ABSTRACT: Porosity Preservation and Occlusion in Mississippian Carbonates, Williston Basin, North Dakota

WETMORE, MICHAEL J., and PATRICIA E. VIDETICH, University of North Dakota, Grand Forks, ND

Hydrocarbon production from Mississippian carbonates in the Williston basin, Bottineau County, North Dakota, is controlled mainly by diagenetic processes rather than depositional environments. Oil production in the area of study is from the Wayne beds in the Alida interval of the Mission Canyon Formation in Roth, Leonard, and Cimbel fields. The main hydrocarbon trapping mechanisms in these fields are structural highs with four-way closure caused by salt solution collapse of the underlying Devonian Prairie Evaporite. The ultimate recoverable oil reserves per well, however, are not structurally controlled. These fields contain structurally high oil wells that are very poor producers and structurally low wells that are excellent producers.

Destruction of porosity is the main control of the ultimate oil production from wells within these fields. Vugular and intergranular porosity in pisolitic packstones, skeletal wackestones, and mudstones are being occluded by anhydrite, calcite, and dolomite cements. Of these, anhydrite is by far the most important porosity destroyer. Most of these cements were precipitated during formation of the pre-Mesozoic unconformity, which is represented by the missing section between the Mission Canyon Formation and the overlying Triassic Spearfish Formation.

Sulfur stable isotope analysis of the anhydrite cements has enabled the relative ages of the anhydrite cementation events to be established. By analyzing ancient topography, depositional environments, and groundwater flow paths present at the time of anhydrite cementation, an exploration model can predict the location of preserved porosity in these Mission Canyon Formation structures.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)