Bluebell-Altamont Field: Core from the Fractured Green River-Wasatch Reservoir, Uinta Basin, Utah
Arthur Baclawski, John South, John Lorenz, and Scott Cooper
Bluebell-Altamont Field (along with the adjacent Cedar Rim Field) covers an area of about 500 sq-mi and has produced over 300 MMBO and 500 BFCG from Tertiary age Wasatch and Green River formations. From the core of the basin along the north side of the field, stacked reservoirs pinchout updip to the south from sandstones and carbonates into open lacustrine mudstones, forming major transgressive wedges. These wedges create multiple opportunities for stratigraphic traps in the over-pressured accumulation. Core description and analysis from eleven cores and of several formation image logs have documented the existence of two fracture domains in the reservoir interval: one striking approximately N20-45W and another striking approximately N90E, with the change in orientation generally occurring in the lower part of the Lower Green River interval (about the TGR3 level). Fractures are generally confined to brittle sandstones and carbonates terminating or becoming narrower where the lithology becomes more clay-rich. These naturally occurring fractures range in width from <.1 mm to >5 mm. They are generally lined with calcite, which occludes all or some of the original fracture, but also provide a mechanism to retain the remaining fracture aperture at reservoir conditions. Apparent fracture spacing from core is a few feet although image log data indicates that closer spaced fractures are common in the subsurface. Given these parameters, calculated fracture porosity in the pervasively fractured sandstones and limestones is at least 1% and results in large effective drainage areas and high recovery factors from this porosity system. It appears that the tight (<4% porosity, <.01 md) matrix in the Wasatch and Lower Green River reservoir contributes a negligible amount to the recoverable reserves in the field.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013