PETROGRAPHY OF THE TRENTON AND BLACK RIVER GROUP CARBONATE ROCKS IN THE APPALACHIAN BASIN

DOLOMITE TEXTURES, DIAGENESIS, AND POROSITY

Several prolific petroleum reservoirs are hosted in dolostones in the Black River Formation in south central New York and north central Pennsylvania. Equally productive dolostone reservoirs in the Trenton Formation were developed in northwestern Ohio in the late 1800's and early 1900's and produced that state's only giant field (Wickstrom and others, 1992). Recent exploration in Ohio has discovered Trenton and Black River dolostone reservoirs in the north central and northeastern parts of the state. Similar dolostone reservoirs in these rocks and their equivalents produce petroleum in Kentucky, Tennessee, Michigan, Ontario and elsewhere. The petrographic and related petrophysical characteristics of the dolomite reservoir rocks are of great interest to geologists and engineers working in this play. It is clear that these petroleum reservoirs developed in hydrothermal dolomites, that these hydrothermal dolomites formed in the subsurface during the interaction of hot saline brines with the carbonate country rock, and that the hydrothermal brines migrated into the rocks they altered via faults, specifically basement-related faults with some strike-slip component (Wickstrom and others, 1992; Smith and others, 2005). It also is clear from petrographic studies that the hydrothermal fluid flux, which generated the dolomite, both created and destroyed reservoir porosity in these essentially tight rocks. The porosity and permeability distribution and evolution in the Black River and Trenton dolostone reservoirs is a direct function of 1) fractures and 2) variable reaction stoichiometry during limestone replacement and/or dolomite recrystallization as hydrothermal brines moved through these rocks via fault-related fractures.

Two basic dolomite textures exist in sedimentary rocks- planar dolomite and nonplanar dolomite (Woody and others, 1996). Planar dolomite crystals have straight boundaries, whereas nonplanar dolomite crystals have curved, lobate, serrated, indistinct, or otherwise irregular boundaries, and often have undulatory extinction (Sibley and Gregg, 1987, p. 970). Planar dolomite forms in both near-surface and burial diagenetic environments. Near-surface diagenetic environments extend to a few hundred meters of depth and are influenced by local groundwater flow systems; burial environments extend from the near surface to depths in excess of 3000 m and are influenced by intermediate to regional groundwater flow systems (Machel, 1999). Nonplanar dolomite develops at temperatures greater than 50°C in burial environments (Woody and others, 1996). Hydrothermal dolomites represent a special case of dolomitization in the burial environment where the dolomite formed at a temperature at least 5 to 10°C higher than the temperature of the surrounding strata (Machel, 2004). Geochemical data presented by Smith and others (2005, and in the reports of this research effort) demonstrate that the reservoir dolomites of the Black River and Trenton play in the Appalachian basin satisfy this criterion. The massive, mostly nonplanar hydrothermal dolomites of the Black River and Trenton reservoirs discussed in this report formed locally around faults in burial environments (Smith and others, 2005).

For this report we chose to use the dolomite textural classification of Wright (2001). Wright's classification combines those of Sibley and Gregg (1997) and Gregg and Sibley (1994) with the recognition of a transitional texture between planar and nonplanar dolomite. The classification has two principal categories: 1) crystal size distribution - unimodal or polymodal, and 2) crystal boundary shape - planar or nonplanar. Planar crystal boundaries are further subdivided as euhedral (planar-e) or subhedral (planar-s). Planar-c dolomite is cement that lines or fills pores and planar-p dolomite is porphyrotopic. Nonplanar dolomite occurs as anhedral dolomite mosaics (nonplanar-a), saddle dolomite (nonplanar), and porphyrotopic crystals (nonplanar-p). Our complete textural description includes recognizable grains, matrix, and cement. Allochems and cements may be unreplaced, partially replaced, or completely replaced. Replacement may be mimetic or non-mimetic. Figures A3-1 through A3-9 show some of the various types of dolomite textures we observed in the Trenton and Black River rocks of the Appalachian basin.

