Abstract

Highly variable porosity and lithology within Hunton Group Limestones contributes to highly heterogeneous production. Apart from depositional circumstances, variable rock and petrophysical properties may arise from diagenetic and structural phenomena related to karsting and resulting collapse breccia. To investigate this effect, two horizontally bedded outcrops of the non-brecciated Chimneyhill Subgroup in Southern Oklahoma were studied for natural fracture orientation, aperture, and spacing. In the outcrops, three to four stratabound macrofracture (opening displacement > 0.05 mm) sets, i.e., N-S (175° azimuth), E-W (95° azimuth), NW-SE (300–320° azimuth), and NE-SW (40–70° azimuth), were identified. Fracture aperture and spacing generally exhibit a characteristic, i.e., exponential and lognormal, distribution. For comparison, we described a partly-brecciated core of the Chimneyhill Subgroup of the Hunton Group from the Pottawatomie County in Oklahoma. The core was used for petrographic analysis and measuring fracture aperture, intensity, porosity, and permeability in both the brecciated and non-brecciated portions. In the core, fractures occur strictly in the brecciated zones. The brecciated zones show high rebound-hardness values and have relatively high permeability and porosity values compared to the non-brecciated portions. Fracture intensity is higher in the brecciated part of the core compared to that in the outcrop. The average fracture aperture in the brecciated part of the core is lower compared that in the outcrops. Understanding fractures may help to explain hydrocarbon production trends from the Hunton Group wells.