Polygonal Fault Systems - A New Structural Style for the Niobrara Formation, Denver Basin, CO
Stephen Sonnenberg and David Underwood
Polygonal fault systems (PFS) are present in the Niobrara Formation of the Denver Basin. This type of fault system is recognized primarily on 3D seismic. The PFS is detached from basement faults and occurs in distinct layer-bounded systems (Niobrara and lower Pierre Shale). The faults are minor extensional faults, randomly oriented and form polygonal networks in map view. Faults dip 30 to 80 deg; have throws of 30 to 70 ft; and lengths of generally less than 4000 ft. The polygonal fault system deforms the Niobrara over a large part of the Denver Basin. Relatively undeformed sequences occur above and below the Niobrara faulted interval. This type of fault structure may also be present in other Rocky Mountain basins. An additional and separate PFS zone is seen towards the base of the Larimer-Rocky Ridge Member of the Pierre Shale. The faults are similar to ones reported in North Sea shale sequences. PFS are thought to represent volumetric contraction resulting from compaction-driven fluid expulsion. PFS are probably common features in thick shale sequences in sedimentary basins. Layer-bounded faults in the Niobrara and Pierre have previously been interpreted as listric faults. The new interpretation does not support the listric interpretation. This new interpretation explains the random orientation of the faults which previously was not explained. Recognition of this structural style is important for future wells targeting the Niobrara petroleum system.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012