The control of fault-caprock configurations on hydrocarbon accumulation in the Pinghu Slope Belt of the Xihu Depression, East China Sea Shelf Basin (ECSSB)

Hydrocarbon generation and expulsion of tertiary coaly source rocks and hydrocarbon accumulation in the Xihu Sag of the East China Sea Shelf Basin, China

Differences in hydrocarbon accumulation and controlling factors of slope belt in graben basin: A case study of Pinghu Slope Belt in the Xihu sag of the east China Sea Shelf basin (ECSSB)

The Xihu Sag in the East China Sea Shelf Basin contains abundant oil and gas reserves and is a focus for hydrocarbon exploration and development. Source rocks are mainly coals and coal-measures mudstones in the Paleogene Pinghu and Huagang formations. Samples from the Pinghu Formation in the Xihu Sag were collected for petrology, total organic carbon, and Rock-Eval analysis for the purpose of investigating macerals component and their contributions to hydrocarbon generation potential. The coaly source rocks from the Pinghu Formation are dominated by vitrinite (average 86.18%) but have an obviously elevated content of liptinite (average 12.59%) and a much lower amount of inertinite (average 1.23%). Liptinite of the samples is mainly composed of resinite, with a small amount of cutinite, sporinite and alginate in descending order. TOC values are 37.55%–65.58% (average 49.16%). Effective HI values are 167–281 mg HC/g TOC (average 223.5 mg HC/g TOC), suggesting the organic matter is type II kerogen. Relatively high HI values and macerals components suggest that the coaly source rocks can generate both oil and gas. Although the liptinite in the coaly source rocks has a content lower than vitrinite values, it makes a significant contribution to both total hydrocarbon and liquid hydrocarbon generation. The contributions of vitrinite, liptinite and inertinite to the total hydrocarbon generation approximately are 63.21%, 36.46% and 0.33%, respectively. The contributions of vitrinite and liptinite to the liquid hydrocarbon generation are approximately 40.95% and 59.05%, respectively. These results demonstrate that the coaly source rocks are dominated by vitrinite macerals with a relatively higher content of liptinite macerals, especially resinite, and these source rocks are more prone to both total hydrocarbon and liquid hydrocarbon generation. Paleogene coaly source rocks from other parts of the world should be considered for their oil-prone nature.

Astronomical forcing of the Paleogene coal-bearing hydrocarbon source rocks of the East China Sea Shelf Basin

The Pinghu Formation consists primarily of marine-continental transitional deposits. The widely distributed fluvial and tidal transgressive sand bodies comprise the main reservoirs of the Baochu slope zone in the Xihu Sag in the East China Sea Shelf Basin. These sand bodies are deeply buried, laterally discontinuous, and are frequently interrupted by coal-bearing intervals, thereby making it extremely difficult for us to characterize their hydrocarbon potential quantitatively via seismic inversion techniques, such as multi-attribute seismic analysis and post-stack seismic inversion, hindering further hydrocarbon exploration in the Xihu Sag. Here, a prestack seismic inversion approach is applied to the regional seismic data to decipher the spatiotemporal distribution pattern of the sand bodies across the four sequences, i.e., SQ1, SQ2, SQ3, and SQ4, from bottom up, within the Pinghu Formation. In combination with detailed petrology, well log, and seismic facies analysis, the secular evolution of the sedimentary facies distribution pattern during the accumulation of the Pinghu Formation is derived from the sand body prediction results. It is concluded that the sedimentary facies and sand body distribution pattern rely on the interplay between the hydrodynamics of fluvial and tidal driving forces from the continent and open ocean, respectively. Drops in the sea level led to the gradual weakening of tidal driving forces and relative increases in riverine driving forces. The direction of the sand body distribution pattern evolves from NE–SW oriented to NW–SE oriented, and the dominant sand body changes from tidal facies to fluvial facies. In addition, the sea level drop led to the decrease in the water column salinity, redox condition, organic matter composition, and the development of coal seams, all of which directly influenced the quality of reservoir and source rocks. The sand bodies in SQ2 and SQ3 are favorable reservoirs in the Pinghu Formation due to their good reservoir properties and great thickness. The high-quality source rock in SQ1 could provide significant hydrocarbons and get preserved in the sand body within SQ2 and SQ3. This contribution provides an insight into the control of the sea level change over the development of hydrocarbon reservoirs in the petroleum system from marginal-marine environments such as the Xihu Sag.

