ABSTRACT

The Tyler Formation in North Dakota may be divided into upper and lower units, reflecting changes in lithology as well as depositional environment. The lower unit is dominated by shales, mudstones and siltstones, thin coal beds and sandstones. In the Medora field area, the upper unit can be divided into a lower subunit, dominated by argillaceous limestones and calcareous shales, and an upper subunit dominated by anhydritic limestones, calcareous shales and local, thin anhydrite beds. In the vicinity of the Dickinson field, the upper unit is locally dominated by various lithologies, which indicates rapid changes in depositional environments.

The lower unit is interpreted to represent sedimentation on a progradational delta plain. Distribution maps of lower unit sandstones indicate an east-west trend (in Billings and Stark Counties) of medium-grained well-sorted quartz arenites, interpreted as a delta front deposit. To the south the maps indicate a northwest-southeast trend of medium-grained, poorly to moderately-sorted quartz arenites, interpreted to be distributary channel-fill deposits.

Barrier islands controlled deposition of the upper unit in Billings and Stark Counties. Barrier island development was followed by northward progradation of lagoonal and estuarine environments in the Dickinson area (Stark County) and transgression of a shallow anoxic sea in the Medora area (Billings and Golden Valley Counties). Still-stand or northward progradation in the Dickinson area, and regression in the Medora area created similar tidal flat deposits in the uppermost Tyler.

Each sandstone facies exhibits a distinctive diagenetic history. Channel sandstones are characterised by low original porosity and permeability due to large proportions of detrital clay. Porosity and permeability were reduced by authigenic kaolinite and late stage ankerite cement. Delta-front sandstones are tightly cemented by anhydrite: as much as 30 percent of the original porosity has been eliminated by precipitation of calcium sulfate derived from local hypersaline lakes. Porosity in barrier island sandstones has been reduced by quartz overgrowths. Detrital clay coatings on quartz grains inhibited overgrowth and total cementation. Late-stage authigenic kaolinite has further reduced primary intergranular porosity and permeability.