Lithofacies and Reservoir Characteristics of Lower Wolfcamp and Upper Virgil Strata, Amazon Field, Cheyenne County, Nebraska

John C. Webb

Amazon field was discovered in 1980 by Diamond Shamrock. The discovery well (McMillan Ranch 1) produced 115 BOPD and some gas. Cumulative production for three wells (July 1987) from lower Wolfcamp and upper Virgil zones reached 53,000 bbl oil and 140,000 bbl water. Producing zones occur within three carbonate cycles. In ascending order, these are a breccia/grainstone facies (upper Virgil), the McMillan Ranch (upper Virgil) zone, and Christenson (lower Wolfcamp) zone.

The breccia/grainstone facies is dolomitic, peloidal lime packstone to intraclastic peloidal grainstone, overlain by peloid, intraclastic, ooid lime grainstone. The McMillan Ranch is very fine dolomitic mudstone, overlain by dolomitic, intraclastic, peloid skeletal lime packstone to lime mudstone. The McMillan Ranch is capped by stromatolitic lime and dolomite boundstone and laminated fenestral dolomite mudstone. The Christenson is laminated, intraclastic peloid dolomite packstone to grainstone, stromatolitic ostracod dolomite boundstone, fenestral anhydritic dolomite mudstone, and skeletal intraclastic lime packstone to wackestone. Gross productive zones range from 8 to 10 ft. Matrix porosity and permeability are low in all three zones. Maximum porosity and permeability are 17% and 2 5 md in the Christenson. Subaerial exposure of the breccia/grainstone resulted in dissolution of allochems and in irregular fractures. Rare primary intergranular and fenestral porosity are preserved in the McMillan Ranch. Dolomitization and minor dissolution have created intercrystalline, skeletal-moldic, and microvuggy porosity in the McMillan Ranch and Christenson. Vertical fractures are common. Production comes from a few feet of matrix and fracture porosity in each zone.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988