Mid-Continent Section

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Revisiting Vertically Producing Fields for Horizontal Exploitation: A Case Study of the Atokan “Tussy” Sandstone in the Western Anadarko Basin


The Atoka Formation in the Anadarko Basin produces from ∼2,000 wells, yet only 91 of those are horizontal wells. Historical production within the Atoka Formation has resulted in 1.7 TCF of gas and 39 MMBO, with most of the production coming from vertically drilled shale and lime wells. Utilizing an interdisciplinary approach, the Atokan ‘Tussy’ sand was assessed for its horizontal development potential. The Tussy sand is a set of two ∼42 km long and ∼ 9 km wide NW-SE trending 10′–24′ thick sand bodies in central Custer Co., Oklahoma. Predominately a mixed shale and limestone deposit, the Atoka does have local isolated sand deposits, which were part of a greater mixed marine and fluvial/deltaic influenced environment. Sourced from the ancestral Rockies to the west and the Canadian Shield to the north, the Tussy sand was deposited during an overall transgressive systems tract. Wireline log character interpretations suggest the sands have a lobate geometry with beds ranging between 2′–10′ thick and can form up to four stacked beds, usually separated by a 2′–3′ silty bed. The mapped isopach profile and depositional environment would suggest that the Tussy sand bodies are representative of barrier islands drowned by a transgressive system. Over 30 vertical wells produce from the two sand bodies and no Tussy horizontal wells have been drilled to date. With neutron density logs estimating typical porosity range between 10–18%, the Tussy sands remain an enticing future prospect.