Mechanical stratigraphy and layer-bound normal faulting in the Upper Cretaceous Niobrara Formation, Wattenberg Field, Colorado

Abstract

Layer-bound normal faults are pervasive within the very fine-grained rocks of the Upper Cretaceous Niobrara and Carlile formations in the Denver Basin. 3-D seismic and well log interpretation reveal a complex, segmented fault system that is divided into two discrete tiers: an upper tier located in the Pierre Shale, and a lower tier located in the Niobrara Formation. 3-D fault throw analysis shows maximum throw near the top of the Niobrara Formation with steep, asymmetrical throw gradient down section in the lower Niobrara and Carlile formations. Faults are laterally well-connected in the upper Niobrara Formation and commonly form linear arrays of linked graben systems. In contrast, faults deeper in the stratigraphic section that offset the Carlile and Greenhorn formations are more segmented and commonly form half grabens (as opposed to full, fault-bound grabens). In cross-section, fault planes measured from seismic have a general dip of 45°. However, close inspection reveals that faults consistently change dip angle as they pass through the lower Niobrara Formation, refracting from ~55° to ~35° through the Niobrara C Marl, then back up to ~50° in the Carlile and Greenhorn formations. The fault dip refraction produces a contractional step or bend in the fault plane associated with the lower dip segments. This geometry is investigated further with horizontal image logs and other borehole data to reveal a kinematic relationship between fault dip angle and mechanical stratigraphy. Field examples of normal faults that cut mechanically layered rock help better understand these complex fault geometries and provide reasonable inferences to their development and propagation history. In summary, it is argued that the mechanically layered nature of the Niobrara and Carlile formations is responsible for many of the fault characteristics described and provides valuable insight into understanding the fault system

The mechanics of initiation and development of thrust faults and thrust ramps

“…Mechanical stratigraphy is the stratal expression of the contrasts in the cohesive, compressive, tensile, and frictional strengths of rock, along with the thicknesses of rock units and interface properties between mechanical units (Corbett et al, 1987;Gross et al, 1995;Underwood et al, 2003;Laubach et al, 2009;Ferrill et al, 2017;Hart and Cooper, 2021). Mechanical stratigraphy affects fault nucleation and propagation (Teixell and Koyi, 2003;Underwood et al, 2003;Laubach et al, 2009;Roche et al, 2013;Ferrill et al, 2017;Bracken, 2020), fracture nucleation location (Eisenstadt and De Paor, 1987;, fault length, width, and the aperture across a slip surface (Laubach et al, 2009;McGinnis et al, 2017), fault-growth directions (King et al, 1988;Mitra and Mount, 1998), the proportions of folds and faults that form (Morley, 1994;Erickson, 1996), fold geometry (Fischer and Jackson, 1999;Gutiérrez-Alonso and Gross, 1999), fault-fold interactions (Chester, 2003), and fault shape and related fault zone deformation (Woodward and Rutherford, 1989;Pfiffner, 1993;Ferrill and Morris, 2008). Strong, competent units withstand higher stresses before deforming plastically (permanently) and they accommodate stress loads by brittle failure (Currie et al, 1962).…”