Geothermal regime and source rock thermal evolution in the Chagan sag, Inner Mongolia, northern China

The Chagan sag has the greatest oil and gas exploration potential among other sags in the Yingen Ejinaqi Basin, Inner Mongolia. The average geothermal gradient in the Chagan sag is 33.6 °C/km, whereas the heat flow ranges from 65.9 mW/m2 to approximately 85.5 mW/m2, with an average value of 74.5 mW/m2. Thermal history reveals that the Chagan sag experienced the following 4 stages of thermal evolutions: (1) a rapidly increasing geothermal gradient stage from the Early Cretaceous Bayingebi Formation depositional period to the Early Cretaceous Suhongtu Formation depositional period; (2) a geothermal gradient peak stage during the Early Cretaceous Yingen Formation depositional period; (3) a high geothermal gradient continuation stage during the Late Cretaceous Wulansuhai Formation depositional period; and (4) a thermal subsidence stage during the Cenozoic. Tectonic subsidence analysis reveals that the area experienced an initial synrift subsidence during the Early Cretaceous followed by a subsequent long-term thermal subsidence since Late Cretaceous. Thermal and tectonic subsidence histories of this area are of great significance to petroleum exploration and hydrocarbon resource assessment because they bear directly on issues of source rock maturation. The maturation histories of the 3 sets of source rocks in the sag were modeled on the basis of the present geothermal field, thermal history and tectonic subsidence history. The results reveal that the hydrocarbon generation of the Chagan sag was controlled by the Early Cretaceous geothermal fields, and the source rock maturity reached the maximum at the end of the Yingen Formation depositional period. Moreover, the maturation evolution degree shows a difference for the 3 sets of source rocks. The source rocks of the Bayingebi 1 and 2 Formations reached the middle mature and dry-gas stage. In contrast, the source rocks of the Suhongtu 1 Formation only reached the early mature and middle mature stage. This work may provide new insights for the understanding of the oil and gas exploration potential of the Chagan sag.