Evaluation of the geothermal parameters to decipher the thermal structure of the upper crust of the Longmenshan fault zone derived from borehole data

Abstract

The Wenchuan earthquake Fault Scientific Drilling venture (WFSD) offers an interesting possibility for examining the faulting and thermal system of the Yingxiu-Beichuan fault, Longmenshan fault zone, east edge of the Tibetan Plateau. However, the thermal and structural behaviour of the Yingxiu-Beichuan fault at the deeper level within the Sichuan Basin is still unknown. Therefore, the current study aims to delineate the thermal structure of the Yingxiu-Beichuan fault by employing the geothermal parameters along with the statistical concentration number (CN) model, which has been previously neglected. Two accessible wells, WFSD-2 and WFSD-3, with their common gamma-ray (GR) records, were utilized to portray and measure the heat production (HP) and thermal conductivity (TC) extent in the Wenchuan earthquake zone. Besides, fifteen temperature logging curves run along different time period in WSFD-2 was utilized to measure to equilibrium temperature (ET). The ET was used to estimate the thermal gradient (TG). The TC and TG were multiplied to get the heat flow (HF). Consequently, the examination of 17,501 estimated data points in WFSD-2 through geothermal parameters shows that the Yingxiu-Beichuan fault (YBF) corresponds to low TC, low HF, and high HP values. The HP differs between 0.1 and 5.5 μw/m3, TC changes between 0.23 and 4.32 w/mk, and HF ranges amid 45–55 mW/m2. In addition, a multi-fractal approach with the utilization of concentration number (CN) fractal model technique and log-log plots were utilized for describing and separating the six population zones with different intensities using GR, HP, and TC. These six population zones represent the six formations of Xujiahe sedimentary formation and Pengguan complex igneous formation encountered in the WFSD-2. The comparison of geothermal parameters (GR, HP, and TC) with spectral gamma-ray (SGR) (U, Th, and K) also shows the presence of six population ranges. The results obtained by applying the CN model and SGR are in agreement with each other, which shows the reliability of the research work. The corollary, the proposed methodology for the estimation of the thermal structure of faults, can be further utilized to numerous basins worldwide, which have the same geology of sedimentary and igneous rocks.