The quantitative evaluation of hydrocarbon generating potential of source rocks in the basin is the key problem which should be solved in the oil and gas exploration. Tiancao sag develops many sets of source rocks, but the main source rock is still unclear, and the report about the evaluation of source rock has not been seen. This research takes the dark color mudstone of the Tian 2 well as the dissection object, and takes advantage of many parameters such as organic matter abundance, the type and maturity of organic matter to describe quantitively or qualitatively the hydrocarbon generating potential characteristics of the dark color mudstone. Results indicate that the source rock's organic matter abundance of the lower segment in Lower Crataceous Bayingebi Formation is the highest and the thermal evolution is at the mature stage, the organic matter type isⅡ2—Ⅲ, so it should be the main hydrocarbon source rock of Tiancao sag; The organic matter abundance of Lower Crataceous Suhongtu Formation is higher, whose organic matter type isⅡ2, while its hydrocarbon source rock maturity is not high, therefore, it is the secondary hydrocarbon source rock of Tiancao sag. Although the total organic carbon content of Carboniferous and Jurassic is high, the hydrocarbon generating potential is low and the organic matter type is only Ⅲ, nowadays R O>3.0%, so it is at the overmature stage and belongs to the depletive source rock.
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