Conclusions and Reservoir Quality
Textural and compositional differences between Upper "B" and Lower "B" porous dolomites exhibited in other Red River wells are also demonstrated here. Upper "B" dolomites are skeletal poor and locally cryptalgal or peloidal; Lower "B" dolomites exhibit a greater abundance and diversity of fauna and flora, and become more limy with depth.
Porosity-reducing silica cement is generally concentrated in the tight transition zone, rather than throughout the "B" interval. Minor replacive chert also makes an appearance in deeper sucrosic dolomites and calcareous mudstones (below reservoir facies). Vug- or fracture-filling anhydrite is most abundant in shallower Upper "B" dolomites and deeper sucrosic dolomites. In nearly all samples is halite, a minor authigenic phase spatially associated with anhydrite.
Of further interest is the way in which diagenetic processes have compartmentalized reservoir intervals in this and other studied wells. The top of the Upper "B" is typically not as porous and permeable as slightly deeper beds because of pervasive pore filling by anhydrite, presumably derived from overlying evaporite units. In the case of Lower "B" dolomites, style and distribution of dolomitization greatly influences porosity development: deeper beds exhibit coarser, more tightly packed, and less euhedral dolomite textures or are only partly dolomitized. Both of these processes result in decreased porosity relative to dolomites above. Reservoir compartmentalization is further illustrated by the presence of a thin, laterally continuous, chert-cemented zone separating Upper and Lower "B" units.
Well-developed pore networks characterize both the Upper and Lower "B" intervals, with measured porosities between 16 and 26 percent. Lower "B" dolomites (down to 8904 ft) are somewhat more porous and oil saturated, but slightly less permeable because of the abundance of micro-pores. Likewise, Upper "B" dolomites, though less porous and less oil saturated, are more permeable because of pervasive, well-interconnected inter-crystalline porosity. Relative to other Red River "B" studies permeabilities in both dolomite intervals here are very good. With permeabilities as good as they are in the Lower "B" especially, the operator may want to consider a localized flooding of this interval.
If sample 8901.6 (ft) is representative of the tight transition zone, then inter-communication between Upper and Lower "B" dolomites is possible via thin, open micro-fractures.
Deeper, coarser, less porous dolomites below 8905 ft are dependent on open or re-opened natural fractures for pore communication. Reasonable permeabilities here (1-8 md) reflect fairly good inter-crystalline porosity connected by micro-fractures; however, porosity and reservoir quality in this interval become markedly worse below 8912 ft.