Figure
1.
Structural outline of the Xihu Depression in the East China Sea Shelf Basin (Yu, 2020; Zhu et al., 2019). The whole wells are excessively dense and only key wells are shown.
| Citation: | Yongcai Yang, Xiaojun Xie, Youchuan Li, Gang Guo, Xiaoying Xi, Wenjing Ding. 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[J]. Acta Oceanologica Sinica, 2023, 42(3): 254-269. doi: 10.1007/s13131-023-2176-8
|
Natural gas is a high-quality, efficient, green and clean low-carbon energy. Coal measure source rocks buried deeper than the oil window are important sources of liquid and gaseous hydrocarbons in petroliferous basins (Shanmugam, 1985; Powell and Boreham, 1991; Huang et al., 1995; Bohacs and Suter, 1997; Holdgate et al., 2000; Wang et al., 2021; Wilkins and George, 2002). Hence, it is necessary to predict the occurrence, distribution and volume of coal measure source rocks during the exploration and exploitation of sedimentary basins (Bohacs and Suter, 1997; Wang et al., 2015; Huang et al., 2015; Ren et al., 2015; Zhang et al., 2021; Li et al., 2021; Song et al., 2021).
Crude oil in the Xihu Depression of the East China Sea Shelf Basin is thought to be mainly derived from coal measure source rocks of the Pinghu Formation. However, the source of natural gas in the region has not been clearly elucidated. According to the correlation between the calculated thermal maturity of the natural gas and the extent of thermal evolution exhibited by the corresponding source rocks, the natural gas likely originated from coal measure source rocks of the Huagang and Pinghu formations, or from undrilled Lower-Eocene source rocks of the Baoshi and Pinghu formations (Ye and Guo, 1996; Jia et al., 2000; Zhu et al., 2012; Su et al., 2018, 2020).
Earlier studies have found coal to be the dominant form of source rock in the Pinghu Formation in the Xihu Depression (Fu, 1994; Jia et al., 2000; Tong et al., 2011; Fu et al., 2003; Xie et al., 2018; Wang et al., 2019; Xu et al., 2020; Zhang et al., 2020, 2021; Zhu et al., 2021; Quan et al., 2022; Li et al., 2022; Wang et al., 2022). However, it remains unclear what sedimentary environment offers the most favorable conditions for coal development. In this respect, marshes located in delta plains or delta fronts have previously been reported to be the most conducive to coal deposition (Yancey, 1997; Peters et al., 2000; Saller et al., 2006; Samuel et al., 2009; Deng et al., 2009, 2013, 2021; Deng, 2010, 2016; Xie, 2014). Other sedimentary facies, including tidal flats, abandoned deltas and tide-controlled deltas, have also been proposed as favorable environments for coal formation (Shen et al., 2016; Wei et al., 2013; Zhou et al., 2016). As a result, it is greatly crucial for the exploration of the Xihu Depression to study the geochemical characteristics, development pattern and distribution of the coal measure source rocks of the Pinghu Formation in the Pinghu Slope and predict the distribution of the coal measure source rocks in the Hangzhou and Tiantai slopes.
In the current study, we performed thermal simulation experiments on source rock samples obtained from different locations in the Xihu Depression to establish the typical charts of the oil and gas source correlations. We then determined the key factors that governed the formation of the coal measure source rocks in the Eocene Pinghu Formation. Furthermore, we classified the sedimentary organic facies of the Pinghu Formation in the Pinghu Slope and proposed the favorable exploration areas for the coal measure source rocks in the Hangzhou and the Tiantai slopes. It was a significantly helpful to exploration and exploitation of the oil and gas fields in the Xihu Depression.
The Xihu Depression in the East China Sea Shelf Basin is rich in oil and gas (Gong, 1997; Qiu and Gong, 1999; Zhu et al., 2010; Deng et al., 2013; Zhou et al., 2020). Natural gas exploration in the region has focused mainly on the source rocks including mudstone and coal measure mudstone in the Pinghu Slope and the central inversion zone. The Hangzhou Slope in the north and the Tiantai Slope in the south share similar characteristics in tectonic evolution and sedimentation with the Pinghu Slope, making both areas of interest for petroleum exploration in the Xihu Depression (Fig. 1). The Cenozoic sedimentary strata of the Pinghu Slope consist of Pre-Eocene sediments (age undetermined), the Eocene Baoshi (E2b) and Pinghu (E2p) formations, the Oligocene Huagang Formation (E3h), the Miocene Longjing (N11l), Yuquan (N12y) and Liulang (N13l) formations, the Pliocene Santan Formation (N2s), as well as the Quaternary Donghai Group (Qd) (Fig. 2) (Hao et al., 2018; Cai et al., 2019; Liang and Wang, 2019; Zhu et al., 2019; Quan et al., 2022). Among them, the Pinghu Formation was divided into five members, which are, in a decreasing order of age, E2p5, E2p4, E2p3, E2p2 and E2p1.
The East China Sea Shelf Basin lies in the region where the Indian−Eurasian collision zone and western Pacific subduction zone meet (Zhu Welin et al., 2019; Zhu Xinjian et al.,2021). The collision between the Indian and the Eurasian plates, together with the subduction with the Pacific Plate, turned the East China Sea Shelf Basin into a tectonically complex region and contributed to its lithological heterogeneity (Wang et al., 2017; Zhu et al., 2019). The evolution of the Xihu Depression can be divided into three stages, the Cretaceous−Eocene rifting (56.5−32 Ma), Oligocene−Miocene compression/inversion (32−5.3 Ma), and Pliocene−Quaternary subsidence (5.3 Ma to present) (Du et al., 2020). During the rifting, the Eocene Baoshi and Pinghu Formations were deposited. The Baoshi Formation was mainly composed of intermediate-acidic volcanic rocks, sandstones and mudstones, which were all deposited in a gulf and/or coastal lacustrine environment. Seawater intrusion occurred during the late Eocene, causing the Pinghu Formation to be deposited in a marine environment (Quan et al., 2022). The Oligocene−Miocene compression/inversion witnessed the deposition of the Oligocene Huagang (E3h) Formation, as well as the Miocene Longjing (N11l), Yuquan (N12y) and Liulang (N13l) formations. The Huagang Formation, mainly comprised of siltstones, mudstones, carbonaceous mudstones and coals, was deposited in the fluvial, deltaic, and lacustrine environments (Xu et al., 2020; Quan et al., 2022). The Miocene Longjing Formation, with its lower stratum consisting of sandy conglomerate and/or conglomeratic sand intercalated with mudstone, and upper layer siltstone interbedded mudstone and coal, was deposited in a fluvial and/or lacustrine environment (Ye et al., 2007). Since then, the Xihu Depression underwent thermal subsidence, leading to the deposition of the Pliocene Santan Formation (N2s) and Quaternary Donghai Group (Qd) (Fig. 2).
Briefly, 205 coal measure source rock samples, including coal (TOC≥40%), carbonaceous mudstone (6%≤TOC<40%) and coal measure mudstone (TOC<6%), were collected from the cores and cuttings of the Eocene Pinghu Formation in the Xihu Depression (Fig. 2). The additional 19 oil samples were obtained from the the sandstone reservoirs of the Eocene Pinghu and Oligocene Huagang formations in a total of 13 wells distributed across the Pinghu Slope, the Tiantai Slope and the central inversion zone in the Xihu Depression.
