--> ABSTRACT: Geological and Reservoir Characterization of Shallow-Shelf Carbonate Fields, Southern Paradox Basin, Utah, by Thomas C. Chidsey Jr. and David E. Eby; #91019 (1996)

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Geological and Reservoir Characterization of Shallow-Shelf Carbonate Fields, Southern Paradox Basin, Utah

Thomas C. Chidsey Jr. and David E. Eby

The Paradox basin of Utah, Colorado, and Arizona contains nearly l00 small oil fields producing from carbonate mounds within the Pennsylvanian (Desmoinesian) Paradox Formation. These fields typically have one to three wells with primary per field production ranging from 700 MBO to 2 MMBO at a 15-20% recovery rate. Five fields (Anasazi, Mule, Blue Hogan, Heron North, and Runway) within the Navajo Nation of southeastern Utah have been evaluated for CO2-flood projects based upon geological characterization and reservoir modeling.

Conventional cores from the five fields show that three compositional reservoir types are present: (1) phylloid algal, (2) bioclastic calcarenite, and (3) bryozoan-dominated. Phylloid algal mounds are abundant in four of the five fields, and exhibit the best overall porosity and permeability. This mound type developed where shallow water depths and low energy allowed establishment of calcareous algal colonies possibly on paleohighs. The principal reservoir rock is algal bafflestone composed mostly of the phylloid Ivanovia and occasionally dolomitized. The Heron North field is a bioclastic calcarenite reservoir. It represents high-energy conditions resulting in carbonate beaches developed over foreshore carbonate rubble. The principal reservoir rocks are grainstones and rudstones having grain-selective dissolution and complete dolomitization. Bryozoan-dominated mounds present in Runway field developed in quiet, below wave-base settings that appear to be localized along Mississippian fault blocks trends. The principal reservoir rocks are bindstone and framestone with no dolomitization.

Possible sequence boundaries, lithofacies types, petrophysical variations, and other reservoir parameters have been incorporated into a field-scale, compositional reservoir simulator. The resulting model suggests that CO2 miscible flooding of these and other small carbonate reservoirs in the Paradox basin could significantly increase ultimate recovery of oil.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California