Advances in the Understanding of Key Parameters Determining Carbonate Fault Rock Permeability

Abstract

Faults have been shown to exert significant control on fluid flow within the subsurface. Considerable amounts of research has been directed towards determining the conditions in which faults act as conduits, barriers or partial barriers to flow in siliciclastic reservoirs. This understanding can help to reduce uncertainty when estimating the hydraulic properties of fault zones in the subsurface. However, limited research has been undertaken on the impact of faults on fluid flow in carbonate reservoirs despite their importance in global hydrocarbon reserves; around 60% of global oil reserves and 40% of global gas reserves are stored in carbonates. To assess cross fault flow potential, and consequent reservoir compartmentalisation, the distribution and petrophysical properties of fault rock within a fault zone must be determined. Fault zone architectural models consist of a localised fault core composed of high strain products exhibiting low permeability (i.e. fault rock). However, the porosity of the protolith lithology has a major control on what kind of fault core is observed; in siliciclastics, it has been well documented that high porosity rocks tend to deform via compaction, resulting in a permeability decrease, whereas low porosity rocks tend to deform in a dilatant manor, resulting in a permeability increase. This is not necessarily true for carbonates, where lithotype and pore type also have controls on the deformation mechanism. Accordingly, this research works towards a predictive method to estimate fault rock production in carbonate rocks based upon key lithological and fault parameters. To this goal, samples of fault rocks in both high and low porosity carbonates from a variety of settings have been studied from both outcrop and core. Fault zone mapping is used to assess the continuity of fault rocks and how their spatial distribution is controlled by fault zone architecture. The deformation mechanisms that form such fault rock fabrics are determined using microstructural analyses, allowing for an investigation into the control of porosity and pore type on deformation style. Combining this knowledge with petrophysical properties derived from the lab, fault juxtaposition evolution models are used to show the impact of carbonate lithotype and fault displacement on fault rock production and the consequent cross fault fluid flow potential. Ultimately aiding the development of a predictive model for cross fault fluid flow in carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017