There are two distinct types of dolomite in the Trenton and Black River rocks of the Appalachian basin- planar and nonplanar dolomites. The first type of dolomite (TYPE I) consists of micron- to centimicron-size (usually decimicron-size) planar dolomites. Most of this dolomite is matrix-selective replacement dolomite, but some planar-p and planar-e dolomites locally replace allochems too, and we observed a few examples of void filling planar-c dolomite cement (Figure A3-5). These planar dolomites are pervasively developed in thin beds of supratidal and intertidal carbonate facies (Figure A3-3B), and also occur in all subtidal facies throughout the basin as common, but intermittent lenses, laminae, and very thin beds ( Figure A3-2C). The former planar dolomite probably formed in peritidal environments through reflux and/or mixing zone dolomitization; the latter formed on a local scale in buried subtidal sediments via compaction-driven fluid flow (see Machel, 2004, Figure 19). The planar dolomite textures also occur as a limestone replacement adjacent to faults; these crystals are probably hydrothermal (Smith and others, 2005; Harris, 2005), and clearly predate later nonplanar dolomite.

The second type of dolomite (TYPE II) consists of decimicron- to millimeter-size (usually centimicron-size) transitional and nonplanar dolomites. Planar-s to nonplanar (transitional) dolomite and nonplanar-a dolomite occur as an obliterative replacement of limestone (matrix and allochems) and as neomorphic recrystallization of planar dolomite. Nonplanar (saddle) dolomite occurs as pore-lining and pore-filling cement. The transitional and nonplanar dolomites occur only in association with localized, basement-related faulting and fracturing, and are most likely hydrothermal in origin (see Wickstrom and others, 1992, Harris, 2005, and Smith and others, 2005).

The iron content of both types of dolomite is highly variable throughout the basin. Harris (2005) reports that fault-related, relatively early planar dolomites in central Kentucky are nonferroan, with some iron zoning evident in the crystals, and that later nonplanar (saddle) type dolomites are iron-rich. Conversely, in northwestern Ohio, the so-called "cap dolomite" (texturally a planar type dolomite) at the top of the Trenton Formation there consists of ferroan decimicron-size planar dolomite (Keith, 1988,) while the fracture-associated nonplanar dolomites may be ferroan (Budai and Wilson, 1986), slightly ferroan (Taylor and Sibly, 1986), or nonferroan (Haefner and others, 1988). Other planar dolomites in Ohio (so-called "regional dolomite") reportedly are nonferroan (Taylor and Sibley, 1986; Keith, 1988). Smith and others (2005) report ferroan planar and nonplanar type dolomites in the subsurface of central New York State. Using EDS in conjunction with SEM microscopy, we measured iron concentrations from 2.17 to 4.26 wt. percent in nonplanar dolomites from New York. In Ohio samples, we measured iron concentrations from 0 to 1.5 wt. percent in planar dolomites (nonferroan) and 0.28 to 10.92 wt. percent in nonplanar dolomites. The highest iron concentrations occur in nonplanar (saddle) dolomite cements that line and fill pores.

Rocks recovered from most of the Trenton and Black River dolostone reservoirs in the Appalachian basin exhibit a complex combination of planar and nonplanar crystal textures. It is clear that epigenetic dolomitization and neomorphic dolomite recrystallization took place in response to exposure of the rocks to hydrothermal mineralizing fluids, and the resultant textures overprint both precedent limestone diagenetic fabrics and earlier diagenetic dolomites textures.

Association with Base-Metal Sulfide Mineralization

The formation of dolomite reservoirs in the Black River and Trenton Formations was a direct result of massive geochemical and textural alteration of their precursor carbonate mineralogy and fabric by hydrothermal, basinal saline brines (Davies, 2000; Gregg, 2004; Smith and others, 2005, and data generated during this research). These alterations included 1) dolomitization of limestone and neomorphic recrystallization of planar dolomites, 2) iterative precipitation of pore-filling nonplanar (saddle) dolomite, calcite, quartz, sulfates, halides, sulfides, and supergene metal oxides alternating with intervals of 3) carbonate dissolution. Appendices 3 and 4 and 5 show examples of all of these alterations. Geochemical evidence for the hydrothermal origin of these alterations is given elsewhere in this research by Smith, and will be presented in final form at the conclusion of this project.

