Pore system evolution in arenaceous regoliths - Case study from the Sila Massif (southern Italy)

Abstract

Porous and permeable crystalline rocks have the potential of being hydrocarbon reservoirs. In many cases, reservoir properties are created by rock-altering physico-chemical processes occurring in the near-surface environment. Hydrocarbon reservoirs in altered granitoids may comprise both arenaceous (clay-poor) and argillaceous (clay-rich) palaeoregoliths, and hence outcrop analogues from various environments are required. In the Sila Massif, southern Italy, the arenaceous weathering of medium to coarse grained granodioritic rocks has been studied. The pore system, dominated by microfractures, was described and quantified at each stage in the weathering process (altered coherent rock facies → saprock facies→ lower and upper saprolite facies → soil) in relation to changes in chemical and physical properties. The transformation of rock to saprolite was close to an isochemical process, as demonstrated by the low variations in the bulk mass transfer coefficient (τbulk). At the same time, trans-, intra- and intermineral microfractures increased in frequency and aperture upwards through the regolith profile. It was the main reason for the reduction of bulk densities (ρb) from 2.66 g/cm3 in the altered coherent rock to 1.88 g/cm3 in the saprolite, corresponding to the formation of up to 30% porosity (φc). The results suggest that a volume increase of >50%, represented by the volumetric strain factor (ε), was the main cause for the transformation of solid crystalline rock to porous saprolite. Anisovolumetric arenaceous regoliths with minimal mass loss, are the result of one of the major weathering processes, notwithstanding that they are much less understood than isovolumetric, anisochemical, argillaceous regoliths. Arenaceous saprolites potentially can have excellent porosity and permeability properties, with open and well-connected microfractures. Reservoir quality of arenaceous saprolites is in stark contrast to the poor permeability observed in some argillaceous saprolites where porosity is dominated by isolated dissolution voids within a clay-rich matrix.