Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 243,
Sedimentology and Ichnology of Paleozoic Estuarine and Shoreface Reservoirs, Morrow Sandstone, Lower Pennsylvanian of Southwest Kansas, USA--page 13 of 14
By integrating data from ichnologic, sedimentologic, and stratigraphic studies of four cores from the lower Morrow Sandstone, we were able to distinguish estuarine from open-marine deposits and thus provide a more precise picture of reservoir sandstones in the Lower Pennsylvanian of southwest Kansas. The estuarine assemblage includes facies that have been deposited in fluvial channels, interfluves, upper-estuarine channels, estuary bay, restricted tidal flats, tidal channels, and estuary mouth. The lower Morrow estuarine system was northwest-southeast oriented and displays the classical tripartite division of wave-dominated estuaries, with localized evidence of tidal action. Estuarine deposits contain low-diversity trace-fossil assemblages produced by an opportunistic, depauperate marine infauna, indicative of stressful conditions in a brackish-water setting. Distribution of ichnocoenosis within the estuarine system was essentially controlled by the salinity gradient.
The open-marine assemblage includes upper-, middle-, and lower-shoreface sandstones encased in offshore-transition, offshore, and shelf fine-grained facies. These deposits are regionally extensive and were formed under weak storm influence. Open-marine ichnofaunas are characterized by high diversity of trace fossils produced by the resident benthos under normal-salinity conditions. Trace-fossil data are instrumental to propose an environmental subdivision of the shoreface-offshore packages. An onshore-offshore replacement of the Skolithos ichnofacies by the Cruziana ichnofacies is observed.
Because this study is one of the first ichnologic analyses of a Paleozoic reservoir, it provides original information to test the applicability of models proposed on the basis of observations in Mesozoic and Cenozoic reservoirs. Ichnologic analysis of the Morrow Sandstone suggests that post-Paleozoic trace-fossil models may be applied, albeit with caution, to the study of cores from late Paleozoic reservoirs.
Recognition of valley-fill sandstones in the lower Morrow has implications for hydrocarbon exploration and subsequent production. The emergent picture is one of a heterogeneous and compartmentalized reservoir, displaying high variability in sedimentary facies, and a complex pattern in distribution and connectivity of reservoir sandstones.
We would like to thank Robert Brenner, Howard Feldman, and Andrew Rindsberg for reviewing the manuscript, as well as Scott Beaty for very valuable discussions and extensive help with this project. We would also like to thank Mark Schoneweis for the drawings, John Charlton for photographic work, Pat Acker for digital work with the illustrations, and Liz Brosius for editing the manuscript. This paper was written while LAB and MGM were recipients of a postdoctoral external award by the Argentinian Research Council (CONICET). We are also grateful to Huber and Anadarko Petroleum for providing access to the cores. Work in the Morrow Sandstone was funded by a DOE Grant DE-FG22-97BC15008.
Kansas Geological Survey
Web version November 9,1999