Environment and controlling factors of the Miocene marine source rocks in the Yinggehai Basin

Wells encountering the Miocene marine source rocks in the Yinggehai Basin are mostly confined to the edge and slope areas, leaving the overall forming environment and distribution of these high-quality source rocks in the basin a puzzle to be cracked. This paper, from the perspective of biogeochemistry and environmental geochemistry, reveals the main controlling factors of the formation and distribution of the source rocks through a systematical analysis of paleoproductivity, terrestrial organic matter input and redox conditions of the rocks in the basin. The P/Ti values of the source rocks indicate relatively low paleo-productivity, with only that of the Ledong area showing an increasing trend. The supply of terrestrial organic matter is relatively sufficient because of several rivers around the basin. Oleanane derived from angiosperms is widely occurring in the source rocks, especially those in the Sanya Formation. The Ni/Co ratios of the rocks from the Yingbei, Yingdong Slope, Dongfang and Ledong areas indicate an oxic environment with the redox conditions in the Ledong area better than the rest. The relationship between TOC values and P/Ti and Ni/Co ratios shows that the paleo-productivity and redox conditions only control the formation of the source rocks from L30-1 wellblock in the Ledong area. TOC values of the source rocks have a positive correlation with the oleanane index (oleanane/C₃₀ hopane), showing that terrestrial organic matter input has a significant control effect on the development of marine source rocks. The H29-1 wellblock of the Yingbei area, provided with sufficient supply of terrestrial organic matter from both Hainan Island and the Red River, serves as a perfect location for enrichment of high orgnic marine source rocks. While the D29-2 and L30-1 wellblocks with fine paleo-productivity and redox conditions are relatively good for the formation of endogenous high-quality source rocks.