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

Christopher D. Laughrey - Pennsylvania Geological Survey
Jaime Kostelnik - Pennsylvania Geological Survey

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INTRODUCTION

Carbonate petrography is the study of limestones, dolostones, and associated rocks by optical and electron microscopy (Scholle and Ulmer-Scholle, 2003). Our purpose in the petrographic portion of this study is 1) to enhance field studies and core descriptions of Trenton and Black River Group carbonate rocks in the Appalachian basin, 2) to interpret the diagenetic history of these rocks, particularly with regard to its effects on porosity and permeability, and 3) to provide a frame of reference for the geochemical data and interpretations presented elsewhere in this study.

Petrographic data enhance field and core descriptions of carbonate rocks through the identification of constituent grains, matrix, and depositional texture. This data enable us to develop detailed classifications of the reservoir rocks and interpret their depositional setting.

Interpretations of the diagenetic history of the Trenton and Black River carbonates provides us with a sense of the comparative timing of significant diagenetic events such as cementation or secondary porosity development relative to the emplacement of hydrocarbons in the reservoirs. Such interpretations are particularly critical for understanding Trenton and Black River reservoirs. Cambro-Ordovician carbonate reservoirs worldwide are dominated by meter-scale peritidal, mud-dominated cycles and thin-bedded, heterogeneous layering; the only buildups are thrombolitic or microbial, and reservoir quality is controlled by fracturing, dolomitization, and karst development beneath the top-Saulk unconformity (Markello, 2005). The Trenton and Black River Groups consist of remarkably tight carbonate rocks that were completely lithified throughout the basin during marine and burial diagenesis. Reservoirs only occur where fracturing and dolomitization created adequate storage capacity for commercial hydrocarbon accumulations.

Petrography provides a frame of reference for the geochemical data we use to understand the dolomitization processes that yielded the most important petroleum reservoirs in the Trenton and Black River Groups in the Appalachian basin. The geochemical data include stable carbon and oxygen isotopes, fluid inclusions, ⁸⁷Sr/⁸⁶Sr ratios, and trace element (Fe and Mn) distributions.

The petrographic data presented herein are useful on a broad scale for documenting the distribution of porous and permeable carbonate rock facies in the Trenton and Black River Groups in the Appalachian basin. The data allow us to determine the spatial distribution of reservoir seals, compartmentalization, and diagenetically controlled pore geometry in the productive gas fields. to top

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METHODS

We collected samples from 17 different cores of Trenton and/or Black River carbonate rocks recovered from wells throughout the Appalachian basin. The cores we examined and sampled are listed in Table 1. We described the cores before sampling, and these descriptions will be integrated into the pending reports on stratigraphy. We also collected cuttings samples from one well in Pennsylvania for thin sections, and prepared numerous thin sections from outcrop samples in Pennsylvania and Kentucky. In all, we analyzed 740 thin sections of Trenton and Black River carbonates. Selected samples were further studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy.

We used a Leica DML polarizing microscope equipped with a Leica DFC camera for all transmitted light microscopy. We examined selected unembedded and unpolished samples on a Hitachi S-2600N variable pressure scanning electron microscope (SEM) operating with a pressure of 10 Pascals using the backscattered electron detection imaging system at 20 kV and a working distance of 15 mm. The elemental composition of these samples was determined using Quartz Imaging System's Quartz XOne energy-dispersive X-ray spectroscopy (EDS) package with a Gresham Sirius 10/UTW/SEM detector having a 10 mm² crystal and an ultra thin polymer window. We used the same SEM operating at 20 kV and a working distance of 15 mm when conducting EDS analyses.

We classified all carbonate rock samples using the classification systems of both Dunham (1962) and Folk (1962). We found Dunham's (1962) classification useful for microfacies analysis when we could use it - it is descriptive, objective, and easy to use in the field or laboratory, and it conveys some genetic information. But the Dunham classification fell short in naming many of the Trenton and Black River limestones and dolostones significantly altered by diagenesis. In these instances the Folk (1962) classification system provided us with a useful, objective and descriptive terminology that provided for grain size, allochem composition, and diagenetic alterations. For these reasons, we use carbonate names from both systems interchangeably throughout this report. We classified all dolostones using the classification system of Wright (2001). We classified carbonate porosity using the systems of both Choquette and Pray (1970) and Luo and Machel (1995). to top

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CONSTITUENTS

Skeletal Grains

The skeletal constituents of carbonate rocks reflect the distribution of calcium carbonate-secreting organisms throughout geologic time. Many new carbonate-secreting marine organisms emerged by Middle Ordovician time, some 450 million years ago, and these are well represented in the composition of Trenton and Black River rocks. Fragments of brachiopods, bryozoans, crinoids, corals, trilobites, calcareous algae, and gastropods comprise the principal skeletal grains in the rocks. The distribution of these organisms in the carbonate depositional realm was controlled by environmental factors such as water depth, temperature, salinity, substrate, and turbulence. Thus the correct identification of these skeletal grain types and their depositional texture is critical for correct environmental interpretations.

The original mineralogy of the skeletal grains, i.e., aragonite, low Mg calcite, high Mg calcite, or a mixture of aragonite and calcite, affected the fate of the skeletal grains during diagenesis. Their susceptibility to recrystallization, dissolution, and dolomitization was particularly important to the development of Trenton and Black River carbonate reservoirs in the subsurface.

Skeletal grains are identified on the basis of their size, shape, microstructure, and original mineralogy (Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003). Appendix I provides a comprehensive review and photographic guide of the major skeletal grains that occur in the Trenton and Black River rocks of the Appalachian basin.

Non-skeletal Grains

Non-skeletal carbonate grains in the Trenton and Black River rocks include ooids, peloids, grain aggregates, and clasts. Appendix I includes a general review and photographic guide of the non-skeletal grains found in the Trenton and Black River rocks of the Appalachian basin.

Ooids are a type of coated carbonate grain, spherical to sub spherical in shape, consisting of one or more regular concentric lamellae around a nucleus. The nucleus is often a carbonate particle, but can be a non-carbonate clastic particle too. The term ooid is restricted to grains less than 2 mm in diameter, and most ooids range from 0.2 to 0.5 mm in diameter. This is the size of fine- to medium- grained sand in the Wentworth scale. Recent marine ooids exhibit tangential, radial, or random microfabrics. Ancient marine ooids may have relic tangential microstructures or, more commonly, radial microfabrics. Many ancient ooids are micritic or display replacement with neomorphic spar. We only found ooids in portions of the Black River Group. We did not find any ooids in the Trenton Group rocks that we examined, and we could not find any report of Trenton ooids in the literature.

While ooids are locally important, peloids are the most diverse and abundant non-skeletal grains in the limestones and dolostones that we examined, and their origins are diverse and complex. Indeed, many, if not most, of the peloids in the Trenton and Black River carbonates may not be grains at all but cements instead. The original textures of the peloidal limestones appear to have influenced dolomitization processes and subsequent dolomite fabrics and porosity distributions in the Trenton and Black River petroleum reservoirs throughout the Appalachian basin.

