Coal seams in the Eocene Pinghu Formation in Pingbei slope zone of Xihu Sag: Sedimentary environment and model

Thin coal seams are extensively developed in the Eocene Pinghu Formation of the Pingbei slope zone in the Xihu Sag. However, poor understanding of their sedimentary environment and distribution patterns has constrained the expansion of hydrocarbon exploration toward the lower slope area. Therefore, core, logging, seismic, and paleontological data were integrated to investigate systematically the sedimentary structures, lithological associations, and biogenic assemblages in the coal seam intervals in 16 local key wells. The sedimentary environment types were identified, the development of the depositional systems characterized, and a depositional model established. Three types of coal-bearing lithofacies associations were distinguished: thin sandstone-carbonaceous mudstone-siltstone-mudstone-thin coal interbeds; mudstone interbedded with siltstone and thin coal seams; and thick sandstone and mudstone with thin coal seams. The three types of the assemblages represent collectively a systematic response to sea-level fluctuations and strong tidal dynamics. The sedimentary environment of the Pinghu Formation was interpreted tidal flat and tide-dominated delta facies mainly, in which thin coal seams were formed in intertidal peat flats and delta-front peat swamps. The coal accumulation decreased from the lower to the upper members of the Pinghu Formation. A tidal flat and tide-dominated delta depositional model was established that elucidates the differential responses of various lithofacies associations to sea-level fluctuations. The well-developed thin coal seams serve as important hydrocarbon source rocks in the study area, indicating good exploration potential in the lower slope zone toward the depocenter. This study provided a theoretical foundation for future exploration in the region.