Outcrop-scale fracture analysis and seismic modelling of a basin-bounding normal fault in platform carbonates, central Italy

Abstract

Faults are characterized by a complex internal architecture. In carbonates, the geometry, attitude, and distribution of fault-related fractures and subsidiary faults can largely affect the petrophysical properties and hydraulic behavior of the fault zone. This work investigates the footwall damage zone of a seismic-scale normal fault (throw ∼ 300 m) from a structural, petrophysical and seismic point of view. The studied Venere Fault (VF) bounds the intra-mountain Fucino Basin (central Italy) and crosscuts Lower Cretaceous platform carbonates. A significant portion of the footwall VF damage zone (VF-DZ) is well exposed in the 400 × 200 m Santilli Quarry. There, we assess the amount of outcrop-scale fracture porosity and permeability by in-situ fracture analyses and permeability measurements. The results show a composite power-law decay of fracture intensity away from the main slip surfaces, strongly influenced by subsidiary faults. An outcrop-based, digital 2D model of the VF-DZ is constructed and populated with acoustic properties (Vp, Vs and density) derived from both the matrix and fracture porosities. This model is enlarged five times and used for seismic modelling to investigate the seismic signature of the VF-DZ under different but realistic geological and geophysical conditions. Seismic modelling suggests that within the modelled damage zone and for wave frequencies of 20–40 Hz, seismic impedance contrasts associated with subsidiary faults may be imaged, depending on the degree of fracture porosity, fracture aperture, and the illumination angle (a measure of the maximum dip that can be imaged), the last two parameters being controlled by overburden depth. These results have implications for the seismic interpretation and characterization of fault zones in carbonates, and hence for the evaluation of fluid migration through these structures.