Shirley P. Dutton
The Upper Cretaceous Frontier Formation in the western Green River basin has generally low permeability and has been designated a "tight
gas sandstone," but diagenetic variations contribute to significant reservoir-quality differences within and between fields along the Moxa arch. Core and core-analysis data were available from Tip Top, Hogsback, and Fontenelle fields at the northern end of the arch and from Church Buttes field 50 mi to the south. Present burial depth of the Second Frontier Sandstone in these fields ranges from 6800 ft in the north to 12,100 ft in the south. Petrographic examination of 75 Frontier thin sections from 5 cores is the basis for preliminary investigation of the influence of texture, detrital mineralogy, compaction, and cementation on reservoir quality.
Frontier sandstones are mostly fine- to medium-grained litharenites and sublitharenites. Chert, mudstone, sandstone, and low-rank metamorphic rock fragments are the most abundant lithic grains. The major events in the diagenetic history of Frontier sandstones were (1) mechanical compaction by grain rearrangement and deformation of ductile grains, (2) precipitation of authigenic quartz, calcite, and clay minerals, (3) generation of secondary porosity by dissolution of calcite cement and detrital feldspar, chert, and mudstone, and (4) chemical compaction by intergranular pressure solution and stylolitization.
The most important controls on porosity and permeability in clean sandstones in Fontenelle field are grain size and detrital mineralogy. Best reservoir quality (^phgr = 17%, k = 5 md) occurs in medium-grained sandstones having low (<15%) volume of ductile rock fragments. In contrast, volume of authigenic cement is the most important influence on reservoir quality in the other three fields. Reservoir quality in Tip Top and Hogsback fields is controlled primarily by carbonate cement volume (highest ^phgr = 15%, k = 5 md), whereas permeability in Church Buttes field is a function of the volume of authigenic quartz, illite, and chlorite (highest ^phgr = 10%, k = 1 md). Understanding the diagenetic variability in the Frontier Formation can aid in predicting zones having the best permeab lity and in determining appropriate production methods.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990