This association of hydrothermal dolomitization and base-metal sulfide mineralization adjacent to faults is directly responsible for reservoir porosity and permeability in the Trenton and Black River carbonates of the Appalachian basin. These carbonates were rendered remarkably tight by limestone diagenesis, particularly marine diagenesis on and just below the sea floor, and by neomorphism during early burial. Excellent commercial porosity and permeability developed only where the hot basinal brines invaded the Trenton or Black River limestones via fault-related fractures. Dolomitization did more to reduce and destroy porosity, however, than it did to enhance it (Figure 10). Fractured limestones in these formations are productive in West Virginia and Kentucky, but these fields do not have the same kind of reserves or sustained production as those developed in dolomite in Ohio, New York, or Pennsylvania.

All of the dolomite fabrics observed in these rocks developed before significant chemical compaction took place, an observation that constrains the timing of fracturing and hydrothermal dolomitization. Porosity and Permeability We use two fundamental porosity classifications in this report. The first is that of Choquette and Pray (1970), which defines the basic type of porosity present in a carbonate rock, and then provides an indication of the pore space's genesis and abundance. In this texturally descriptive scheme, the voids are fabric selective, not fabric selective, or some combination thereof. The carbonate components, crystals, or other physical structures in the rock control fabric selective pores - the voids do not cross the boundaries of these features. Pores that are not fabric selective do cross the component boundaries and are larger that the carbonate allochems or crystals that contain them. We also use the carbonate pore size classification of Luo and Machel (1995), which relates pore size to the fluid flow characteristics of the reservoir rock.

There are distinctive pore textures in Trenton and Black River dolograinstones, dolopackstones, dolowackestones, and dolomudstones. In the productive dolograinstones and dolopackstones of northwestern Ohio, the porosity development is partially related to depositional texture. This depositional texture may or may not be readily recognizable to the unaided eye. The reservoir rocks consist of planar-s to nonplanar-a and saddle dolomite. Macroporosity is not fabric selective, and consists of small to medium vugs and fractures. Mesoporosity is fabric selective, and consists of moldic and intercrystalline voids. Microporosity is both intercrystalline and intracrystalline. Porosity developed through a combination of fracturing, selective dissolution of allochems (usually crinoids), and dissolution of both calcite and dolomite. Appendix 5 includes numerous examples of these pore textures.

Dolowackestones and dolomudstones are the most productive reservoirs in the Trenton and Black River play in the Appalachian basin. The depositional texture of these rocks is readily recognizable (see Appendix 5). These rocks consist of planar-s to nonplanar-a dolomites and saddle dolomites. Macroporosity is not fabric selective, and consists of 1) voids associated with zebra and breccia fabrics, 2) small to large vugs, and 3) fractures. Mesoporosity is fabric selective, consisting of intercrystalline and moldic voids. Microporosity is both intercrystalline and intracrystalline. Porosity developed through fracturing, associated brecciation, and dissolution of both calcite and dolomite. Appendix 5 provides several examples of these pore textures. Petrophysical Considerations The dominance of vuggy porosity in Trenton and Black River reservoirs, and their association with intercrystalline and fracture voids, yields a complex pore geometry. The predominance of planar-s to nonplanar dolomite textures means that we can expect bimodal to intermediate Hg capillary pressure curves with steep gradients in these rocks, reflecting poorly interconnected pore networks with large pore to throat size ratios (Woody and others, 1996). The best reservoirs occur where fractures provide interconnections between large dissolution voids yielding collapse breccias and adjacent zebra fabrics. Geologists and engineers should anticipate the fact that the Archie equation is unreliable for geophysical log evaluation in these and use appropriate alternate techniques (Asquith, 1985).
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