ABSTRACT: Heterogeneity of Sandstone Reservoirs in the Fort Union Formation, Fuller Reservoir Field, Wind River Basin, Wyoming

C. W. Keighin, R. M. Flores

The Paleocene Fort Union Formation in the Wind River basin includes fluvio-deltaic sandstone, shale, carbonaceous shale, and minor coal, conglomerate, limestone, and mudstone. Cores, geophysical logs, and petrophysical and petrologic data show wide variations in petrologic properties of shallow (about 2500-5000 ft in depth) reservoir sandstones in the Fort Union Formation, Fuller Reservoir field, Fremont County, Wyoming.

This heterogeneity of reservoir sandstones, which is clearly illustrated in outcrops at the Castle Gardens and Waltman areas, reflects disparity in depositional environments. These sandstones were diagenetically modified by moderate to extensive dissolution of plagioclase and other framework grains and by formation of clay minerals, especially kaolinite and illite. Porosity, as measured on core plugs, ranges from 4 to 19%; permeability ranges from 0.03 to 33 md. Porosity as determined by modal analysis on a different suite of samples ranges from 0.3 to 27%. Although porosity is often good, many pores are partially to extensively occluded by pore-filling clays, especially kaolinite. Locally, porosity has been virtually eliminated through precipitation of iron-bearing carbonate cement. itrinite reflectance data (R_o = 0.55-0.69%), determined from coals of subbituminous rank, suggest a low thermal regime, as supported by the degree of diagenetic alteration seen in thin section. Modal analyses reveal moderate scatter in distribution of quartz, feldspar, and rock fragment framework grains in the sandstones. This uneven distribution is probably a function of the variability of depositional environments.

Heterogeneity in reservoir sandstones within the Fort Union Formation occurs at all scales from outcrop to thin section. This heterogeneity results in many small cells within what appear to be, on geophysical logs, continuous well-defined reservoir sandstones. This study increases our knowledge of the genesis of the reservoir sandstones and may facilitate development and exploitation of the oil and natural gas in several horizons in Fuller Reservoir and other fields throughout the basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990