Peloids are a type of non-skeletal carbonate particle formed of cryptocrystalline and microcrystalline calcium carbonate, and/or carbonate microspar (Scholle and Ulmer-Scholle, 2003, p. 254). Peloids are spherical, cylindrical, or angular particles composed of aggregated carbonate mud and/or precipitated calcium carbonate. They exhibit no internal structure. There is no defined restriction on the size or origin of peloids, thus the term allows reference to allochems composed of micritic material without implying their specific origin (McKee and Gutschick, 1969).

Peloids are polygenetic, and identifying their precise origin is often difficult in carbonate rocks (see discussions by Macintyre, 1985 and Scholle and Ulmer-Scholle, 2003). Some peloids are fecal in origin (carbonate pellets), while others are grains derived from calcareous algae, micritized allochems, and reworked mud clasts (Tucker and Wright, 1990; see the examples presented in Appendix I). Most peloidal textures in carbonate rocks, however, probably are chemical in origin, i.e., cements in which the peloids appear as clots with a flocculent fabric- the structure grumeleuse of Bathurst (1975, p. 511 - 513 and Figure 350). These clots are the nucleation sites of small crystals of high-magnesium calcite (Tucker and Wright, 1990). The nuclei may be organic, possibly microbial matter (Chafetz, 1986) or simply sub-microcrystalline, radial, acicular calcite crystals that grew around a small number of nuclei (Bosak and others, 2004). In either case, the peloids precipitated in situ as marine cement on or just below the sea floor (Tucker and Wright, 1990; Malone and others, 2001; Bosak and others, 2004). The recent work of Bosak and others (2004) recommends that abiotic mechanisms should be the null-hypothesis for peloid formation.

Peloidal textures are ubiquitous in the Trenton and Black River carbonates throughout the Appalachian basin. They occur in all carbonate rock types and their origins are quite diverse. Appendix I contains numerous examples of peloids that clearly are carbonate grains. In many instances, however, we interpreted peloidal fabrics in Trenton and Black River rocks as cement. We discuss peloidal cement textures in detail below in the section on diagenesis.

Most of the dolomitized carbonate rocks of Black River age in the subsurface of west central New York state and north central Pennsylvania exhibit a precursor peloidal fabric that dominated the limestones there. The petrophysical character of microporosity in these precursor peloidal limestones may have been critical in controlling the migration of dolomitizing fluids through the rocks adjacent to faulted and fractured strata (see Cantrell and Hagerty, 1999). These peloidal textures also are common in dolomitized limestones in western Ohio and central Kentucky.

Matrix

Fine-grained matrix in the Trenton and Black River Formations consists of calcite micrite, microspar, pseudospar, and terrigenous clay minerals. Micrite is composed of small calcite crystals 1 to 4 m in diameter. These crystals formed through the breakdown of coarser carbonate grains, such as calcareous algae, or through inorganic precipitation on the seafloor (Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003). Figure 1 shows several examples of micrite matrix in the Trenton and Black River Formations. Microspar consists of calcite crystals 5 to 30 m in diameter. It forms through neomorphic recrystallization of micrite. Pseudospar also is a recrystallization product of finer calcite, but it is even coarser than microspar with crystal diameters of 30 to 50 μm. Clay mineral, predominately illite, mixed-layer illite-chlorite, and smectite, occur in some Trenton Formation samples, particularly in ramp slope carbonates intimately interbedded with terrigenous shales (Figure 2). Some of the matrix material in the Trenton Formation is organic rich (up to 3.74% total organic carbon), and might be important as a petroleum source rock.

Other Components (non-authigenic)

In addition to terrigenous clay minerals, non-carbonate sedimentary components of the Trenton and Black River Formations include detritial quartz silt and very fine sand, bentonite, and rare glauconite. The quartz entered the carbonate environments as air-borne and/or water-borne sediment. Very fine quartz sand and silt dispersed throughout some of the limestones in the Black River Formation (<1% of the rock) might be air-borne, but was more likely reworked from very thin fluvial and paralic siliciclastic accumulations that were deposited on the carbonate ramp during sea level lowstands (Figure 3). The volcaniclastic k-bentonites (Figure 4) are interbedded with the carbonates, and were rapidly deposited below storm wave base by volcanism along the island arc that formed along the southeastern Laurentian continental margin during the Taconic orogeny (Thompson, 1999). Reworked fragments of these k-bentonites occur in some of the limestones. to top

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MICROFACIES AND DEPOSITIONAL ENVIRONMENTS

All of the principal carbonate rock types occur in the Trenton and Black River Formations of the Appalachian basin. The depositional texture of the original limestones correlate directly to the sedimentary environments of the carbonate ramp on which the various rock types accumulated. Tidal flat and lagoonal limestones mostly consist of mudstone and wackestone, with thin packstone/grainstone deposits that collected within tidal channels. These rocks were deposited in peritidal settings as low-energy, shallowing-upward successions and thick lagoonal successions. Although less common, some grainstones and packstones accumulated as high-energy shallowing upward successions deposited as beach carbonates in the peritidal settings too.

Subtidal deposits include boundstones, grainstones, packstones, wackestones, and mudstones deposited in both shallow and deeper carbonate ramp setting. Rocks that formed above wave base on the shallow ramp include hardground-bounded, amalgamated grainstone, and grainstone-capped high energy shoaling successions. Skeletal packstone/wackestone - mudstone successions, also bound by well-developed hardgrounds, formed on the deeper ramp below fair-weather wavebase. Graded carbonate beds - tempestites and turbidites - accumulated in slope and basin margin environments. These consist of upward-fining grainstone/packstone and wackestone/mudstone couplets separated by siliciclastic shales. The carbonate materials were introduced from the carbonate ramp, while the shales came from the Taconic highlands and volcanic arc along the southeast margin of Laurentia (Pope and Steffens, 2003). to top

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DIAGENESIS

The term diagenesis refers to all of the processes that affect sediment from just after deposition up to the lowest grade of metamorphism, the greenschist facies (Pettijohn and others, 1987). It is the sum of physical, chemical, and biochemical changes occurring in a sedimentary deposit after its initial accumulation, excluding metamorphism (Friedman and Saunders, 1978). Diagenesis in the rocks of the Trenton and Black River Groups included seven major processes: 1) microbial micritization; 2) cementation; 3) neomorphism; 4) replacement; 5) physical and chemical compaction; 6) dissolution; and 7) dolomitization (see, Tucker and Wright, 1990, p.314). Dolomitization is the most important diagenetic aspect of the Trenton and Black River petroleum reservoirs and is discussed separately in the next section of this report.

Note: The term diagenesis is used differently by organic geochemists, and we employ their usage in the petroleum geochemistry reports of this research. In petroleum geochemistry, diagenesis is the process involving the biological, physical, and chemical alteration of the organic debris in sediments without a pronounced effect from rising temperature (Hunt, 1996). It covers the range of temperature up to about 50°C (122°F). Readers should bear these different usages in mind when reading this report.

Microbial Micritization

Partial to complete microbial micritization of skeletal grains, ooids, and probable pellets occurs in most carbonate rock types in the Trenton and Black River Groups throughout the Appalachian basin. Microbial micritization is most evident in bioclastic grainstones and packstones where micrite envelops developed on skeletal grains, and in some mixed oolitic/peloidal grainstones and packstones where the peloids show compelling evidence of carbonate grain degradation and replacement with micrite. In both cases, the micritization process may have been mediated by endolithic algae, fungi, or bacteria and associated biochemical or physiochemical processes.

