Hydrocarbon Accumulation Model In Typical Steep Slope Zones Of Bohai Bay Basin

As an important part of graben basin,steep slope zone has various reservoir forming conditions and processes,which affect the distribution of oil and gas.Taking Bohai Bay Basin as an example,based on the classification of steep slope zones and the establishment of oil and gas distribution patterns,combined with the data of geology,geophysics,geochemistry,the analysis of hydrocarbon supply capacity of source rocks and fault activity during reservoir formation,revealing the mechanism of oil and gas accumulation,establishing the model of oil and gas accumulation and defining the main controlling factors of oil and gas accumulation in steep slope zones.Based on the development characteristics of main faults,steep slope zones in Bohai Bay Basin are divided into listric steep slope zone,slope-flat steep slope zone and faulted terrace type steep slope zone.The listric steep slope zone mainly distributes in the deep and large faults around the basin,dominated by lithology and structural reservoirs;the slope-flat steep slope zone mainly distributes between deep and large faults,and lithology,structure and stratigraphic reservoirs are developed from sag to the edge of steep slope in turn;the faulted terrace type steep slope zone mainly distributes in the Bohai Sea,and structural reservoirs are developed.The three types of steep slope zones have obvious differences in Ar and Dr ratios of source rock area and distance,fault-source rock contact thickness,matching between reservoir-forming period and fault activity,and overpressure development characteristics.The matching of reservoir-forming elements is the best in the slope-flat steep slope zone,and hydrocarbon is the easiest to form reservoirs.Oil and gas accounts for the largest proportion of total resources in the sag.The matching of reservoir-forming elements in the listric steep slope zone is the worst,and the proportion of resources is the smallest.The listric steep slope zone has weak hydrocarbon supply capacity,low activity rate of main faults on steep slope,and faults are sealed.Oil and gas migrate laterally along adjacent sand bodies and then close to main faults to form reservoirs,which is a "single-source weak drive-sand lateral transport type" reservoir-forming model.The slope-flat steep slope zone has strong hydrocarbon supply capacity,strong fault activity and semi-closed nature.Oil and gas migrate vertically and laterally along secondary faults and lateral docking sand bodies,which is a "multi-source strong driving-fault vertical conduction type" reservoir-forming model.The faulted terrace type steep slope zone has strong hydrocarbon supply capacity,strong fault activity and faults are conductive.Oil and gas mainly migrate vertically along main and secondary fractures,which is a reservoir-forming model of "multi-source strong drive-multi-disconnected transmission type".Oil and gas accumulation in steep slope zone is mainly controlled by hydrocarbon supply capacity and fault transmission capacity.The hydrocarbon supply ability of source rocks determines the degree of oil and gas enrichment in steep slope zone,and the activity of faults determines the oil and gas enrichment strata in steep slope zone.