Faults and Fractures in Carbonates a report by Emanuele Tondi,1 Fabrizio Agosta,1,2 Andrea Rustichelli1 and Antonino Cilona1 1. Geology Division, School of Science and Technology, University of Camerino; 2. Department of Geological Sciences, University of Basilicata
Worldwide, about 50 % of natural geofluids (i.e. mineral and hydrothermal waters, geothermal fluids, oil and gas) are hosted in carbonate reservoirs.1
consist of a great variety of lithotypes. Based upon the nature and organisation/shape of the constituent elements (grains, pores, cement, minerals, etc.), which are strongly related to the depositional setting and in relation to their diagenetic evolution, carbonate rocks are characterised by a wide range of porosity and permeability.2,3
composition) which control the diagenetic processes.23–25 Unlike other types of rocks, carbonates
The interplay
between boundary conditions and rock rheology is responsible for the development of compressive structures within the rock under lithification. These structures, oriented parallel to bedding, are explained as anti-cracks26
and/or compaction bands (CB).17,27 Besides
the aforementioned primary controls on the petrophysical properties of carbonate rocks, the containment and migration of geofluids in carbonate rocks are strongly influenced by fault zones and fractures.4–6 In the last few years, numerous researches were aimed at understanding the faulting and fracturing processes in carbonate rocks, as well as the quantification of their spatial and dimensional distributions in relation to the physical-chemical properties of the various types of carbonates.7–15 Following these lines of research, the Faults and Fractures in Carbonates (F&FC) project of the University of Camerino, currently sponsored by a consortium of oil companies, is aimed at significantly improving current knowledge on the role of faults and fractures in the fluid flow properties of carbonate rocks.
In several geological contexts, carbonate rocks are characterised by both diffused and localised strain, which can be represented by background deformation and fault zones, respectively. The combination of diffused and localised deformation may form a discontinuity network that affects the hydraulic rock properties. With regards to background deformation, the primary control on fracture types, spacing and connectivity is provided by the skeletal grain assemblage, heterogeneities such as bedding or inclusions and lateral/vertical variations in cementation and porosity. For what concerns the fault zones, in low-porosity carbonate
rocks they form preferential sites for fluid migration or accumulation, 16,17
whereas they represent barriers to fluid migration in high-porosity
and (iii) faulting of low-porosity carbonates, in which we report a synthesis of our studies focusing on the deformation mechanisms, architecture, multiscale and petrophysical properties of various types of fault zones.16,20–22
Background Deformation
The progressive burial of carbonate sediments, subsequently to deposition, is responsible for the progressive change of the boundary conditions (i.e. stress field, pressure and temperature, fluid
© TOUCH BRIEFINGS 2011
In the present contribution, we group the results of our research activities into three main subjects: (i) background deformation, in which we report an examples of studies aimed at investigating the control exerted by host rock heterogeneity on the development of background structures in layered carbonates;18,19 (ii) faulting of high-porosity carbonates, characterised by a faulting mechanism which determines both volumetric and shear strain localisation into narrow tabular bands that constitute seals for fluid flow;10,11
carbonates.10,11,14
The main results of this research are summarised below. •
and consist on pressure solution seams (PS) Also related to background
deformation, bed-perpendicular (BP) structures such as joints and/or PS are also widespread in layered carbonates.8,12,14,22
The frequency
and distribution of BP structures (i.e. length and spacing) is controlled by bed-parallel mechanical interfaces formed by bed surfaces, CB and PS (see Figures 1A, B and C). In other words, bed-parallel depositional and/or tectonic structures act as surfaces against which the various types of fractures may abut (see Figure 1D).28
For this reason, their
distribution plays a critical role controlling both length and spacing of BP structures.17
form barriers to fluid flow.10,11,29
Most of the overburden-related bed-parallel structures This behaviour is due to the fact that
PS include residual clay, insoluble material, whereas CB are characterised by lower values of porosity with respect to the surrounding host rock.27
In this contribution we report a synthesis of a study conducted in the Maiella Mountain of central Italy, specifically on the Oligo-Miocene skeletal grainstones to the hemipelagic mudstones of the Bolognano Formation. These carbonates were deposited in a non-tropical carbonate ramp pertaining at that time to the Apulian platform realm. With the objective to assess the control exerted by environmental, sedimentological, diagenetic and petrophysical properties on the development of bed-parallel PS and CB (see Figure 2), we integrate stratigraphic, sedimentological and structural field analyses with laboratory tests such as optical microscopy, HCl testing, diffrattometric and digital image analyses).
Under a vertical loading, intergranular pressure solution was strongly enhanced by the presence of small amounts of clay (2–4 % in
Emanuele Tondi is Associate Professor in the Geology Division of the School of Science and Technology of the University of Camerino, Italy. His research activity is mainly focused on the study of brittle deformation and on its applications to solving regional and seismotectonic problems, as well as the recovery of geofluids from the subsurface. Specifically, he has carried out investigations on active tectonics of central and southern Italy, focusing on the seismogenic
potential of active faults and nucleation and growth processes of faults and fractures in carbonate rocks. He is a Co-director of the Faults and Fractures in Carbonates research project funded by a consortium of oil companies.
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Geology
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