Tepee Buttes, Methane Seeps, and Polygonal Fault Systems, 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).
AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section
Meeting, Cheyenne, Wyoming, September 15-18, 2019