Thermophysical rock properties of the crystalline Gonghe Basin Complex (Northeastern Qinghai–Tibet-Plateau, China) basement rocks

Abstract

The basement of the Gonghe Basin complex (GBC) mainly consists of plutonic rocks, which, in general are suitable for geothermal applications. Knowledge of the rock properties of the deep basement formations is of fundamental importance for unconventional geothermal applications such as enhanced geothermal systems. An outcrop analogue study at the margin of the GBC was conducted to improve the understanding of the petrophysical rock properties and enhance the data availability for numeric simulation and resource assessment approaches. In total 148 samples were derived from 21 sampling locations at the margin of the GBC area and mountain ranges within. Lithologically, the sample set was divided in three sample types: (1) syenogranite, (2) granite and biotite granite, (3) granodiorite. Petrophysical properties such as grain density, bulk density, porosity, intrinsic matrix permeability, compressional and shear wave velocities as well as thermal properties like thermal conductivity and thermal diffusivity were analyzed on oven-dry specimens under laboratory conditions (ambient temperature, atmospheric pressure). Unconfined compressive strength was additionally measured on selected samples. The resulting dataset shows averaged bulk densities ranging between 2.59 and 2.73 g cm−3 and porosities from 0.2 to 1.7%. Matrix permeability is lower than 1 × 10–18 m2. Averaged thermal conductivity ranges from 2.34 to 3.19 W m−1 K−1, compressional wave velocity from 3.6 to 6.2 km s−1 and unconfined compressive strength from 128 to 241 MPa. Petrophysical data are correlated with mineral content and grain size to show the influence of petrography on petrophysical properties. Although the petrophysical rock properties were analyzed at laboratory conditions and therefore deviate from in situ properties at reservoir conditions, the presented dataset enhances the knowledge of petrophysical rock properties within the study area for further geothermal applications. A first prediction of in situ reservoir conditions was performed on laboratory data based on empirically determined pressure and temperature dependencies of thermal conductivity, thermal diffusivity, specific heat capacity and compressional wave velocity.