Sedimentation and Petrology of Fanshawe Sand, Red Oak Field, Arkoma Basin, Oklahoma

Edward D. Pittman, Laura L. Wray

The Fanshawe sand, a very fine to fine-grained lithic sandstone, probably formed in moderate to deep water downslope from a delta system to the east. Sediment-laden discharge flowed from east to west as channelized, bottom-hugging density currents. Deposition of the Fanshawe sand seems to have been restricted to a west-southwest-trending zone approximately 2 mi wide on the north side of Red Oak field. The sand is a composite of a series of narrow, shifting, meandering submarine channels that often are stacked. Stratigraphic cross sections show extreme variability, even along depositional strike, and individual channels typically are narrower than the distance between development wells. Reservoir quality is enhanced where these narrow channels coalesce horizontally and ver ically. Net sand thickness ranges from 36 to 180 ft with associated reserves of up to 14 bcf/well. Completion rates can reach 8 mmcf/day with decline rates averaging 6%.

Preliminary results of an increased density drilling program further substantiate the narrow, sinuous nature of these fan channels. Air drilling causes severe hole washouts, making net pay determinations questionable. But by mapping overall net sand trends, it is possible to high-grade drilling prospects. Prediction of porosity development, however, remains difficult.

Porosity in the Fanshawe is due to (1) precipitation of pore-lining chlorite, which retarded quartz cementation by blocking potential nucleation sites on detrital quartz grains and preserved primary porosity, and (2) dissolution of feldspars and lithic fragments. The better reservoir rock has both porosity types. Where pore-lining chlorite was absent, thin, or discontinuous, quartz overgrowths developed and intergranular porosity decreased. This created a pore geometry consisting of poorly interconnected, disseminated, intragranular/moldic, dissolution pores and low permeability.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.