Abstract

Some normal faults developed in poorly lithified sediments in Miocene-Pliocene deposits of NW Borneo in the vicinity of Brunei display regular zonations of deformation bands. For fault displacements of a few metres the zones of deformation bands extend up to about 10 m into both the hanging wall and footwall. They range from closely spaced anastomosing seams within or adjacent to the main slip planes, to more widely spaced sub-parallel and parallel seams passing away from the fault zone. They reduce porosity and permeability, and if the faults are closely spaced, are likely to impact reservoir production characteristics and reserve estimates. In cross-section and map view fault zones are commonly composed of several important gouge and cataclasis zones, which branch and join, display listric detachments and various types of hard and soft linkage. Some of these geometries have been described as common characteristics of faults, others are comparatively rare. They have significant implications for the interpretation of seismic data.

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