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The Effect of Karsting on Natural Fracture, Hardness, and Brittleness of the Hunton Limestone and Paleo-Deposition of the Woodford Shale: A Study Using 3-D Seismic, Outcrop, Well Log, and Core Data

Abstract

Much of the oil and gas production in Oklahoma is from carbonates and shales, such as the Viola limestone, Sylvan Shale, Hunton Limestone, and Woodford Shales. Karsting is a common feature of carbonate environments, which causes topographic irregularities on an unconformity surface. In parts of the Cherokee platform (Central Oklahoma), the Hunton Limestone lies beneath the Woodford Shale and it might have controlled Woodford deposition. The study is undertaken to 1) understand the impact of karsting on the natural fractures in the Hunton Limestone 2) study the effect of the unconformity karst surface on the overlying Woodford Shale, 3) evaluate the hardness of the Hunton Limestone and the Woodford Shale for artificial fracture stimulation, 4) determine the geomechanical properties, such as young's modulus, poisson's ratios, and brittleness index from well logs, and 5) describe the lithology and nature of the boundary contact between the Hunton Carbonate and the Woodford Shale. To accomplish this study, we used thin sections, core, outcrop, well logs, and a 3-D seismic survey data.

A 3-D seismic survey and well logs were used to map the structure and thickness of the Viola Limestone, Sylvan Shale, Hunton Limestone, and the overlying Woodford Shale. The Hunton unconformity in Central Oklahoma are affected by karst features, such as collapse and sinkholes on the Hunton unconformity surface. These karst features are prominent factors controlling the paleotopography and deposition of the Woodford Shale, as was observed from 3D structural maps. Sinkhole features in 3-D structural maps range in diameter from 1150 to 2300 ft. and extend vertically to almost 300 ft. These sinkholes have a potential effect on the deposition of the overlaying Woodford Shale. Similarly, there might have been a potential effect of the Viola Limestone karstification on the overlying Sylvan Shale. The thickness variations of the Woodford Shale are controlled by paleotopography of the underlying Hunton Group where thicker Woodford is observed in the karst lows (sinkholes). Additionally, the structural maps show an inverse correlation between the thickness of limestones (Viola and Hunton) and shales (Sylvan and Woodford).

We quantified the fracture intensity and apertures manually using comparator and hand lens at core site as well as using Image J softwareTM. Fractures in the Hunton core are affected by the karstification on the Hunton unconformity. Core data reveals that fractures exist only in the karstified section of the uppermost 15 ft. of the Hunton due to karstification. The fracture aperture (sealed) ranges from 0.003 to 0.01 in. and the fracture intensity ranges from 8 to 30 fractures/ ft. every 6 inches (on average) along the core length. The higher hardness measurements correspond to areas with higher fracture abundance due to more brittle rocks in the karsted zone. Additionally, a core that consists of the Hunton Group, boundary contact, and Woodford shale was studied to quantify the fracture aperture and intensity, measure the hardness using a Rebound HammerTM, and describe the lithology and nature of the boundary contact. The studied core exhibits an erosional unconformity surface between the Hunton Limestone and the overlying Woodford shale with possible Misener Sandstone.

In addition, bulk density as well as shear and compressional wave travel times were used to determine young's modulus, poisson's ratios, and brittleness index for the Viola Carbonate, Sylvan Shale, Hunton Carbonate, and Woodford Shale to aid in picking brittle areas for fracturing/ refracturing plans. The Hunton interval has the highest brittleness index (average of 0.8) among the Viola, Sylvan, and Woodford formations. However, Sylvan Shale has the lowest brittleness index (average 0.35). The Viola Limestone has brittleness index (average 0.6). Additionally, the brittleness index for the Woodford Shale stratigraphic interval in the studied area vary from 0.4 to 0.8. This study predicts the locations of thick Woodford Shale sections suggesting possible spots for landing horizontal wells in the Woodford Shale. Also, the high brittleness index from well logs can be used as a proxy to target a zone for hydraulic fracturing through the Viola Carbonate, Sylvan Shale, Hunton Group Carbonate, and the Woodford Shale.