Petrographic Description
Carbonate classification and assignment of lithologic facies are largely based on sedimentary structure, dolomite and/or limestone texture, recognizable grain types, stratigraphic position, and comparison with previous Red River "B" studies in nearby wells. Lower "A" limestone, "B" anhydrite, Upper "B" porous dolomite, "B" transition dolomite, Lower "B" porous dolomite, and Lower "B" limestone and dolomite have distinct lithologic and reservoir characteristics, and will, therefore, be discussed separately below.
Lower "A" Limestone
Sample 8892.0 ft taken near the top of the cored interval, is a wavy-laminated minimally dolomitized, tight calcareous mudstone to wackestone and represents the lower portion of the cycle "A" limestone. Ostracode fragments are the most commonly encountered skeletal grains, with sparse bivalve or brachiopod fragments and possible codiacean algae also present. Intraclasts, micritic peloids, and rare phosphatic grains are nonskeletal components. Wavy and compacted calcareous mudstone laminae are also characterized by wispy organic stringers and several horizontal and vertical stylolites defined by concentrations of micro-crystalline pyrite, organic residuum, and phosphatic material (Plate 1).
Partial dolomitization has created replacive dolomicrite and micro-crystalline, euhedral dolomite rhombohedra "floating" in and replacing calcareous matrix (Plate 1); coarser ankerite is a minor, later-forming, pore-filling mineral. Fossil fragments remain calcareous, though many are filled or replaced by secondary calcite spar. Anhydrite is also an important authigenic mineral, forming large, coarsely crystalline, replacive patches and mold fillings.
Compared to porous "B" dolomites below, porosity development in this sample is generally poor. Small, isolated patches of micro-porosity exist in muddy matrix, and are spatially associated with numerous short, open micro-fractures originating at stylolite seams. Other similar micro-fractures exist within laminae or anhydrite patches, but vertical inter-connectivity among fractures is limited overall.
"B" Anhydrite/Dolomite Contact
Thin-section depth 8898.0 ft represents the stratigraphic and lithologic transition from cycle "B" anhydrite and underlying "B" porous dolomites (reservoir). As such, this sample is characterized by wavy and thinly interbedded, interlaminated anhydrite and dolomitic mudstone. Some beds are composed of strictly nonporous, nodular, microcrystalline anhydrite, whereas adjacent beds may be composed of admixed anhydrite and dolomite mud, loosely packed and highly porous dolomite, or dolomitized cryptalgal(?) laminites (Plate 2). Thin inter-laminae or stringers of organic material with pyrite are also present. Dolomite texture is dominated by microcrystalline, subhedral, loosely packed crystals, with intercrystalline porosity locally filled by anhydrite.
Porosity is definitely better developed than above, but is clearly lamination controlled: anhydrite beds are markedly less porous than dolomite and cryptalgal dolomite beds. Interconnected micro-intercrystalline pores are pervasive in porous dolomite beds, however, vertical inter-connectivity of porosity between beds does not appear good. Several layer-parallel, open fractures may enhance horizontal permeability in less porous beds.
Upper "B" Porous Dolomite
Two samples from depths 8900.0 ft and 8900.5 ft represent the highly porous beds of Upper "B" dolomite. Both samples are completely dolomitized, crudely or wavy laminated mudstones to wackestones the shallower sample (8900.0 ft), however, exhibits an irregular, intra-clastic texture of ripped-up or compacted beds of filamentous algae (Plate 3). Better preserved algae of this type at equivalent stratigraphic positions in other nearby wells are identified as the codiacean Dimorphosiphon. Peloids and peloidal molds, with possible relict algal features, dominate texture in the deeper sample (8900.5 ft, Plate 4). Open or anhydrite-filled skeletal molds of unknown affinity are present in both samples; overall, however, abundance and diversity of biogenic constituents are limited in both samples-typical of the Upper "B" facies.
Dolomite exists largely as micro-crystalline subhedral-to-euhedral rhombs replacing grains and matrix. The well-formed and loosely packed nature of dolomite texture likely indicates early, shallow dolomitization. More finely crystalline and tightly packed dolomicrite and coarser, euhedral, pore-filing dolomite are present locally. Coarsely crystalline anhydrite forms large (up to several mm in diameter) void- and mold-filling patches in both samples, though is somewhat more abundant in sample 8900.5 ft. In addition, minor amounts of an isotropic halite fill small vugs (Plate 3, and Plate 4). Textural relationships in thin section clearly illustrate that both anhydrite and halite precipitated after dolomitization. Trace organic residue and pyrite are also recognized in intercrystalline pores in both samples.
Approximately 20 percent porosity in this interval includes contributions by intercrystalline voids, interparticle pores, moldic pores, and micro-pores. Well-developed and well-interconnected intercrystalline porosity (5-l0u) is the largest contributor in both samples. Round dissolution molds left by leached peloids or other grains are particularly abundant and form a "Swiss cheese" texture in sample 8900.5 ft. Larger than inter-crystalline pores are inter-particle voids between algal rip-up clasts or intraclasts in sample 8900.0 ft. Peloids, algal fragments, and other features composed of dolomicrite commonly contain micro-pores of less than 5u size.
