Terrestrial heat flow and crustal thermal structure in the northern slope of Tazhong uplift in Tarim Basin

Abstract

Considerable breakthroughs have been obtained recently in deep and ultra-deep carbonate strata of the northern slope of Tazhong uplift, which is characterized by a burial depth of more than 6300 m and is characterized by high burial temperature. However, the lack of systematic temperature research and effective temperature prediction for the ultra-deep strata in northern slope of the Tazhong has seriously restricted the petroleum exploration in the future. The continuous steady-state temperature logs and well test temperature were obtained, and the thermal conductivity were measured to analyze the current thermal state and deep thermal structure characteristics of the northern slope of Tazhong uplift. The geothermal gradient is generally between 19.4 and 28.5 ℃/km, and the geothermal heat flow is 34.3–59.2 mW/m2 with an average of 45.4 mW/m2, which is typical of low heat flow background of the Precambrian craton basin. In addition, the spatial distributions of the estimated formation temperature at the depth of 6˜8 km are also similar and are mainly controlled by the basement structural pattern. The temperature at the depth of 6000 m is 118˜172 ℃, the temperature at 7000 m is 136˜198 ℃, and the temperature at 8000 m is 190˜224 ℃. The temperature gradually increases from Shunbei to Shunnan area, but the temperature near SN4 fault zone is about 10˜20 °C higher than the surrounding rock. This thermal disturbances is possibly caused by hydrothermal fluids passing through the fracture system from the deep strata to the Ordovician strata, with hot alkaline brine from the Lower Cambrian providing heat source, and fault activities providing power and acting as channels for fluids upwelling. Furthermore, the 1-D theoretical crustal thermal structures of the Shunbei and Shunnan area were constructed by solving steady-state heat conduction equation. The results show that the thickness of the “thermal” lithosphere and the Moho temperature in Shunbei area are 177 km and 580 °C, respectively. However, the thickness of the “thermal” lithosphere in Shunnan area is relatively thin, with an average of 149 km. The heat flow distribution on the northern slope of Tazhong indicates the crustal heat flow contributes more to the surface heat flow and has the typical thermal structure characteristics of “hot crust and cold mantle” model.