Main controlling factors for the formation and evolution of hydrocarbon source rocks in the Bozhong depression,Bohai Bay Basin
The Bohai oil field is currently China's largest oil field.The Bozhong depression is the most abundant hydrocarbon-generating depression in Bohai Sea.Based on seismic,drilling,and analytical data,the quality of source rocks and their differences in the planar and vertical distributions in the Bozhong depression are evaluated in this work.The main controlling factors for the formation and evolution of source rocks are analyzed from geological aspects such as faults,provenance,and thermal subsidence of the strata,providing natural explanations for why the Bozhong depression is the most hydrocarbon-rich depression.The results show that the average total organic carbon values of the Third member of the Shahejie Formation(Es3),the First member of the Shahejie Formation(Es1),and the Third member of the Dongying Formation(Ed3)are 2.06％,1.95％,and 1.59％,respectively,and the organic matter types of Es3,Es1,and Ed3 are mainly type Ⅱ1,type Ⅰ-Ⅱ 1,and type Ⅱ 1-Ⅱ 2 kerogens,respectively.The source rocks of Es3 and Esy are in a high overmaturity stage,while the source rocks of Ed3 are extensively in a stage of high maturity.The scale and activity intensity of faults control the formation and distribution of the source rocks,resulting in the different distributions of the source rocks in various parts of the Bozhong depression.Located in the center of Bohai Bay Basin,the Bozhong depression is far away from external provenance.Before the middle Oligocene epoch,small scale provenances were widely distributed within the basin.Long-term uncompensated sedimentation has contributed to the area an increase of semi-deep and deep lake facies,in which the sources rocks of the Bozhong depression formed in multiple layers with high thickness.Due to the large subsidence range and sedimentary thickness that occurred during the Neogene period,as well as the deep burial depth and high evolution degree of hydrocarbon source rocks deposited during the Paleogene period,the oil and gas in the Bozhong depression are more enriched compared to other depressions in Bohai Bay Basin.The results of this study not only provide a good basis for further hydrocarbon exploration in the Bozhong depression,but also supply a reference for the evaluation of other similar depressions.
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