"B" Transition Dolomite
Marking the base of Upper "B" porous dolomites is a thin zone of tightly cemented, siliceous dolomitic packstone to wackestone commonly referred to as the "tight streak" or transitional lithofacies. Peloids and ostracodes are commonly encountered grains, with interspersed intraclasts and dasycladacean (Rhabdoporella) fragments. Thinly bedded structure is defined by varying proportions of dolomitic mud, peloids, ostracodes, and algae.
All grains and matrix are completely dolomitized and composed of subhedral-to-anhedral, micro-crystalline dolomite or dolomicrite. Dolomite crystal size generally reflects texture of material being replaced, e.g., peloids ale replaced by dolomicrite but rimmed by coarser dolomite. A diagnostic feature of this thin interval is the presence of pervasive authigenic silica: chert lines and fills smaller molds and voids, whereas coarser quartz lines larger vugs (Plate 5). Though distribution of chert is somewhat patchy, this mineral effectively reduces inter-crystalline and inter-particle porosity; measured porosity of associated plug sample is less than 6 percent. Porosity not occluded by silica typically exists as micro, inter-crystalline pores in peloids and other grains, as well as isolated dissolution pores or lined vugs. Pore communication in matrix is limited; however, open, vertical micro-fractures and associated micro-porosity along fracture walls (Plate 5) likely enhance permeability here, and may allow inter-communication between Upper and Lower "B" facies.
Lower "B" Porous Dolomite
Sample 8902.7 (ft) is a dolomitized, fossiliferous wackestone than represents the Lower "B" porous dolomite in this well. As is typical of the Lower "B" lithofacies in other wells, faunal and floral diversity and abundance are notably higher than the Upper "B", with skeletal grains comprising brachiopods ostracodes bryozoan fragments, filamentous algae (Hedstroemia?), pelmatozoan echinoderms possible tabulate coral fragments, and platy, porous features interpreted as stromatoporoid fragments. Crude layering is defined by platy stromatoporoids and other flattened skeletal debris.
Completely dolomitized texture reflects original, varied textural character of grains and matrix: dolomicrite micro-crystalline and finely crystalline dolomite rhombs and coarse pore-filling baroque dolomite are all present (Plate 6). Dolomicrite commonly replaces peloids, other muddy grains, and mud matrix, whereas micro- and finely crystalline dolomite replaces skeletal grains and fills porosity. Coarse, baroque dolomite is strictly pore filling, and precipitated after (and deeper than) the main phase of dolomitization. Anhydrite and halite are minor pore-filling cements after dolomite, and traces of organic residue are observed in intercrystalline porosity.
Greater than 20 percent porosity is typical of the porous Lower "B" unit. Loose dolomite texture creates a wealth of inter-crystalline porosity; micro, inter-crystalline pores 5u or less in size are most abundant overall and are found throughout the mud-rich matrix. Other recognized pore types include larger inter-crystalline pores in skeletal grains, isolated moldic and skelmoldic pores and vugs, and a few non-penetrative, open micro-fractures cross-cutting matrix. Good pore inter-connectivity is shown by horizontal and vertical permeabilities of 11-12 md and 2.3 - 7.7 md, respectively, which are appreciably higher than measurements in equivalent Lower "B" dolomites in other studied wells.
Lower "B" Dolomite and Limestone
Fractured sucrosic dolomite represented by samples 8908 and 8914.3 (ft) contrasts with overlying porous, fossiliferous Lower "B" dolomites. Uniformly finely crystalline (50-100u), subhedral to euhedral, variably porous dolospar dominates texture in sample 8908 (ft) and most of sample 8914.3 (ft), and is distinctly coarser than dolomite above. The other part of sample 8914.3 (ft) is characterized by tight, fossiliferous, calcareous wackestone likely representing the non-dolomitized deeper beds of the Lower "B" facies; contact between limestone and dolomite is locally stylolitic. In dolomites, sparse dolomitized or calcareous (8914.3 ft) pelmatozoan and bivalve fragments are the only skeletal constituents. The calcareous wackestone, in contrast, contains pelmatozoans, brachiopods, bryozoan hash, gastropods algae and possible ostracodes nearly all of which remain calcareous or are partly silicified.
In both samples, dolomite rhombohedra commonly exhibit "cloudy" centers (where replacive) and "cleaner" rims (Plate 7 and Plate 8), with Fe-rich dolomite rims present in some cases. In addition, coarser, "cleaner" dolomite commonly fills molds, vugs and other pores. Textural differences between the two samples include: 1) more euhedral and loosely packed dolomite at 8908 ft; 2) more intercrystalline porosity at 8908 ft; 3) the presence of remnant calcareous clasts and matrix exhibiting partial consumption by replacive dolomite at 8914.3 ft; and 4) a greater abundance of patchy, replacive chert at 8914.3 ft.
Inter-crystalline porosity of (l0-l00u) size is generally well developed in patches of loosely packed dolomite at 8908 ft. Such pores are rare in the deeper sample, where limited porosity exists as patchy micro-porosity in chert and calcareous mud. Open and filled, cross-cutting fractures characterize both samples, and are especially prevalent at 8908 ft. Here, numerous anhydrite plus halite-filled veins are reopened or cross-cut by a network of oven micro-fractures (Plate 7). Inter-connected micro-fractures likely enhance permeability (1.8-3.2 md) at this depth, given that porosity is only 7.5 percent.