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 first half of the fourth project year (April 6 through October 5, 2003). The work included (1) analysis of well-test data and oil production from Cherokee and Bug fields, San Juan County, Utah, and (2) diagenetic evaluation of stable isotopes from the upper Ismay and lower Desert Creek zones of the Paradox Formation in the Blanding sub-basin, Utah. Production ''sweet spots''and potential horizontal drilling candidates were identified for Cherokee and Bug fields. In Cherokee field, the most productive wells are located in the thickest part of the mound facies of the upper Ismay zone, where microporosity is well developed. In Bug field, the most productive wells are located structurally downdip from the updip porosity pinch out in the dolomitized lower Desert Creek zone, where micro-box-work porosity is well developed. Microporosity and micro-box-work porosity have the greatest hydrocarbon storage and flow capacity, and potential horizontal drilling target in these fields. Diagenesis is the main control on the quality of Ismay and Desert Creek reservoirs. Most of the carbonates present within the lower Desert Creek and Ismay have retained a marine-influenced carbon isotope geochemistry throughout marine cementation as well as through post-burial recycling of marine carbonate components during dolomitization, stylolitization, dissolution, and late cementation. Meteoric waters do not appear to have had any effect on the composition of the dolomites in these zones. Light oxygen values obtained from reservoir samples for wells located along the margins or flanks of Bug field may be indicative of exposure to higher temperatures, to fluids depleted in {sup 18}O relative to sea water, or to hypersaline waters during burial diagenesis. The samples from Bug field with the lightest oxygen isotope compositions are from wells that have produced significantly greater amounts of hydrocarbons. There is no significant difference between the oxygen isotope compositions from lower Desert Creek dolomite samples in Bug field and the upper Ismay limestones and dolomites from Cherokee field. Carbon isotopic compositions for samples from Patterson Canyon field can be divided into two populations: isotopically heavier mound cement and isotopically lighter oolite and banded cement. Technology transfer activities consisted of exhibiting a booth display of project materials at the annual national convention of the American Association of Petroleum Geologists, a technical presentation, a core workshop, and publications. The project home page was updated on the Utah Geological Survey Internet web site.

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