Characterising the structure and petrophysical properties of deformation band fault cores in reservoir sandstone
Abstract
Deformation band faults in porous sandstone typically have a "core" of intense deformation associated with the localisation of strain onto discrete slip surfaces. In outcrop, the core is recognisable by its glassy appearance and its increased resistance to erosion with respect to the host rock. Samples of fault core have been collected from the San Rafael Swell, Utah, USA (Navajo Sandstone) and the Cheshire Basin, England (Sherwood Sandstone). Outcrop mapping in both localities reveals that the fault core thickness varies from < 3 cm to 30 cm along strike. Thickness variability does not correlate with displacement. The fault core is composed of closely spaced deformation bands and the host rock between these deformation bands is highly crushed. The mean grain size in the fault core is lower than in single deformation bands and there is a greater spread of particle sizes. Quartz grains in the fault core tend to be aligned. Fault core porosity (1-5%) is decreased with respect to the host sandstone (17-22%) and individual deformation bands (7-8%). However, microstructural observations suggest that fluids have moved through these structures during deformation. Cement phases are preferentially located along deformation band boundaries. Clay rims, which are common in the host rock, are absent in the fault core. Additionally preliminary fluid inclusion data show elevated temperatures in healed fault core microfractures. It is essential to characterise the architecture, composition and porosity/permeability of fault core to accurately predict fluid flow parallel and perpendicular to deformation band fault zones.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11122B