Abstract

The Paradox Basin of Utah, Colorado, and Arizona contains more than 50 small oil fields producing from carbonate buildups within the Pennsylvanian (Desmoinesian) Paradox Formation. These fields typically have one to four wells with primary production ranging from 700,000 to 2,000,000 barrels (112,000-318,000 m³) of oil per field at a 15- to 20- percent recovery rate. Five of these fields (Heron North, Runway, Mule, Blue Hogan, and Anasazi) within the Navajo Nation of southeastern Utah, were chosen for case study to develop geologic reservoir models to be used for waterflood and carbon-dioxide- miscible flood simulations. Conventional cores, modern geophysical logs, reservoir data, closely spaced seismic lines, and outcrop analogs for the five case-study fields were used to determine the reservoir facies, geometric relationships, flow units, and areal extent of regional facies belts and localized hydrocarbon traps.

Local scale, phylloid algal buildup facies are the dominant hydrocarbon traps. These buildups typically consist of basal platform, middle mound-core, and upper supra-mound intervals. Reservoir heterogeneity is caused by combinations of lithotypes, mound relief, flooding surfaces, and diagenesis. The basal platform contains combinations of low-permeability carbonate fabrics and is rarely productive. The mound cores are generally homogenous, with porous and permeable algal lime bafflestones. The supramounds are very heterogeneous, with lower overall permeability but higher average porosity than the underlying mound cores. This heterogeneity is due to the mixed mineralogy as well as the wide range of depositional fabrics.

Primary recovery is estimated to be approximately 40 percent from the mound-core and less than 15 percent from the supra-mound intervals. Oil production rates are highest from permeable algal-lime bafflestones of the mound cores. However, most of the oil reserves reside in porous dolomites of the overlying supra-mound intervals. As oil is produced from algal-lime bafflestones, oil from the overlying dolomites continually replenishes the bafflestone pore system, resulting in a production capacity far greater than can be attributed to the mound cores alone.

This study provides the basis for several areas of future work. First, waterflood and/or carbon-dioxide-miscible flood projects can be designed using models and reservoir simulations based on this study. The objective is to drive oil trapped in the supra-mound rocks into the mound-core reservoir where recovery is highest. Second, the results of this study can be used to increase recoverable reserves by identifying untapped compartments created by reservoir heterogeneity and prevent premature abandonment of numerous small fields throughout the Paradox Basin. Third, the results of this study may apply to small carbonate- buildup reservoirs in other areas in the Rocky Mountain Region, the Michigan and Illinois basins, and the Midcontinent.