Abstract
The fabric of pores in sedimentary rocks around fault zones can be subject to significant modification. Knowledge of how pore fabrics vary during and after faulting is important for understanding how rocks transmit fluids around fault zones, and can help to predict mechanical instability due to changes in pore fluid pressure. Datasets detailing the geometry of pore fabrics in faulted porous rocks are lacking. This paper describes pore fabrics quantified from two outcrops of normally faulted sandstone. The porosity and the size, shape and geometry of pores were quantified from core plugs and thin sections. Results were mapped within a framework of the faults to better illustrate how these datasets may be used to improve understanding of fluid flow around fault zones. Results from a mature, quartz-rich arenite show a change in pore fabric from pores oriented horizontally and parallel to laminations to pores oriented at a low angle to σ1. Pore fabrics quantified from a clay-rich, quartz sub-arkose changed from moderate aspect ratio pores with no preferred orientation, to high aspect ratio pores oriented dominantly sub-parallel to the fault surface. Permeabilities measured on corresponding core plugs showed anisotropy of permeability with maximum permeability oriented down fault dip around both faults.
Highlights
Porosity is a measure of a rock's capacity to store fluid, and is defined as a scalar value equal to the ratio of the void volume to the total rock volume
To consider the role these pore fabrics have on controlling fluid flow around faults, corresponding permeabilities of arenites and arkoses are presented (Section 4.4)
Petrophysical and geometrical analyses have been conducted to examine the relationship between porosity and pore fabrics to fluid flow and permeability anisotropy, on samples collected from two normal fault zone outcrops hosted in a ‘clean’ quartz-rich arenite and a ‘muddy’ quartz sub-arkose
Summary
Porosity is a measure of a rock's capacity to store fluid, and is defined as a scalar value equal to the ratio of the void volume to the total rock volume. In geology, ‘rock fabric’ describes the organisation and geometries of crystals or grains that make up the rock and is commonly quantified to deduce the distribution of pores and interpret rock petrophysical properties such as permeability (Lucia, 1995). Inverting the rock fabric yields summary information about the distribution of pores it fails to describe the structure and spatial organisation of the pore network, termed the ‘pore fabric’ in this study. Previous studies modelling fluid flow have produced large datasets detailing pore networks of fractures (Schild et al, 2000; Takemura et al, 2003) e but little or no pore fabric information has been quantified from volumetrically significant intergranular porosity. In granular rocks like sandstones, where intergranular pores make up 98% of porosity, this must be addressed
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