The source rocks were crushed in a rotary mill and extracted in a Soxhlet apparatus in dichloromethane (CH2Cl2) (24 h). The rock extracts and the oil samples were deasphaltened in n-hexane and factionated by column chromatography with a stationary phase of silica gel and alumina in a 3-to-2 ratio (w:w). Saturate, aromatic and polar fractions were sequentially eluted by n-hexane, dischloromethane: n-hexane (2:1, v:v), and chloroform: ethanol (1:1, v:v), respectively. All solvents were distilled before use.
The saturates were analyzed by gas chromatography-mass spectrometry (GC-MS) on a Thermo-Finnigan Trace-DSQ instrument equipped with a HP-5MS fused silica capillary column (60 m×0.25 mm×0.25 μm) in full-scan mode, with helium (99.999% purity) as carrier gas and injector temperature at 300℃. The oven temperature was programmed as follows: at 50℃ for 1 min, from 50℃ to 120℃ at a rate of 20℃/min, from 120℃ to 250℃ at a rate of at 4℃/min, from 250℃ to 310℃ at a rate of 3℃/min, and finally at 310℃ for 30 min. The mass spectrometer was operated in EI (electron impact) mode at 70 eV with a scan range from 35 Da to 550 Da.
Two coal samples and two coal measure mudstone cuttings were selected from the Eocene Pinghu Formation. TOC of the samples ranged from 0.48% to 63.15%, with vitrinite reflectance (Ro) of 0.50%–0.76% (Table 1). Each sample was extracted in a Soxhlet extractor for 72 h with an azeotropic mixture of dichloromethane and methanol (93:7) to yield a soluble organic fraction (1% extractability) and kerogen residue. Roughly 5 mg to 50 mg of the kerogen residue from each sample were divided equally and loaded into two identical gold tubes (40 mm length, 5 mm inner diameter and 0.25 mm wall thickness) that were pre-welded on one end.
| No. | Well | Depth/m | Sample type | Lithology | TOC/% | Ro/% |
| 1 | A1 | 3678−3681 | cutting | coal measure mudstone | 0.68 | 0.73 |
| 2 | A35 | 3570 | cutting | coal | 63.15 | 0.63 |
| 3 | A36 | 3008 | cutting | coal measure mudstone | 0.48 | 0.76 |
| 4 | A37 | 2624 | cutting | coal | 48.64 | 0.50 |
DownLoad:
CSV
The kerogen-loaded gold tubes were purged with argon gas for 15 min, and the open ends were subsequently sealed by welding. The internal pressure of the gold tubes was maintained at (50±0.1) MPa. Each pair of gold tubes that contained the same sample were first heated in a furnace from room temperature to 200℃ over 10 h, and then from 200℃ to 600℃, but at different rates (2℃/h and 20℃/h). Sampling was conducted 15 times from 300℃ to 600℃ at an interval of about 20℃ (Table 1). Two thermocouples at a precision of ±1℃ were used for temperature measurement .
The gaseous hydrocarbon products were analyzed on an Agilent 7890A GC instrument equipped with a HP-5MS fused silica capillary column (60 m×0.25 mm×0.25 μm) in full-scan mode, with injector temperature at 300℃ and nitrogen (99.999% purity) as carrier gas. The oven temperature was programmed as followed: at 40℃ for 1 min, from 40℃ to 70℃ at a rate of 4℃/min, from 70℃ to 300℃ at a rate of 8℃/min, and finally at 300℃ for 20 min.
After the pyrolysis experiments ended, the gaseous hydrocarbons were released from the gold tubes into a collector, and then directed into a GC9160 instrument equipped with a HP-PONA fused aluminium-sesquioxide capillary column. The GC analyses were conducted in full-scan mode, with injector temperature at 300℃ and nitrogen (99.999% purity) as carrier gas. The oven temperature was programmed as follows: at 35℃ for 5 min, from 35℃ to 150℃ at a rate of 5℃/min, from 150℃ to 220℃ at a rate of 10℃/min, and finally at 220℃ for 2 min. The liquid hydrocarbon products were analyzed on an Agilent 7890A equipped with a HP-5MS fused silica capillary column (60 m×0.25 mm×0.25 μm) in full-scan mode, with injector temperature at 300℃ and nitrogen (99.999% purity) as carrier gas. The oven temperature was programmed at follows: at 40℃ for 1 min, from 40℃ to 70℃ at a rate of 4℃/min, from 70℃ to 300℃ at a rate of 8℃/min, and finally at 300℃ for 20 min.
For stable carbon-isotope analysis, a portion of the gaseous products released from the pierced gold tubes was drawn into a syringe and then injected into a GC/C/TC/Delta Plus XP mass spectrometer equipped with a C-2000 column coupled to an aluminium sesquioxide capillary column. Helium was used as the carrier gas. The oven temperature was programmed at follows: at 35℃ for 5 min, from 35℃ to 150℃ at a rate of 5℃/min, from 150℃ to 220℃ at a rate of 10℃/min, and finally at 220℃ for 2 min. Subsequently, GC analyses were conducted in full-scan mode on a GC9160 instrument equipped with a HP-PONA fused aluminium sesquioxide capillary column, with injector temperature at 300℃ and nitrogen (99.999% purity) as carrier gas. The oven temperature was programmed as follows: at 30℃ for 5 min, from 30℃ to 200℃ at a rate of 10℃/min, and finally 200℃ for 5 min. The stable carbon isotope value of each sample was measured three times within a calculated precision of ±0.2‰.
An additional 75 cuttings were collected from the coal measure mudstone of the Eocene Pinghu Formation and the lower member of the the Oligocene Huagang Formation in a total of ten wells across the Xihu Depression. Calcium carbonate and silicon were removed from the samples by hydrochloric acid (HCl) and hydrofluoric acid (HF), respectively. Then, all floating particles, including spores, pollens, planktic algae, fossilized plant microbodies and various sedimentary organic debris, were separated with heavy liquid (specific gravity 2.20), washed, and subsequently imaged under a transmission light microscope. No oxidant was used throughout the above procedures so as to preserve the organic content in the samples.
The aforementioned 75 samples were visualized under a back-scattered electron microscope, and 20 of them were also subjected to energy spectrum analysis. Back-scattered electron imaging achieves high-resolution sample visualization and analysis based on the principle that elastic collision between an incident electron beam and target atoms generates back-scattered electrons in an quantity (i.e., backscatter coefficient η) proportional to the number of atoms in a given area. In X-ray energy spectrum analysis, the atoms are bombarded by an accelerating electron beam, leading to the excitation of their inner electrons and the subsequent emission of X-ray photons. Interpretation of the resultant energy spectra can help reveal the identity and composition of the constituent elements.
Based on our thermal simulation experiments, the coal and carbonaceous mudstone in the Pinghu Formation generated hydrocarbons at a yield of 100−175 mg/g. In contrast, the coal measure mudstone was significantly less productive (18−50 mg/g), and the neritic or variegated mudstone produced very little hydrocarbon (less than 5 mg/g). The coal seams in the Pinghu Slope were relatively thin and gradually transitioned in both vertical directions into a gradient of increasingly carbon-depleted lithologies, including carbonaceous mudstone, dark mudstone, gray mudstone or sandstone, with a shell-shaped section. The core of each coal seam was composed mostly of semi-bright coal, with occasional observation of coal lines (Fig. 3).