U–Pb detrital zircon ages from late Eocene to early Oligocene Pinghu formation of the East China Sea Shelf Basin: Inferences on synergistic effects between East Asia offshore sedimentation and major rivers supply

Improved method for quantitative evaluation of fault vertical sealing: A case study from the eastern Pinghu Slope Belt of the Xihu Depression, East China Sea Shelf Basin

Petroleum accumulation: from the continuous to discontinuous

The origins of overpressure in the central Xihu depression have been accepted to result from disequilibrium compaction and hydrocarbon generation. This study uses organic matter correction of the log data in source rocks and five empirical methods (the multilogging combination method, Bowers’ method, the sonic velocity–density cross-plot method, the porosity method, and the pressure inversion method) to understand the origins of overpressure in the central Xihu depression. Overpressure strata are mainly distributed in the following two areas: 1) the Pinghu Formation on the western slope of the central Xihu depression; 2) the Pinghu Formation and lower part of the Huagang Formation in the western sag and the central Inversion anticline belt. The areas contain two types of pressure profiles: normal pressure–overpressure and normal pressure–overpressure–normal pressure. At the western slope and the western sag, overpressure in source rock and nonsource rock was caused by hydrocarbon generation and pressure transfer, respectively. At the central inversion anticline belt, however, overpressure was caused by hydrocarbon generation and tectonic compression in source rock and by pressure transfer and tectonic compression in nonsource rock. We also propose criteria for confirming the combination of tectonic compression and hydrocarbon generation as the origin of overpressure, where the porosity distribution in the overpressured mudstone is consistent with the normal compaction trend or conforms to a higher level of normal compaction as the depth increases. In the sonic velocity–effective stress diagram, sonic velocity increases with decreasing effective stress, whereas density and sonic velocity both increase with increasing normal compaction in the density–sonic velocity diagram.

Xihu Sag is the largest petroliferous sag in China offshore area, which is also the main oil and gas exploration battlefield in the East China Sea shelf basin. Source rock is the main material basis for oil and gas generation, its study has important significance on oil and gas exploration in Xihu Sag. By taking source rock of the Eocene Pinghu Formation and Oligocene Huagang Formation of Pingbei area in Xihu Sag as the research object, static characteristics of source rock in Pingbei Area are evaluated and analyzed in the following three aspects: organic matter abundance, types and maturity. The results show that, based on the organic matter abundance, lower member of Pinghu Formation has a high organic matter abundance, mainly gas, while lower member of Huagang Formation is mainly medium-better source rock, with a common hydrocarbon generating potential. Based on the organic matter types, Huagang Formation and Pinghu Formation are mainly type II and type III kerogen, having a certain hydrocarbon generating potential. While, from the point of view of maturity, lower member of Pinghu Formation is under low mature - mature stage, and the lower part of Huagang Formation mainly under mature to high mature stage. Its hydrocarbon generation peak is basically at present.

The transport system plays an important role in hydrocarbon accumulation in slope area. The geochemical characteristics of the reservoir indicate that the oil and gas in this area generally have the tendency of migration and accumulation from the hydrocarbon generation center to the high part of slope. According to the relationship between source kitchens and traps, the hydrocarbon migration is divided into three types: inside source accumulation, near source vertical migration and outside source accumulation, controlled by the differential pressure between source rock and reservoir, dominant pathway-cap assembly and dominant migration phase respectively. The main types of migration pathway in the slope area are sand bodies, faults and unconformities, and the matching relationship between them controlled the direction and mode of oil and gas migration and accumulation in the slope area. The hydrocarbon migration and accumulation of middle-shallow layer in the low-middle slope are controlled by the dominant pathways and the matching relationship between faults and cap rocks, the hydrocarbon migration and accumulation in the middle slope are controlled by the favorable sedimentary facies, and the hydrocarbon migration and accumulation in the high slope are controlled by the distribution of sand bodies and favorable structures.

