Granite Rock Mass Identification and Geothermal Well Location Deployment Based on the Wide Field Electromagnetic Method: A Case Study of Sanshui Basin

Abstract

ABSTRACT: Due to the special geological environment, two sets of geothermal reservoirs of Buxin Formation with lithology of sandstone and deep volcanic rocks have been formed in Sanshui Basin. Although a lot of research progress has been made in the geological conditions such as heat source, channel, reservoir and cover layer in this area, the depth and range of deep dry hot rock mass are not clear. Therefore, effective exploration methods are needed to provide technical support for geothermal well deployment and resource assessment. In this paper, based on the principle of wide field electromagnetic method (WFEM), four survey lines are laid to explore the geothermal resources in the study area. Secondly, the underground resistivity data are obtained by the location information of the field source and measurement points on the surface and the observed electric field signals. Finally, the electrical cross-section is interpreted with geological data. The results show that the longitudinal section can be roughly divided into four electrical layers of "low-high-low-high", which are Paleogene, Cretaceous, Permian-Carboniferous, and granite basement. According to the distribution of granite, the range of deep granite body with high resistance has been delineated, which provides a prerequisite for the geothermal well deployment. 1. INTRODUCTION The special geological environment of Sanshui Basin leads to the formation and enrichment of CO2 gas, which indicates that there may be deep geothermal resource potential in this area. At present, oil and gas drilling is mainly concentrated in the depth range of 700∼1300 m (Chen, et al., 2008). The drilling temperature is not high, but the geothermal gradient is about 3.1 °C/100m. Based on the geothermal gradient, it is inferred that there may be carbonate or igneous bedrock reservoirs in the bedrock uplift structure of the deep concave area to form the accumulation of medium-high temperature geothermal resources. From the perspective of sedimentary evolution, there are two sets of geothermal reservoirs in the Sanshui Basin. The first set of reservoirs are limestone and dolomite deposited in Carboniferous-Permian-Triassic period, and the second set of reservoirs are sandstone deposited in Buxin Formation and rhyolite and trachyte formed by volcanic eruption. (Chen, 2018). At the same time, the mudstone of Buxin Formation provides good capping conditions for the sandstone reservoir, volcanic rock reservoir and limestone reservoir Triassic-Carboniferous. The development of faults in this area provides a good migration channel for deep geothermal fluid and heat flow, so the sedimentary evolution and fault development characteristics determine the vertical development of two sets of deep geothermal reservoir-cap assemblage systems.