Geochemical Characteristics and Thermal Evolution History Recovery of Cretaceous Source Rocks in Bayanhushu Sag, Hailar Basin

The Lower Cretaceous source rocks in Bayanhushu Sag have good oil generating conditions and great potential for oil and gas resources. This article analyzes the geochemical characteristics and thermal evolution history of Lower Cretaceous source rocks using 290 pyrolysis data and 107 vitrinite reflectance data from 11 drilling wells in the study area. The organic matter types of the Lower Cretaceous source rocks are mainly type I-II kerogen. Thermal history simulation results show that the Cretaceous source rocks in the Bayanhushu Depression have undergone two main thermal evolution stages since sedimentation: a rapid warming stage from the Early Cretaceous to the end of the Early Cretaceous, and a slow cooling stage from the end of the Early Cretaceous to the present day. The Tongbomiao Formation belongs to a medium hydrocarbon source rock, which entered the hydrocarbon generation threshold at around 137 Ma and is currently in the high maturity gas generation stage; The main source rock layer, Nantun Formation, belongs to the good source rock, which entered the hydrocarbon generation threshold at about 135 Ma and is currently in the high maturity gas generation stage; The Damoguaihe Formation is a medium to good source rock, which entered the hydrocarbon generation threshold at around 126 Ma and is currently in the immature to low maturity stage. The second stage source rock of the Damoguaihe Formation has not yet reached the peak of hydrocarbon generation. Thermal history simulation shows that the Bayan Hushu Depression reached its maximum paleotemperature during the late stage of the Yimin Formation sedimentation, with a maximum paleotemperature exceeding 150 ℃. The geothermal evolution has a characteristic of first increasing and then decreasing. Since the deposition of the Tongbomiao Formation, the geothermal gradient has first increased to 47 ℃~55 ℃/km and gradually decreased to the current 37.5 ℃/km. The main hydrocarbon generation period in the Bayan Hushu Depression is 125~105 Ma, with a maximum oil production rate of 70 mg/(g · TOC · Ma) and a maximum gas production rate of 13 mg/(g · TOC · Ma).