Redox conditions of the depositional environment were analyzed by calculating the ratio of pristane (Pr) to phytane (Ph) (Didyk et al., 1978; Peters et al., 2005). In general, high Pr/Ph suggests an input of terrigenous organic matter under oxic conditions, whereas low Pr/Ph (< 0.8) implies an anoxic, usually hypersaline or carbonate depositonal environment. The source rocks of the Pinghu Formation were characterized by high Pr/Ph in the range of 3.66−10.3 (Fig. 4), whereas the oil samples exhibited similarly high Pr/Ph (3.08−12.73) (Fig. 4), together with a wide distribution of Pr/nC17 (0.47−4.95) and low Ph/nC18 (0.08−0.40). These data provided evidence that the oil-producing source rocks were enriched in terrigenous, Type III organic matter that was deposited under relative oxic conditions in the Xihu Depression.
Pentacyclic triterpanes (hopanes) are common biomarkers in source rocks and crude oils, with wide application in organic geochemistry (Peters et al., 2005). The Xihu Depression featured remarkably low relative abundances of tricyclic terpanes and pentacyclic triterpanes (hopanes) (Fig. 5).
Tetracyclic diterpanes are a highly useful indicator of terrigenous organic matter input (Killop et al., 1995; Peters et al., 2005). In all coal extracts and oil samples prepared in this study, tetracyclic diterpanes were found to be significantly more abundant than triterpanes (Fig. 5) and share a similar, distinctive distribution pattern (norpimarane, isopimarane, phyllocladane) (Fig. 6). These features are characteristic of oil from source rocks enriched in terrigenous organic matter.
Stable-carbon isotope analysis indicated that the proportion of 13C in the kerogen-derived methane, denoted as δ13C1, first briefly declined but then rose steadily with increasing easy Ro values (%) (Fig. 7). The initial decrease of δ13C1 was consistent with the results of many previous pyrolysis studies, and could generally be attributed to the mixing of gas precursors (Tang et al., 2000; Tian et al., 2010; Wang et al., 2013; Jia et al., 2014).
Despite the fact that coal, coal measure mudstone and neritic mudstone shared similar yield curves for gaseous hydrocarbons, coal-derived kerogen consistently showed higher δ13C1 compared to that produced from the neritic mudstone (Fig. 8). These results suggested that the initial structural precursors of the methane were isotopically more depleted than the more thermally stable aromatic structures of the kerogen (Galimov, 2006; Wang et al., 2013). Therefore, the origin and source of natural gas in the Xihu Depression can be determined by comparing the carbon isotope values of source rock-derived methane.
The liquid hydrocarbons yielded from the coal in Well A3 had high Pr/Ph that fell within fairly narrow ranges, regardless of whether the gold tubes were heated at a rate of 2℃/h (Pr/Ph: 5.00−7.00) or 20℃/h (Pr/Ph: 5.33−7.50) (Table 2, Fig. 9). Moreover, Pr/nC17−Ph/nC18 plot indicated that the thermal evolution of both parameters with thermal maturity followed similar trends in the coal samples, which differed significantly from those observed in the neritic mudstone (Fig. 9) (Albrecht et al., 1976; Vuković et al., 2016), thereby offering a useful method for hydrocarbon-source rock correlation.
| No. | Sample type | Heating rate/ (℃·h−1) | Pr/nC17 ratio | Ph/nC18 ratio | Pr/Ph ratio |
| 1 | coal | 20 | 3.20 | 0.58 | 7.00 |
| 2 | coal | 20 | 2.85 | 0.58 | 6.50 |
| 3 | coal | 20 | 2.94 | 0.57 | 7.05 |
| 4 | coal | 20 | 3.51 | 0.59 | 7.58 |
| 5 | coal | 20 | 3.47 | 0.59 | 7.38 |
| 6 | coal | 20 | 3.31 | 0.55 | 7.44 |
| 7 | coal | 20 | 2.95 | 0.55 | 6.39 |
| 8 | coal | 20 | 2.30 | 0.42 | 5.75 |
| 9 | coal | 20 | 1.86 | 0.35 | 5.78 |
| 10 | coal | 20 | 1.28 | 0.25 | 5.38 |
| 11 | coal | 20 | 0.93 | 0.18 | 5.46 |
| 12 | coal | 20 | 0.68 | 0.15 | 5.00 |
| 13 | coal | 2 | 3.00 | 0.52 | 7.50 |
| 14 | coal | 2 | 3.43 | 0.64 | 6.67 |
| 15 | coal | 2 | 3.75 | 0.65 | 7.06 |
| 16 | coal | 2 | 3.64 | 0.61 | 7.06 |
| 17 | coal | 2 | 2.90 | 0.53 | 5.95 |
| 18 | coal | 2 | 2.33 | 0.42 | 5.67 |
| 19 | coal | 2 | 1.66 | 0.31 | 5.42 |
| 20 | coal | 2 | 0.99 | 0.18 | 5.92 |
| 21 | coal | 2 | 0.69 | 0.12 | 5.73 |
| 22 | coal | 2 | 0.50 | 0.09 | 5.67 |
| 23 | coal | 2 | 0.21 | 0.04 | 6.25 |
| 24 | coal | 2 | 0.07 | 0.02 | 5.33 |
| Note: Sample informations are showed in Table 1. | |||||
DownLoad:
CSV
Sequence stratigraphic, paleogeomorphological and sedimentological data obtained from specimens in the boreholes and the core of the Xihu Depression suggested that the Haijiao Uplift drove the transport and deposition of sedimentary materials that resulted in the development of the Pinghu Formation in the Xihu Depression. Based on petrological, logging and geochemical characteristics, representative wells were selected to for single-well facies analysis, which led to identification of several distinct types of sedimentary facies. These facies were further divided into subfacies and microfacies.
The 2nd member of the Pinghu Formation in Well A25 mainly comprised strata of gray or dark-gray mudstones and dark gray silty mudstones that are interbedded with thick, yellow, fine-grained or thin, medium-grained sandstone, sandwiched with eight thin layers of coal seams. The sedimentary structure vertically overlapped with stacking river channels, and the sedimentary fillings showed an overall positive cycle. The gamma-ray logs of the member were box-shaped. The member was classified as deltaic depositional subfacies under tidal influence (Fig. 10), with distributary channels and interdistributary bays as the main microfacies. The TOCs of the carbonaceous mudstones in the member were determined to be 18.47%−35.45% and the hydrogen indices 317−377 mg/g, where those of the mudstones were in the ranges of 0.79%−4.90% and 218−324 mg/g, respectively.
The 3th member of the Pinghu Formation in Well A13 consisted mainly of dark-gray silty mudstones or mudstones, with four thin layers of coal seams and scattered presence of yellow, fine-grained calcareous sandstones (Fig. 11). The gamma-ray logs of the member assumed a jagged pattern. The member was characterized as deposits of tidal-flat and lagoon subfacies, which mainly developed into mixed flat-lagoon mud microfacies. The TOCs of the coal seams, carbonaceous mudstones and mudstones in the member were determined to be 18.47%−35.45%, 28.79% and 0.47%−4.21%, respectively, where their hydrogen indices were 317−377 mg/g, 427mg/g and 67−356 mg/g, respectively. On the other hand, the 2th member of the same well was lithologically composed of grey mudstones and thick, yellow, fine-grained sandstones, with scattered presence of locally developed coal seams. The gamma-ray logs of the member were box-shaped, and the sedimentary fillings were the positive cycles. These findings suggested that the member could be classified as delta-front depositional subfacies, from which microfacies of subaqueous distributary and interdistributary channels were formed. The TOCs and hydrogen indices of the mudstones in the member were calculatd to be 0.52%−0.73% and 110−286 mg/g, respectively.