The micrite envelops shown in Appendix II (Micritization Examples #1 - 4) formed around many of the bioclasts in mixed-fossil grainstones. These envelopes are essentially identical to those shown and discussed by Milliman (1974), Bathurst (1975), Tucker and Wright (1990), and Scholle and Ulmer-Scholle (2003). Bathurst (1975, p.381 - 389) suggested such envelopes formed through the filling of altered grains rather than precipitation of a new rind around the grain: algae, fungi, or bacteria bore into the grain, die, and the subsequent alteration of the organic material provides a chemical environment conducive to calcium carbonate precipitation, thus filling the voids.

Intense activity by endolithic microbes can lead to complete micritization of carbonate grains. This was relatively common during deposition of both Trenton and Black River carbonates. ( Figure 5) shows wholly micritized ooids in the Black River Formation in the subsurface of western New York.

The micrite envelope surrounding the echinoid fragment in Figure 6 originated in the same way as those shown Appendix II, i.e., through marine cementation within spaces created by endolithic boring organisms. The micrite cement shown in Figure 6, however, displays a distinctive clotted or peloidal texture Figure ( 6b and 6c). Higher magnification ( Figure 6c) reveals that the "clots" of microcrystalline calcite in the micrite rind consist of unimodal, decimicron-size spherical clusters composed of even smaller (micron- and sub micron-size) calcite crystals. Each clot or peloid consists of a brownish, cloudy nucleus and a rim of clear, well-developed euhedral crystals. These clots grade into adjacent patches of coarser, centimicron-size microcrystalline matrix with an identical peloidal fabric or into coarser calcite spar cement. These micrite envelopes and contiguous patches of clotted microcrystalline calcite may be bacterially induced calcite precipitates, or strictly abiotic cement (Lighty, 1985; Macintyre, 1985; Chafetz, 1986; Tucker and Wright, 1990; Scholle and Ulmer-Scholle, 2003; Bosak and others, 2004).

Cementation

The precipitation of calcium carbonate cements in the Trenton and Black River limestones was a major diagenetic process during and shortly after deposition on the sea floor in Ordovician time. Burial cementation also affected the rocks. The principal calcite cements in the rocks are 1) peloidal calcite, 2) prismatic fibrous to bladed calcite rinds on allochems, 3) meniscus calcite cement, 4) syntaxial calcite overgrowths, and 5) calcite spar. Dolomite cements are important, but we discuss these later in the section on dolomite textures. Several late-stage non-carbonate cements are associated with dolomite cements and we discuss these in the section on dolomitization.

Peloidal Cements

While microbial micrite envelopes around allochems and micrite replacement of grains are common in the Trenton and Black River carbonates of the Appalachian basin, most of the microcrystalline calcite, microspar, and pseudospar in these rocks occur as matrix in packstones, wackestones, and mudstones. As discussed above, much of this matrix originally formed in situ as carbonate mud derived from the breakdown of organisms, and it now exists in the rocks as neomorphic, recrystallized calcite. A great deal of the carbonate matrix in all of the limestone types, however, also formed in situ as peloidal cement. The clotted fabric of peloidal cement is ubiquitous in the Trenton and Black River limestones, and can usually be discerned even through most dolomite fabrics.

Figure 7 shows core and thin section photographs of Black River Formation limestone recovered in the Gray #1 well core, Steuben County, NY. Figure 7a shows the macroscopic appearance of this limestone - a seeming medium gray, bioturbated and burrowed sparse biomicrite (Folk, 1962) or wackestone (Dunham, 1962). There are problems with using the Dunham (1962) classification for this sample (see discussion below) so we prefer the Folk (1962) name. Dissolution structures (solution seams) indicate moderate chemical compaction.

Thin section examination of this sample reveals that skeletal grains comprise about 20 percent of the limestone, and include crinoids, bryozoans, brachiopods, trilobites, alga, and gastropods ( Figure 7b). Most allochems were altered by neomorphism, specifically recrystallization and degrading recrystallization. Authigenic pyrite, quartz, feldspar, and anhydrite along with dolomite make up about 5.5 percent of the rock. About 75 percent of this limestone, however, consists of decimicron-size peloids that could be interpreted as either framework grains or cement (Scholle and Ulmer-Scholle, 2003; Figure 7b). If framework grains, then the peloids might have originated as 1) algal material, 2) detritial sediment, 3) pellets, or 4) a replacement of other framework grains (Tucker and Wright, 1990, p.321 and references therein). If the peloids are cement then they are in situ precipitates.

Careful examination of the peloids at higher magnifications (Figure 7c and 7d) reveals that they most probably are cement and not carbonate grains. Individual peloids are 50 to 100 μm in diameter and consist of 1) a dark brown nucleus composed of micron-size calcite surrounded by 2) a rim of euhedral, dentate to blocky microspar. The average crystal size of the later is ~25μm. The highest magnification view ( Figure 7d) shows the nuclei consist of clots of submicron-size opaque material; this material might be organic, possibly microbial matter (Chafetz, 1986) or simply submicrocrystalline, radial, acicular calcite crystals that grew around a small number of nuclei (Bosak and others, 2004). If the nuclei are organic, the peloids likely originated as in situ precipitates around clumps of bacteria and the microspar likely is a neomorphic recrystallization product of earlier micrite matrix (Lighty, 1985; Macintyre, 1985; Chafetz, 1986; Tucker and Wright, 1990). If the nuclei are inorganic, the opaque appearance of the peloids probably is a consequence of the small crystal grain size relative to the thickness of the thin section (Bosak and others, 2004). In this case, the peloidal nuclei are strictly abiotic in origin and formed as calcite cement from suspension and geopetal settling (Bosak and others, 2004).

The peloidal fabric in this limestone occurs as: 1) dominant groundmass or "matrix" (Figure 7a, 7b, 7c, 7d); 2) as an internal cement within the zooecia of bryozoan skeletal grains and the lumens of crinoid fragments ( Figure 7e); 3) as cement filling fabric-selective pores, i.e., intraparticle voids ( Figure 7f); and 4) as mimic replacement of bryozoan grains ( Figure 7g).

All of the characteristics of the peloids - their uniform crystal size, restricted size range, consistent texture, monomineralogy, opaque nuclei, and euhedral outer rims- suggest that they are in situ precipitates which formed through cementation on or just below the sea floor (Tucker and Wright, 1990; Malone and others, 2001; Bosak and others, 2004. This peloidal fabric characterizes most of the fine-grained rocks in the Trenton and Black River carbonates that we examined.

The most compelling evidence for interpreting these peloidal textures as marine cement is the fact that this fabric is ubiquitous in Trenton and Black River Group hardgrounds throughout the basin (Figure 8). Hardgrounds are synsedimentarily lithified carbonate seafloors that become hardened in situ by the precipitation of carbonate cement in the primary pore space (Wilson and Palmer, 1992, p.3). They are sedimentary horizons in marine carbonates that exhibit evidence of exposure on the sea floor as lithified rock. Detailed discussions of hardgrounds in the Trenton and Black River rocks of the Appalachian basin can be found in Brett and Brookfield (1984) and Laughrey and others (2003). All of the fine-grained or finely crystalline hardground lithologies that we examined petrographically have a peloidal fabric.

