Counterintuitive fracturing in a multilayer of more or less competent rocks: Examples in porous carbonates and metamorphic rocks, and explanation with numerical modelling

Characterizing and modelling geometrical and topological characteristics of fracture networks, both in fault zones and in the less-fractured background, is essential for the analysis and modelling of mechanical and hydraulic properties of rock masses (i.e. rock plus fractures). Here we present evidence of a counterintuitive behavior in mechanically layered sequences of different kinds of rocks, from porous carbonates to metamorphic rocks. In our case studies the more intense fracturing, both in terms of fracture density/intensity and number of fracture sets, is observed in the more competent layers, that can be therefore considered the more permeable ones.

In large outcrops of the Island of Gozo (Malta) we have characterized several damage zones in the Lower Globigerina Member (LGM) and Lower Coralline Limestone (LCL). A complete petrophysical and geomechanical characterization of these rocks shows the following properties (LGM vs. LCL): porosity 33% vs. 23%; Young’s modulus 2.4GPa vs. 5.5GPa; Poisson’s coefficient 0.18 vs. 0.15; UCS 14MPa vs. 36MPa; tensile strength 2.3MPa vs. 4.4MPa. Despite the LGM being by far the “softer” mechanical layer, we see that the thickness of the damage zone is about 1/30 in this unit with respect to the LCL, and that, comparing sections at the same distance to the fault core, fracture intensity is about 1/10.

In large outcrops in the Breuil-Cervinia area, at the foots of the Italian side of the Cervino-Matterhorn, we have observed a strikingly similar situation in uniformly fractured rocks (no major fault here) of the Dent Blanche and Combin Nappes. These are, in order of decreasing competence, greenschist facies (possibly formerly blueschist) meta-gabbros and meta-granitoids (Dent Blanche), and prasinites and calcschists (Combin). As in the Gozo case study, our quantitative characterization of fracturing reveals an inverse correlation between competence and fracturing parameters.

To understand the physics behind these observations, we have performed simulations with a geomechanical finite element code. During horizontal extension of a multilayer with variable elastic properties, deviatoric stresses build up much more quickly in less compliant, stiffer rocks. This is because all the different layers are subject to the same strain (horizontal stretching), and stress is controlled by the elastic moduli, resulting in higher deviatoric stresses in more rigid layers. At some point, brittle failure (simulated as plastic yield in continuous FEM codes) takes place in the stiff layers, well in advance with respect to failure in the soft ones. At this point, the simulation reveals a situation where fracturing is confined in the stiff layers. As horizontal stretching continues, failure can occur also in the soft layers, but always in a more limited way.

Even if in the Cervino-Matterhorn case study also pressure solution should have played a role in inhibiting fracturing in calcschists, we feel that this mechanical behavior, observed in very different tectonic environments and lithological units, can be of general relevance and might result in a reevaluation of paradigms used to predict fracturing and hydraulic properties of mechanically layered reservoirs in general.