Repurposing the Inyan Kara Group of the northern Great Plains: outcrop studies for reservoir management in the energy transition

The Lower Cretaceous Inyan Kara Group (IKG; Lakota and Fall River Formations) of the Northern Great Plains demonstrates the complexity of sedimentary systems important to the energy transition. Spanning the Black Hills (BH) uplift and the adjacent subsurface Powder River Basin (PRB) and Williston Basin (WB), this interval exhibits a distinctive, twofold stratigraphic subdivision reflecting the eustatic and tectonic evolution of this portion of the Western Interior Seaway during the early Cretaceous, While the sequences of the Lakota Formation exhibit mainly non-marine depositional environments with local syn-kinematic influences, the sequences of the overlying Fall River Formation reflect progressively more shallow marine influences. Despite this predictable two-fold character, strata of the IKG are extremely heterogeneous over short distances.

Historically, the stratigraphic architecture of these units defined significant economic fairways, including fluvial sandstone-hosted uranium deposits in the southern BH and stratigraphic hydrocarbon traps in the PRB. More recently, the IKG has emerged as a premier target for saltwater disposal (SWD) and carbon capture and storage (CCS) in the WB. Assessments indicate CCS potential exists in the PRB but is constrained by water salinity considerations prohibiting injection under environmental regulations. Subsurface mapping reveals that incised valley-fill (IVF) complexes within the group offer optimal storage parameters, characterized by porosities exceeding 20%, Darcy-scale permeability and high connectivity. The pinch-outs of IVFs provide stratigraphic traps, while the overlying Skull Creek Shale provides a widespread and robust regional top seal. Together these form the elements essential for secure fluid containment.

By utilizing the Black Hills outcrops as high-resolution analogs for subsurface reservoirs, operators can better predict injectivity and storage capacity. This study highlights how traditional geologic descriptions of the outcrops can be refined with newly acquired 3D photogrammetry models (60+) to develop concepts to de-risk commercial-scale SWD and CCS projects and support sustainable reservoir management.