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Hydrocarbon reserves in Red River limestones are typically found in porosity zones created by paragenetic dolomitization and diagenetic calcite solution and microfracturing. The porosity developed on the tops and flanks of low-relief carbonate highs in a shallow marine environment. Since basement structure and topography controlled the location and limits of porosity development, we attempt to reconstruct Ordovician topography (structure) in our search for Red River prospects. Our attempts at mapping prospects with seismic data are hampered by velocity errors and data resolution.
Most Red River fields are composed of small topographic bumps with associated pods of porosity. Clusters of bumps may be aligned locally and may form regional trends reflecting paleoshorelines. Exploration along these trends can be very successful. However, direct offsets to good discovery wells may be disappointing for several reasons: seismic data may not be able to adequately define the structure, the structure or reservoir may be too small to provide offset potential, porosity may not be developed, or acreage ownership or spacing regulations may restrict optimum well positioning. Unfortunately, lease or drilling deadlines generally have the habit of compounding these problems.
Examples of seismic structures and "porosity" anomalies illustrate the problems of offset drilling. Extension prospecting appears to be a more practical approach to development in the Williston basin.
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