Creating a Geologic Play Book for Trenton-Black River Appalachian Basin Exploration

Preliminary isopach and facies maps, combined with a literature review, were used to develop a sequence of basin geometry, architecture and facies development during Cambrian and Ordovician time. The main architectural features--basins, sub basins and platforms--were identified and mapped as their positions shifted with time. This is significant because a better understanding of the control of basin geometry and architecture on the distribution of key facies and on subsequent reservoir development in Ordovician carbonates within the Trenton and Black River is essential for future exploration planning. Good exploration potential is thought to exist along the entire platform margin, where clean grainstones were deposited in skeletal shoals from Indiana thorough Ohio and Ontario into Pennsylvania. The best reservoir facies for the development of hydrothermal dolomites appears to be these clean carbonates. This conclusion is supported by observations taken in existing fields in Indiana, Ontario, Ohio and New York. In contrast, Trenton-Black River production in Kentucky and West Virginia has been from fractured, but non-dolomitized, limestone reservoirs. Facies maps indicate that these limestones were deposited under conditions that led to a higher argillaceous content than the cleaner limestones deposited in higher-energy environments along platform margins. However, even in the broad area of argillaceous limestones, clean limestone buildups have been observed in eastern outcrops and, if present and dolomitized in the subsurface, may provide additional exploration targets. Structure and isopach maps developed as part of the structural and seismic study supported the basin architecture and geometry conclusions, and from them some structural control on the location of architectural features may be inferred. This portion of the study eventually will lead to a determination of the timing relative to fracturing, dolomitization and hydrocarbon charging of reservoirs in the Trenton and Black River carbonates. The focus of this effort will shift in the next few months from regional to more detailed structural analyses. This new effort will include topics such as the determination of the source of the hot, dolomitizing fluids that created hydrothermal dolomite reservoirs in the Black River, and the probable migration paths of these fluids. Faults of suitable age, orientation and location to be relevant for hydrothermal dolomite creation in the Trenton-Black River play will be isolated and mapped, and potential fairways delineated. A detailed study of hydrothermal alteration of carbonate reservoirs was completed and is discussed at length in this report. New ideas that were developed from this research were combined with a literature review and existing concepts to develop a model for the development of hydrothermal dolomite reservoirs in the study area. Fault-related hydrothermal alteration is a key component of this model. Hydrothermal alteration produces a spectrum of features in reservoirs, ranging from leached limestone and microporosity to matrix dolomite, saddle dolomite-lined breccias, zebra fabrics and fractures. Mineralization probably occurred during the pressure drop associated with the rise of fluids up the fault system, and is due to the mixing of hydrothermal fluids with cooler, in situ fluids. Once they began to cool themselves, the hydrothermal fluids, which had a lower pH and higher salinity than formation fluids, were capable of leaching the host limestones. Microporosity is common in leached limestones, and it is likely that it was formed, in some cases, during hydrothermal alteration. Dolomite leaching occurs near the end of the paragenetic sequence, and may significantly enhance porosity. However, leaching of dolomite typically is followed by the precipitation of calcite or anhydrite, which reduces porosity. A final conclusion is that hydrothermal alteration may be more common than previously thought, and some features previously attributed to other processes may be in fact be hydrothermal in origin. Production data are being collected from all project partners. These data will be used to predict ultimate production from existing fields for which a complete production history is known, and for remaining reserves in fields for which the complete production history is unavailable. A plan to estimate the gas resource in the entire play area is being developed. Enhancement of the project database, GIS and website was emphasized during the report period. An effort is being made to add all logs - approximately 1800 - that have been scanned in Tiff format, and another 500 logs that have been converted to vector (LAS) format, to the database and website. Tables for production data, log data, core data and cross sections, all linked to the well header information, were created.

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