Red River B Zone
The Red River B zone is found at approximately 12.2 m (40 ft) from the base of the Stony Mountain Shale. The gross porous interval ranges in thickness from 1.8 to 5.5 m (6 to 18 ft). The base of the Red River B zone is tight, skeletal, burrowed and nodular bedded limestone which is capped by laminated, massive and burrowed porous dolomites. The laminations are flat to wavy and are millimeter-scale. The laminations are similar to those in the C zone interval, but are thinner with locally well-developed permeability. The base of the B zone was deposited in open-marine environments that were sparsely to moderately burrowed. Effective porosity is absent in these beds. The overlying porous dolomites were deposited in low-energy environments where increased salinity or loss of oxygen, along with trapping and binding activities of cyanobacteria, were common. These laminated and locally burrowed sediments were deposited in shallow subaqueous, salinity-stressed settings.
Initial dolomitization probably occurred by seepage of magnesium-rich brines from the overlying and proximal anhydrite (gypsum). This type of dolomitization produced cryptocrystalline to fine-crystalline dolomite (10 to 20 microns with poor to good permeability. Pore interconnection is variable resulting in very poor to fair reservoir development. Unlike the C zone interval, early dolomitization occurred across a terrain that appears to have less original vertical variation based on the thickness of the B zone. This might have reduced the effects from seepage of magnesium-rich brines thereby slowing processes of dolomitization and producing greater permeability. Also, the thickness of the B anhydrite is variable and locally absent which may have resulted in less extensive and rapid early dolomitization.
Pore occlusion in the Red River B zone occurs by finely dispersed, early and late paragenetic anhydrite. Sparse, clear 20 to 50-microns dolomite rhombohedrons along stylolites and pressure-solution seams and white to light brown, late paragenetic crystals of baroque or saddle dolomite occlude intercrystalline pores. In cores, fractures are absent to sparse and appear to have no control on permeability.