Mesozoic and Cenozoic tectono-thermal evolution history in the Chagan Sag,Inner Mongolia

The Chagan sag has the greatest oil and gas exploration potential among the sags in the Yingen-Ejinaqi Basin, Inner Mongolia. To reveal the tectono-thermal evolution history of the Chagan sag, this article utilized a combination of forward and inversion methods on the basis of 7 apatite fission track and 119 vitrinite reflectance data to reconstruct the Mesozoic and Cenozoic tectono-ihermal evolution history in the Chagan sag. The results show that the Chagan sag had a high geothermal gradient during the Cretaceous, and it experienced the following 4 stages of tectono-thermal evolutions: (1) a rapid geothermal gradient increase stage from the Bayingebi Formation depositional period to the Suhongtu Formation. depositional period, during which the geothermal gradient increased to 46 similar to 52 degrees C/km at the end of the Suhongtu Formation depositional period; (2) a geothermal gradient peak stage during the Yingen Formation depositional period, with maximum geothermal gradient ranged from 50 to 58 degrees C/km; (3) a high geothermal gradient continuation stage during the Wulansuhai Formation depositional period, with maximum geothermal gradient ranged from 39 to 48 degrees C/km; and (4) a thermal subsidence stage during the Cenozoic, during which the Chagan sag is in the uplift and erosion stage due to the Himalayan movement and the geothermal gradient gradually decreased to 31 similar to 34 degrees C/km at the present day. Moreover, the tectono-thermal evolution was matched with the tectonic evolution and volcanic activities in the Chagan sag. During the Early Cretaceous, the intraplate rift was developed, lithosphere was thinned and multi-phase intense volcanoes were erupted in the Chagan sag, so that a large amount of energy was released to the surface from the deep crust, resulting in a high geothermal gradient during this period. In addition, the high geothermal gradient during the Cretaceous was favorable for hydrocarbon generation, corresponding to a shallow paleo-generation threshold, and the Early Cretaceous geothermal fields controlled the hydrocarbon generation of the Chagan sag. This work may provide new insights for the understanding of the oil and gas exploration potential of the Chagan sag.