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The Establishment of Source Rock Models and Their Characteristics of Hydrocarbon Generation and Expulsion in Bozhong Subbasion, Bohai Bay Basin, China


There are three organic-rich, oil-prone terrestrial source rock intervals in Bozhong Subbasin, Bohai Bay Basin: the upper Eocene third member of Shahejie Formation (E2s3), the lower Oligocene first member of Shahejie Formation (E3s1), and the upper Oligocene Dongying Formation (E3d). Terrestrial source rocks are closely related to sedimentary facies, sedimentation rate, paleoclimate, sedimentary water chemical conditions et al. From uplift to deep sag, source rocks gradually change from delta and shallow lacustrine facies to semi-deep and deep lacustrine facies, with organic matter abundance being higher, kerogen type better and thickness larger. Due to high drilling cost offshore, most of the wells are drilled around salients and end up shallow into the Neogene. To predict source rocks quality in the deep sag with no wells and establish appropriate source rock models in the basin modeling, we analyzed over 1100 source rock pyrolysis and total organic carbon results, over 600 kerogen macerals from all of the 56 important wells drilling into paleogene source rocks in Bozhong Subbasin. Over 90% percent of the samples are immature or low mature with vitrinite reflectance less than 0.9%. We get the total organic carbon and hydrogen index distribution intervals of E2s3, E3s1 and E3d source rocks. There are four subbasins in Bohai Sea, including the Bozhong Subbasin. Source rock characteristics of hydrocarbon generation and expulsion are determined by their unique hydrocarbon generation kinetics and thermal history in each subbasin. We systematicaly sampled source rocks for the only scientific exploration well (BZ21-2-1) with total depth over 5100m (over 16000ft) in Bozhong Subbasin, then set up source rock thermal evolution sequence according to hydrocarbon generation and expulsion characteristics, with which we calibrate the 1D basin modeling. Original total organic carbon and hydrocarbon index of source rock samples in high thermal evolution stage are restored based on the linear relationship of TOC and HI from the source rock models discussed above. Hydrocarbon generation and yield amount can be got when present residual hydrocarbon indices and free hydrocarbon are deducted from the original. Hydrocarbon expulsion intensity reaches up to 884 million tons oil equivalent per square kilometers within 287m excellent source rock interval with TOC being over 2%. The research results can help for hydrocarbon resources prediction and sag appraisal.