A note on classification: The Dunham (1962) classification of the sample shown in Figure 7 as a wackestone based on hand sample description or a mixed-fossil/peloidal grainstone based on thin section description is wrong because the fine calcite crystal size and peloidal fabric do not reflect the limestone's depositional texture. The Folk (1962) classification sparse biopelmicrite provides a better name for the rock, although one might argue for sparse biopelsparite instead (see Scholle and Ulmer-Scholle, 2003, p.266 - 271). We prefer the former Folk (1962) name because the peloidal nuclei volumetrically dominate the rock and their micron-size calcite crystals can be properly called micrite (see Scholle and Ulmer-Scholle, 2003, p. 266). An alternate name, from the classification scheme of Wright (1992) is cementstone.

Other Calcite Cements

In addition to the peloidal calcite cements just discussed, calcite cement also occurs as prismatic fibrous to bladed rinds, meniscus-type cement, syntaxial overgrowths on allochems, poikilotopic calcite spar, and void-filling spar. We interpret these various calcite cements as products of both marine and burial diagenesis.

Prismatic fibrous to bladed rinds of calcite are common on all allochems we observed in the Trenton and Black River limestones. These crystals may grow directly on the allochems, or atop micrite envelopes. Good examples are shown Appendix II (Prismatic fringe examples 1 - 3). These morphologies are typical of modern high-Mg calcite and aragonite cements (Scholle and Ulmer-Scholle, 2003), but probably precipitated as calcite in the Ordovician sea (Lowenstein and others, 2001).

We observed unique meniscus-type cement (Hillgartner and others, 2001) in oolitic grainstones of the Black River Formation at Union Furnace, Pennsylvania (Appendix I, nonskeletal grains, ooid example #5). Meniscus cements usually are cited as evidence for meteoric diagenesis in the vadose zone (James and Choquette, 1984; Scholle and Ulmer-Scholle, 2003), but Hillgartner and others (2001) demonstrated a marine diagenetic origin for microbially-induced meniscus cements in carbonate sands of the Bahamas and Mesozoic platform carbonates of the Swiss and French Jura Mountains. These authors cautioned that an interpretation of early vadose diagenesis should not be based on meniscus cement alone. They suggested the term meniscus-type cement for these unique marine cements that form in association with the calcification of microbial filaments and the trapping of percolating micrite in subtidal settings. The meniscus-type cements in the Black River oolitic grainstones occur along with grapestone, oolitic intraclasts, micritized grains, bladed calcite rinds, and hardgrounds all suggesting sea floor lithification.

Syntaxial calcite overgrowths are common on echinoid fragments in the Trenton and Black River rocks of the Appalachian basin (Appendix II, Syntaxial Overgrowths Examples #1 and #2). Such cements are reported from meteoric, marine, and burial diagenetic environments (Scholle and Ulmer-Scholle, 2003), ands require careful geochemical and petrographic arguments in order to be diagnostic (Tucker and Wright, 1990). We interpret the syntaxial overgrowths in the Trenton and Black River Formations as products of marine and/or burial diagenesis because of their association with other marine cements in the rocks and the lack of any evidence for exposure to the meteoric environment (i.e., lack of grain dissolution and no evidence whatsoever of karst processes). Poikilotopic spar is likewise common (Appendix II, Poikilotopic spar Examples 1#1 and #2) and we interpret it to be of marine and/or burial diagenetic origin too.

Drusy mosaics of calcite spar fill most primary pore space in the Trenton and Black River Formations in the Appalachian basin (Appendix II, Equant Calcite Spar Examples #1 and #2, Drusy Spar Examples #1 and #2). The precipitation of this calcite spar followed chemical compaction of the limestones (Appendix II, Compaction Examples #5 and #6) indicating that these are clearly burial diagenetic environment cements (Wright and Tucker, 1990, p.352). Additional observations supporting this conclusion include broken and collapsed micrite envelopes within the spar and fractured grains cemented by the spar.

Neomorphism

A number of neomorphic fabrics occur in the Trenton and Black River rocks of the Appalachian Basin (Appendix II). Microspar and pseudospar commonly replace micrite in muddy carbonate rocks. Most of this is aggrading neomorphism (Appendix II, Neomorphism examples #1 - #3). Microspar and pseudospar also commonly replace allochems (see Appendix I, Coral Example #2 and #3).

Replacement

Numerous noncarbonate minerals replace both limestone and dolomite in the Trenton and Black River Formations. These include chert, chalcedony and quartz, feldspar, iron sulfides and oxides, sphalerite, fluorite, phosphate, sulfates, and chlorides. Some examples follow.

Chert and chalcedony replace both limestone and dolomite in the Trenton and Black River rocks throughout the Appalachian basin. Appendix II (Silicification Examples 31 - #4) and Figure 9 show examples of this replacement. Chalcedony that replaces hydrothermal dolomite is length-slow (Figure 9), possibly implying that the replacement took place in a sulfate-rich aqueous environment (Folk and Pittman, 1971; Scholle and Ulmer-Scholle, 2003).

Compaction (including pressure solution)

Compaction fabrics in the Trenton and Black River Formations are widespread throughout the basin. They include mechanical compaction features such as plastic deformation of soft grains and brittle fractures in grains, and chemical compaction features such as cocavo-convex and sutured contacts between grains, dissolution seams, and stylolites. Examples of these are provided in Appendix II.

Dissolution

There is very little evidence for dissolution features in the limestones of the Trenton and Black River Formations in the basin. This is consistent with what is known of the Ordovician carbonate systems on a global scale (Markello and others, 2005), and with recent work on the local scale by Harris (2005) and Smith and others (2005). Limestone dissolution adequate for creating commercial reservoirs is restricted to processes associated with dolomitization adjacent to fractures. These are discussed below. to top

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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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31. Markello, J. R., R. B. Koepnick, and L. E. Waite (2005), The carbonate analogues through time (CATT) hypothesis - a systematic and predictive look at Phanerozoic carbonate reservoirs, 2005 AAPG Annual Convention, Technical Program and Abstracts, http://aapg.confex.com/aapg/cal2005/techprogram/meeting_cal2005.htm>.
    
    
32. McKee, E. D. and R. C. Gutschick (1969), History of Redwall Limestone of northern Arizona, Boulder, CO., Geological Society of America Memoir 114, 726p.
    
    
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34. Pope, M. and J. B. Steffen (2003), Widespread, prolonged late Middle to Late Ordovician upwelling in North America: a proxy record of glaciation?, Geology, 31, p. 63 - 66.
    
    
35. Scholle, P. A. and D. S. Ulmer-Scholle (2003), A color guide to the petrography of carbonate rocks: grains, textures, porosity, diagenesis, Tulsa, OK, American Association of Petroleum Geologists Memoir 77, 474 p.
    
    
36. Sibley, D. F. and J. M. Gregg (1987), Classification of dolomite rock textures, Journal of Sedimentary Petrology, 57, p. 967 - 975.
    
    
37. Smith, L., R. Nyahay, and B. Slater (2005), Hydrothermal dolomite core workshop and field trip, June 7, 2005, Albany, NY, U. S. Department of Energy, Petroleum Technology and Transfer Council, Appalachian Region and NYSERDA, short course notes.
    
