--> --> Characterization of layer-bound normal faults within the Upper Cretaceous Niobrara Formation, Wattenberg Field, Denver Basin, Colorado

AAPG Rocky Mountain Section Meeting

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Characterization of layer-bound normal faults within the Upper Cretaceous Niobrara Formation, Wattenberg Field, Denver Basin, Colorado

Abstract

Layer-bound, low displacement normal faults are pervasive within the very fine-grained rocks of the Upper Cretaceous Niobrara and Carlile Formations in the Denver Basin. These faults pose a challenge to drilling horizontal oil and gas development wells by making it difficult to place the laterals completely within the thin (<30’ thick) target reservoir zones. Detailed characterization of this fault system is essential for accurate well planning and execution. 3D seismic and well log interpretation reveal a complex, segmented fault system, where individual fault segments display various stages of linkage in both map and section view. Fault throw (vertical separation) distributions demonstrate maximum throw near the top of the Niobrara with a steep, asymmetrical throw gradient down section in the lower Niobrara and Codell. The faults are best expressed in the upper Niobrara and commonly form linked graben systems where parallel, antithetic fault arrays have joined together with overlapping synthetic segments to breach relay zones and form continuous, hardlinked fault systems. In contrast, faults deeper in section that offset the Codell and Ft. Hays are sparse and commonly form half grabens (as opposed to full, fault-bound grabens) and un-breached relay ramps. In cross-section, the faults are planar, but refract to a lower dip angle (from ~55 to ~35˚) through the lower Niobrara and return to a steeper dip angle (~60˚) down section into the Ft. Hays and Codell. This contractional step, or bend, in the fault plane is investigated further with horizontal image logs (XRMI) and other borehole data, to reveal a kinematic relationship between fault dip angle and rock strength. Field examples of normal faults that cut mechanically layered stratigraphy 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 as a whole.