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Integrated 3-D Seismic and Core Data for Characterization of Natural Fractures of the Hunton Limestone and the Woodford Shale in Central Oklahoma

Abstract

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 underlies the Woodford Shale and might have controlled Woodford deposition. The main objectives of this study are 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, and 3) evaluate the hardness of the Hunton Limestone and the Woodford Shale for artificial fracture stimulation. To achieve this understanding, we used a 3D seismic survey and well logs to map the structure and thickness of the Viola Limestone, Sylvan Shale, Hunton Limestone, and the overlying Woodford Shale. In addition, we used core analyses 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 karst features, such as collapse and sinkholes on the Hunton unconformity surface are prominent factors controlling the paleotopography and deposition of the Woodford Shale, as was observed from 3D structural maps. Similarly, there might be 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) (Infante et al, in press). Sinkhole features range in diameter from 1150 to 2300 ft. and extend vertically to almost 300 ft. Additionally, there is an inverse correlation between the thickness of limestones (Viola and Hunton) and shales (Sylvan and Woodford). 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. An erosional unconformity surface between the Hunton Limestone and the overlying Woodford shale, with possibly some Misener Sandstone occurs in one core. This study predicts the locations of thick Woodford Shale sections suggesting possible spots for landing horizontal wells in the Woodford Shale.