    
38. Taylor, T. R. and D. F. Sibley (1986), Petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the Trenton Formation, Ordovician, Michigan basin, USA, Sedimentology, 33, p. 61 - 86.
    
    
39. Thompson, A. M. (1999), Chapter 5. Ordovician, p.74 to 89, in Schultz, C. H., ed., The Geology of Pennsylvania, Part II, Stratigraphy. Pennsylvania Geological Survey, 4th Series, Special Publication 1, 888 p.
    
    
40. Tucker, M. E. and V. P. Wright (1990), Carbonate sedimentology, Oxford, UK, Blackwell Science Ltd., 482 p.
    
    
41. Wickstrom, L. H., J. D. Gray, and R. D. Stieglitz (1992), Stratigraphy, structure, and production history of the Trenton Limestone (Ordovician) and adjacent strata in northwestern Ohio, Ohio Division of Geological Survey, Report of Investigations No. 143, 78 p.
    
    
42. Wilson, M. A. and T. J. Palmer (1992), Hardgrounds and hardground faunas, University of Wales, Aberystwyth Institute of Earth Studies Publications No. 9, 131 p.
    
    
43. Woody, R. E., J. M. Gregg, and L. F. Koedertz (1996), Effect of texture on petrophysical properties of dolomite: evidence from the Cambrian-Ordovician of southeastern Missouri, AAPG Bulletin, 80, p.119 - 132.
    
    
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FIGURES AND APPENDICIES

Figure Captions

(Click on thumbnail image for an expanded view.)

Figure 1. Carbonate matrix in the Trenton and Black River Formations.
	Figure 1A. Skeletal wackestone (Dunham, 1962) or a packed biomicrite (Folk, 1962) from the Black River Formation, Union Furnace, Huntingdon County, PA. The matrix consists of microcrystalline calcite, or micrite, which is presumed to have formed through the breakdown of coarser carbonate grains (Scholle and Ulmer-Scholle, 2003). Note that most of the skeletal material has undergone neomorphic recrystallization or solution followed by cavity filling with calcite cement.
	Figure 1B. Dark organic-rich matrix in a skeletal wackestone or packed biomicrite from the Trenton Formation at Union Furnace, PA. The carbonate matrix material consists of microspar (crystals 5 - 20 m in size) presumably recrystallized from micrite. The dark, clotted organic matrix matter is kerogen.
Figure 2. Trenton Formation at Union Furnace, PA.
	Figure 2. Limestone tempesites or turbidites interbedded with siliciclastic shales in the Trenton Formation at Union Furnace, PA.
Figure 3. Very fine-grained quartz arenite from a very thin sandstone bed in the Black River Formation, Union Furnace, Huntington County, PA.
	Figure 3A. Low magnification view (crossed polars) showing well sorted subrounded grains of quartz and minor feldspar cemented by calcite microspar and some anhydrite.
	Figure 3B. Higher magnification view of sandstone shown in Figure 3A showing calcite and anhydrite cements and feldspar grains.
Figure 4. K-bentonites in the Trenton and Black River Formations.
	Figure 4A. Deicke k-bentonite exposed at Union Furnace, PA.
	Figure 4B. Millbrig k-bentonite recovered in core of the Trenton Formation cut 500 ft. west of the Union Furnace outcrop.
	Figure 4C. Millbrig k-bentonite at 1550 ft. in the Chevron 1A Prudential core, Marion County, OH.
Figure 5. Completely micritized ooids in the Black River Formation.
	Figure 5. Completely micritized ooids in the Black River Formation.
Figure 6. Micrite envelope on echinoid grain and peloidal cement.
	Figure 6A. The large echinoid fragment in the left-center of the photomicrograph exhibits a bored rim and well-developed micrite envelope.
	Figure 6B. The same photomicrograph in cross-polarized light reveals that the micrite envelope actually consists of calcite cement with a peloidal texture. Note that this peloidal cement grades into coarser peloidal cement that fills some of the pore spaces adjacent to the grain in the southeast and northwest quadrants. Identical peloidal cement fills the zooecia of bryozoan grains.
	Figure 6C. High magnification view of the peloidal cement that fills both the bored edge of the echinoid grain and the void space immediately below it. The rest of the void space is filled with calcite spar. Montgomery #4 well, Mercer County, PA. Trenton Formation, 8495 ft.
Figure 7. Peloidal cement fabric in the Black River Formation.
	Figure 7A. Core sample of apparent burrowed and bioturbated wackestone (Dunham, 1962) or sparse biomicrite (Folk, 1962). Gray #1 well, Steuben County, NY, 7823 ft.).
	Figure 7B. Thin section photomicrograph of the same sample. Peloids comprise 75% of the limestone. Based on this information we can modify the Folk (1962) name for this rock to a sparse biopelmicrite. Using the Dunham (1962) classification we could rename the rock a peloidal/mixed-fossil grainstone, which is erroneous because the peloids are not grains but are cement.
	Figures 7C and 7D. Progressively higher magnification views of the same sample. Note that the peloids consist of 1) a dark nucleus of micron-size calcite (opaque clots) surrounded by 2) rims of euhedral microspar.
	Figure 7E. Another thin section view of peloidal cement in the same sample. Here peloidal cement fills the zooecia of a bryozoan fragment as well as lithifies the matrix.
	Figure 7F. Peloidal cement fills a void formed earlier through dissolution of part of a bryozoan fragment.
	Figure 7G. Peloidal cement that once filled all of the zooecia of a bryozoan grain now mimics the original fossil structure.
Figure 8. Hardgrounds and peloidal cements.
	Figure 8A. Stacked, amalgamated hardground in the Trenton Formation exposed southeast of Lexington in central Kentucky. The finger points to the surface of a smooth and rolling hardground, but those beneath exhibit hummocky, undercut, pebbly, and reworked morphologies (see Brett and Brookfield, 1984, Wilson and Palmer, 1992, and Laughrey and others, 2003).
	Figure 8B. Plan view of bryozoans encrusting the top of a hardground, Trenton Formation, central Kentucky.
	Figure 8C. Thin section photomicrograph of the hardground surface that the geologist's finger points to in A. Note the peloidal cement texture of the limestone. Also note the authigenic pyrite at and just above the hardground surface.
	Figure 8D. A planar and undercut hardground in the Black River Formation exposed at Union Furnace, Pennsylvania.
	Figures 8E and 8F. Thin section photomicrographs of the hardground shown in D. The clotted fabric characteristic of peloidal cements is evident in both photomicrographs.
Figure 9. High magnification view of chalcedony replacing nonplanar dolomite that mimically replaces a crinoid grain (see Figure A3-8 in Appendix III for additional views of this sample).
	Figure 9A. View under crossed polars showing the typical radiating habit of chalcedony.
	Figure 9B. Same view, but with the gypsum plate inserted into the microscope. The birefringence colors in the northeast and southwest quadrants increased, and the birefringence colors in the northwest and southeast quadrants decreased, indicating that the chalcedony is length-slow.
Figure 10. Macropores and mesopores in the Black River Formation
.	Figure 10. Macropores and mesopores in the Black River Formation, Whiteman #1 well, Chemung County, NY, 9529 ft. Porosity consists of isolated small vugs (not fabric selective), minor intercrystalline pores (fabric selective), and fractures (not fabric selective). The rock consists of nonplanar-a dolomite, and nonplanar (saddle) dolomite cement partially fills the vugs. The vugs formed through dissolution of both calcite and dolomite. Note that dolomitization mostly obliterated porosity in this rock.

