Organic geochemical characteristics and hydrocarbon generation potential of Jurassic shales and mudstones in Qixiang Co area southern Qiangtang Basin, Tibet

The Qixiang Co area is located in the South Qiangtang Depression Zone. Jurassic shales and mudstones with large thicknesses are widely developed in this area. The Jurassic was a potential exploration horizon in the Qiangtang Basin; however, few detailed analyses have been conducted on the geochemical characteristics of mudstones and shales in this area. In this paper, the hydrocarbon generation potential and sedimentary environment of shales and mudstones are analyzed using organic geochemical data. The results show that the abundance of organic matter is low, as indicated by the low TOC value (averaging 0.70%) and hydrocarbon potential (S-1+S-2) (0.13 mg/g). The main macerals of kemgen are exinites with an average value of 82%, and they are mainly type II2 kerogen. A summary of the results is presented: the thermal maturity of organic matter is high; the average Ro value is more than 1.2%; n-alkanes are dominated by a high carbon number and have no obvious odd-even advantage; the average value of Pr/Ph is 0.74; the distribution of C-23 triterpenoids satisfies C-20 < C-21 < C-23; the content of the gammacerane index is moderate (0.21) the variation range is small; and the C-27 content in regular steranes is dominant. It can be concluded that the organic matter precursors in this area were a mix of higher plants and marine phytoplankton. Organic matter was deposited under a reductive environment with stable water and slightly high salinity. The hydrocarbon generation potential of the Jurassic shale and mudstone in this area is poor, which is unfavorable to oil and gas exploration.