Sleepy Hollow oil field, Red Willow County, Nebraska, discovered in 1960, has produced nearly 30 million bbl of oil from a “basal” Desmoinesian (Pennsylvanian) sandstone. The trap for this substantial oil accumulation is complex, related to (1) sandstone characteristics and distribution, and (2) gentle late Paleozoic structure.
Examination of reservoir core samples from 23 Sleepy Hollow field wells reveals sedimentary characteristics which, when combined with observations of the stratigraphic sequences in and adjacent to the sandstone, suggest that the following sequence of sedimentary events governed the distribution of this reservoir rock. The Precambrian surface that had been exposed at least since Early Pennsylvanian time was incised by fluvial channels that were choked with granite detritus (quartz and feldspar in various stages of weathering). During the Middle Pennsylvanian marine transgression of the Precambrian surface, quartz fragments derived from the granite wash were reworked by westward longshore drift in the shallow sea but rarely were transported more than a few miles from the drainage system from which they were derived. Accumulation of relatively pure quartz sand is presumed to have occurred as a result of the removal of feldspar through weathering of the exposed crystalline rocks and by winnowing by shallow-marine currents. After several episodes of shoreline and shallow-marine sand deposition and accompanying winnowing of clay, lobate sand bodies coalesced to form the present continuous reservoir. Sand deposition did not occur north and south of Sleepy Hollow because of the absence of a nearshore high-energy environment for clastic deposits. Sand deposition did not occur east of Sleepy Hollow because of the relative position of the fluvial channel and the direction of longshore drift.