HETEROGENEOUS SHALLOW-SHELF CARBONATE BUILDUPS IN THE PARADOX BASIN, UTAH AND COLORADO: TARGETS FOR INCREASED OIL PRODUCTION AND RESERVES USING HORIZONTAL DRILLING TECHNIQUES

The Paradox Basin of Utah, Colorado, Arizona, and New Mexico contains nearly 100 small oil fields producing from carbonate buildups within the Pennsylvanian (Desmoinesian) Paradox Formation. These fields typically have one to 10 wells with primary production ranging from 700,000 to 2,000,000 barrels (111,300-318,000 m{sup 3}) of oil per field and a 15 to 20 percent recovery rate. At least 200 million barrels (31.8 million m{sup 3}) of oil will not be recovered from these small fields because of inefficient recovery practices and undrained heterogeneous reservoirs. Several fields in southeastern Utah and southwestern Colorado are being evaluated as candidates for horizontal drilling and enhanced oil recovery from existing vertical wells based upon geological characterization and reservoir modeling case studies. Geological characterization on a local scale is focused on reservoir heterogeneity, quality, and lateral continuity, as well as possible reservoir compartmentalization, within these fields. This study utilizes representative cores, geophysical logs, and thin sections to characterize and grade each field's potential for drilling horizontal laterals from existing development wells. The results of these studies can be applied to similar fields elsewhere in the Paradox Basin and the Rocky Mountain region, the Michigan and Illinois Basins, and the Midcontinent region. This report covers research activities for the second half of the third project year (October 6, 2002, through April 5, 2003). The primary work included describing and mapping regional facies of the upper Ismay and lower Desert Creek zones of the Paradox Formation in the Blanding sub-basin, Utah. Regional cross sections show the development of ''clean carbonate''packages that contain all of the productive reservoir facies. These clean carbonates abruptly change laterally into thick anhydrite packages that filled several small intra-shelf basins in the upper Ismay zone. Examination of upper Ismay cores identified seven depositional facies: open marine, middle shelf, inner shelf/tidal flat, bryozoan mounds, phylloid-algal mounds, quartz sand dunes, and anhydritic salinas. Lower Desert Creek facies include open marine, middle shelf, protomounds/collapse breccia, and phylloid-algal mounds. Mapping the upper Ismay zone facies delineates very prospective reservoir trends that contain porous, productive buildups around the anhydrite-filled intra-shelf basins. Facies and reservoir controls imposed by the anhydritic intra-shelf basins should be considered when selecting the optimal location and orientation of any horizontal drilling from known phylloidalgal reservoirs to undrained reserves, as well as identifying new exploration trends. Although intra-shelf basins are not present in the lower Desert Creek zone of the Blanding sub-basin, drilling horizontally along linear shoreline trends could also encounter previously undrilled, porous intervals and buildups. Technology transfer activities consisted of a technical presentation at a Class II Review conference sponsored by the National Energy Technology Laboratory at the Center for Energy and Economic Diversification in Odessa, Texas. The project home page was updated on the Utah Geological Survey Internet web site.

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