Controls of the Himalayan deformation on hydrocarbon accumulation in the western Qaidam Basin, Northwest China

The East China Sea shelf basin, which is a fault subsidence during the Cenozoic Era, locates in the East China Sea continental shelf. In this paper, balanced section technique has been applied to analyzing the differential evolution in the East China Sea shelf basin south of Cenozoic tectonic and summarizing the control factors of tectonic activities on the petroleum accumulation. Our study results will provide essential data and basis for the distribution of the Cenozoic oil and gas and promote the development of the petroleum exploration in the East China Sea shelf basin.

The coal‐bearing series of the Eocene Pinghu Formation is one of the focused reservoir exploration in the Xihu Sag. This study conducted a cyclostratigraphy time series analysis using natural gamma‐ray (GR) logging data from four boreholes of the Xihu Sag. The spectral analysis shows a good match with the La2004 orbital solution. The sedimentation duration calculated by the astronomical age models spans at least 4.31–5.94 Myr, with an average sedimentation rate of 13.47–18.57 cm/kyr. The Pinghu Formation is subdivided into 1 second‐order, 3 third‐order and 14 fourth‐order sequences within seismic, well logging and 405 kyr‐long cycles constraints. Based on available age‐control data, the Pinghu Formation is corresponding to the middle‐late Eocene. The floating astronomical timescale of XH‐3 borehole now provides a new age constraint for the Pinghu Formation. There are four shorter‐term (<2 Myr) regional sea‐level cycles in the Pinghu Formation, which are similar to the calibrated global sea‐level fluctuations. Inconsistencies between reconstructed sea‐level curve by sedimentary noise modelling and true sea level inferred from lithology and sequence reveal various sources of noise in the sedimentary record. Variations in the distribution and thickness of coal seams in the Pinghu Formation appear to be a stratigraphic response to astronomical and non‐astronomical forcing such as regional tectonicssm and facies changes.

Hydrocarbon migration patterns and pathways were studied on the basis of three-dimensional seismic interpretation, drilling, geochemistry, production performance, and other data. Using these findings, the main factors controlling hydrocarbon migration and accumulation in the Lower Paleozoic carbonate rocks of the Tazhong Uplift were discussed. The spatiotemporal relationship between the hydrocarbon kitchens and pathway systems of the Tazhong Uplift and the spatial pattern of pathway systems were considered the main factors causing differences in hydrocarbon enrichment. Results also revealed that the Lower Paleozoic carbonates of the Tazhong Uplift have two hydrocarbon accumulation systems (inside and outside the source rocks). For the accumulation system within the source rocks, hydrocarbon migration and enrichment are vertically differentiated. Middle Cambrian gypsum salt rocks serve as the boundary, above which thrust and strike-slip faults mainly allow vertical transport of hydrocarbons. A multistage superposition pattern of strike-slip faults controls the differences in hydrocarbon enrichment on the periphery of the fault zone. Beneath the gypsum-salt rocks, hydrocarbon migration and enrichment is controlled by the topography of paleostructures and paleogeomorphology. For the hydrocarbon accumulation system outside the source rocks, hydrocarbon migration and enrichment are restricted by the layered pathway system, and the topography of the paleostructures and paleogeomorphology is the key factor controlling hydrocarbon enrichment. The Tazhong No. 1 Fault is the main vertical pathway system in the area underlain by no source rocks, and hydrocarbons are enriched at the periphery of the Middle-Lower Cambrian and No. 1 Fault Zone.