SEQUENCE STRATIGRAPHY AND CLAY MINERALOGY OF TYLER FORMATION DEPOSITS, WILLISTON BASIN, WESTERN NORTH DAKOTA WITH COMPARISON TO SIMILAR DEPOSITS OF THE CYPRESS SANDSTONE OF THE ILLINOIS BASIN FOR THE CHARACTERIZATION OF EOR POTENTIAL

The analysis of clay minerals within the Pennsylvanian Tyler Formation, Williston Basin of Western North Dakota, led to advances in understanding enhanced oil recovery (EOR) possibilities of the interval, provided an improved understanding of Pennsylvanian climate change and aided in more complete spatial framework. The Tyler Formation exhibits meter-scale, cyclical facies characteristic of Pennsylvanian rocks found throughout North America. Thicker lithofacies occur towards the center of the basin, and thin as they extend toward the basin margin. A weakly developed incised valley fill (IVF) cuts the basin margin resulting in loss of some facies. The mineral results, synthesized with gamma ray logs and sequence stratigraphy, improves correlation of the cycle within the Tyler via association of units of similar clay mineral content.

Eight data points were taken from a central basin core, six from a basin margin core. Each sample set represents a complete lithological cycle in the Tyler bound by brecciated carbonates interpreted as paleosols that represent a maximum regressive surface. Mineral data in both data sets show changing depositional environment. The presence of kaolinite in the paleosols demonstrate a tropical and humid climate up section in the lower Tyler, reinforcing conclusions of the interpreted environment of shallow tropical marine sea with a transgressive and regressive cycle, but also present the possibility to correlate units via semi-quantitative clay mineralogy. The basin margin demonstrates a drastic difference in the stratigraphic sequence compared to the central basin core as a result of missing facies, which are replaced by the IVF sandstone. The IVF has been a conventional reservoir, but is being considered for unconventional plays. It contains Fe rich chlorite, not seen in the central basin core, which could react with acidic EOR fluids, reducing permeability and oil recovery. This work demonstrated some mineralogical similarity with an oil-producing Illinois Basin Cypress Sandstone facies, also interpreted as an IVF where Fe-rich chlorite was found. The IVF sandstones may be depositionally and diagentically similar. The Cypress may provide insight to the temporal and spatial framework of the Tyler to construct a system of evaluating similar basin deposits elsewhere.