Mesozoic and Cenozoic thermal history and source rock thermal evolution of the Baiyinchagan sag, Erlian Basin, Northern China

Abstract

Abstract The hydrocarbon-bearing Baiyinchagan sag is located in the west of the Erlian Basin in the Inner Mongolia, Northern China. Its potential source rocks include the Lower Cretaceous Aershan, Tenggeer and Duhongmu 1 Formations. The former two formations are major source rocks. They are dominated by dark mudstone with the largest thickness of 560-600 m, and mainly distributed in the western sub-sag. In this study, temperature data from three wells used to calculate the present-day geothermal gradient in the sag, and used 144 vitrinite reflectance measurements from 35 wells together with seven apatite fission track data from seven wells to reconstruct the Mesozoic and Cenozoic thermal history. The results show that the present-day geothermal gradient is 35.1 °C/km. In the Early Cretaceous, the geothermal gradient was 40.0–42.1 °C/km during the early deposition of the Aershan Formation (135–110 Ma), and then increased to 49.9–56.4 °C/km at the end deposition of the Saihantala Formation (100–95 Ma). The geothermal gradient decreased to a present-day value of 32.0–35.4 °C/km. Using this model of thermal history, combined with the source rock geochemistry, the maturation histories of three source rock intervals, including the Aershan, Tenggeer and Duhongmu 1 Formations, were modeled. The modeled results suggest that source rock maturation was controlled by palaeo geothermal gradient, and that source rocks in the eastern sub-sag have not reached hydrocarbon generation threshold (0.5% R o ). In the western sub-sag, Aershan Formation source rocks reached a high mature (1.0% o o o n possesses the least hydrocarbon generati o n potential (0.5% o