Abstract

The Grassy Trail Creek field produces 40 gravity oil and minor gas from shallow-marine sandstones of the Triassic Moenkopi Formation on the north-plunging nose of the San Rafael Swell in east-central Utah. Production is controlled by a combination of stratigraphic heterogeneities, fractures, and minor north-south trending faults. Although fracture permeability enhances production of the reservoir, some faults appear to act as barriers to fluid migration, segmenting the area into productive and dry fault blocks. The C zone in the Black Dragon Member of the Moenkopi contains graded, dolomite-cemented sandstones interpreted to represent estuarine channel deposits. The C zone can be correlated to oil-impregnated sandstones in both outcrop and in the subsurface throughout the northern San Rafael Swell. The regional distribution of hydrocarbon stained sandstones in the Black Dragon Member suggests a north to northwest trending channel to mouthbar sequence in a large tidal delta.

Log analyses indicate that the main reservoir is a complex stack of thin tidal channel sandstones. Isochore maps of the A and B zones in the Torrey Member indicate thickened meanders that form localized pods which are vertically offset. The channelized sandstone bodies are concentrated parallel to a northwest-southeast trending shoreline. The location of the channels appears to have been controlled by the distribution of the highs of the underlying Sinbad Limestone Member, an oolitic barrier island complex within the Moenkopi Formation.