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 Fonnation. These fields typically have one to 10 wells with primary production ranging from 700,000 to 2,000,000 barrels (111,300-318,000 m3) of oil per field and a 15 to 20 percent recovery rate. At least 200 million barrels (31.8 million m3) 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 for horizontal drilling from existing vertical field wells based upon geological characterization and reservoir modeling case studies. The results of these studies can be applied to similar fields 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 first project year (September 6, 2000, through April 5, 2001). This work includes description and analysis of cores, correlation of geophysical well logs, reservoir mapping, petrographic description of thin sections, cross plotting of penneability and porosity data, and development of horizontal drilling strategies for the Cherokee and Bug fields in San Juan County, Utah. Geological characterization on a local scale foc\lt)ed on reservoir  heterogeneity, quality, and lateral continuity, as well as possible compartmentalization, within these fields. This study utilizes representative core, geophysical logs, and thin sections to characterize and grade each field's potential for drilling horizontal laterals from existing development wells. The typical vertical sequence or lithofacies from the Cherokee and Bug fields, as determined from conventional core, was tied to its corresponding log response to identify reservoir and non-reservoir rock and detennine potential units suitable for horizontal drilling projects. Structure contour maps on the top of the upper Ismay zone and the Chimney Rock shale and isochore maps of the upper Ismay and lower Desert Creek for Cherokee and Bug fields, respectively, were constructed to show carbonate buildup trends, define limits of field potential, and also indicate possible horizontal drilling targets. In order to detennine the diagenetic histories of the various Ismay and Desert Creek reservoirs, petrographic descriptions of 44 thin sections were completed from representative core samples. The diagenetic fabrics and porosity types found at Cherokee and Bug fields are indicators of reservoir flow capacity, storage capacity, and potential for horizontal drilling. The reservoir quality of Cherokee and Bug fields has been affected by multiple generations of dissolution, anhydrite plugging, and various types of cementation which act as barriers or baffles to fluid flow. The most significant and unique diagenetic characteristics were intense, late-stage microporosity and early-stage micro-box-work porosity. Based on cross plots of penneability and porosity data, the reservoir quality of the rocks in Cherokee and Bugs fields is most dependant on pore types and diagenesis. Strategies for horizontal drilling include the following targets: depositional facies in the Ismay and Desert Creek zones, microporosity in the Ismay zone, and micro-box-work porosity in the Desert Creek zone.

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