Hydrocarbon accumulation process and reservoir-forming models of structure A in the central inversion tectonic belt of the Xihu Depression, East China Sea Basin

The Huagang Formation of the central inverted structural tectonic belt structure A in the Xihu Depression has abundant hydrocarbon resources. This is an important area for exploration and development in the East China Sea Basin (ECSB). Due to the high drilling costs in offshore basins and limited drilling in Structure A, the mechanism of hydrocarbon accumulation is complex, and the laws and processes of hydrocarbon accumulation in structure A are still unclear. To clarify the hydrocarbon accumulation process in the Huagang Formation of structure A, this study used thin section observation, scanning electron microscopy, natural gas carbon isotopes, fluid inclusions, and micro thermometry. Combined with structural evolution and burial history, the origin and source of natural gas, hydrocarbon accumulation periods, and processes were systematically analyzed. The research shows that the sandstone reservoir in the Huagang Formation has satisfactory reservoir, trapping, and preservation conditions; the primary hydrocarbon accumulation period was the Longjing Movement. Therefore, early natural gas filling existed before the Longjing Movement. Based on the evolutionary and fluid potential characteristics of the top tectonic peak of the Pinghu Formation, the natural gas generated in the early stage of structure A is stored in the paleo-structural high point of the Pinghu Formation (paleo convergence area). In the early stage of the Longjing Movement, natural gas migrated from the paleo convergence area of the Pinghu Formation to the current tectonic high point and accumulated in the trap. Therefore, three different “convergence ahead of accumulation” reservoir formation models were developed. These reservoir formation models are critical for guiding the exploration of a series of large traps, including structure A and its surrounding structures, and they have important reference significance for natural gas exploration in other basins.