Tectonic control on the petrophysical properties of foredeep sandstone in the Central Apennines, Italy

Abstract

Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by boundary conditions, rock fabric/microstructure, and tectonics that require a multiscale approach to be properly defined. Here we (1) report laboratory measurements of density, porosity, permeability, and P wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Formation); (2) compare the laboratory results with larger-scale geophysical investigations; and (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P wave velocities (Vp) from 1.5 km/s to 2.7 km/s. The P wave velocity increased with confining pressure, reaching between 3.3 km/s and 4.7 km/s at 100 MPa. In situ Vp profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10-15 m2 and 3.9 × 10-15 m2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano-Antrodoco fault plane progressively decreased with distance while P wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65%, and 20% for porosity, permeability, and P wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.