Geochemical Characteristics and Distribution of the Subsalt Source Rocks in the Santos Basin, Brazil

[Image: see text] The Santos Basin in Brazil is a hot area of oil and gas exploration in recent years, and its subsalt lacustrine mudstones are the main source rock of the basin. However, there is a lack of studies on the source rocks of the subsalt Picarras and Itapema formations, which is not conducive to the accurate evaluation of the source rock characteristics. Based on logging data of 51 wells and geochemical data of 16 wells, this paper makes detailed evaluations of the organic matter abundance, type, maturity, and distribution characteristics of source rocks of the subsalt Picarras Formation and Itapema Formation in the Santos Basin. The results show that the characteristics of source rocks of the Itapema and Picarras formations are similar, both of which have a high abundance of organic matter. The types of organic matter are mainly type I and II(1), and the maturities are in the low-maturity to the high-maturity stage, which meets the standard of good source rocks. The total organic carbon content of the source rocks of the Picarras Formation ranges from 0.4 to 4.0%, much lower than that of the source rocks of the Itapema Formation, 0.8–5.6%. In addition, the hydrogen index average value of the source rocks of the Itapema Formation is 712.8 mg/g TOC, higher than that of the Picarras Formation, 697.5 mg/g TOC, both revealing a great hydrocarbon potential. The quality of source rocks of the Itapema Formation is better than that of the Picarras Formation. The two sets of source rocks have great hydrocarbon generation potential and are mainly developed in the eastern and western sags of the central depression. Therefore, the surrounding uplift areas will be the target for further oil and gas exploration.