BERG, ROBERT R., Texas A&M University, Geology Department, College Station, TX; and ALANA L. HAVEMAN, Texaco, Inc., New Orleans, LA

ABSTRACT: Sealing Properties of Growth Faults, Texas Gulf Coast

Growth faults consist of non-sealing fault planes and sealing shear zones that developed on the moving or hanging wall. The properties of shear zones are assumed to be identical to those of ductile de-formation of soft sediment during mass movement. In cores, the shear zones display fabrics similar to Riedel shears and are termed: wispy, crenulate, crossed, and miniscate, in order of increasing deformation. Permeabilities and porosities range from 0.1 md and 18% to less than 0.01 md and 8%. Based on limited measurements, the mercury-injection capillary-pressures range from 400 to 550 psia, sufficient to trap an average oil column of 320 ft. Normally bedded, non-sheared siltstones have injection pressures that range from 100 to 400 psia and will trap lesser columns that average only 17 ft.

Shear zones are effective seals because ductile deformation homogenized the original sediments, destroyed primary structures, and resulted in small but uniformly distributed pores. In contrast, the fault plane is a region of extension which is presumed to result in higher premeabilities, low displacement pressures, and the ability to transmit migrating oil and gas from deep source beds to shallow traps. Thus, growth faults can seal on the downthrown, sheared wall and leak along the fault plane. An important consequence is that faults without shear zones are onlyleaks. Shear zones are distinctive on dipmeter logs. Dips through shear zones have variable magnitudes and directions whereas dips adjacent to faults exhibit more uniform drag patterns.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.