Polygonal Fault Systems Upper Cretaceous Strata, Denver Basin

Abstract

Polygonal fault systems (PFS) are observed in two distinct Cretaceous levels in the Denver Basin. One level extends from the lower Pierre Shale Formation through the Niobrara Formation into the Greenhorn Formation and the other is located just beneath the Hygiene Sandstone Member of the Pierre Shale. These two faulted intervals are separated by a relatively undeformed interval. PFS are layer-bounded normal fault systems. The PFS have throws of 30 to 150 ft and fault planes dip 30 to 70 degrees. The faults aggregate into polygons or partial polygons in map or plan view. Five separate 3D seismic surveys illustrate the widespread distribution of PFS across the Denver Basin. These surveys illustrate the following: regional presence of PFS; PFS form early as evidenced by growth strata; PFS are slightly listric. The faults in the PFS generally are intersected by antithetic faults. These fault pairs create complex horst and graben systems in the Niobrara Formation. These surveys also illustrate that other structure styles cause faults in the Niobrara (e.g., basement faults, compaction features, etc.). Other published 3D surveys support the PFS model when reinterpreted. Fractures intensity increases dramatically close to fault systems. Thus PFS are important contributors to production sweet spots in the Niobrara.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016