The 4th member of the Pinghu Formation in Well A40 displayed a lithological composition of dark-gray mudstone and gray silty mudstones, mixed with brown-yellow argillaceous or yellow siltstones. No coal measure source rocks were observed (Fig. 12). The member experienced sedimentation in a deeper aqueous environment, resulting primarily in the developent of neritic subfacies. The TOCs of the mudstone samples were mostly below 2.51%, with the lone exception at 6.13%, and the hydrogen indices were in the range of 32−115 mg/g.
The sedimentary facies of the source rocks from the Pinghu Formation were subdivided into sedimentary subfacies and microfacies (Fig. 10, Fig. 11 and Fig. 12). Their geochemical characteristics were then examined and compared to yield predictions of where in the Pinghu Formation had favorable conditions for source rock development.
Palynological analysis of the source rocks in the Pinghu Formation focused on fossilized plant microbodies, plant detritus and amorphous organic matter, which differ in mechanism of genesis and morphology. In general, plant microbodies include spores, pollen and planktonic algae, while plant detritus refers to morphologically distinct sedimentary organic matter that often assumes a herbaceous, woody or coaly appearance or texture, which signifies terrestrial origins. Amorphous organic matter is generally of phytoplankton origin and constitutes the majority of the biological fraction in lacustrine and marine facies. Based on microscopic analysis, around 60%−95% of the macerals in the Pinghu Formation were identified as woody or coaly detritus of terrestrial higher plants (Fig. 13, Fig. 14), and the rest was attributed to amorphous organic matter, which lent convincing evidence of a fourishing terrestrial flora and abundant production of organic materials in the sedimentary basin during the sedimentary period.
Characterization of the mudstones in the Xihu Depression via back-scattered electron microscopy confirmed its organic content to be predominantly coal in nature, appearing as continuous or intermittent carbonaceous belts, or as dispersed carbon dusts. The continuous carbonaceous belts comprised alternating dark and light layers with presence of spherical pyrite. The dark layers further consisted of flat, wavy or slightly wrinkled carbonaceous bands, and were shown by energy spectrum analysis to contain mostly carbon (C), and to a minor extent, silicon (Si), aluminum (Al), oxygen (O) and sulfur (S) (Fig. 15). On the other hand, the light layers were dominated by clay minerals, such as chlorite or illite. The carbonaceous mudstone debris in the light layers exhibited no stratified structure, with dispersed carbon as the main form of organic matter.
The oil and gas fields in the Xihu Depression of the East China Sea Shelf Basin are mainly in the Pinghu Slope and the central inversion zone. The hydrocarbon resource is primarily composed of natural gases and condensates, as well as some light oils, with the density of crude oils in the range of 0.77−0.84 g/cm3.
The cross-plot of Pr/nC17 and Ph/nC18 was generated to determine oil-source correlation based on the results of thermal simulation experiments in confined gold tubes under high P-T conditions (Fig. 9). Both the Pr/nC17 and Ph/nC18 of the coal-derived liquid hydrocarbon products were inversely correlated with thermal maturity. Importantly, coal could be easily differentiated from the neritic mudstone based on the Pr/nC17-to-Ph/nC18 ratio (Fig. 9), which offered a novel oil-source correlations method. The crude oil in the Pinghu Slope, western sub-depression and central inversion zone showed very similar characteristics as the coal measure source rocks in the Pinghu Formation after reaching maturity (Fig. 9, Fig. 16), suggesting a derivative relationship.
Steranes and terpenoids were found to be substantially less abundant than diterpanes in the crude oil from the Pinghu Slope (Fig. 5), indicating that the organic matter in the source rocks of the Pinghu Formation mainly originated from gymnosperm plants (Killop et al.,1995; Peters et al., 2005). The crude oil and source rocks in the Pinghu Slope exhibited similar relative abundance and distribution of diterpanes, including norpimarane, isopimarane and phyllocladane (Fig. 6), providing further experimental support that the oils were likely derived from gymnosperm-rich source rocks in the Pinghu Formation in the Xihu Depression.
Carbon isotope composition analysis of methane and ethane showed that the natural gas in the study area was derived from coal. A positive correlation was established between the thermal evolution of the natural gas in the Pinghu Slope and the coal measure source rocks of the Pinghu Formation, based on the data from the gold-tube pyrolysis experiments (Fig. 17). However, the natural gas mentioned above differed considerably in carbon isotope composition from those obtained experimentally from the neritic mudstone in the Pinghu Formation. These findings demonstrated the natural gas in the Pinghu Slope also originated from coal measure source rocks in the Pinghu Formation.
Back-scattered electron microscopy and energy spectrum analysis were used to illustrate the differences in sedimentary structure, the composition and distribution pattern of organic matter, and the mineral composition of the mudstone. The experimental data suggested that the organic matter was mainly from coals or carbonaceous mudstones in nature based on visual observation both with the unaided eye (Fig. 3) and under a back-scattered electron microscope, and appeared as continuous carbonaceous strips, intermittent carbonaceous strips, or dispersed carbonaceous debris (Fig. 15, Fig. 18).
Energy spectrum analysis provides a powerful method to deduce the depositional environment of organic matter by measuring the proportions of various elements. Specifically, a higher sulfur content is commonly associated with a strong reducing environment. In addition, abundances of aluminum, oxygen and silicon have also been shown to serve as useful indicators of an oxidative sedimentary environment. Aluminum and silicon contents in the Pinghu Formation were inversely correlated with those of carbon and sulfur (Fig. 18).
Coal formation and accumulation occurred predominantly in delta-plain, delta-front, and barrier coast-tidal flat-lagoon facies during the deposition of the Pinghu Formation in the Pinghu Slope (Fig. 19). Among them, the lagoon subfacies developed continuous laminar carbonaceous mudstone with high abundances of organic matter, carbon and sulfur, as well as low levels of aluminum, oxygen and silicon (Fig. 18, Fig. 19). Coals and carbonaceous mudstones (TOC>6%) containing a substantial quantity of reduced sulfur (S>0.5%) were widely present in the tidal flat-lagoon subfacies from the Pinghu Slope and Tiantai Slope (Fig. 19). These findings implied a strongly reducing sedimentary environment in the lagoon subfacies. The delta-front subfacies formed massive mudstone that contained carbonaceous debris, with a low abundance of sulfur and organic matter, a medium-to-high carbon content, high levels of aluminum, oxygen and silicon, and strong oxidizing properties (Fig. 15). The mudstones (TOC<2.0%) originating from the delta-plain and delta-front subfacies in the Pinghu Slope and Tiantai Slope were mostly depleted in sulfur (S<0.5%) (Fig. 19). Overall, the reducing environment created favorable conditions for the development of coal measure source rocks.
The coal seams and carbonaceous mudstones were formed in the reducing lagoon subfacies, whereas a low-abundance of sulfur-poor and oxygen-rich massive mudstone with disordered carbonaceous layers and under-developed coal seams was present in the partially oxidized delta-front subfacies. Theses findings provided corroborating evidence that the reducing aqueous environment in the Pinghu Formation of the Pinghu Slope is conducive to the development of coal measure source rocks.