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Appendix 3 Figure Captions.

Dolomite textures in the Trenton and Black River carbonate rocks of the Appalachian basin.
(Click on thumbnail image for an expanded view.)

Figure A3-1. Type I planar-p dolomite.

All of the illustrated dolomites are matrix-selective and occur in subtidal carbonate facies. These dolomites are not associated with fractures, faults, or rock alteration by hydrothermal basinal brine. They probably formed through burial dolomitization at a local scale via compaction-driven fluid flow.

	A. Planar-p dolomite replacing bioturbated, fossiliferous pelmicrite. The dolomite crystals are polymodal, ranging in size from 40 to 200 m in diameter. The crystals have planar boundaries and are porphyrotopic, floating in the limestone matrix they replaced. Most dolomite crystals are cloudy due to inclusions, and some indistinct zoning is evident in most crystals. The matrix is peloidal micrite that we interpret as marine cement. Note the incipient solution seams, outlined by micron-size pyrite crystals, which impart a somewhat lenticular habit to the limestone. Some of the planar-p dolomite is partially dissolved along these seams indicating the dolomite formed before chemical compaction. Black River Formation, Power Oil Company #9634 well (Sandhill well), Wood County, WV, 9906 ft.
	B. Finely to medium crystalline, polymodal, planar-p dolomite replacing decimicron-size neomorphic calcite crystals. The dolomite crystals are cloudy due to abundant inclusions, and predate fractures in the rock. . Black River Formation, Power Oil Company #9634 well (Sandhill well), Wood County, WV, 10,162 ft.
	C. Polymodal, planar-p dolomite replacing matrix in a sparse biopelmicrite in the Black River Formation. Outcrop sample from Union Furnace, Huntingdon County, PA.

Figure A3-2. Type I planar-p and planar-e dolomite.

All of the illustrated dolomites are matrix-selective and occur in subtidal carbonate facies. These dolomites are not associated with fractures, faults, or rock alteration by hydrothermal basinal brine. They probably formed through burial dolomitization at a local scale via compaction-driven fluid flow.

	A. Polymodal planar-e dolomite replacing bioturbated, fossiliferous pelmicrite adjacent to stylolites. The dolomite crystals are 2 to 100 m in diameter. Note the abundant inclusions in the dolomite crystals. This planar-e dolomite resembles the planar-p textures shown in the last three photomicrographs above, but the euhedral dolomite now forms a mosaic that almost entirely replaces limestone matrix. Note, however, the planar-p dolomite replacing limestone away from the solution seam. Chemical compaction might have concentrated the planar dolomite crystals along the solution seams. Black River Formation, Power Oil Company #9634 well (Sandhill well), Wood County, WV, 10,162 ft.
	B. Higher magnification view of the planar-e dolomite in Figure A3-2 A. Note that some of the dolomite floats within the insoluble material of the seam, and is partially dissolved along it too. Dolomitization predates chemical compaction.
	C. Laminae of brown planar-e dolomite interbedded with thicker beds of skeletal limestone (wackestone and packstone) in the Black River Formation exposed at the Union Furnace outcrop in central PA.
	D. Thin section photomicrograph of matrix-selective, polymodal planar-e dolomite replacing micrite in one of the brown laminae shown previously in Figure A3-2C above.

Figure A3-3. Type I planar-e and planar-s dolomites.

	A. Thin section photomicrograph of decimicron- to centimicron-size polymodal, planar-e to planar-s dolomite replacing peloidal calcite cement in a grainstone. Trenton Formation, Melben Oil Company Emma McKnight #1 well, Mercer County, PA, 6845.2 ft. This dolomite is matrix-selective and occurs in a subtidal carbonate ramp facies. This dolomite is not associated with fractures, faults, or rock alteration by hydrothermal basinal brine.
	B. Micron- and decimicron-size polymodal planar-e to planar-s dolomite in laminated peritidal carbonates. 1 and 2 are stromatolites exposed in the basal Black River Formation at Union Furnace in Huntingdon County, PA in which the dolomite replaced the original limestone. 3 and 4 show respective core and backscattered electron SEM photographs of probable peritidal stromatolites in the Ohio #2854 well, Delaware County, Ohio (Trenton Formation, 2901 ft.). These planar dolomites probably formed through reflux and/or mixing zone dolomitization. 1. 2. 3. 4.
	C. A hydrothermal dolomitization front in the Black River Formation. Polymodal decimicron-size planar-e dolomite partially replaces pelmicrite. Intercrystalline porosity in the dolomite is filled by bitumen. This planar dolomite fabric occurs in a zone near the top of a faulted and fractured subsurface interval altered by hydrothermal dolomitization. The dolomite formed through rock alteration by saline basinal brines (the dolomite 18O = -9 permil, Smith, 2005, personal communication). Black River Formation, Dominion Exploration and Production Company Bayles #1 well, Bradford County, PA, 12,425 ft.
	D. Polymodal decimicron- and centimicron-size planar-e to planar-s dolomite completely replacing pelmicrite. The apparent intercrystalline porosity is filled with bitumen. This planar fabric is associated with fracturing, faulting, and hydrothermal dolomitization. Black River Formation, Dominion Exploration and Production Company Bayles #1 well, Bradford County, PA, 12,425 ft.
	E. Hydrothermal decimicron- to centimicron-size polymodal planar-s dolomite. Bitumen fills intercrystalline void space. Also note the cutting composed of centimicron-size planar-s dolomite in the upper left corner of the photograph. Black River Formation, Dominion Exploration and Production Company Bayles #1 well, Bradford County, PA, 12,425 ft.

Figure A3-4. Type I planar-c dolomite.

	A. SEM photomicrograph of polymodal (60 m to 300 m crystals) planar-c dolomite lining and partially filling a small vug in the Trenton Limestone. Prudential 1A well, Marion County, OH, 2013.5 ft.
	B. Higher magnification view of planar-c dolomite shown in Figure A3-4A. This dolomite is in the upper right corner of the vug. The cross on the crystal just right of the center of the SEM photo shows the spot of the EDS analysis presented next in Figure A3-4C.
	C. EDS spectra of the planar-c dolomite shown in Figure A3-4B. The dolomite is not stoichiometric, having an excess of Ca (Ca = 17.77%, Mg = 11.50 %), and it is ferroan (Fe = 3.53 wt.%), indicating formation from saline basinal fluid (see Allan and Wiggins, 1993).
	D. SEM photomicrograph of polymodal decimicron- to centimicron-size planar-c dolomite lining a medium vug in the Trenton Formation. The Fe concentration of this dolomite is 8.98 wt.%, indicating saline basinal brine as the dolomitizing fluid. Prudential 1A well, Marion County, OH, 1855 ft.

Figure A3-5. Type II planar-s to nonplanar (transitional) dolomites.

