Processes controlling the hydrochemical composition of geothermal fluids in the sandstone and dolostone reservoirs beneath the sedimentary basin in north China

Abstract

The Tianjin and Kaifeng geothermal areas are typical sedimentary geothermal systems located in North China Plain. These systems have great potential for geothermal development because the main reservoirs comprise dolomite and sandstone lithologies, distribute widely and have thick aquifers. This study investigated the water-mineral reaction processes that determine the composition of the thermal fluids in the dolomite and sandstone reservoirs. The results indicate that the reservoirs in Kaifeng and Tianjin are mid-low geothermal systems, with the highest reservoir temperatures 76.88 °C and 107.2 °C, respectively. The Kaifeng Neogene sandstone reservoirs could be treated as conductive geothermal systems, while the Mesoproterozoic dolomite reservoirs in the Tianjin geothermal area may be convective geothermal systems. Also, we conclude that the geothermal waters in the Tianjin and Kaifeng geothermal areas mainly originate from meteoric water, and isochemical dissolution of rocks is the main hydrochemical process in the dolomite or sandstone reservoirs, even though partial equilibrium existed in some cases. In the sandstone reservoirs, the K–Mg ion exchange system equilibrates more easily. However, the Ca–Mg equilibrium, determined by calcite-dolomite and chalcedony equilibria, seems common in the dolomite reservoirs. Additionally, in Kaifeng, apart from rocks dissolution, Ca and Mg are exchanged by Na adsorbed in the clay minerals, resulting in Na enrichment of the thermal fluids. However, in Tianjin's reservoirs, both alteration of Na-bearing silicate mineral and halite dissolution are the main sources of Na, although the Na-bearing minerals are rare in the dolomite reservoirs. Unlike Kaifeng, Ca and Mg in the geothermal fluids of Tianjin are primarily from the dissolution of calcite, dolomite, gypsum and fluorite. These minerals are also the main sources of F and SO4 in geothermal water, but the fluorite dissolution equilibrium is attained more quickly.