trilobite Isotelus gigas
Trenton Black River Project

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Introduction | Methods | Constituents | Microfacies and Depositional Environments | Diagenesis | Dolomite Textures, Diagenesis, and Porosity |
References | Text Figures | Appendix I [Skeletal (PDF) - Nonskeletal (PDF)] | Appendix II (PDF) |
Appendix III-Figure Captions | Appendix IV-Figure Captions | Appendix V-Figure Captions |
Table 1 - TBR Core and Outcrop Samples (PDF)


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