	A. Planar-s to nonplanar (transitional) dolomite replacing a peloidal or oolitic limestone. Compare the ghosts of the peloids or ooids in this dolomite with the peloids (micritized ooids and grains) in the limestone cutting from the same sample shown in Figure A3-5B. Black River Formation, Dominion Exploration and Production Company Bayles #1 well, Bradford County, PA, 12,425 ft.
	B. Peloidal limestone (pelsparite) cutting from 12,425 ft. in the Bayles #1 well, Bradford County, PA. The peloids, possibly micritized ooids, are cemented by calcite spar. Compare with the dolomite in Figure A3-5A.
	C. Polymodal decimicron- to centimicron-size planar-s to nonplanar-a (transitional) dolomite in the Black River Formation, CNR Gray #1 well, Steuben County, NY, 7793 ft.
	D. Polymodal decimicron- to centimicron-size planar-s to nonplanar-a (transitional) dolomite in the Black River Formation, CNR Gray #1 well, Steuben County, NY, 7793 ft.

Figure A3-6. Type II nonplanar-a dolomite textures.

All of the illustrated dolostones show fabric obliteration, and are fault and fracture-related, hydrothermal, nonplanar dolomites.

	A. Unimodal, medium crystalline nonplanar-a dolomite (Black River Formation, Columbia Natural Resources Gray 24468 well, Steuben County, NY, 7796 ft.).
	B. Higher magnification view of nonplanar-a dolomite shown in Figure A3-6A. Note the tightly interlocked crystal fabric and consequent lack of intercrystalline pore space.
	C. Backscattered SEM photomicrograph of nonplanar-a dolomite texture. This sample is from the same well core as those shown in Figure A3-6A and B, but is from a slightly deeper interval at 7799.9 ft. The predominant medium gray is nonplanar-a dolomite, which is not stoichiometric (Ca = 23.54 wt.%, Mg = 11.87 wt.%) and is ferroan (Fe = 2.17 wt.%). The white material is a chloride (Cl = 20.35 wt.%) with notable sodium and potassium (Na = 7.10 wt.%, K = 6.30 wt.%). The material does not appear in thin sections, probably due to high solubility in water and loss during preparation of the slide. It does appear as a dark, reddish, compact and granular mass in the hand samples of the core. The material might be an admixture of halite and sylvite, or a precipitate in the core that formed through evaporation of a residual K-Na-Cl brine.
	D. EDS spectra of the nonplanar-a dolomite in Figure A3-6C.

Figure A3-7. Nonplanar (saddle) dolomite textures.

	A. Nonplanar (saddle) dolomite cement partially filling a small vug in the Trenton Formation (OH 3479, Anderson well, Hancock Co., OH, 1337.8 ft.). Note the diagnostic spearhead crystal shape, mottled salt and pepper appearance (due to inclusions), and sweeping extinction under crossed polars.
	B. Backscattered SEM photomicrograph of nonplanar (saddle) dolomite cement filling a small vug in the Trenton Formation. This is the same sample shown in Figure A3-7A. The saddle dolomite crystals exhibit the same spearhead shape observed in thin section, and curved crystal faces are easier to see in this three dimensional view.
	C. High magnification SEM view of nonplanar (saddle) dolomite filling a small vug in the same Ohio sample shown in Figure A3-7A and B. The cross on the crystal in the upper center of the photograph is the spot of the EDS analysis shown in Figure A3-7D. The saddle dolomite is not stoichiometric, with calcium excess (Ca = 17.05 wt.%, Mg = 12.68 wt.%), and it is nonferroan (Fe = 1.01 wt.%).
	D. EDS spectra of nonplanar (saddle) dolomite shown in Figure A3-7C.
	E. Pore-filling nonplanar (saddle) dolomite with oil staining in the Trenton Limestone. Note the curved crystal faces. (OH 2549 core, Wood Co., OH, 1168.25 ft.).
	F. Nonplanar (saddle) dolomite lining a large vug in the Black River Formation. (Whiteman #1 well, Steuben Co., NY, 9531 ft.).
	G. Backscattered SEM view of the same sample shown in Figure A3-7F. Nonplanar (saddle) dolomite cement lines a large vug. Note the bitumen coatings on some of the dolomite.
	H. Nonplanar (saddle) dolomite cement completely fills a small vug in the same sample shown in Figures A3-7F and G.

Figure A3-8. Limestone replacement textures.

Mimetic and non-mimetic replacement of original crinoid bioclasts (originally composed of high-magnesium calcite) by planar-s to nonplanar-a dolomite. All examples from the Trenton Formation, OH 3479, Anderson well, Hancock Co., OH, 1337.8 ft.

	A. Core sample of vuggy crinoid/bryozoan dolograinstone with mimetically replaced crinoid visible near the top center of the photograph. A fabric selective intraparticle pore occurs in the crinoid's central lumen. Vague circular rinds and halos around some mesopores suggest they are non-mimetically replaced allochems.
	B. Thin section photomicrograph of a mimetically replaced crinoid in the same sample shown in (1). Planar-s to nonplanar-a dolomite replaced the original crinoid grain. Note chert replacing the dolomitized crinoid in the upper left corner of the fossil.

Figure A3-9. Hybrid dolomite fabrics.

	A. A small pocket of polymodal, decimicron-sized planar-s dolomite surrounded by coarser (centimicron- to millimeter-sized) planar and nonplanar dolomite. Centimicron-sized planar-s to nonplanar-a dolomite surrounds the finer crystalline planar-s dolomite. Centimicron-sized planar-e and centimicron- to millimeter-sized saddle dolomite both partially fill large mesopores and medium vugs. Nonplanar (saddle) dolomite also mimically replaces a large echinoderm fragment (lower left) in this former skeletal grainstone. Trenton Formation, Strayer #1 well, (OH 3478 core), Allen County, OH, 1211.25 ft.
	B. Higher magnification view of the pocket of decimicron-sized planar-s dolomite shown in Figure A3-9A.
	C. Contact between decimicron-sized planar-s dolomite (right) and centimicron-sized nonplanar (saddle) dolomite (left). Trenton Formation, Strayer #1 well, (OH 3478 core), Allen County, OH, 1211.25 ft.
	D. Same thin section photomicrograph as Figure A3-9C, but with white index card inserted beneath the thin section. Note the ghosts of the decimicron-sized planar-s dolomite (identical to the same dolomite texture on the right) within the saddle dolomite. This suggests that the saddle dolomite is a neomorphic recrystallization product of the earlier diagenetic planar-s dolomite.

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Appendix 4 Figure Captions.

Base metal sulfide mineralization.

Figure A4-1.

Late-stage calcite replacing nonplanar (saddle) dolomite. Note the abundant pyrite associated with the calcite and dolomite. OH3267 core, Strayer #1 well, Allen County, OH, 1230.3 ft.

Figure A4-2.

A. Calcite replacing nonplanar (saddle) dolomite. Although the large, almost white crystal on the right side of the SEM photo exhibits the characteristic morphology of saddle dolomite, it's lighter color in the backscattered electron mode means that it's composition is different than the adjacent saddle dolomites in the picture. The EDS spectrum shown in B shows calcium excess in the crystal. Trenton Formation, OH 3479, Anderson well, Hancock Co., OH, 1337.8 ft.

B. EDS spectra of the crystal shown in A.

Figure A4-3.

Authigenic quartz replacing anhydrite in the Trenton Formation, northwest Ohio.

Figure A4-4.

