Controlling factors of organic matter enrichment in source rocks in delta-tidal flat environment of Tiantai block, Xihu sag of the East China Sea Basin

The source rocks of Pinghu Formation in Tiantai block of Xihu sag are developed in the marine-continental transitional facies environment, and the distribution of high-quality source rocks is unclear, which limits the exploration deployment. Based on the analysis of pyrolysis, biomarker compounds, and major and trace elements of source rock samples from wells and cores, the abundance and type of organic matter in source rocks were evaluated. Combined with the identification of organic matter source and sedimentary preservation environment, the controlling factors of organic matter enrichment in source rocks were analyzed, and the development model of source rocks was established. Result shows that: The source rocks of the Pinghu Formation are medium-good source rocks as a whole. The organic matter type is mainly type II2-III, and some coal seams are developed. The source rocks are mainly gas-generating and a small amount of oil-generating. The quality of source rocks becomes better from the slope zone to the near-sag zone. The source of organic matter in source rocks is dominated by terrestrial higher plants, followed by aquatic organisms. From the slope zone to the near-sag zone, the contribution of aquatic organisms gradually increases. The ancient sedimentary environment of the source rock of the Pinghu Formation was a humid-semi-arid climate, and the salinity of the water body was fresh water to brackish water. It was an oxidation-weak reduction environment with low primary productivity. The input degree of terrestrial higher plants was the main controlling factor of organic matter enrichment in marine-continental transitional source rocks, while the influence of other factors was not significant. The development model of source rock in Tiantai block was established, and near-sag area is the favorable exploration direction of Tiantai block at present.