Inorganic mineral series in the transitional facies belt also serve as a useful redox indicator. For example, mudstone and siltstone in shallow oxidative deposits usually appear brown or variegated due to the high content of ferric oxides, including limonite, hematite and iron silicate. Indeed, the dark-brownish mudstone in the delta-plain subfacies from the first and second members of Well A1 showed high abundances of hematite and limonite, whereas the variegated mudstone in the tidal-flat subfacies from the first and second members of Well A35 was enriched in oolitic chlorite. Meanwhile, iron sulfides, such as pyrite and hematite, were readily produced in the sulfide belt within the deep sediments (Fig. 20). This is consistent with our discovery of rich marcasite and pyrite in the coal seams of the tidal-flat subfacies from the first and second members of Well A32, and of the tidal flat-lagoon subfacies from the third member of Well A12. In fact, coal measure source rocks are frequently located in sulfur-rich and reducing sedimentary environment due to the tendency of coal and pyrite to form colloidal co-precipitates. Taken together, the distribution patterns of redox-sensitive inorganic mineral deposits enabled us to establish a metallogenic model based on colloidal chemistry to elucidate the formation and deposition of coal measure source rocks in the Pinghu Formation (Fig. 20).
A continental-marine transitional sedimentary system consisting of deltaic, tidal-flat, lagoon and neritic facies was developed in the Pinghu Formation of the Pinghu Slope. The tidal flat-lagoon subfacies experienced the largest expansion during the deposition of the 4th member of the Pinghu Formation, when the largest marine transgression occurred.
The macerals in the source rocks of the Pinghu Formation, regardless of which facies they were obtained from, comprised around 60%−90% of coaly or woody detritus based on the results of palynological studies (Fig. 14), implying that the organic matter in the region was predominantly derived from higher plants. This is echoed by further micropaleontological evidence that the porportion of pollen in various subfacies of the Pinghu Formation remained overwhelmingly in the range of 80%−95% (Fig. 21).
Coal measure source rocks were confirmed to be widely distributed across all studied subfacies in each member of the Pinghu Formation and contain a prolific input of organic matter derived from terrigenous higher plants. In general, coal seams in the tidal flat and lagoon subfacies were more abundant and showed greater thickness relative to that of the Pinghu Formation, compared to those in the delta-plain and delta-front subfacies (Fig. 22). In contrast, no development of coal measure source rocks was observed in neritic subfacies. As the delta-plain subfacies gradually transitioned into the neritic subfacies, the input of organic matter from terrigenous higher plants increased at first but then decreased.
The source rocks that originated from the tidal-flat and lagoon subfacies had high TOC contents, hydrogen indices, and inputs of terrigenous organic matter (Fig. 19, Fig. 23). In contrast, those in the delta-plain and delta-front subfacies exhibited fluctuating hydrogen indices, and were low in other two metrics. No coal measure source rocks were found in the inner neritic subfacies, which was depleted in carbon and hydrogen.
Organic facies in the sedimentary basins were classified on the basis of geochemical characteristics of the source rocks in various sedimentary facies (Habib and Miller, 1989; Requejo et al., 1994; Tribovillard et al., 2001). In this study, we categorized the coal measure source rocks in the Pinghu Formation based on the sedimentary environment, preservation conditions, distribution, geochemistry and seismic response, resulting in the delineation of four predominantly terrigenous organic facies, including (1) carbon- and hydrogen-rich or (2) carbon-rich and hydrogen-neutral tidal flat-lagoon, (3) carbon-rich and hydrogen-neutral delta plain, and (4) carbon-rich and hydrogen-neutral delta front, as well as one mixed-sourced neritic facies containing a medium level of carbon and low content of hydrogen. In general, the source rocks in the tidal flat-lagoon facies and in the necritic facies showed the highest and the lowest hydrocarbon generation potentials, respectively. Furthermore, the sedimentary and redox environment of the tidal flat-lagoon facies proved to be the most conducive to the development of coal measure source rocks, due to an abundant input of terrigenous organic matter and favorable preservation conditions (Fig. 24).
In contrast to the Hangzhou, the Pinghu, and the Tiantai slopes, coal measure source rocks also developed in the 5th to 3rd members of the Pinghu Formation in the Hangzhou Slope, and their formation time was relatively early. The coal measure source rocks occur from the fifth member to the first member of the Pinghu Formation in the Pinghu Slope. Moreover, the coal measure source rocks had developed in the Pinghu Slope for a long time. However, the coal measure source rocks were developed in the third to the first members of the Pinghu Formation in the Tiantai Slope, and their formation time was relatively late. The Pinghu Formation coal measure source rocks were the most developed on the Pinghu Slope. It is predicted that the coal measure source rocks in the Pinghu Formation were also heavily developed in the Hangzhou and the Tiantai slopes. Therefore, it is likely for the large- and medium-sized oil and gas fields to be discovered due to the great hydrocarbon generation potential for oil and gas exploration in the Hangzhou and the Tiantai slopes. It provides an important guiding significance for the formation and distribution of the Cenozoic coal measure source rocks and the promotion of the vital discovery of the large and medium-sized oil and gas fields in the offshore China.
(1) The results of our gold-tube pyrolysis experiments indicated that the oil and gas fields discovered in the Pinghu Slope and the central inversion zone were mainly derived from the coal measure source rocks in the Eocene Pinghu Formation. The coal and carbonaceous mudstone in the studied region demonstrated great hydrocarbon generation potential.
(2) The distribution of coal measure source rocks in the Pinghu Formation was mainly governed by the input of terrigenous organic matter and the reducing depositional environment. Studies on the sedimentary and geochemical characteristics of coal measure source rocks in five different organic facies of the Pinghu Formation found those in the terrigenous, carbon- and hydrogen-rich tidal-flat and lagoon facies to display the highest hydrocarbon generation potential. In contrast, the source rocks derived from the neritic mudstone contained a moderate content of organic matter and exhibited the lowest hydrocarbon generation potential.
(3) The coal measure source rocks of the Pinghu Formation were more developed in the Pinghu Slope than those in the Hangzhou and Tiantai slopes. Hence, the coal measure source rocks in the Pinghu Formation also have great hydrocarbon generation potential for oil and gas fields in the Hangzhou and the Tiantai slopes. It reveals a good exploration prospect of the large- and medium-sized oil and gas fields in the Hangzhou and the Tiantai slopes.
