--> Tectonic Versus Non-Tectonic Origin of Complex Fault and Fracture Patterns in the Niobrara Formation, DJ Basin, CO

AAPG ACE 2018

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Tectonic Versus Non-Tectonic Origin of Complex Fault and Fracture Patterns in the Niobrara Formation, DJ Basin, CO

Abstract

The Upper Cretaceous Niobrara Fm is emerging as a prolific HC resource play in Colorado. The Niobrara Petroleum System (TPS) is a self-sourced inverted tight HC system producing oil and gas mainly from fractured reservoirs.

The Niobrara Fm. forms part of the late Paleozoic through early Tertiary sedimentary sequence of the Rocky Mountain foreland basins that were uplifted during the post-Cretaceous Laramide Orogeny. The Denver-Jules (DJ) Basin and others, are asymmetric basins with a steeply dipping western flank bounded by the Rocky Mountains Front Range and a gently dipping eastern flank.

The Niobrara reservoir rocks consist mainly of interbedded chalks, marls, organic-rich shales and sandstones that thicken from west to east. The fine-grained tight character of these reservoir rocks, makes them unconventional plays with production depending on development of natural fractures systems.

The structural evolution of the DJ basin and the Niobrara exhibits Laramide compression, large-scale SW-NE basement-involved right-lateral wrench faults, and Neogene extension characterized by layer-bound small normal faults with varying strike. These faults however vary from random to more organized along basement structures, representing a peculiarity of the Niobrara and subject of debate due to their significance in the HC system. The faults are minor extensional faults with throws of 10-60 m and dips of 30-80 degrees. They are distinct layer-bounded systems concentrating in the brittle calcareous rocks over- and underlain and interbedded with soft ductile marine shales sealing the HCs.

The faults have been interpreted as being listric tectonic faults to polygonal non-tectonic faults. Polygonal faults have been observed in basins worldwide and are commonly interpreted to be formed by mechanisms such as volumetric contraction due to compaction-driven fluid expulsion, shallow overpressure, or differential compaction.

This study takes a structural analysis and balancing approach in order to distinguish tectonic and non-tectonic fault systems and establish the stress distribution in the Niobrara Fm., around the larger regional and basement. Using trishear to model the deformation of the Niobrara above reactivated basement steps in conjunction with mechanical stratigraphy where multi-directional extensional strains in the competent layers create layer-bound faults with varying strike, is suggested as a tectonic model for the Niobrara fault and fracture enigma.