Phase evolution and accumulation mode of hydrocarbons in deep coarse-grained clastic reservoirs in the Yanjia area, Dongying Sag, Bohai Bay Basin

Deep hydrocarbon reservoirs in the Yanjia area in the northern steep slope zone of the Dongying Sag experienced complex thermal evolution. The resultant multiple hydrocarbon phases pose challenges for the exploration and exploitation of deep hydrocarbons. Using basin modeling, pressure-volume-temperature (pVT) phase simulation, and the analysis of hydrocarbon fluid inclusions, we reveal the evolutionary process of deep hydrocarbon phases in the Yanjia area and establish the hydrocarbon accumulation modes of different types of reservoirs. Findings indicate that deep hydrocarbons in the 4^th member of the Shahejie Formation in the Yanjia area exhibit an orderly phase distribution in the vertical direction, with reservoirs of light oil, condensate gas, and dry gas found from shallow to deep. While thermal evolution predominantly governed the formation of hydrocarbon phases in deep coarse-grained clastic rocks, the phase evolution in condensate gas reservoirs stemmed from hydrocarbon generation from kerogen, crude oil cracking, and external natural gas charging. By integrating insights into the hydrocarbon phases, characteristics, and temperature and pressure evolutionary histories of different hydrocarbon reservoirs, we propose a hydrocarbon accumulation mode for deep coarse-grained clastic rocks in the Yanjia area. This mode encompasses multi-stage hydrocarbon generation, migration via superimposed sand beds within middle fans while sealed by mudstone within root fans, and the orderly phase distribution of hydrocarbons. This study seeks to provide theoretical guidance for the efficient exploration of deep hydrocarbons in continental downfaulted basins.