Evolution of the Pristane/Phytane Ratio with Maturity of Lacustrine Source Rocks: Implications of a Thermal Simulation Experiment

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.