Fluvial, Deltaic, and Slope Reservoir Sandstone in West-Central Texas: ABSTRACT

Abstract

More than 300 Lower Permian sandstone oil and gas fields are present in an area of about 7,000 sq mi on the Eastern shelf and slope of the Midland basin. All these fields appear to be in stratigraphic traps in fluvial, deltaic, and slope depositional environments. Two Lower Permian Cook fluvial sandstone systems extend in a downdip direction across the Eastern shelf to the shelf edge, where progradational Cook sandstone facies form two high-constructive deltas. These are the proximal parts of a thick terrigenous clastic wedge of sediments covering the eastern slope. Outcrops of Cook channel-fill deposits grade upward from conglomerate and quartz sandstone, to gray shale. Downdip from outcrop to the shelf edge, Cook sandstone is present in one meander belt system 82 mi long. The upper 30 mi is about five mi wide and bounded by shale and two thin lenticular limestone beds. The belt narrows to about one mi where adjacent limestone beds are thicker and were more resistant to lateral channel erosion. About 58 mi downdip from outcrop, the meander belt divides into two belts. Each belt is about one mi wide and bounded by shale and thick limestone beds to the shelf edge. Point bars (with characteristic electric-log SP curves), crevasse splays, and/or natural levees are discernible in the meander belts. Many stratigraphic traps are present in point bars, in sandstone belts perpendicular to regional dip, and in belts of sandstone that overlie buried reefs, hills, structures, and other channel sandstones. Paleocreekology is applicable in locating topographic features on the paleoshelf. Both Cook sandstone deltas, nearly joining, are in an area of 295 sq mi. Delta-plain facies consist of interbedded dark gray shale, fine-grained, lenticular sandstones, and thin coal lenses underlain by thick, medium-grained sandstone facies in distributary channels. Electric-log SP curves of the channel sandstones are characteristic. Delta front distributary mouth bar and sheet-sandstone facies overlie thick prodelta shale and thin, lenticular sandstone. The Group 4000 Cisco sandstone field is the only oil field of any significance in the two deltas. It produces from delta-plain sandstone facies. West of the shelf edge, the Cook wedge of shale and sandstone lenses reaches a maximum thickness of about 1,300 ft at midslope and thins basinward. The base is a maximum gradient of about 3° at midslope. The Northeast Bloodworth Canyon sandstone field produces from Cook sandstone, probably a turbidite and slump deposit, evidently deposited in a submarine channel. The Jameson Strawn sandstone field, about 35 sq mi in area, produced from Cook lower slope sandstone lenses, probably part of a submarine fan. End_of_Article - Last_Page 420------------