Trapping capacity of fault zones, downdip Yegua Formation, Texas Gulf Coast basin

Abstract

The homogenization of sediment from shearing forms traps in both the hanging wall and footwall due to capillary pressure differences. The sheared zone associated with large faults can form traps. Sheared zones associated with small faults can compartmentalize reservoirs leading to unreliable estimates of reserves. Measured values of capillary pressure were compared with estimated capillary pressure values from four correlation methods. Based on comparison of estimated pressure with measured pressure, Pittman's method gave the most reliable results for predicting capillary pressure and therefore the height of the oil column. However, all methods for predicting capillary pressure worked well for samples with porosities above 20% and permeabilities above 100 md. Hydrocarbon column heights were then determined from the capillary pressure values. The large sheared zone of the Cities Service Dincans1 (CD1) well, Toro Grande field, Jackson County, has a predicted trapping capacity of up to 1,961 ft of oil at 10,000 ft depth and 35% saturation of oil. Shearing is mostly at the hand specimen scale in the General Crude Kirby Lumber 2 (GCK2) well, East Sour Lake field, Hardin County, which has a maximum trapping capacity of 111 ft. From this study, it is then concluded that a homogenized sheared zone, formed from ductile deformation of sand and shale layers can form a trap to hydrocarbons. Sheared zones may be thick zones, on the order of feet, or smaller zones at microscopic or hand specimen scale. The larger the sheared zone is, the greater the trapping capacity of the fault.