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Abstract:

The Dongying depression is one of the regions with the high petroleum production in the Bohai Bay basin. The third member of the Eocene Shahejie Formation (Es3, 42.5-38.0 Ma) is one of principal reservoir rock section in the region. This paper reports a new paleo-fluid dataset obtained from diagenetic cements, in an attempt to reveal the fluid source and evolution of the Shahejie Formation. The petrology, mineralogy, stable isotopic compositions, and fluid inclusions in carbonate cements in the Es3 sandstone reservoirs in the Dongying depression were investigated. Group I calcite cement display red luminescence. In contrast, Group II calcite cements have undergone dolomitization and is not luminescent. Furthermore, fluid inclusions in quartz overgrowth are characterized by low salinity (4.1%–4.8%), whereas Group II cement preserves high-salinity fluids (up to 22.6%). The C-O isotopic compositions of the carbonate cements are also distinct. The δ¹⁸OPDB values of Group I and Group II range from –17.4‰ to –7.3‰ and –11.9‰ to –4.9‰, respectively. The carbon isotopic composition (δ¹³CPDB) of Group II is more negative than that of Group I, and both are more negative than the lacustrine carbonates from the same basin. Our data suggest that Group I and Group II cements were produced from fluids derived from Es3 and Es4, respectively. Furthermore, the petroleum inclusions display two fluorescence colors (bright-yellow and khaki) under UV light. The homogenization temperatures of aqueous inclusions coexisting with two kinds of petroleum inclusions are consistent with each other. It suggests that the hydrocarbon fluid of the reservoir in Es3 was almost charged in the same time. The different fluorescence colors are caused by difference between source rocks. The integrated studies are useful to understand the fluid origin and evolution of the Eocene reservoir, and would improve the accuracy of reservoir prediction in Dongying depression.