Red River C Zone
The Red River C zone is found at approximately 25.9 m (85 ft) from the base of the Stony Mountain Shale. The Red River C zone porosity typically exhibits a shallowing-upward sequence that culminates with a thick anhydrite. The gross porous interval ranges in thickness from 6.1 to 15.2 m (20 ft to 50 ft). Rocks in the C zone are dolomites with millimeter-scale, flat to wavy laminations and stromatolites which are laterally linked hemispheroids and low-relief, vertically-stacked hemispheroids. The C zone has significantly reduced permeability from the crypto-crystalline nature of the dolomite and finely dispersed anhydrite.
This facies was deposited in low energy, shallow subaqueous, and salinity-stressed environments where the trapping and binding activities of cyanobacteria were common. Desiccation features are sparse, and fossils are missing, with the exception of sparse ostracods. Thin interbeds of bioclastic and peloidal sediments are storm deposits within these restricted subaqueous settings. Single-burrow mottling by small Planolites is common, especially near the base of these laminated beds.
Dolomitization of laminated and burrowed sediments produced porosity in the C interval. Proximity to the overlying C zone anhydrite (gypsum) produced early dolomitization by seepage of magnesium-rich brines. Syndepositional precipitation of dolomite may have also occurred in this interval. These types of early or syndepositional dolomitization produced abundant cryptocrystalline dolomite (10 microns) with poor permeability.
The C zone has moderate variation in thickness, generally fair to good porosity, but poor permeability. It is inferred that early dolomitization, with its concomitant loss of permeability produced by extensive intergrowth of dolomite rhombohedrons, prohibited the C zone interval from being an effective petroleum reservoir. Because of low permeability, these dolomites appear to have been poorly affected by hydrothermal fluids and late diagenetic dolomite replacement or recrystallization like the underlying D zone.
Pore occlusion in the C 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. Fractures are absent to sparse and appear to have no control on permeability.