Microstructural, petrophysical, and mechanical properties of compactive shear bands associated to calcite cement concretions in arkose sandstone

摘要

Diagenetic concretions are common in sandstones, but little is known about a possible structural control on such features, and even less so in arkose. We studied compactive shear bands (CSB) with cataclasis in high-porosity arkose associated with spatially heterogeneous carbonate cementation. We investigated the mineralogy, microstructures, grain-size and grain-shape, porosity and pore-size, for both host rock and CSBs. We documented the relationship between CSBs and concretions and quantified the effect of both deformation and cementation on petrophysical and mechanical properties of the assemblage bands-nodule with respect to the pristine rock. Microstructural observations on this particular mineralogy allowed us to identify different fracturing mechanism that affect feldspar (intragranular fracturing) and quartz (spalling/flaking of edges). Cleavage-facilitated fracturing of feldspar results in i) preferential cataclasis of feldspar with respect to quartz, and ii) cleavage control on cataclasis, grain-shape and grain-organization within the CSB. The spatial association between concretions and CSBs suggests that CSBs played a role in cement precipitation. The combined effect of deformation and cementation on mechanical properties is reflected on the rock-strength measurements that show a rock strengthening in the assemblage bands-nodule. Our findings represent new insights into the deformation mechanism in arkose, the relationship between structures and diagenesis as well as their effect on petrophysical and geomechanic characteristics.