Abstract

The Green River Formation in the Uinta Basin of east-central Utah has had a long oil and gas exploration and production history, beginning primarily with the discovery of Bluebell/Altamont Field by Humble in 1949. Logging and petrophysical practices have varied widely as have the exploration concepts used to exploit oil and gas in the Green River Formation. While Bluebell/Altamont has long been considered an overpressured fractured play, fields such as Brundage Canyon, Antelope Creek, and Monument Butte are primarily conventional matrix porosity reservoirs.

Geophysical logging and log interpretation provide insight into the reservoir characteristics that are important to exploration and development of Green River hydrocarbon resources. Open-hole logging in the south-central Uinta Basin through the Brundage Canyon-Monument Butte trend is fraught with pitfalls that, when examined and corrected, can lead to questions as to whether these fields are truly conventional matrix reservoir fields or are, in part, an extension of the fracture play present at Altamont/Bluebell.

Comparison of lower Green River Formation porosity and permeability data from cores with well logs not only shows that open-hole log porosities commonly read higher than the true formation porosity, but also that the permeabilities within these reservoirs can be insufficient to provide economic hydrocarbon flow. Thus, fracturing must play at least a limited role in the recovery of oil within the Brundage Canyon-Monument trend. Petrophysical evaluation of a conventional suite of gamma-ray, neutron-density porosity, and resistivity logs involves significant assumptions including the matrix densities on porosity logs and mud salinities on resistivity devices.

A key factor in characterizing the Green River Formation reservoirs is recognizing the diverse lithologies. Virtually every sedimentary rock type is encountered in the Green River Formation and cements range from carbonates and silicates to solid hydrocarbons. These lithologies and cements are intermixed, and, when combined with the uncertainty of formation water resistivity, recognition of economic reservoirs can be difficult. Examples of formation analysis, pay characteristics, permeability trapping, and fluid evaluation and typing are all examined to support the conclusion that many, if not most, of the zones completed in the lower Green River Formation contribute only marginally to the total observed production unless naturally fractured to some extent.