Analysis of Main Controlling Factors of Natural CO2 Based on 3D Geological Modelling in the Northern Wuerxun Sag, Hailar Basin, China

Natural CO2 was discovered in Nantun Formation in the northern Wuerxun Sag. A better understanding of the main controlling factors of CO2 distribution in this area is necessary. In this paper, a 3D geological model of Nantun Formation is established based on well-logging and seismic interpretation results, constrained by dual facies conditions. The 3D geological model includes structural, lithologic, physical property information and the spatial distribution of magmatic intrusions. This comprehensive model can display the stratigraphic framework, fault distribution and spatial characteristics of sandstone reservoirs and physical properties. Modelling results show that the sandstone reservoirs of Nantun Formation are of low permeability and low porosity, with N1 Member characterized by a higher sandstone content than N2 Member. The CO2 is of inorganic origin degassed from intruded magma, and most CO2 is stored at structural traps in the west of the Surennuoer strike-slip fault zone. There is a spatial relationship among the CO2 distribution, Surennuoer strike-slip fault and magmatic intrusion bodies, indicating a genetic link. Therefore, we propose that the strike-slip fault and magmatic bodies are the main controlling factors of CO2 distribution and that reservoir quality plays a secondary role. The CO2 accumulation model is established showing the CO2 degassed from magmatic intrusion body, migrated upward along deep faults and was stored in shallower structure traps. Generally, this modelling method is helpful to understand the controlling factors of CO2 accumulation, which can be widely applicable in other areas with natural CO2 occurrence.