Relationship Between Paleogene Reservoir Densification and Hydrocarbon Accumulation in the Xihu Depression

Fluid inclusion analysis and testing were conducted to clarify the relationship between reservoir densification and hydrocarbon accumulation in the Paleogene Pinghu and Huagang formations in the Xihu Depression. The hydrocarbon accumulation stages of the reservoirs were studied in combination with the reconstruction results of burial and thermal evolution histories. Furthermore, the relationship between reservoir densification and accumulation charging was clarified in combination with the pore evolutionary history. In accordance with the time relation between reservoir densification and hydrocarbon charging, the reservoirs were classified into three types: pre-charging, syn-charging, and after-charging densification. Results indicated that large-scale hydrocarbon charging occurred in 11–0Myr. Reservoir densification was mainly caused by strong mechanical compaction and pore filling by well-developed siliceous and carbonate cements. Entering the middle diagenetic stage A1, the reservoir was under an acidic-diagenetic environment, resulting in the development of secondary dissolution pores. If large-scale hydrocarbon charging occurred during this period, then an after-charging densification reservoir, which is the most suitable type for hydrocarbon accumulation, might have developed. Entering the middle diagenetic stage A2, the reservoir was under an acidic-alkaline transitional diagenetic environment. During this stage, dissolution became weak, and compaction and cementation were enhanced, resulting in the continuous loss of pore space and reservoir densification. Entering the middle diagenetic period B, the reservoir was under an alkaline-diagenetic environment, and the reservoir had been largely densified. If large-scale hydrocarbon charging occurred during this period, a pre-charging densified reservoir, which is the worst reservoir type for hydrocarbon accumulation, might have developed.