Fissility analysis in Vaca Muerta Formation (Neuquén Basin, Argentina) from well core and log data

Abstract

Fissility is commonly defined in outcrops as the ability to split parallel to the stratification across relatively smooth, finely spaced surfaces. This structural property is revealed by rock decompression and weathering through the parting along weakness planes. The present study analyzes this property in subsurface rocks by means of the DAD index, a semi-quantitative methodology, applied in well cores. Different degrees of rock fissility are analyzed in terms of composition and fabric using petrographic thin sections, X-Ray Diffraction, Total Organic Carbon (TOC) together with well log data. Finally, petrophysic and geomechanical laboratory tests are included in the study to describe the impact of fissility in the geomechanical behavior of the rocks. Moreover, a predictive model built to reproduce this index by means of cluster analysis from well logs is presented.The main controls on fissility development are closely linked to syn-sedimentary conditions (promoting primary fissility) and to post-depositional processes (developing secondary fissility). Hence, it is possible to analyze and, ultimately, predict this property from a sequence stratigraphy point of view, from the parasequence to several orders of transgressive-regressive cycles scales. Mudstones display the highest fissility at the parasequence base and near the maximum flooding surface, becoming less fissile or massive towards the top of the parasequences and at the end of the regressive hemicycles. Highly fissile mudstones tend to be clay and organic matter-rich, moderately to highly laminated and carbonate-poor, in opposition to massive rocks, characterized by high carbonate content, low clay and organic matter content and occasionally, very bioturbated. Highly fissile rocks display systematically higher porosity and permeability values and lower density measurements than their massive counterparts. The geomechanical laboratory tests indicate a clear positive correlation between fissility, anisotropy and rock weakness. Consequently, this property might have a strong impact on the hydraulic fracture efficiency.