Fracture Networks Development, Fluid Flow, and Diagenetic Processes in Sandstones and Carbonate Rocks
Leonardo Del Sole
A Al lm ma a M Ma at te er r S St tu ud di io or ru um m --U Un ni iv ve er rs si it tà à d di i B Bo ol lo og gn na a DOTTORATO DI RICERCA IN All Rights Reserved iii iv ABSTRACT Porous sandstone and carbonate rocks form important geofluid reservoirs. Interaction therein among deformation and diagenesis is critical since both processes can deteriorate or enhance reservoir quality as well as affect its texture and mechanical properties. Deformation can focus diagenetic processes and control the
... istribution of diagenetic products. Diagenetic processes likewise can control deformation characteristics and distribution in a sedimentary sequence. Deciphering the interaction between these processes is thus a critical prerequisite to the evaluation and prediction of the architecture and permeability structure of fault zones as well as their mechanics, to the assessment of reservoir petrophysical properties uncertainties and, ultimately, to the understanding of the structural diagenesis evolution of rock volumes. Whilst this work concerns a common general subject and objective, its content is two-fold. In the first part (Chapters 2 through 4), we have made a comprehensive study of the textural, (micro)structural, petrophysical, and mechanical properties of deformation bands (DBs) associated with calcite nodules in high-porosity arkosic sandstone (Loiano, Northern Apennines -Italy). We investigate the mineralogy, texture, and microstructure of the host rock and DBs via optical and scanning electron microscopy, and digital image analysis to elucidate how sandstone mineralogy control the deformation mechanisms, cataclasis intensity, and the final band structure in arkosic sandstone. Microstructural observations on this particular mineralogy allowed us to identify different fracturing mechanisms 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 DB. Our findings represent new insights into the deformation mechanism in arkosic sandstone. We also aim to quantify how DBs and nodules affect the petrophysical and mechanical properties of the host rock. Porosity and permeability of the host rock and DBs are quantified via digital image analysis and the Kozeny-Carman relation. Air-permeability and uniaxial compressive strength of the host rock, DBs, and nodules are quantified through minipermeametry and the Schmidt hammer. Porosity and permeability in the DB are 1 and 3 orders of magnitude lower than in the host rock, respectively. Porosity drops by 1 order of magnitude from the host rock to the nodules. DBs and nodules degrade porosity and permeability and produce a strengthening effect of the rock volume, imparting a strong petrophysical and mechanical heterogeneity. We used the ground penetrating radar (GPR) to detect and characterize the network of subseismic scale structural and diagenetic heterogeneities (SDH; DBs and nodules). GPR surveys allowed the description of the SDH spatial organization and their geometry in the subsurface. v Different textural, petrophysical, and geomechanical properties between host rock, DBs, and nodules result in different GPR response (relative permittivity, instantaneous attributes). Such response can be thus used to characterize variations in petrophysical and mechanical properties together with the organization and geometry of SDH in the subsurface, in a way to reconstruct realistic and detailed conceptual models of outcrop analogs of faulted aquifers and reservoirs in porous sandstone. Finally, we have focused on the impact of DBs on fluid flow and diagenesis in porous sandstones in two different case studies (Loiano, Italy; Bollène, France) by combining a variety of multiscalar mapping techniques, detailed field and microstructural observations, and stable isotope analysis. We show that DBs buffer and compartmentalize fluid flow and foster and localize diagenesis, recorded by carbonate cement nodules spatially associated with the bands. Our work shows that DBs control flow patterns within a porous sandstone reservoir and this, in turn, affects how diagenetic heterogeneities are distributed. This information is invaluable to assess the uncertainties in reservoir petrophysical properties, especially where structural and diagenetic heterogeneities are below seismic resolution. In the second part (Chapter 5), we have made a thorough study of the distribution heterogeneity of joints concentrated in chert nodules within pelagic limestones (Northern Apennines, Italy), by means of detailed field observations, photo mapping, and 3D geomechanical modelling. The objective of this study is to explain the occurrence and paleo-stress significance of 3D joint clustering in chert nodules (inclusions) within a layered carbonate sequence. The difference in stiffness between chert and limestone is about one order of magnitude. Field observations show that fracture localization occurs mostly in chert nodules as opposed to the limestone matrix. We show with a novel 3D geomechanical modelling analysis how the inclusion (ellipsoid) axes ratio influences fracture intensity and propagation within and outside the chert nodules and how the nodules record different deformation phases under different remote stress conditions. From field observations, we recognize two joint sets in the chert nodules: joints parallel (older) and normal (younger) to the plane containing the two major axes of the nodule (bedding plane). The modelling of the 3D Eshelby solution for the stress field inside the chert nodule and in the surrounding matrix is consistent with our field observations and it suggests a strong differential stress during deformation (σ r min/σ r max < 0.3). Chert nodules in a deformed carbonate sequence, therefore, can provide important clues on the paleo-stress conditions, the temporal sequence of events, and fracture distribution heterogeneity.