Present-day geothermal regime of the Jizhong depression in Bohai Bay basin, East China

As a second major tectonic unit of the Bohai Bay basin, the Jizhong depression is a very typical area in China with enrichment of buried-hill hydrocarbon reservoirs. Based on 117 borehole temperature data and 45 measured thermal conductivity data, we studied present-day geothermal characteristics of the Jizhong depression systematically such as geothermal gradient, terrestrial heat flow, thermal lithosphere thickness and lithospheric thermal structure. The results show that the present-day geothermal gradients at depths 0~3000 m in this region range from 20.8 ℃·km^-1 to 41.0 ℃·km^-1 with a mean of 31.6 ℃·km^-1, lower 1~3 ℃·km^-1 than uncorrected value. The heat flow varies between 48.7~79.7 mW·m^-2 with an average of 59.2 mW·m^-2. Present-day geothermal gradients and heat flow of this depression gradually increase from west (basin margin) to east (basin interior) horizontally. These values in sub-uplift areas of the Jizhong depression are usually higher than those of the sub-depression regions, and their variations correspond to the basement relief very well. Meanwhile, the uplift areas with high heat flow in the central Jizhong depression host amounts of geothermal fields. In addition, we calculated the thickness of present-day thermal lithosphere of study area to be 98~109 km, and its lithospheric thermal structure is a “cold crust and hot mantle” type now. This study has an important guiding significance for hydrocarbon exploration and geothermal energy exploitation in the Jizhong depression, and provides new insights for deep lithosphere research (especially for the scientific problem-North China Craton destruction).