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Hunton Limestone: A Highly Fractured, Natural Gas-Driven, Multiple Pay Carbonate Reservoir Within the STACK Play, Central Oklahoma

Abstract

The Hunton Limestone formation lies within the STACK play in central Oklahoma, as well as

beyond the defined boundary in north-central and northwest Oklahoma. Structurally, the productive

formation lies along the border of the Anadarko basin to the southwest and the Anadarko shelf to the

northeast. The Hunton Limestone does not contain primarily conventional porosity as viewed through

electric logs from thousands of historic vertical wells. However, this carbonate reservoir, sourced from

the overlying Devonian Woodford shale, internally provides multiple characteristics of secondary

porosity, such as: diagenesis, karsting, vugular porosity, dolomite, and regional fracturing. From 70+

horizontal wells drilled in central Oklahoma, image log analysis has determined that the entire Hunton

interval contains a large network of partial and open fractures that span across the region. These

fractures are the primary feature that provides the pooling of hydrocarbons within the reservoir. The

Hunton formation is both truncated structurally by the Anadarko shelf and stratigraphically by pinching

out to the northeast by the underlying Sylvan Shale. The Hunton formation can be sub-divided into

three different producing formation: Upper Hunton (Frisco and Bois d’arc limestones), Middle Hunton,

and Lower Hunton (Chimney Hill formation composed of the Clarita and Cochrane limestones).

As a carbonate formation with unconventional porosity, the completion process must also be

viewed unconventionally. The fracture network within the formation must be exploited to obtain a long

term producing and naturally flowing horizontal well. With a higher than average recovery efficiencies

for STACK horizontal wells, EUR’s for the Hunton Limestone intervals range from 125k BO to 385k BO

and 2.5 BCFG to 3.5 BCFG. The secondary porosity inherent within the formation, the regional fracture

network, and custom fracturing design, ultimately provides evidence for a large and economically viable

hydrocarbon reservoir within the STACK play of central Oklahoma.