Abstract

Ellenburger hydrocarbon reservoirs produce from fractured and brecciated carbonate strata resulting from the collapse of extensive cavern systems formed during the middle Ordovician. Because similar fractured and brecciated carbonates are exposed in the El Paso Group in the Franklin Mountains, this study uses the outcrop as an analog in order to discuss optimal exploration and development strategies.

Both the outcrop and subsurface carbonates contain little matrix porosity. In the subsurface, permeability is confined to fracture, mosaic breccia, and clast-supported chaotic breccia. The principal pore type found in the outcrop is fracture and mosaic breccia; the collapse (chaotic) breccias are generally matrix filled and contain little permeability. Fracture and mosaic porosity is commonly found in two locations relative to the collapse breccia: (1) at the top of the El Paso Group and in the lower formation of the Montoya Group (Upham) overlying the brecciated El Paso Group and (2) adjacent to and within lower cave breccias of the El Paso Group. From the outcrop model, these are optimal locations for exploiting Ellenburger hydrocarbon reservoirs.

The upper cave breccias are laterally discontinuous and have unbrecciated pillars. Brecciated areas are 50 ft lower structurally than unbrecciated areas. The lower cave breccias are both circular and linear, extend 1,000 ft below the El Paso Group upper surface, and have 200 ft of structural offset at this surface. Subsurface structural maps typically interpret the offset as faulting. Drilling near these interpreted faults would be a good strategy if fracturing and brecciation associated with faulting were the reservoir model. However, the outcrop model suggests that the interpreted faults are actually major cave collapse features having little or no permeability. The outcrop model also suggests that the structurally highest areas may be unbrecciated pillars having little reservoir-quality rock. Therefore, an optimal drilling strategy might be to locate wells away from the faults but not on the structural highs.