Along-Strike Variations of Fault Zone Architecture and Deformation Mechanisms of Normal Faults in Poorly Lithified Sediments, Miri (Malaysia)

Abstract

Shale smearing has long been recognized as one of the key mechanisms for membrane fault sealing. The study site in Miri (Malaysia) offers unprecedented exposure of a composite shale smear fault architecture. We use this site as a natural laboratory to investigate the linkage of multiple smaller smears into a composite smear and to quantify the risk of discontinuities in the smear, allowing potential leakage across the fault.

The extensive along-strike exposure permits analysis of detailed along-strike variation in fault structure and fault rock properties, and the impact this variation can have on hydraulic properties. The outcrop contains a normal fault trending ENE-WSW, offset can not be determined, but the thickness of the hanging wall offers a minimum constraint of 20 m.

The damage zone of the major fault is characterised by deformation bands, zones of shear and gentle folding. The damage zone also contains fractures that postdate the faulting. The fault core is composed of dark grey, foliated clay. The foliation is marked by white sandstone lenses up to 40 cm long embedded in the matrix and elongated sub-parallel to the fault core edges. Clay smears are incorporated into the fault core through folding and shearing, in a stair-stepping, or telescoping geometry. The continuity within the fault zone of a single clay smear originating from a 20-30 cm thick footwall bed can be traced up to 2.5 m down-dip and up to 7 m along-strike.

Microstructural analysis of the samples collected in Miri show particulate flow as the dominant deformation mechanism, combined with minor cataclasis, pressure-solution and growth of authigenic clays. Both the secondary shear zones and the fault core are dominated by compositional banding driven by grain rotation and rearrangement, while there is little evidence of mixing at the grain scale.

Across the length of the exposure there are considerable fault core thickness variations over short distances, while throw variation is deemed much smaller. The fault core thickness variation is influenced by stratigraphic changes (bed composition and thickness), by fault wall irregularities and by secondary shears. Eight areas that could represent potential cross-fault pathways have been identified over the 56 m of exposed fault. Predicting the occurrence of these discontinuities in the shale smears is key to predicting the upscaled permeability and sealing characteristics of the fault zone.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018