Deformation bands characterization in porous carbonates: a case study from the Matera High (Southern Italy)

Faults and fractures have a crucial role in controlling the permeability in carbonate reservoirs, as they can act as a conduit or barrier for fluid flow. Reservoir-scale outcrop analogue studies provide a useful tool to investigate their spatial distribution and connectivity and to establish the relationships between small-scale structures with larger structures that can be identified in the subsurface.

In this contribution, we describe the preliminary results of a structural study carried out in the Matera's High, South Italy, as an analogue for porous carbonate structures that could be used as CO2 storage fields. Matera High is located on the western side of the Murge region, at the boundary between the Apulian foreland and the foredeep of the southern Apennines thrust belt. It consists of an asymmetrical horst structure involving the Cretaceous carbonates of the Apulian platform (Calcare di Altamura). The Calcare di Altamura is unconformably overlain by Plio-Pleistocene shallow-marine coarse-grained carbonates (Calcarenite di Gravina). The Calcare di Altamura is moderately tilted and is characterised by NW-SE striking normal faults with a throw variable from centimetres to tens of meters. The Cretaceous sequence is also characterised by widespread joints, whose intensity increases approaching faults. The Plio-Pleistocene carbonate succession has very few faults. It is dominated by deformation bands organized into 3 main sets dipping at high angles and striking N-S, NW-SE, and NE-SW. This geological setting allows us to conduct a detailed structural study on an area of about 80 km2, investigating how deformation structures affect the secondary porosity in tight limestone and porous calcarenites. The study was conducted at multiple scales in the field and laboratory and includes (1) geological mapping and structural measurements of faults, fractures and deformation bands; (2) use of linear scan-lines to characterise the deformation bands density across faults; (3) use of photogrammetric techniques to obtain Virtual Outcrop Models (VOMs); (4) development of 3D model based on statistical and topological analysis obtained from scan lines and scan areas in the field and VOMs, (5) petrophysical logging (uniaxial strength, in situ permeability, gamma ray) to highlight the factors that control the formation of the deformation bands, (6) image analysis of blue-resin impregnated thin section and optical cathodoluminescence images, and (7) He-density and Hg-intrusion porosimetry to quantify host rock and deformation bands porosity and pore size distribution.

The preliminary results suggest that the combination of fieldwork, VOMs and laboratory measurements allow the characterization of the deformation bands with more confidence to obtain conceptual and quantitative models about its effects on the fluid flow which can be used for reservoirs characterization for CO2 sequestration.