Sleepy Hollow field, Red Willow County, Nebraska, discovered in 1960, has produced more than 28 MMBO from a “basal” Pennsylvanian sandstone. The trap for this substantial oil accumulation is complex, related to sandstone characteristics and distribution, low-relief late Paleozoic tectonism, and very subtle post-Paleozoic folding.

Examination of reservoir core samples from 23 field wells reveals the following sedimentologic characteristics.

  1. Despite some difficulty of grain-size measurement resulting from erratic sorting, sand grains are consistently coarser at the top and finer at the base of producing sandstone intervals.

  2. Sandstone in the producing interval is rarely cemented, and often disaggregates completely when cored or drilled. Core samples of the reservoir sandstone that have been preserved are usually consolidated with an oil residue, and crumble when handled. Uncommon interlaminated, less permeable sandstone beds have argillaceous (kaolinitic?) cement.

  3. Mineralogy of the producing sandstone is quite simple; essentially pure quartz with very scarce clasts and cobbles of lime mudstone, weathered feldspar, and chert.

  4. Coarse quartz grains in the sandstone are nearly spherical, and commonly frosted and pitted; fine quartz grains are usually subangular.

  5. When sedimentary structures can be observed (in the scarce argillaceous beds), subplanar lamination and low-angle cross-lamination can be observed. No moderate or high-angle cross-lamination has been found.

The aforementioned characteristics, when combined with observations of the stratigraphic sequences adjacent to the subject sandstone, suggest the following sequence of sedimentary events governing the distribution of this reservoir rock. First, the Precambrian surface that had been exposed since Early Pennsylvanian time (and perhaps before) was incised by fluvial channels which were choked with granite detritus (quartz, and feldspar in various stages of weathering decay). During mid-Pennsylvanian marine transgression of the crystalline surface, quartz fragments derived from the (channel) granite wash were reworked by westward longshore drift in the shallow sea, but rarely transported more than a mile or two from the drainage system from which they were derived. Accumulation of relatively pure quartz sand is presumed to have been accomplished by final oxidation and destruction of decayed feldspar and then removal through winnowing by marine currents of the clay minerals thus generated. Because of low inclination of the transgressed Precambrian surface (less than 10 ft per mi), subtle eustatic sea-level changes had a profound effect on strand movement and position. Therefore, sand accumulation occurred in an erratic pattern when compared to modern shoreline sand deposits. After three episodes of shoreline and shallow-marine sand deposition and accompanying clay-winnowing, the lobate sand bodies coalesced to form present reservoir distribution. Sand deposition did not occur north and south of Sleepy Hollow because of absence of a nearshore high-energy clastic environment. Sand deposition did not occur east of Sleepy Hollow because of the relative position of the fluvial channel and the direction of longshore drift.

The Desmoines oil accumulation at Sleepy Hollow field is controlled by sandstone reservoir distribution as described above, and by two increments of gentle tectonic uplift, modified by subtle differential compaction of overlying shales.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.