The Fractured Green River and Wasatch Formations of the Uinta Basins, Utah: Ideal Targets for Horizontal Drilling

CHIDSEY, THOMAS C., and CRAIG D. MORGAN, Utah Geological & Mineral Survey, Salt Lake City, UT, and MICHAEL D. LAINE, Utah Division of Oil, Gas, & Mining, Salt Lake City, UT

The fractured Tertiary (Eocene) Green River and Wasatch Formations of the Uinta basin represent ideal targets for exploration and development using horizontal drilling techniques. These formations have the following characteristics favorable for horizontal drilling: (1) predictable orientations of natural fractures which control production; (2) multiple, low-permeability sandstone and siltstone reservoirs; (3) overpressured zones; and (4) thick, heterogeneous oil-bearing and oil-generating sequences. The Green River and Wasatch formations have produced over 200 million bbl of oil in the Uinta basin from conventional vertical wells.

In the Greater Altamont-Bluebell field, the Wasatch and Green River formations represent an 8000-ft-thick oil-bearing section, of which 2500 ft is overpressured. Production occurs in multiple, low-matrix porosity, thin-bedded sandstones and siltstones which are widely distributed in the vertical sequence. Permeability is enhanced by vertical fractures exhibiting a single dominant north-northwest trend. These zones are excellent targets for horizontal (or high-angle) drilling perpendicular to the fractures. Untapped reserves may exist in neighboring abandoned fields, such as Star Flat, Cottonwood Wash, and Gusher fields, due to the lateral heterogeneity of the reservoirs.

To the south, several fields occur along the east-west-trending Duchesne fracture zone with production from subtle, fractured structural noses such as the Duchesne field. One of the Duchesne field's 27 wells, the Ute Tribal 1, penetrated a greater fracture density in the Wasatch Formation than other wells in the field and accounts for nearly one-third of the total production. Similar results may be achieved with horizontal drilling techniques in existing fields and future discoveries in the Uinta basin.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)