavy.
Formation and distribution of coal measure source rocks in the Eocene Pinghu Formation in the Pinghu Slope of the Xihu Depression, East China Sea Shelf Basin
Yongcai Yang, Xiaojun Xie, Youchuan Li, Gang Guo, Xiaoying Xi, Wenjing Ding
2023, 42(3): 254-269. doi: 10.1007/s13131-023-2176-8
The Xihu Depression in the East China Sea Shelf Basin is a large petroliferous sedimentary depression, in which oil and gas reservoirs were mainly discovered in the Pinghu Slope and the central inversion zone. The oil-gas source correlation in the Xihu Depression was analyzed by hydrocarbon generating thermal simulation data via gold-tube pyrolysis experiments. The results indicated that the oil and gas in the Xihu Depression were mainly derived from coal measure source rocks of the Eocene Pinghu Formation. Therefore, the identification of coal seams is extremely crucial for evaluating coal measure source rocks in the Pinghu Formation in the Xihu Depression. Geochemical and petrological characterization pointed to input of terrigenous organic matter and redox conditions of the depositional environment as factors that govern the ability of the coal measure source rocks in hydrocarbon generation in the Xihu Depression. In this regard, the sedimentary organic facies in the Pinghu Formation were classified into four predominantly terrigenous and one mixed-source subfacies, which all varied in carbon and hydrogen content. The coal measure source rocks in the carbon- and hydrogen-rich tidal flat-lagoon exhibited the highest hydrocarbon generation potential, whereas the mudstone in the neritic facies was the poorest in its hydrocarbon yield. These results suggested that the coal measure source rocks in the Pinghu Formation likely developed in the Hangzhou Slope and the Tiantai Slope, both representing promising sources for oil and gas exploration.