A New Depositional Model for the Lower Cretaceous Inyan Kara Formation at Halliday Dome, North Dakota, and Its Implications for the Site’s Carbon Dioxide Sequestration Potential

The Halliday Dome is a large (~400 km²), low-relief anticline in western North Dakota that is being developed for geologic CO₂ sequestration. The injection target is the Early Cretaceous Inyan Kara Formation, which is regionally equivalent to the lower unit of the Dakota Aquifer, a laterally extensive saline aquifer that extends across the Williston Basin and is over 200 m thick in the site area. Because of its excellent reservoir properties, the Inyan Kara Formation is often used for wastewater injection in North Dakota and was identified by PCOR (Plains CO₂ Reduction Partnership) as a suitable formation for CO₂ sequestration. To characterize the reservoir and develop 3D geologic models for simulations of CO₂ injection, we examined geologic data from 15 wells, regional core data, and field analogs. The interpreted depositional model of the Inyan Kara Formation is a fluvial-dominated valley-fill system that transitions upward to a marginal marine system, reflecting the overall global sea level rise of the Early Cretaceous. The well log data exhibit a distinct sharp-based, fining upward, blocky pattern, which is interpreted as fluvial valley-fills that grade upward into estuarine and marine deposits. The lowermost member of the Inyan Kara can be separated into two units that are capped by a laterally continuous shale unit. These marine shale units indicate transgressive flooding surfaces that can be correlated regionally. The best reservoir properties are found in the valley-fill fluvial sandstones, which have an average porosity of 20-21% and Darcy-scale permeability. Outcrop studies in the laterally equivalent Early Cretaceous Fall River Formation in and around the Black Hills Region of South Dakota show that these deposits are 10s of meters thick and are generally well connected with other valley-fills within the sequence. Capturing the vertical and horizontal connectivity of the reservoir in the 3D geologic model is critical in predicting the CO₂ plume migration. While correlating individual valley-fills across the project area is difficult due to sparse well control, the consistency in the valley-fill well log signature, in addition to the characteristics of field analogs, suggests they are laterally extensive and may occur as well-connected sands at the scale of the project area. The laterally continuous marine shales would likely impede vertical connectivity and prevent vertical migration of the CO₂ plume.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California