This paper identifies formation interfaces that would appear to be “weak interfaces”, in the Jurassic, Early-Cretaceous Vaca Muerta formation, in the Neuquén Basin, Argentina. Significant distribution of the weak interfaces was observed including contacts between volcanic ash layers of varying thicknesses, calcite-filled veins, and contacts between lithology changes, such as between argillaceous mudstone and mottled carbonates. The paper shows mineral and organic matter constituents of selected interfaces and suggests that interface weakness depends on certain composition and morphology of mineral and organic constituents associated with the interfaces. The understanding presented in this paper of the composition of interfaces is a first step toward both identifying the location and frequency of the weak interfaces throughout the formation and ultimately of determining the shear strength of these interfaces. These interfaces have a significant effect on hydraulic fractures used for completion stimulation.
Hydraulic fracturing is required to create large surface area to allow oil or gas to be extracted from unconventional formations, and particularly mudstone reservoirs. Recent work has shown the importance of rock fabric and particularly weak interfaces on hydraulic fracture propagation and geometry (Detournay, 2004; Dargaud et al., 2005; Jeffrey et al., 2009; Suarez-Rivera et al. 2006; Lecampion et al., 2013; Green, 2013). Early analysis and observations suggested that hydraulic fracture growth is affected by weak interfaces and other localized heterogeneities in the formation. They promote the development of complex fracture networks, may restrict the propped fracture surface area in contact with the reservoir, and thus influence the production potential of the play (Warpinski, 1987; Suarez-Rivera, 2011 & 2013; Green, 2013 & 2014).
The weak interfaces as discussed here tend to have low shear strength, and are thus more susceptible to local shear deformation as the fracture intersects the interface during the fracture propagation (Suarez-Rivera, 2013). The propagating fracture can either be arrested at the interface or can reorient and propagate in a different direction along the weak interface or may branch into two fractures (Thiercelin, 2005, Suarez-Rivera et al., 2006 & 2013; Green, 2014).