Faulted Shoreline and Tidal Deposits in the Moenkopi Formation of the Grassy Trail Creek Field, Utah

ALLISON, M. LEE, Utah Geological and Mineral Survey, Salt Lake City, UT, and SUSAN JUCH LUTZ, University of Utah Research Institute, Salt Lake City, UT

The Grassy Trail Creek field produces 40 degree API oil and minor gas from shallow marine sandstones of the Triassic Moenkopi Formation on the north-plunging nose of the San Rafael swell in central Utah. Production is controlled by a combination of stratigraphic variations and minor north-south-trending faults. The faults are normal, near vertical, and discontinuous, and run roughly parallel to the axis of the San Rafael swell. Although fracture permeability enhances production of the reservoir, some faults act as barriers to fluid migration, segmenting the area into productive and dry fault blocks. Horizontal drilling techniques developed in this field in the early 1980s resulted in significantly better production.

Cores from productive intervals in the Torrey Member of the Moenkopi Formation contain fractured, cross-stratified, dolomite-cemented sandstone beds, 2-16 ft (0.7-5.3 m) thick. The fining-upward (medium to fine-grained) beds have massive to intraclast-rich bases that scour into underlying rippled to bioturbated micaceous siltstones and mudstones. The sandstones are interpreted to represent tidal channels which cut into intertidal and subtidal deposits.

Log analyses indicate the main reservoir is a complex stack of these thin tidal channel sandstones. Isochore maps of the A and B zones indicate thickened meanders that form localized reservoir pods that are vertically offset. The distribution of isochore thicks appears to represent deposition along a northwest-southeast-trending shoreline fed by sediments from the northeast. There is potential for field extensions in similar deposits along this paleoshoreline.

The Moenkopi Formation, long thought to be self-sourcing, may contain oil generated in Precambrian sediments equivalent to the Late Proterozoic Chuar Group. Presence of this older oil would have required migration from Precambrian sedimentary rocks surrounding the San Rafael swell.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)