The natural fracture network in unconventional reservoirs is key to productivity but challenging to model and predict. Therefore, fracture proxies such as dip and curvature are commonly used. This study explores a less common proxy, (cumulative) strain in a pilot study of the Niobrara Fm. In the DJ Basin, CO.
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 vary from random to more organized along basement structures, representing a peculiarityof 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 either listric tectonic faults or 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. The faults however vary from random to more organized representing a peculiarity of the Niobrara and raising the question of tectonic versus non-tectonic origin.
This study takes a structural analysis and balancing approach in order to distinguish tectonic and non-tectonic fault systems and establish the strain distribution in the Niobrara Fm., around the larger regional faults and basement. Basement structures often influence the structural architecture and strain distribution of overlying cover sediments by providing a pre-existing geometry or by reactivation. While structural overprinting can be directly observed, the cumulation of strain can have significant influence on the fault and fracture network and geologic scenarios and related HC reservoirs.
LithoTect Software was used to model the evolution of the Niobrara Fmand related structures. The forward model includes two steps representing pre-Laramide extension and normal faults and Laramide compression and fault reactivation. Since the basement faults are blind, they can be best modeled using trishear, which deforms also the layers above the fault tip. Pre-existing extensional strains are cumulated with compressional strains to calculate the cumulative strain, which is the summation of the individual finite strains at every modeling step and therefore best represents the total damage (and fractures) to the rocks over the course of the deformation history.
The results show that highest strain develop in the fold limbs, where tectonic faults develop with geometrical relationship to deformation directions ,representing the more organized and oriented sections of the fault pattern.Lowest strains develop in the hinges where the pre-existing uni-axial extensional strains come through and non-tectonic polygonal faults develop, representing the more randomly arranged sections of the fault pattern.
Balancing and forward modeling workflows in combination with tracking of cumulative strains is a powerful technique to understand and predict present-day fault and fracture patterns. Cumulative strain is a valid proxy for understanding the effects of superposed deformation and total rock damage (fractures) caused.
From: AAPG 2018 “Tectonic versus non-tectonic origin ofcomplex fault and fracture patterns in the Niobrara Fm., DJ Basin, CO” byCatalina Luneburg
