Dolomite Types
Red River oil is most typically entrapped within dolomitized burrow-mottled mudstones and wackestones, and within laminated and cryptalgal dolomites. The best matrix porosity occurs within burrow-mottled dolomites which are commonly thick and well developed within the Red River D zone.
There are four types of dolomite present within the study area, and these are, from earliest to latest in the paragenetic sequence, DOLOMITE-1 through DOLOMITE-4. DOLOMITE-1 is the most abundant and occurs as early diagenetic and fabric preserving, microcrystalline to cryptocrystalline (<10 µ) dolomite replacement in which most of the visible porosity is associated with depositional and early diagenetic processes (interparticle, intraparticle, and moldic pores). Porosity in this type of dolomite is commonly good, but associated permeability is generally poor. DOLOMITE-1 is abundant in the A, B, and C zones. The Red River D zone contains sparse to common DOLOMITE-1 in low-permeability strata. This dolomite probably developed by the seepage of magnesium-rich brines into muddy sediments from overlying and proximate gypsum beds.
DOLOMITE-2 is characterized by fabric destructive, medium-sized (60-200 µ), euhedral to subhedral replacement crystals. It typically displays sucrosic intercrystalline porosity and moderate to high permeability because of well-developed, uniform intercrystalline pore space. This dolomite appears to account for the best conventional reservoirs. This type of dolomite was formed by late diagenetic replacement or recrystallization of earlier dolomites (DOLOMITE-1) by saline hydrothermal fluids. This type of late replacement is relatively sparse in the Red River B zone and absent in the C zone, but abundant in the D zone.
The third type of Red River dolomite is scattered clear, 20 to 50-µ dolomite rhombohedrons along stylolites and pressure-solution seams. DOLOMITE-3 clearly formed in the subsurface during, or after, significant compaction and matrix pressure-solution, and is generally associated with loss of pre-existing matrix porosity due to its precipitation as an important reactant mineral during pressure solution. It appears to be most abundant in the Red River A, B, and C zones.
The fourth type of dolomite consists of pore-filling cements in the form of very coarse (up to 1 cm across), white to light brown crystals of dolospar and baroque or saddle dolomite (figure 36). This dolomite formed under moderate to high temperatures (>90°C). Baroque dolomite and dolospar are present in the Red River B zone where early and late pore space is plugged by this cement. DOLOMITE-4 is relatively sparse within Red River D zone beds.