Sealing Properties of Tertiary Growth Faults, Texas Gulf Coast

Abstract

Growth faults consist of nonsealing fault surfaces and sealing sheared zones that may occur on either the footwall or hanging wall. The properties of sheared zones are assumed to be identical to those of soft sediment that has undergone ductile deformation during mass movement. In cores, the sheared zones display fabrics similar to Riedel shears and are termed wispy, crenulate, conjugate, and meniscate, 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, initial mercury-injection capillary pressures range from 400 to 550 psia, sufficient to trap an average oil column of 98 m (320 ft). Sheared zones are effective seals because ductile deformation has homogenized the original sediments and resulted in a uniform distribution of small pores. In contrast, the fault surface is a region of extension that is presumed to result in higher permeabilities, low displacement pressures, and the ability to transmit migrating oil and gas from deep source beds to shallow traps. Thus, growth faults can seal in the sheared zone and leak along the fault surface. Sheared zones are distinctive on dip logs. Dips within sheared zones have variable magnitudes and directions, whereas dips adjacent to faults exhibit more uniform patterns resulting from normal drag.