AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies and Porosity-Permeability Relationships in Red River A and B Intervals, North Dakota
(1) Geology, St. Lawrence University, Canton, NY.
The Late Ordovician (Katian) A and B intervals of the subsurface Red River Formation in North Dakota are a 67 to 80 foot thick succession of mixed carbonate-evaporite rocks that formed in the intracratonic Williston Basin. For this study, detailed bed-by-bed analysis of nine selected cores through the Red River Formation was used to describe facies and their stacking patterns in detail. The available NDGS core plug data and 218 thin-sections were analyzed for relationships between porosity-permeability and facies type. Microfacies include, from deepest to shallowest: 1) Fossiliferous wackestone to packstone (shallow subtidal), weakly burrowed, locally containing dolomitized kerogenous laminae. This microfacies was observed in the lower B and upper-middle A interval. Its porosity is primarily vug with some between-crystal, and ranges from 0.04-4.9%. Permeability ranges from 0.01-124mD; 2) Brachiopod-crinoid-gastropod packstone to grainstone (high energy sand shoal). Porosity ranges from 1.1-5.9%, permeability from 0.01-1.4mD; 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 portions of both the A and B intervals; it is highly dolomitized and contains between-crystal and vug porosity which varies between 1.5 and 33.1%. Permeability ranges from 0.01-44mD. Data from this facies shows a distinct correlation between porosity and permeability; 4) Sabhka-type flat-laminated and nodular (chicken-wire) anhydrite capping the B interval. This facies is locally interbedded with flat-laminated fine grained dolomite. Porosity ranges from 0.4-1.9%, Permeability from 0.01-0.7mD.
The relationship between facies type and porosity vs. permeability is strong in the laminated and thrombolytic facies. In all other facies, this relationship is much more complex, likely due to a large variety within the facies present as well as the dolomitization pattern. The stoichiometry of dolomites is analyzed using the x-ray diffraction; the permeability-porosity relationship to facies and dolomite diagenesis are tested using the cathodoluminescence and energy-dispersive X-ray spectroscopy (SEM-EDX).