Red River A Zone
The Red River A zone is found at approximately 3.0 m (10 ft) from the base of the Stony Mountain Shale and it is sometimes absent by result of erosion or non-deposition. The A zone is similar to the B zone in that porosity is found in one contiguous bench. The A zone is a laminated, massive, and burrowed dolomite. The dolomites are similar to rocks in the B zone interval, but well developed permeability is absent. Below and above the A zone porosity, tight skeletal and sparsely burrowed limestones are present. The base and top of the A zone were deposited in open marine environments. Effective porosity is absent in these beds. Between these subtidal beds, burrowed mudstones were deposited in moderately restricted subtidal environments. These sediments were poorly dolomitized.
Original dolomitization probably occurred by seepage of magnesium-rich brines from an overlying or proximal Red River A zone anhydrite (gypsum) which was subsequently eroded and capped by open marine sediments. This early dolomitization produced abundant cryptocrystalline dolomite (10 microns) with poor permeability. Low permeabilities reduced the effects of late diagenetic replacement or recrystallization. It is inferred that capping anhydrite in the A interval, if present, was very thin and was not an abundant source for magnesium-rich brines. Dolomitization, therefore, is discontinuous and ineffective.
Pore occlusion in the red River A zone occurs by finely dispersed, early and late paragenetic anhydrite and sparsely scattered, clear 20 to 50-microns dolomite rhombohedrons along stylolites and pressure-solution seams. In core, fractures are sparse to absent.