Acknowledgements: We gratefully acknowledge the Shanghai Branch of the China National Offshore Oil Corporation (CNOOC) Co., Ltd. for samples and data collection, and the State Key Laboratory of Heavy Oil Processing of China University of Petroleum (Beijing) for the GC-MS analyses. We are grateful to Gongcheng Zhang and Jianyong Xu of the Exploration and Development Research Department of CNOOC Research Institute Co., Ltd., and Meijun Li of China University of Petroleum (Beijing) for their helpful suggestions relating to the manuscript.|
Albrecht P, Vandenbroucke M, Mandengué M. 1976. Geochemical studies on the organic matter from the Douala Basin (Cameroon)—I. Evolution of the extractable organic matter and the formation of petroleum. Geochimica et Cosmochimica Acta, 40: 791–799. doi: 10.1016/0016-7037(76)90031-4
|
|
Bohacs K, Suter J. 1997. Sequence stratigraphic distribution of coaly rocks: fundamental controls and paralic examples. AAPG Bulletin, 81(10): 1612–1639
|
|
Cai Hua, Qin Lanzhi, Liu Yinghui. 2019. Differentiation and coupling model of source-to-sink systems with transitional facies in Pingbei Slope of Xihu Sag. Earth Science (in Chinese), 44(3): 880–897
|
|
Deng Yunhua. 2009. Analysis on differences of petroleum type and geological conditions between two depression belts in China offshore. Acta Petrolei Sinica (in Chinese), 30(1): 1–8
|
|
Deng Yunhua. 2010. Analysis on correlation of river and petroleum. Acta Petrolei Sinica (in Chinese), 31(1): 12–17
|
|
Deng Yunhua. 2016. River-delta systems: a significant deposition location of global coal-measure source rocks. Journal of Earth Science, 27(4): 631–641. doi: 10.1007/s12583-016-0710-8
|
|
Deng Yunhua, Yang Yongcai, Yang Ting. 2021. Three Systems of Oil and Gas Formation in the World (in Chinese). Beijing: Science Press, 156–227
|
|
Deng Yunhua, Zhang Gongcheng, Liu Chuncheng, et al. 2013. Petroleum Geological Theory and Exporation Practice of the Two Oil and Gas Depression Belts in Offshore China (in Chinese). Beijing: Petroleum Industry Press, 36–101
|
|
Didyk B M, Simoneit B R T, Brassell S C, et al. 1978. Organic geochemical indicators of palaeoenvironmental conditions of sedimentation. Nature, 272: 216–222. doi: 10.1038/272216a0
|
|
Du Xuebin, Lu Yongchao, Cao Qiang, et al. 2020. Grading evaluation of deep reservoir in Xihu Depression, East China Sea Basin. Bulletin of Geological Science and Technology (in Chinese), 39(3): 10–19
|
|
Fu Ning. 1994. Diterpenoid compounds in coal and condesates in Xihu Sag of East China Sea. China Offshore Oil and Gas (Geology) (in Chinese), 8(1): 21–28
|
|
Fu Ning, Li Youchuan, Chen Guihua, et al. 2003. Pooling mechanisms of “evaporating fractionation” of oil and gas in the Xihu depression‚ East China Sea. Petroleum Exploration and Development (in Chinese), 30(2): 39–42
|
|
Galimov E M. 2006. Isotope organic geochemistry. Organic Geochemistry, 37(10): 1200–1262. doi: 10.1016/j.orggeochem.2006.04.009
|
|
Gong Zaisheng. 1997. The Major Oil and Gas Fields of China Offshore (in Chinese). Beijing: Petroleum Industry Press, 7–69
|
|
Habib D, Miller J A. 1989. Dinoflagellate species and organic facies evidence of marine transgression and regression in the Atlantic coastal plain. Palaeogeography, Palaeoclimatology, Palaeoecology, 74(1–2): 23–47
|
|
Hao Lewei, Wang Qi, Guo Ruiliang, et al. 2018. Diagenetic fluids evolution of Oligocene Huagang Formation sandstone reservoir in the south of Xihu Sag, the East China Sea Shelf Basin: constraints from petrology, mineralogy, and isotope geochemistry. Acta Oceanologica Sinica, 37(2): 25–34. doi: 10.1007/s13131-017-1126-8
|
|
Holdgate G R, Wallace M W, Gallagher S J, et al. 2000. A review of the Traralgon Formation in the Gippsland Basin—a world class brown coal resource. International Journal of Coal Geology, 45(1): 55–84. doi: 10.1016/S0166-5162(00)00020-3
|
|
Huang Baojia, Huang Hao, Wang Zhenfeng, et al. 2015. Kinetics and model of gas generation of source rocks in the deepwater area, Qiongdongnan Basin. Acta Oceanologica Sinica, 34(4): 11–18. doi: 10.1007/s13131-015-0646-3
|
|
Huang Difan, Qin Kuangzong, Wang Tieguan, et al. 1995. Oil from Coal: Formation and Mechanism (in Chinese). Beijing: Petroleum Industry Press
|
|
Jia Wanglu, Wang Qiuling, Liu Jinzhong, et al. 2014. The effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage: a pyrolysis study of Type II kerogen from Pingliang Shale, China. Organic Geochemistry, 71: 17–29. doi: 10.1016/j.orggeochem.2014.03.009
|
|
Jia Jianyi, Xu Xuehao, Sun Boqiang. 2000. Oil/gas geochemical character in the Xihu trough of the East China Sea. Offshore Oil (in Chinese), (2): 1–7
|
|
Killops S D, Raine J I, Woolhouse A D, et al. 1995. Chemostratigraphic evidence of higher-plant evolution in the Taranaki Basin, New Zealand. Organic Geochemistry, 23(5): 429–445. doi: 10.1016/0146-6380(95)00019-B
|
|
Li Shuxia, Shao Longyi, Liu Jinshui, et al. 2022. Oil generation model of the liptinite-rich coals: Palaeogene in the Xihu Sag, East China Sea Shelf Basin. Journal of Petroleum Science and Engineering, 209: 109844. doi: 10.1016/j.petrol.2021.109844
|
|
Li Zengxue, Zeng Qingbo, Xu Meng, et al. 2021. Peat formation and accumulation mechanism in northern marginal basin of South China Sea. Acta Oceanologica Sinica, 40(2): 95–106. doi: 10.1007/s13131-021-1748-8
|
|
Liang Jintong, Wang Hongliang. 2019. Cenozoic tectonic evolution of the East China Sea Shelf Basin and its coupling relationships with the Pacific Plate subduction. Journal of Asian Earth Sciences, 171: 376–387. doi: 10.1016/j.jseaes.2018.08.030
|
|
Peters K E, Snedden J W, Sulaeman A, et al. 2000. A new geochemical-sequence stratigraphic model for the Mahakam delta and Makassar slope, Kalimantan, Indonesia. AAPG Bulletin, 84(1): 12–44
|
|
Peters K E, Walters C C, Moldowan J M. 2005. The Biomarker Guide, Biomarkers and Isotopes in Petroleum Exploration and Earth History. Cambridge: Cambridge University Press, 475–587
|
|
Powell T G, Boreham C J. 1991. Petroleum generation and source rock assessment in terrigenous sequences: an update. The APPEA Journal, 31(1): 297–311. doi: 10.1071/AJ90023
|
|