Anhydrite and quartz replace nonplanar (saddle) dolomite and fill a large vug in the Trenton Formation in the subsurface of northwestern Ohio.

Figure A4-5.

Authigenic quartz partially filling a small vug in the Black River Formation, Gray #1 well, Steuben County, NY, 7803 ft.

Figure A4-6.

Supergene mineralization in the Black River Formation in the subsurface of south-central NY. Gray #1 well, 7799.9 ft., Steuben County.

	A. Bitumen coats a crystal of delafossite, a copper-iron oxide that commonly occurs as a secondary mineral near the base of the oxidized zone of copper deposits. Black crystals of delafossite fill many small vugs in the Grey well core (it appears white in this backscattered electron photomicrograph due to the high atomic number of the copper and iron).
	B. and C. show the location of EDS analysis in this sample and the EDS spectra for the white mineral.

Figure A4-7.

Nonplanar (saddle dolomite fills this small vug in the Black River Formation (Gray #1 well, Steuben County, NY, 7803 ft.). White specks throughout the SEM photo are anhydrite.

Figure A4-8.

Opal partially fills a small vug in peloidal limestone in the Black River Formation in the subsurface of north central Pennsylvania. This sample is from a zone about 70 ft. above an extensively dolomitized section in the well. Identification of the white mineral as opal is based on its isotropic behavior under crossed polars and its index of refraction less than epoxy. Bayles #1 well, Bradford County, Pa, 12,300 ft.

Figure A4-9.

Bitumen filling intercrystalline pore space in dolostone, Grey #1 well, Steuben County, NY, 7803 ft.

Figure A4-10.

Bitumen filling intercrystalline pore space in dolostone, Whiteman #1 well, Chemung County, NY, 9531 ft.

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Appendix 5 Figure Captions.

Dolomite porosity in the Trenton and Black River Formations.

Figure A5-1. Macroporosity in dolograinstone from northwestern Ohio.

A. Small and medium vugs in dolostone from the Trenton Formation. The rock's depositional texture is not recognizable, but its origin as a crinoidal grainstone can be inferred from adjacent core. OH3267 well core, Auglaize County, 1167.6 ft.

B. Small and medium vugs in crinoidal grainstone from the Trenton Formation. Much of this sample's original depositional texture was obliterated by dolomitization, but mimic relics of some crinoids can be discerned. Voids in the crinoid lumen (most likely moldic after calcite dissolution) provided the starting porosity for subsequent enlargement into vugs. OH 3479, Anderson well, Hancock Co., OH, 1337.8 ft.

Figure A5-2.

Thin section photomicrograph of the same crinoidal dolograinstone shown in Figure A5-1B. Several types of pores are present in the rock - small to medium vugs, molds, intercrystalline pores, and intracrystalline voids. Several dolomite textures are evident, including nonplanar saddle dolomite cement, planar-s to nonplanar-a dolomite, and planar-c dolomite.

Figure A5-3.

Thin section photomicrograph of the same crinoidal dolograinstone shown in Figure A5-1B. Porosity developed through dissolution of dolomite and late-stage calcite cement.

Figure A5-4.

Thin section photomicrograph of the same crinoidal dolograinstone shown in Figure A5-1B. Calcite is stained red, and some of the nonplanar (saddle) dolomite cement appears pink due to partial replacement by calcite. The vugs formed mostly through the dissolution of calcite, but dolomite dissolution also contributed to the development of void space.

Figure A5-5.

	A. Thin section photomicrograph of the same crinoidal dolograinstone shown in Figure A5-1B. Vugs and molds formed by dissolution of calcite and dolomite. The area outlined by the yellow box is shown in Figure A5-5B.
	B. Residue of calcite remaining after dissolution created a small vug.

Figure A5-6.

Backscattered SEM image of vuggy pore space in the same crinoidal dolograinstone shown in Figure A5-1B. Planar-s to nonplanar-a dolomite and nonplanar (saddle) dolomite surround the void, which also contains pyrite left after calcite dissolution created the pore.

Figure A5-7.

SEM image of a small triangular intercrystalline pore space in the same crinoidal dolograinstone shown in Figure A5-1B.

Figure A5-8.

SEM image of intercrystalline microporosity lining a small vug space in the same crinoidal dolograinstone shown in Figure A5-1B.

Figure A5-9.

Core sample of Black River dolomudstone with recognizable depositional fabric. The matrix is quite tight, but fracturing, brecciation, and dissolution created good reservoir porosity. Prudential #1A well (OH3372), Marion County, OH, 1840 ft.

Figure A5-10.

Core sample of Black River dolomudstone with recognizable subtidal depositional texture. The matrix is tight, but both vuggy and fracture porosity are evident. Nonplanar (saddle) dolomite and late-stage calcite reduce the porosity in these secondary voids. Gray # 1 well, Steuben County, NY, 7800 ft.

Figures A5-11 and A5-12.

Thin section photomicrographs of the dolomudstone shown in Figure A5-10.

Figure A5-13.

Backscattered SEM image of the dolomudstone shown in Figure A5-10. Vuggy porosity developed through dissolution of nonplanar-s to nonplanar-a dolomite groundmass, but nonplanar (saddle) dolomite later reduced this secondary mesoporosity.

Figure A5-14 and A5-15.

SEM views of nonplanar (saddle) dolomite reducing vuggy pore space dolomudstone shown in Figure A5-10.. Bitumen stains the crystals in both photomicrographs.

Figure A5-16.

Good mesoporosity and macroporosity development in bioturbated dolomudstone, Whiteman #1 well, Chemung County, NY, 9529.3 ft. The matrix is tight, but molds and small to medium vugs formed through dissolution. Nonplanar (saddle) dolomite partially fills the vugs and molds.

Figure A5-17.

Close up view of nonplanar (saddle) dolomite partially filling a medium vug in the same sample shown in Figure A5-16.

Figure A5-18.

Thin section photomicrograph of the nonplanar (saddle) dolomite partially filling vugs in the same sample shown in Figure A5-16.

Figures A5-19 and A5-20.

SEM photographs of nonplanar (saddle) dolomite partially filling vugs in the same sample shown in Figure A5-16.

Figure A5-21.

Zebra and breccia fabric in dolomudstone in pay zone of the Whiteman #1well, Chemung County, NY, 9531 ft.

Figure A5-22.

Thin section photomicrograph of medium vug in the same core sample shown in Figure A5-21. Nonplanar (saddle) dolomite cement partially fills the void space.

Figures A5-23 and A5-24.

Thin section photomicrographs of macroporosity in the same core sample shown in Figures A5-21 and A5-22. Note the partially dissolved late-stage calcite crystals (red), and their residue along corroded dolomite crystal boundaries and in intercrystalline void space.

Figure A5-25.

This medium vug is in the same core sample shown in Figure A5-21. The bitumen residue in the vug once lined nonplanar (saddle) dolomite crystals (see Figure A5-26). It is not clear if the crystals were plucked during thin section preparation or actually dissolved to create the void space. Corroded remnants of the dolomite within the void suggest the latter might be true.

Figure A5-26.

Backscattered SEM photograph of bitumen coating nonplanar (saddle) dolomite that partially fills a medium vug in the Black River Formation (Whiteman #1 well, Chemung County, NY, 9531 ft.).

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