Structural styles of the Niobrara Formation: a study of Kansas and Colorado outcrops using an unmanned aerial vehicle (UAV)

Abstract

The Late Cretaceous Niobrara Formation is a productive unconventional hydrocarbon system throughout the Denver Basin. Alternating chalk and marl units of the Smoky Hill Member act as a source, reservoir, and seal for the Niobrara petroleum system. The Niobrara has attracted attention from operators after advances in drilling and production technologies have made Niobrara oil and gas economic. The focus of this study aims at achieving a complete understanding of fracture types, fault characteristics, and fault systems in Niobrara outcrop using modern technology. Outcrop analysis across western Kansas indicates the presence of a polygonal fault system (PFS) within the Niobrara. Unmanned aerial vehicle (UAV) data was collected, developed into 3D models, and interpreted to identify fault and joint orientations, geometries, and relations across the study area. Polygonal and structurally-reactivated faults were observed in outcrop. Non-tectonic polygonal faults display random orientations, dip-slip normal faults, are characteristically layer-bound, and are unaffected by geomechanical variation. Limited outcrop and exposure bias result in a general SE-NW trend for polygonal strike orientations; however, increased data and hangingwall slip directions confirms the presence of a non-tectonic fault system at Castle Rock, Kansas. Structurally-reactivated polygonal faults originate from fourth-order structural influence or tectonic events and display common antithetic faults, brecciation zones, fracture swarms, and changes in fault plane geometries across geomechanical boundaries in the chalk and marl intervals. Data indicates the presence of two or three joint sets at most field locations. Stable isotope results of host rock and calcite fault fill samples indicates meteoric influence. Structural analysis of the UAV models resulted in accurate measurements. Applications for UAVs should be incorporated into all facets of geological research including structural geology, stratigraphy, sequence stratigraphy, sedimentology, mineralogy, and field mapping.