ABSTRACT

Deadwood and Winnipeg sandstones are oil and,gas productive in Newporte Field, Renville County, North Dakota (T163N and T164N, R87W). The Deadwood Formation in this area is sandstone and interbedded gray-green shale. Sandstone reservoirs are subarkosic and mafic arenites with local accumulations of glauconite and garnets. Informally the formation is subdivided into 5 sandstone units. The basal, medium to very coarse-grained sandstone (unit 1) sits disconformably on Precambrian schist and gneiss which were intruded by cross cutting pegmatites. This basal sandstone reservoir is cross-stratified and massive, and has well developed and preserved original and secondary porosity. Overlying lower sandstones comprise units 2-4. Sandstones within these units are generally finer-grained and commonly cross-stratified. Porosity ranges from very good to poor. Interbedded shales are gray-green and locally burrowed and bioturbated. The uppermost Deadwood (unit 5) is dominated by shale with thin ripple laminated sandstone beds. Near the top of this unit, Precambrian fault breccia is common and has been incorrectly described as impact ejecta. The top of the Deadwood is reworked fault breccia, conglomerates, and sandstones associated with regional Winnipeg deepening. The overlying Winnipeg sandstones are highly burrowed and bioturbated and were deposited in near-shore marine environments. Within the northeastern portion of the field, thin Winnipeg sandstones cap large basement structures where Deadwood sediments are absent by erosion or non-deposition. These Winnipeg sandstones are coarse-grained and locally productive.

An extensive seismic grid of over 120 miles (193 kilometres) across the field indicates that basement structures originated by recurrent tectonic processes. Various isochron maps confirm this hypothesis and discount the notion that this field is associated with an impact event.