HYDROCARBON ACCUMULATION OF SHAHEJIE FORMATION, BOHAI BAY BASIN, CHINA
Meng, Fanjin 1989-
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Only a few basin modeling studies have been completed at a depression-scale for the southern Western Depression, Liaohe Basin, Bohai Bay Basin Group. Such studies are essential for advancing our understanding of the hydrocarbon accumulation history and to predicting potential reservoirs. This study analyzed the petroleum system of the Shahejie Formation within the southern Western Depression using techniques of one- and two-dimensional basin modeling and structural restorations. The results showed that the mudstones in the fourth and third member of the Eocene Shahejie Formation (Es4 and Es3) are the primary source rocks for the Shahejie tight gas sand system in the study area, which is in agreement with earlier findings. They are rich in organic matter with TOC content up to 2.83%. Tight sandstones with measured porosity values ranging from 2% to 10% in the Es3 member are the deepest proven hydrocarbon reservoir rocks. To further analyze the petroleum generation and accumulation history of Shahejie Formation, forward basin models were developed based on reconstructions of the burial, thermal, maturity, structural, and migration histories. The present-day and paleo heat flows of the studies basin in 1D models were calibrated with measured borehole temperature and vitrinite-reflectance data. Results indicate that the present-day heat flow is ca. 50 mW/m2. Paleo heat flow increased from 43 to 36 Ma, with a peak of 83.68 mW/m2 at approximately 36 Ma and then decreased till present. The source rocks are presently at a thermal maturity stage of condensate and gas generation window, while some deep-buried areas in the central Depression are in the dry gas threshold. In order to assess the geometry and displacement of listric normal faults and growth faults in the study area, a 2D balanced cross section was reconstructed. This structural model was used in 2D basin modeling to depict the structure of the basin and therefore provide a more robust estimation of the hydrocarbon migration history. The expulsion and accumulation results were calibrated using the homogenization temperatures of fluid inclusion in rock samples from 7 wells in the study area. The model suggests that hydrocarbon generation started at 40 Ma and peaked during 38 Ma to 36.5 Ma. Moreover, major expulsion, migration, and accumulation of hydrocarbon occurred during 36.5 Ma to 20 Ma.