Ordered distribution and differential hydrocarbon enrichment mechanisms of hydrocarbon reservoirs in the whole petroleum system of lacustrine rift basin: A case study of the Paleogene Shahejie Formation, Dongpu Sag, Bohai Bay Basin

This study aims to delve into the ordered distribution and the mechanisms underlying differential hydrocarbon enrichment of conventional and unconventional hydrocarbon reservoirs in the whole petroleum system (WPS) of a petroliferous basin. Focusing on the Dongpu Sag of the Bohai Bay Basin, we analyze the ordered distribution characteristics of hydrocarbon reservoirs in the WPS of the Paleogene Shahejie Formation, and delineate the dynamic fields for hydrocarbon accumulation and reveal the mechanisms behind differential hydrocarbon enrichment. The results indicate that the hydrocarbon reservoirs in the WPS of the Shahejie Formation exhibit an ordered distribution pattern, with conventional oil reservoirs, conventional volatile oil reservoirs, and tight gas reservoirs distributed sequentially from the structural high to the slope zone and then to the sub-sag zone. The buoyancy-driven hydrocarbon accumulation depth (BHAD) in the Shahejie Formation corresponds to average porosities ranging from 10.0% and 12.0% and an average burial depth of 3580 m. In contrast, the hydrocarbon accumulation depth limit (HADL) in this formation is associated with an average porosity of 1.9% and an average burial depth of 5580 m. Based on their genetic mechanisms, hydrocarbon reservoirs in the Shahejie Formation can be categorized into three types: (1) conventional hydrocarbon reservoirs formed in free dynamic field dominated by buoyancy; (2) tight and reformed hydrocarbon reservoirs formed in confined dynamic field dominated by non-buoyancy such as hydrocarbon generation-induced expansion force; and (3) shale hydrocarbon reservoirs formed in bound dynamic field. The hydrocarbon accumulation pattern varies across different structural locations. In the structural highs, hydrocarbon accumulation patterns include “early generation-single source (dominant)-upper accumulation,” “late generation-mixed sources (dominant)-middle accumulation,” and “late generation-self-sourced reservoir-lower accumulation.” In the slope zone, the hydrocarbon accumulation pattern proves to be of “early generation-transport along fault-slope-salt-mud sealing-mixed accumulation of crude oils with the same source but varying maturities.” In contrast, the sub-sag zone is dominated by self-sourced gas reservoirs.