Red River D Zone
The Red River D zone is found at approximately 53 m (175 ft) from the base of the Stony Mountain Shale. The gross porous interval ranges in thickness from 12.1 to 25.9 m (40 ft to 85 ft). Rocks of this interval are burrowed dolomites and sparse to common, skeletal limestone interbeds. Thin beds of black organic-rich shales (kerogenites) are locally present and occur at the base or tops of skeletal limestones. Red River D zone sediments are skeletal limestones and burrowed dolomites which were deposited in open-marine environments. These sediments typically contain crinoids with associated bryozoans, brachiopods, mollusks, rugose and tabulate corals, ostracods, and primitive stromatoporoids. Commonly, skeletal packstones, grainstones, and sparse framestones occur at the base of paracycles and represent flooding or deepening events. Maximum flooding surfaces and interbedded deepening events are represented by shaley lime mudstones with sparse, skeletal fragments and possible cyanobacteria which are informally named kerogenites. Open-marine sediments are interbedded with burrow-mottled sediments indicating minor oscillations in bathymetry or salinity associated with basin filling.
Burrow sediments commonly contain normal marine fauna. These sediments were deposited in environments that were slightly to highly stressed by elevated salinity or low oxygen levels. Thalassinoides and Planolites trace fossils are common, and are part of the Cruziana ichnofacies which has been described along shallow-shelf environments. Infauna burrowing probably increased the initial transmissibility within sediments for subsequent dolomitizing fluids.
Dolomitization of burrowed sediments produced porosity in the Red River D zone. Original dolomitization probably occurred by seepage of magnesium-rich brines. This early dolomitization produced cryptocrystalline dolomite (10 microns). Dolomites in the Red River D zone are characterized by sparse cryptocrystalline and abundant fabric destructive, medium-crystalline (60-200 microns), euhedral to subhedral replacement crystals. Coarser crystalline dolomites typically display sucrosic intercrystalline porosity and moderate to high permeability because of well-developed, uniform intercrystalline pore space. These dolomites produce the best conventional reservoirs within the project area and were formed by late diagenetic replacement or recrystallization of cryptocrystalline dolomites in the deep subsurface. Recrystallization probably occurred by the interaction of original dolomites with saline and dolomitic-rich hydrothermal fluids.
Pore occlusion in the D interval occurs by finely dispersed, early and late paragenetic anhydrite, sparse calcite spar, and sparse to moderate amounts of solid hydrocarbons (bitumen). Fractures are absent to sparse and appear to have little if any control on permeability and productive potential.