Qiu Zhongjian, Gong Zaisheng. 1999. Petroleum Exploration in China, Volume Ⅳ: Offshore Petroleum Province (in Chinese). Beijing: Geological Publishing House, Petroleum Industry Press, 1054–1087
|
|
Quan Yongbin, Chen Zhongyun, Jiang Yiming, et al. 2022. Hydrocarbon generation potential, geochemical characteristics, and accumulation contribution of coal-bearing source rocks in the Xihu Sag, East China Sea Shelf Basin. Marine and Petroleum Geology, 136: 105465. doi: 10.1016/j.marpetgeo.2021.105465
|
|
Ren Jinfeng, Zhang Yingzhao, Wang Hua, et al. 2015. Identification methods of coal-bearing source rocks for Yacheng Formation in the western deepwater area of South China Sea. Acta Oceanologica Sinica, 34(4): 19–31. doi: 10.1007/s13131-015-0647-2
|
|
Requejo A G, Wielchowsky C C, Klosterman M J, et al. 1994. Geochemical characterization of lithofacies and organic facies in Cretaceous organic-rich rocks from Trinidad, East Venezuela Basin. Organic Geochemistry, 22(3–5): 441–459
|
|
Saller A, Lin R, Dunham J. 2006. Leaves in turbidite sands: the main source of oil and gas in the deep-water Kutei Basin, Indonesia. AAPG Bulletin, 90(10): 1585–1608. doi: 10.1306/04110605127
|
|
Samuel O J, Cornford C, Jones M, et al. 2009. Improved understanding of the petroleum systems of the Niger Delta Basin, Nigeria. Organic Geochemistry, 40(4): 461–483. doi: 10.1016/j.orggeochem.2009.01.009
|
|
Shanmugam G. 1985. Significance of coniferous rain forests and related organic matter in generating commercial quantities of oil, Gippsland Basin, Australia. AAPG Bulletin, 69(8): 1241–1254
|
|
Shen Yulin, Qin Yong, Guo Yinghai, et al. 2016. Development characteristics of coal-measure source rocks divided on the basis of Milankovich coal accumulation cycle in Pinghu Formation, Xihu Sag. Acta Petrolei Sinica (in Chinese), 37(6): 706–714
|
|
Song Guangzeng, Li Zengxue, Yang Haizhang, et al. 2021. Control effects of the synsedimentary faults on the basin-marginal fans in the central part of the deep-water area of early Oligocene Qiongdongnan Basin, South China Sea. Acta Oceanologica Sinica, 40(2): 54–64. doi: 10.1007/s13131-021-1749-7
|
|
Su Ao, Chen Honghan, Chen Xu, et al. 2018. The characteristics of low permeability reservoirs, gas origin, generation and charge in the central and western Xihu Depression, East China Sea Basin. Journal of Natural Gas Science and Engineering, 53: 94–109. doi: 10.1016/j.jngse.2018.01.034
|
|
Su Ao, Chen Honghan, Zhao Jianxin, et al. 2020. Natural gas washing induces condensate formation from coal measures in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin: insights from fluid inclusion, geochemistry, and rock gold-tube pyrolysis. Marine and Petroleum Geology, 118: 104450. doi: 10.1016/j.marpetgeo.2020.104450
|
|
Tang Y, Perry J K, Jenden P D, et al. 2000. Mathematical modeling of stable carbon isotope ratios in natural gases. Geochimica et Cosmochimica Acta, 64(15): 2673–2687. doi: 10.1016/S0016-7037(00)00377-X
|
|
Tian Hui, Xiao Xianming, Wilkins R W T, et al. 2010. Genetic origins of marine gases in the Tazhong area of the Tarim basin, NW China: implications from the pyrolysis of marine kerogens and crude oil. International Journal of Coal Geology, 82(1–2): 17–26
|
|
Tong Zhigang, He Qing, Zhao Zhigang, et al. 2011. Analyzing hydrocarbon charges from hydrocarbon occurrences: a case of Pinghu oil and gas field in Xihu Sag, East China Sea. China Offshore Oil and Gas (in Chinese), 23(3): 154–157
|
|
Tribovillard N, Bialkowski A, Tyson R V, et al. 2001. Organic facies variation in the late Kimmeridgian of the Boulonnais area (northernmost France). Marine and Petroleum Geology, 18(3): 371–389. doi: 10.1016/S0264-8172(01)00006-X
|
|
Vuković N, Životić D, Mendonça Filho J G, et al. 2016. The assessment of maturation changes of humic coal organic matter—Insights from closed-system pyrolysis experiments. International Journal of Coal Geology, 154–155: 213–239
|
|
Wang Dongdong, Zhang Gongcheng, Li Zengxue, et al. 2021. The development characteristics and distribution predictions of the Paleogene coal-measure source rock in the Qiongdongnan Basin, Northern South China Sea. Acta Geologica Sinica (English Edition), 95(1): 105–120. doi: 10.1111/1755-6724.14625
|
|
Wang Qian, Li Sanzhong, Guo Lingli, et al. 2017. Analogue modelling and mechanism of tectonic inversion of the Xihu Sag, East China Sea Shelf basin. Journal of Asian Earth Sciences, 139: 129–141. doi: 10.1016/j.jseaes.2017.01.026
|
|
Wang Qingtao, Lu Hong, Greenwood P, et al. 2013. Gas evolution during kerogen pyrolysis of Estonian Kukersite shale in confined gold tube system. Organic Geochemistry, 65: 74–82. doi: 10.1016/j.orggeochem.2013.10.006
|
|
Wang Yonggang, Tian Yankuan, Zhan Zhaowen, et al. 2019. Characteristics and implications of diamondoids in crude oils from the Xihu Depression, East China Sea Basin, China. Natural Gas Geoscience, 30(4): 582–592
|
|
Wang Yingxun, Chen Jianfa, Pang Xiongqi, et al. 2022. Hydrocarbon generation and expulsion of Tertiary coaly source rocks and hydrocarbon accumulation in the Xihu Sag of the East China Sea Shelf Basin, China. Journal of Asian Earth Sciences, 229: 105170. doi: 10.1016/j.jseaes.2022.105170
|
|
Wang Zhenfeng, Sun Zhipeng, Zhang Daojun, et al. 2015. Geology and hydrocarbon accumulations in the deepwater of the northwestern South China Sea—with focus on natural gas. Acta Oceanologica Sinica, 34(10): 57–70. doi: 10.1007/s13131-015-0715-7
|
|
Wei Hengfei, Chen Jianfa, Chen Xiaodong, et al. 2013. The controlling factors and sedimentary environment for developing coastal coal-bearing source rock of Pinghu Formation in Xihu Depression. Geology in China (in Chinese), 40(2): 487–497
|
|
Wilkins R W T, George S C. 2002. Coal as a source rock for oil: a review. International Journal of Coal Geology, 50(1–4): 317–361
|
|
Xie Guoliang. 2014. The coal accumulation patterns of Pinghu formation in Pingbei area, Xihu depression (in Chinese) [dissertation]. Xuzhou: China University of Mining and Technology
|
|
Xie Guoliang, Shen Yulin, Liu Shugen, et al. 2018. Trace and rare earth element (REE) characteristics of mudstones from Eocene Pinghu Formation and Oligocene Huagang Formation in Xihu Sag, East China Sea Basin: implications for provenance, depositional conditions and paleoclimate. Marine and Petroleum Geology, 92: 20–36. doi: 10.1016/j.marpetgeo.2018.02.019
