Altamont-Bluebell—A Major, Naturally Fractured Stratigraphic Trap, Uinta Basin, Utah1
The Altamont-Bluebell trend is composed of a highly overpressured series of oil accumulations in naturally fractured, low-porosity, Tertiary lacustrine sandstones. It now covers more than 350 sq mi (907 km2) located across the deeper part of the Uinta basin of northeastern Utah. Postdepositional shift of the structural axis of the basin in late Tertiary time produced a regional updip pinchout of northerly derived sandstones into a lacustrine “oil-shale” sequence. Facies shifts during the deposition of more than 15,000 ft (4,570 m) of lacustrine sediments have resulted in a changing pattern of reservoir distribution and hydrocarbon charge at various stratigraphic levels. About 8,000 ft (2,440 m) of stratigraphic section is oil bearing, and up to 2,500 ft (760 m) of section contains overpressured producing zones in the fairway wells.
Reservoir performance is significantly enhanced by vertical fractures and initial fluid-pressure gradients, some of which exceed 0.8 psi/ft. The crude has a high paraffin content resulting in pour points above 100°F (37.78°C), gravities of 30-50° API, and an average GOR of 1,000 cu ft/bbl. This unique combination of geologic and hydrocarbon conditions makes it difficult to evaluate the ultimate recovery of the field, which could be more than 250 million bbl.
Figures & Tables
With three previous volumes published by AAPG on structure of American oil fields, this publication takes 17 of these oil fields and describes them in detail. The reservoirs described in these 17 papers range in age from Devonian to Pleistocene; their litholgies are standstone, limestone, or dolomite; and the trapping mechanisms are structural or stratigraphic or a combination of the two. The North American oil fields described are distributed from Alaska and the McKenzie Delta area of Canada on the northwest, to the Gulf of Mexico and Southern Floriday on the southeast. This publication also includes an index to those North American oil and gas fields which have been described in previous AAPG publications.