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In the Qinhuangdao 29 (QHD29) oil field, oil generated from the first member of the Shahejie source rocks is mainly contained in Paleogene reservoirs, whereas deeper oil sourced from the third member of the Shahejie (Es3) source rocks is generally accumulated in Neogene reservoirs. The present study was undertaken to better understand the differences in petroleum accumulation in the QHD29 oil field and to provide suggestions for future petroleum exploration on the Shijiutuo uplift.
Laser Raman spectroscopy reveals that carbon dioxide (CO2) exists with hydrocarbon gas in the same fluid inclusions. Measured homogenization temperatures of aqueous inclusions range from 80°C to 160°C (176°F to 320°F), indicating late-stage rapid petroleum accumulation with a charging time of no earlier than 5.1 Ma. The results of grains containing oil inclusions measurements reveal the presence of paleo-oil accumulation in the current natural gas column. In terms of the boundary fault activity rate (FAR), CO2 distribution is quite relevant to the late-stage activity of boundary faults, with high content of CO2 corresponding to the section of the F1 fault with high-intensity activity (higher FAR values). Both Neogene and Paleogene reservoirs in well A1 contained predominantly Es3-derived oil and were accompanied by abundant mantle CO2. This reveals the segmented vertical transport of petroleum in the fault: both the mantle-derived CO2 and hydrocarbons vertically migrated and accumulated in shallower reservoirs in the high-activity intensity section of the boundary fault. This may account for the occurrence of predominantly Es3-derived oil in the reservoirs near the section of the fault with high activity intensity. In the eastern part of the QHD29 oil field, vertical migration may have been limited because of the relatively low intensity of fault activity, and the distribution of sandstones seems to dominate the petroleum accumulation. Our research reveals that lithologic traps in the Es3 stratum may still have great potential for exploration along the slope of the Shijiutuo uplift.