Sunnyside bitumen-impregnated sandstone reservoirs at Bruin Point, southwest Uinta Basin, Utah

Abstract

The south flank of the Uinta Basin in northeast Utah is the high Tavaputs Plateau that dips gently northward towards the basin center and is deeply dissected by the Green River and its tributaries. Where not removed by canyon incision, bitumen-impregnated sandstones in the Green River Formation (lower Eocene) occur along nearly the full length of the plateau. The bituminous sandstones encompass an area greater than 600 square miles and hold an estimated 11.6 to 14.0 billion barrels of bitumen, but the net thickness of bituminous sandstone and the OOIP rarely exceed 70 feet and 80 thousand barrels per acre (MBO/ac), respectively. However, in an exceptional 4.6 square mile area centered on Bruin Point (elev. 10,184 ft) on the southwest basin rim, the net thickness of bituminous sandstone and OOIP are measured in hundreds of feet and MBO/ac, respectively. The estimated bitumen in place in just this small area is 1.16 billion barrels. Bruin Point is an erosional remnant of a structural-stratigraphic trap formed by the superposition of a subtle flexure on a thick stack of deltaiclittoral sandstones. In the 1970s and 1980s, this unique area was extensively investigated to characterize the reservoirs and delineate the hydrocarbon resource. Over 117 test wells with cores were drilled and analyzed. As many as 32 stacked bituminous sandstone bodies were encountered, 15 of which hold nearly all the bitumen. The sandstones were deposited in constantly shifting deltaic lobes and along inter-delta shorelines. They are encased in marsh and floodplain mudstone and littoral-lacustrine calcareous mudstone and bioclastics. The sandstones are poorly sorted, fine-grained feldspathic arenites up to 115 ft thick in distributary channels, but less than 10 ft thick in beach deposits. Average porosity and permeability are 23% and 570 md, respectively, but values vary widely between, and even within, depositional settings. The bitumen at Bruin Point is heavy (8.6o API) and highly viscous (106 cP). Just 25 miles to the north, these same amalgamated deltaic sandstones are the reservoirs in the Greater Monument Butte conventional oil field. Although many operators have attempted to exploit the Bruin Point site for liquid hydrocarbons using both in situ steam flood and mining with solvent extraction, so far only small-scale mining of the bituminous sandstones for road construction has been commercially successful. This paper presents seven cores, several exceeding 1000 ft in length, that graphically display the distribution of bitumen resource as related to stratigraphic heterogeneity and location within the Bruin Point sector of the Sunnyside deposit.