ANATOMY OF GROWTH FAULT ZONES IN POORLY LITHIFIED SANDSTONES AND SHALES - IMPLICATIONS FOR RESERVOIR STUDIES AND SEISMIC INTERPRETATION .1.OUTCROP STUDY

Some normal faults developed in poorly lithified sediments in Miocene- Pliocene deposits of NW Borneo in the vicinity of Brunei display regul ar zonations of deformation bands. For fault displacements of a few me tres the zones of deformation bands extend up to about 10 m into both the hanging wall and footwall. They range from closely spaced anastomo sing seams within or adjacent to the main slip planes, to more widely spaced sub-parallel and parallel seams passing away from the fault zon e. They reduce porosity and permeability, and if the faults are closel y spaced, are likely to impact reservoir production characteristics an d reserve estimates. In cross-section and map view fault zones are com monly 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 com mon characteristics of faults, others are comparatively rare. They hav e significant implications for the interpretation of seismic data.