The mechanism of “ convergence ahead of accumulation” and its geological significance for gas reservoirs in Paleogene Huagang Formation across the central inverted structural zone of Xihu Depression, East China Sea Shelf Basin

Inverted structural zones in many onshore basins around the world are frequently found to be home to oil and gas traps and have been the targets for studies for years. However, offshore inverted structural zones, in the contrary, are seldom explored because of less drilling due to high cost. Taking the central inverted structural zone of Xihu Depression as an example, this paper focuses on the original capacity of hydrocarbon convergence in the source rock series and the dynamic accumulation of hydrocarbons from source to reservoir, with the aim of revealing the hydrocarbon enrichment law. The correlation of structural evolution, hydrocarbon generation and reservoir evolution as well as hydrocarbon charging history reveals that the massive hydrocarbon generation and charging of source rocks preceded the formation of traps in the northern part of the zone. The early-generated gas formed “semi accumulation” with high abundance within the series, i. e., “convergence first”; and then migrated upwards and accumulated in traps formed later during tectonic movements, i. e., “later accumulation”. The reservoirs show a trend of increasing natural gas maturity from deep to shallow. Based on these results, the paper proposes the new model of “convergence ahead of accumulation”, which not only reasonably explains the mismatch of timing among the hydrocarbon generation, trap formation and hydrocarbon charging in the Huagang Formation in the series, but also reveals the control effect of ancient tectonic background on the current accumulation of oil and gas. Under the guidance of this model, the key factors for the formation of large-scale natural gas reservoirs in the zone are revealed, and the B structure in the zone is pinpointed to be a perfect place for “convergence ahead of accumulation”. The new model can be applied to other offshore petroliferous basins with similar geological conditions.