Development pattern of coal-measure source rocks and division of hydrocarbon accumulation units: a case study of Paleogene Pinghu Formation in Guoxi Subsag, Xihu Sag, East China Sea Basin

The source rocks from Paleogene Pinghu Formation in the Guoxi Subsag, Xihu Sag, East China Sea Basin are characterized by great thickness, high maturity, and good hydrocarbon generation potential. However, the overall exploration degree is low, resulting in unclear hydrocarbon generation potential and oil and gas exploration direction. Therefore, the study aims to reveal the characteristics, distribution patterns, and hydrocarbon accumulation potential of source rocks through multi-parameter geochemical analysis to guide subsequent exploration deployment. Based on geochemical parameters such as total organic carbon (TOC), maximum pyrolysis temperature (Tmax), and vitrinite reflectance (Ro), and sedimentary environment element indicators (Sr/Ba, B/Ga), the types of source rocks were systematically classified. The development model of source rocks and the division of hydrocarbon accumulation units were studied using multi-dimensional methods such as sedimentary facies, seismic facies, fault sealing (SGR model), and paleofluid potential analysis. The results showed that: (1) the coal-measure source rocks in the Pinghu Formation of the Guoxi Subsag had medium to good organic matter abundance, with terrestrial higher plants as the main biogenic source.The kerogen types were mainly type Ⅱ1 (coal and carbonaceous mudstone) and types Ⅱ2-Ⅲ (mudstone), and the rocks were overall in the mature stage. (2) The source rocks were mainly developed in tidal flat-lagoon environments, forming a distribution pattern from west to east of coal-carbonaceous mudstone-mudstone. Planar prediction indicated that mudstone and carbonaceous mudstone were mainly distributed in the deep sag area, while coal seams were mainly developed in the western part. (3) Based on fluid potential and migration-accumulation characteristics, the study area was divided into five hydrocarbon accumulation units. Among them, unit 3, adjacent to the hydrocarbon generation center with favorable migration pathways, exhibited the most favorable conditions for hydrocarbon accumulation. This study, for the first time, establishes the developmental model of coal-measure source rocks in the Guoxi Subsag and clarifies the planar distribution patterns of high-quality source rocks. This area has a hydrocarbon generation foundation from coal-measure source rocks, but oil and gas enrichment exhibits distinct zonation. The low belt is the most favorable exploration area, while the middle-high belt has relatively less favorable conditions for hydrocarbon accumulation.