Clay Mineralogy of the Pennsylvanian Tyler Formation in Western North Dakota for EOR and comparison with similar Illinois Basin deposits

Abstract

The analysis of clay minerals within the Pennsylvanian Tyler Formation, located in the Williston Basin of Western North Dakota, has led to advances in understanding enhanced oil recovery (EOR) possibilities of this interval and improved understanding of climate change during the Pennsylvanian. The Tyler Formation exhibits meter-scale, cyclical facies characteristic of Pennsylvanian rocks found throughout North America. Thicker facies deposits occur in the central basin, thinning as they extend towards the basin margin, with complete loss of some facies as the result of an incised valley fill (IVF) near the basin margin. Previous research established a sequence stratigraphic framework for the Tyler Formation. The mineral results, synthesized with gamma ray logs and sequence stratigraphy, improves correlation of this cycle within the Tyler Formation via association of units of similar clay mineral content.

Thirteen samples representing one Tyler cycle were taken from two cores within the basin (one from the inner basin setting, one from the marginal basin setting) for analysis of bulk and clay mineral content via x-ray diffraction (XRD). The inner basin samples displayed a typical cycle within the lower Tyler with calcareous mudstones overlying a brecciated carbonate, then carbonaceous shales and interbedded mudstones and limestone, which are capped with another brecciated carbonate interval. The mixed-layered illite/smectite of shales and mudstones provides evidence of the burial depth and temperature. The brecciated carbonates, interpreted as paleosols, represent a maximum regressive surface. The presence of kaolinite in the paleosols demonstrate a more 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.

The palesols are also found in the basin margin core, but the IVF destroys much of the evidence of typical cyclical facies. The IVF sandstone unit has been a conventional reservoir. It contains Fe rich chlorite which could react with acidic EOR fluids, reducing permeability and oil recovery. This work demonstrated some mineralogical similarity with recent research in the oil-producing Cypress Sandstone of the Illinois Basin where Fe-rich chlorite was found in IVF sandstones similar to those in the Tyler Formation.

AAPG Datapages/Search and Discovery Article #90218 © 2015 Eastern Section Meeting, Indianapolis, Indiana, September 20-22, 2015