The South Yellow Sea Basin (SYSB) is an extension of the Lower Yangtze Block to the sea. The basin has undergone a complex tectonic-sedimentary evolution, and there has been no breakthrough in oil and gas exploration. Based on newly collected and processed well drilling data, cores, and seismic data of the central-southern SYSB and adjacent areas, the understudy area is set in the Lower Yangtze Region (LYR) to compare the overall sequence division and sedimentary facies. The Permian isochronous stratigraphic framework is methodically established in the central-southern SYSB, and the corresponding sedimentary facies and their distribution are examined in some detail. Finally, together with the previous investigations of tectonic evolution, the potential Permian oil-gas accumulation modes are proposed. The achieved results reveal that the second-order Permian sequence developed in the central-southern SYSB is consistent with the LYR. There exists a relatively complete second-order sequence I developed in the middle-bottom section and the transgressive systems tract of sequence II in the upper section. The Permian sedimentary system in the central-southern SYSB can be compared to the LYR and is generally distributed in a northeast direction. The transgressive systems tract of sequence I is broadly developed with the restricted platform and open platform as the dominant facies. The highstand systems tract of sequence I has been only developed in the northwest of the understudy area, with the shelf as the dominant facies. The transgressive systems tract of sequence II is mostly dominated by delta facies and swamp-lagoon facies with thick coal-measure layers, which provide a material basis for the formation of large-scale oil and gas fields. The thermal evolution of Permian source rocks was controlled by a combination of tectonic evolution and paleothermal gradient. Permian source rocks may have experienced secondary hydrocarbon generation in the western part of the southern Depression, whereas source rocks elsewhere experience only one hydrocarbon generation in the Early Triassic. The obtained results reveal that there are significant differences in the accumulation mode of different structural units.

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Permian sedimentary evolution and hydrocarbon accumulation effects in the central-southern South Yellow Sea Basin

Based on the drilling results of the Taishen-1 well in the Sanhu area in the Qaidam Basin, the study proposed new findings on the conditions of source rock, reservoir, and overlaying strata, as well as accumulation characteristics for biogenic gas. Further, the controlling factors of biogenic gas accumulation as well as two favorable zones for exploration were identified based on the comparison of drilling results and geological findings in adjacent areas. The result of the study showed that: (1) the biogenic gas source rock in the Sanhu area is lacustrine dark mudstone, and the LLTOC (lower limit of total organic carbon) in Q1+2 (the Qigequan Formation) and N (the Neogene) is 0.25% and 0.40%; (2) the ‘self-generation and self-storage’ source-reservoir combination developed across all the formations due to the absence of faults and the development of mudstone; (3) the controlling factors of gas accumulation in the studied area include the limit of burial depth, the methane yield, and climate conditions. The failure of the Taishen-1 well owes to the fact that the gas generated failed to support the demand for gas accumulation; (4) influenced by the southward movement of gas-containing groundwater from the high potential area in the north, the biogenic gas reservoirs are distributed in the northern slope and the central sag of the Sanhu depression, and the lower limit of exploration depth of the biogenic gas reservoirs is about 2119 m; (5) two favorable zones were selected for natural gas exploration: the Upper Tertiary thermogenic gas-bearing Yahu-Sebei area and the biogenic gas-bearing Tainan-Sebei area.

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Mingjian Wang

Papers

Permian sedimentary evolution and hydrocarbon accumulation effects in the central-southern South Yellow Sea Basin

Distribution of Biogenic Gas Reservoirs and Optimization of Favorable Zones in the Sanhu Area, Qaidam Basin, China