Calcite-cement precipitation mediated by cataclastic shear bands in arkosic sand: petrophysical and mechanical considerations

Porous sandstones are important reservoirs for geofluids. In this kind of granular materials, the interplay between deformation localization and cementation is critical since both processes can strongly reduce rock permeability thus, baffling the fluid flow. We study cataclastic shear bands (CSB), in a high-porosity sandstone unit (Loiano Fm. - LS) exposed in the Northern Apennines that presents a spatially heterogeneous carbonate cementation. The Middle Eocene LS are proximal turbidite deposits of poorly consolidated and immature sandstones. CSB localize carbonate diagenetic structures in the form of isolated spheroidal or irregular-shape nodules and tabular concretions evident thanks to differential erodibility respect to the poorly cemented host rock (Hr). The primary research questions that we address in this work are: (1) How did the CSB localize the cementation? (2) How did the CSB and cementation affect the Hr in terms of petrophysical and mechanical properties? Answering these questions is critical to assess the relationships among structures, cementation, and fluid flow. Combining optical and electron microscope analysis with digital image processing, we assessed that the LS are medium-grained and mostly made-up by Quartz (Qz: 49-60%) and Feldspars (Fsp: 39-48%) with a few rock fragments and minor accessory. Manual point counting validated this composition. Thus, LS can be classified as an arkose. Hr total 2D optical porosity is between 20 and 24%; in the cement nodules, total porosity is 0.70- 1.30% and skeleton porosity is 20-23%. Grain size and porosity reduction start affecting Hr porosity approaching the CSB. The skeleton porosity reduces to 4-8% in the CSB. The Fsp grains within the CSB show a grain-size reduction mainly by intragranular fracturing preferentially oriented along cleavage planes. Qz grains, on the other hand, do not show any preferential fracture direction. Thus, Fsp grains tend to preserve/increase their aspect ratio (roundness shows opposite trend) in the CSB. Fsp grains contribution to the increase in fine grains content within the CSB prevails, although the Hr grain size distribution is similar for Qz and Fsp. Analysis of grain angle in the CSB, shows that some grains, mostly Fsp, tend to align with the shear direction. Rock compressive strength [MPa] evaluated with the Schmidt hammer shows higher value for cement nodules [61] with respect to Hr without nodules [24]. This is mostly caused by cementation. The cataclasis in the CSB affects flow and may have been critical in focusing cementation. Different scenarios are possible; we think, however, that the carbonate cement precipitation occurred where the fluid flow slows down (low permeability and advective velocity) and where a major grain surface area is available (fine grain-size). These two conditions occur in the CSB due to cataclasis development. CSBs, therefore, could be a place of preferential nucleation of the carbonate nodules as observed in the field.