Petrophysical characterization and thermal conductivity prediction of serpentinized peridotites

Abstract

SUMMARY We investigated 22 different serpentinization level peridotite samples from the North Pyrenean Zone (NPZ) and along the North Pyrenean Fault (NPF) to characterize their mineralogical composition and petrophysical properties (bulk and grain density, porosity, compressional and shear wave velocities, and thermal conductivity and diffusivity). Peridotites have a percentage of serpentine that varies between 5 and 97 per cent for fresh samples and totally altered ones, respectively. The mineralogical changes by serpentinization induced a wide variety of petrophysical properties. The key indicators of serpentinization from petrophysical properties in serpentinized peridotite include an increase in porosity of up to 10 per cent and the associated decrease in the solid density ranging from 3.29 g cm–1 for fresh peridotites to 2.5 g cm–3 for the totally serpentinized peridotites. Thermo-physical properties were found to follow the same deceasing trend as the serpentinization increases up to 3000 m s–1 and 1.85 W m–1 K–1 for P-wave velocity and thermal conductivity (TC), respectively. For TC estimation, empirical relationships were produced using multiple linear regression (MLR) and a mineralogy model. For the mineralogy model, we used 10 mixing models commonly used in indirect TC estimation based on mineral composition. For the mean TC of individual mineral phases, the TC estimation shows that in case of non-availability of the proper samples for direct measurement, the TC of peridotite can be inferred with an acceptable level of error from the geometric mean model and harmonic mean models for the dry and saturated conditions, respectively. Based on the results of MLR, the study also confirms that the TC of dry rocks can be predicted through acoustic velocity, density and porosity with a prediction accuracy of ±0.24 W m−1 K−1 and confidence of >80 per cent.