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Tepee Buttes, Methane Seeps, and Polygonal Faults, Denver Basin

Abstract

Tepee Buttes are conical mounds found in the Upper Cretaceous Pierre Shale. The origin of the mounds is generally accepted based on biotic zonation and geochemical data to be due do nutrient-rich submarine springs and methane-seeps on the ancient Pierre Shale seafloor. The mounds have dipping flank beds (3-34 degrees) and a limestone core. They resemble carbonate-cemented lithoherms around modern hydrocarbon seeps. The buttes range in height from 5 to 75 feet. The circumference of the buttes lies between 200 and 900 feet. The mounds are held up by an irregular pipe-like vertical to sub vertical limestone core. Facies outwards from a central vuggy limestone core include: limestone with lucinid bivalves; limestone with diverse mollusks; limestone with inoceramids; shale with stromatolites (Kauffman et al., 1996). The Tepee Buttes are common in Cretaceous shale outcrops from south of Colorado Springs to Pueblo. Hundreds of buttes or mounds are present in this area. The buttes are thought to form along fracture systems that vented methane-charged fluids. The source of the methane is from the Sharon Springs member of the Pierre Shale and the Niobrara Formation (biogenic gas from microbial degradation of organic-rich beds). The evidence for the fracture systems is the very linear arrangement of the buttes. Reinterpretation of the linear features suggest they may form polygons and originate from polygonal fault systems. Recent work in the Denver Basin suggests that these polygonal fault systems are quite abundant and form early in burial history. These fault systems are abundant in zones above the Niobrara Formation. The Tepee Buttes zones are easily recognized in well logs in the subsurface by relatively clean gamma ray signature and high resistivities. The carbonate build-ups range in height from a few feet to approximately 75 feet. The buttes often have hydrocarbon shows but their small size suggests very limited accumulations. Maps and cross sections in the Wattenberg field area illustrate the common occurrence of the Tepee Buttes. Tepee Buttes or mounds associated with methane seeps and fracture systems are present in other shale plays (e.g., Vaca Muerta, Argentina).