|
|
Xu Huiyuan, George S C, Hou Dujie, et al. 2020. Petroleum sources in the Xihu Depression, East China Sea: evidence from stable carbon isotopic compositions of individual n-alkanes and isoprenoids. Journal of Petroleum Science and Engineering, 190: 107073. doi: 10.1016/j.petrol.2020.107073
|
|
Yancey T E. 1997. Depositional environments of late Eocene lignite-bearing strata, East-Central Texas. International Journal of Coal Geology, 34(3–4): 261–275
|
|
Ye Jun, Guo Dixiao. 1996. Geochemical characters of the natural gas in West Lake Depression, the East China. Experimental Petroleum Geology (in Chinese), 18(2): 174–181
|
|
Ye Jiaren, Qing Hairuo, Bend S L, et al. 2007. Petroleum systems in the offshore Xihu Basin on the continental shelf of the East China Sea. AAPG Bulletin, 91(8): 1167–1188. doi: 10.1306/02220705158
|
|
Yu Shui. 2020. Depositional genesis analysis of source rock in Pinghu Formation of western slope, Xihu Depression. Earth Science (in Chinese), 45(5): 1722–1736
|
|
Zhang Jingyu, Pas D, Krijgsman W, et al. 2020. Astronomical forcing of the Paleogene coal-bearing hydrocarbon source rocks of the East China Sea Shelf Basin. Sedimentary Geology, 406: 105715. doi: 10.1016/j.sedgeo.2020.105715
|
|
Zhang Gongcheng, Wang Dongdong, Lan Lei, et al. 2021. The geological characteristics of the large- and medium-sized gas fields in the South China Sea. Acta Oceanologica Sinica, 40(2): 1–12. doi: 10.1007/s13131-021-1754-x
|
|
Zhou Qianyu, Shen Wenchao, Zhang Xin, et al. 2016. The coal-accumulating environments characteristics and coal-forming pattern of Pinghu Formation (Paleogene) in Xihu Depression. Journal of Hebei University of Engineering (Natural Science Edition) (in Chinese), 33(1): 105–107, 112
|
|
Zhou Xinhuai, Xu Guosheng, Cui Hengyuan, et al. 2020. Fracture development and hydrocarbon accumulation in tight sandstone reservoirs of the Paleogene Huagang Formation in the central reversal tectonic belt of the Xihu Sag, East China Sea. Petroleum Exploration and Development, 47(3): 499–512. doi: 10.1016/S1876-3804(20)60068-4
|
|
Zhu Xinjian, Chen Jianfa, Zhang Chao, et al. 2021. Effects of evaporative fractionation on diamondoid hydrocarbons in condensates from the Xihu Sag, East China Sea Shelf Basin. Marine and Petroleum Geology, 126: 104929. doi: 10.1016/j.marpetgeo.2021.104929
|
|
Zhu Yangming, Li Ying, Zhou Jie, et al. 2012. Geochemical characteristics of Tertiary coal-bearing source rocks in Xihu Depression, East China Sea basin. Marine and Petroleum Geology, 35(1): 154–165. doi: 10.1016/j.marpetgeo.2012.01.005
|
|
Zhu Weilin, Mi Lijun, Zhang Houhe, et al. 2010. Atlas of Oil and Gas Basins, China Sea (in Chinese). Beijing: Petroleum Industry Press, 68–86
|
|
Zhu Weilin, Zhong Kai, Fu Xiaowei, et al. 2019. The formation and evolution of the East China Sea Shelf Basin: a new view. Earth-Science Reviews, 190: 89–111. doi: 10.1016/j.earscirev.2018.12.009
|
Figures(24) / Tables(2)
Supported by:
Beijing Renhe Information Technology Co. Ltd
Yongcai Yang, Xiaojun Xie, Youchuan Li, Gang Guo, Xiaoying Xi, Wenjing Ding. 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[J]. Acta Oceanologica Sinica, 2023, 42(3): 254-269. doi: 10.1007/s13131-023-2176-8
| No. | Well | Depth/m | Sample type | Lithology | TOC/% | Ro/% |
| 1 | A1 | 3678−3681 | cutting | coal measure mudstone | 0.68 | 0.73 |
| 2 | A35 | 3570 | cutting | coal | 63.15 | 0.63 |
| 3 | A36 | 3008 | cutting | coal measure mudstone | 0.48 | 0.76 |
| 4 | A37 | 2624 | cutting | coal | 48.64 | 0.50 |
DownLoad:
CSV
| No. | Sample type | Heating rate/ (℃·h−1) | Pr/nC17 ratio | Ph/nC18 ratio | Pr/Ph ratio |
| 1 | coal | 20 | 3.20 | 0.58 | 7.00 |
| 2 | coal | 20 | 2.85 | 0.58 | 6.50 |
| 3 | coal | 20 | 2.94 | 0.57 | 7.05 |
| 4 | coal | 20 | 3.51 | 0.59 | 7.58 |
| 5 | coal | 20 | 3.47 | 0.59 | 7.38 |
| 6 | coal | 20 | 3.31 | 0.55 | 7.44 |
| 7 | coal | 20 | 2.95 | 0.55 | 6.39 |
| 8 | coal | 20 | 2.30 | 0.42 | 5.75 |
| 9 | coal | 20 | 1.86 | 0.35 | 5.78 |
| 10 | coal | 20 | 1.28 | 0.25 | 5.38 |
| 11 | coal | 20 | 0.93 | 0.18 | 5.46 |
| 12 | coal | 20 | 0.68 | 0.15 | 5.00 |
| 13 | coal | 2 | 3.00 | 0.52 | 7.50 |
| 14 | coal | 2 | 3.43 | 0.64 | 6.67 |
| 15 | coal | 2 | 3.75 | 0.65 | 7.06 |
| 16 | coal | 2 | 3.64 | 0.61 | 7.06 |
| 17 | coal | 2 | 2.90 | 0.53 | 5.95 |
| 18 | coal | 2 | 2.33 | 0.42 | 5.67 |
| 19 | coal | 2 | 1.66 | 0.31 | 5.42 |
| 20 | coal | 2 | 0.99 | 0.18 | 5.92 |
| 21 | coal | 2 | 0.69 | 0.12 | 5.73 |
| 22 | coal | 2 | 0.50 | 0.09 | 5.67 |
| 23 | coal | 2 | 0.21 | 0.04 | 6.25 |
| 24 | coal | 2 | 0.07 | 0.02 | 5.33 |
| Note: Sample informations are showed in Table 1. | |||||
DownLoad:
CSV
| No. | Well | Depth/m | Sample type | Lithology | TOC/% | Ro/% |
| 1 | A1 | 3678−3681 | cutting | coal measure mudstone | 0.68 | 0.73 |
| 2 | A35 | 3570 | cutting | coal | 63.15 | 0.63 |
| 3 | A36 | 3008 | cutting | coal measure mudstone | 0.48 | 0.76 |
| 4 | A37 | 2624 | cutting | coal | 48.64 | 0.50 |
| No. | Sample type | Heating rate/ (℃·h−1) | Pr/nC17 ratio | Ph/nC18 ratio | Pr/Ph ratio |
| 1 | coal | 20 | 3.20 | 0.58 | 7.00 |
| 2 | coal | 20 | 2.85 | 0.58 | 6.50 |
| 3 | coal | 20 | 2.94 | 0.57 | 7.05 |
| 4 | coal | 20 | 3.51 | 0.59 | 7.58 |
| 5 | coal | 20 | 3.47 | 0.59 | 7.38 |
| 6 | coal | 20 | 3.31 | 0.55 | 7.44 |
| 7 | coal | 20 | 2.95 | 0.55 | 6.39 |
| 8 | coal | 20 | 2.30 | 0.42 | 5.75 |
| 9 | coal | 20 | 1.86 | 0.35 | 5.78 |
| 10 | coal | 20 | 1.28 | 0.25 | 5.38 |
| 11 | coal | 20 | 0.93 | 0.18 | 5.46 |
| 12 | coal | 20 | 0.68 | 0.15 | 5.00 |
| 13 | coal | 2 | 3.00 | 0.52 | 7.50 |
| 14 | coal | 2 | 3.43 | 0.64 | 6.67 |
| 15 | coal | 2 | 3.75 | 0.65 | 7.06 |
| 16 | coal | 2 | 3.64 | 0.61 | 7.06 |
| 17 | coal | 2 | 2.90 | 0.53 | 5.95 |
| 18 | coal | 2 | 2.33 | 0.42 | 5.67 |
| 19 | coal | 2 | 1.66 | 0.31 | 5.42 |
| 20 | coal | 2 | 0.99 | 0.18 | 5.92 |
| 21 | coal | 2 | 0.69 | 0.12 | 5.73 |
| 22 | coal | 2 | 0.50 | 0.09 | 5.67 |
| 23 | coal | 2 | 0.21 | 0.04 | 6.25 |
| 24 | coal | 2 | 0.07 | 0.02 | 5.33 |
| Note: Sample informations are showed in Table 1. | |||||
DownLoad:
DownLoad:
