Geothermal effects of salt structures on marine sedimentary basins and implications for hydrocarbon thermal evolution

Abstract

Evaporitic salt is prevalent in marine sedimentary basins, and many discovered hydrocarbon reservoirs are generally associated with salt structures in the world; accordingly salt structures have attracted much attention from academia and industry during the past decade. The Tarim Basin, located in northwest China, is the largest marine sedimentary basin in China with great hydrocarbon resource potential. Previous studies of salt structures in this basin mainly focused on its strong sealing capacity and structural traps created by salt structures. However, besides its extreme impermeability and low viscosity, rock salt has other unique thermal properties, including a large thermal conductivity as high as 5–6 W/(m K), usually 2–3 times greater than that of other common sedimentary rocks, but a relatively low radiogenic heat production. This strong contrast in thermal properties could change the evolving thermal regime and associated thermal history of the source rocks around salt bodies, but it has not been understood well. Herein based on the theoretical models and interpreted salt-bearing seismic profiles from the Kuqa Foreland Basin, northern Tarim Basin, we use 2D finite element numerical experiments to investigate the impacts of salt structures on the basin geothermal regime and associated hydrocarbon thermal evolution. Our results show that, owing to their high efficiency in heat conduction, the salt rocks would result in obviously positive temperature anomalies (3%–13%) above the salt body and negative temperature anomalies (11%–35%) in the subsalt, enhancing and restraining the thermal maturation of source rocks above and below the salt body, respectively. The amplitude and extent of geothermal effects of salt structures depend on the thermal conductivity, geometry, thickness and burial depth of the salt bodies. The thermally affected area around the salt body can be 2 times the salt radius laterally and 2–3 times the salt thickness vertically. Salt structures in the Kuqa Foreland Basin can prominently cool the subsalt formation temperature and accordingly reduce the thermal maturity (Ro) of Jurassic source rocks as much as 18%, enabling the source rocks to stay in the range of gas generation rather than reaching an over-mature stage as expected previously; this situation is favorable for deep hydrocarbon preservation below salt. Because the salt structures in the west and east Kuqa Foreland Basin show strong differences in their thickness, geometric pattern, burial depth and composition, the thermal effects of salt structures on thermal maturation of subsalt source rocks should differ accordingly, which is supported by the observed temporal-spatial variation of Ro for Jurassic source rocks in this basin. Finally, we propose that the geothermal effects of salt structures will be of great importance in the deep hydrocarbon resources potential assessment and exploration in marine sedimentary basins in China.