Altamont--A Major Fractured and Overpressured Stratigraphic Trap: ABSTRACT

Abstract

Altamont field in the Uinta basin of Utah is a much-overpressured accumulation of high pour-point crude, producing from multiple, thin, Tertiary reservoirs in a 40-mi long stratigraphic trap. Postdepositional shift of the structural axis of the basin created an updip pinchout of low-porosity sandstones into a lacustrine shale sequence. Reservoir performance is enhanced significantly by vertical fractures and initial pressure gradients which sometimes exceed 0.8 psi/ft. Field limits in large part are undetermined, and few dry holes have been drilled. Average well drilling and completion costs approach $1 million. The initial potential of field wells ranges from 500 to 2,500 BOPD with an average GOR of about 1,000 cu ft/bbl. Matrix permeability generally is less than 0.2 md; therefore, high well performance is a function of fracture permeability. Inability to define reservoir parameters and oil in place causes individual well reserve estimates to depend mainly on pressure decline-cumulative relations. Significant engineering problems are related to evaluation and completion of these thin, low-porosity, fractured and overpressured pay intervals, which span up to 2,000 ft of stratigraphic section in some wells. These problems are compounded by the problem of handling 110°F pour-point waxy crude. Inability to pipeline the crude conventionally from the relatively remote area has delayed full production of the field. About 40 drilling rigs are active in development of the trend, and a field potential is suggested of in excess of 250 million bbl with significant delineation of field limits yet to be accomplished. End_of_Article - Last_Page 791------------