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CALDWELL, CRAIG D., Phillips Petroleum Company, Bartlesville, OK, and JOHN C. OSTERGREN, Phillips Petroleum Company, Borger, TX

ABSTRACT: Upper Morrowan Valley-Fill, Sandstone Reservoirs at South Guymon Field, Texas County, Oklahoma: An Integration of Core and Wireline-Log Stratigraphic Studies

South Guymon field, located in south-central Texas County, Oklahoma panhandle, has produced more than 238 bcf of gas and 430 MBO and condensate from sandstone reservoirs in the Lower Pennsylvanian, Upper Morrow and Lower Morrow formations. The field was developed primarily during the early 1960s, but drilling to extend production and replace damaged wells has continued to date. Upper Morrowan hydrocarbon production in the southern part of South Guymon field is from two conglomeratic sandstone units. The integration of core and wireline-log stratigraphic studies indicates that the two sandstones record valley-fill deposition in two distinct, depositional sequences separated by a major, lowstand surface of erosion unconformity.

During periods of subaerial exposure and erosion associated with Upper Morrowan eustatic drops in sea level, fluvial valleys were incised in older, Upper Morrowan marine shales and previously deposited Upper Morrowan valley-fill sequences. With the initiation of sea level rise, incised valleys began to fill with fluvial sediments. Fluvial deposits in the southern part of South Guymon field are light-gray, medium-scale, cross-stratified and horizontally stratified, coarse-grained sandstones and conglomeratic sandstones. Core study, dipmeter analysis, and detailed mapping of the two Upper Morrowan fluvial sandstone reservoirs in the southern part of South Guymon field suggest these sandstones were deposited by braided and coarse-grained meandering streams.

Continued transgression resulted in marine drowning of the incised valleys and development of restricted, quiet-water bays. Bay deposition is recorded by light-gray to gray, wavy parallel-laminated, muddy sandstones and sandy mudstones overlying the coarse-grained, fluvial sandstones. Quiet-water bay deposits contain common, sand-size, uranium-enriched, carbonaceous plant debris that results in high, gamma-ray log values, allowing wireline log recognition of these deposits. Isopach mapping of the cumulative thickness of fluvial and bay valley-fill deposits shows an approximate width of 0.7 to 3 mi (1 to 5 km), and a north-northeast and west-northwest trend for the Upper Morrowan incised valleys in the southern part of South Guymon field.During maximum transgression or sea level highst nd, offshore marine muds were deposited across the region inside and outside of incised valleys. Offshore marine deposition is recored by black, calcareous, fissle shales with agglutinated forams and negligible siliciclastic sand.

AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.