Small Faults as Barriers to Permeability in Sandstone Gas Reservoirs, Pattani Basin, Thailand

A. S. Trevena

A set of small conjugate faults in a sandstone conventional core from a Pattani basin, Thailand, gas well has reduced the horizontal permeability of the rock by about 45%. The faults recovered in core have displacements of several millimeters to several centimeters, and the associated deformation zones are 1.2-9 mm wide. Fault traces consist of zones of disturbed sandstone that contain fragmented sand grains, collapsed pores, and abundant admixed detrital clay, silt, carbonaceous debris, and micas. Sandstone within the disturbed zone is finer grained, more poorly sorted, and contains smaller pores than sandstone outside the fault zones. Sedimentary textures indicate that comminution and rotation of quartz and feldspar grains have taken place along the small faults. Locall , authigenic kaolinite fills fractures within deformation zones. Similar faults of somewhat larger displacement may act as barriers to permeability within Pattani basin gas reservoirs, a contention supported by the rapid pressure decline and early depletion of some reservoirs. Permeability impairment has been associated with small faults in sandstones from other areas, most notably those of the Simpson Group and the New Red sandstone. In some Pattani basin gas fields, major trap-forming normal faults (those having displacements greater than 10 m) are closely spaced. As many as eight faults per mile are present in some areas. The spacing of small seismically unresolvable faults may be even closer in some areas, perhaps attaining tens of faults per mile. Some of these small faults may be p rmeability barriers, and their presence may explain the small size of some Pattani basin gas reservoirs.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.