MICROFACIES AND POROSITY OF THE LATE ORDOVICIAN RED RIVER A AND B INTERVALS IN CENTRAL NORTH DAKOTA

The Late Ordovician (Katian) A and B intervals of the subsurface Red River Formation in central North Dakota are a 67 to 70 foot thick succession of mixed carbonate-evaporite sediments that formed in the intracratonic Williston Basin. Three cores (Wunderlich #1, McHenry County; Klain #1, McLean County; and Flemmer #1, McLean County) were logged, and 172 thin-sections from 12 nearby wells were analyzed using a standard petrographic microscope. A binocular microscope was used to analyze porosity type and estimate percentage porosity. Microfacies include, from deepest to shallowest: 1) Fossiliferous wackestone to packstone (shallow subtidal), weakly burrowed, locally with kerogenous laminae. This microfacies was observed in the lower B and upper-middle A interval. Its porosity is between-crystal and vug in type, and ranges from 0 to 2.5%.; 2) Thin (1 to 30cm) sheets of brachiopod-crinoid-gastropod grainstone to rudstone (high energy sand shoal). This low-porosity (<5%) facies occurs in both intervals, but is more common in the A interval; 3) High-porosity low-relief subtidal buildups (thrombolites) and laminated (flat and crinkly) carbonate formed on tidal flats. This facies is found in the middle parts of both the A and B intervals. Its porosity varies between 10 and 20% (mostly between-crystal and vug porosity). The only allochems are sparse ostracods; anhydrite laths and blebs are common. No connection between pore type, total porosity, and lamination type has been observed. 4) Sabhka-type flat-laminated and nodular (chicken-wire) anhydrite capping the B interval. This facies is locally interbedded with flat-laminated dolomite; 5) Thin intraclast grainstone to rudstone (subaerial exposure). This facies is found both in the A and B intervals. It contains various sizes and shapes of intraclasts, and beds vary in thickness from 1 to 30cm. Estimated porosity varies from 0 to 10%. Estimated percentage porosity values are checked by using the available core plug data. The relationship between facies type and porosity vs. permeability is tested.