Subtle Sea-Level and Paleotopographic Controls from Heterogeneity of from Oolite-Bearing System

Justin M. Fairchild¹, Evan K. Franseen², and Robert H. Goldstein^1
1Department of Geology, University of Kansas, Lawrence, KS
²Kansas Geological Survey, University of Kansas, Lawrence, KS

Oolitic reservoirs are important hydrocarbon targets, but can be notoriously heterogeneous. Dynamic interactions between currents, minor fluctuations in sea level, and subtleties of the paleotopography appear to be major causes of heterogeneity. This study uses extensive exposures to illustrate the environmental process and sedimentologic response of such systems. The Westerville Limestone (Upper Pennsylvanian, Kansas City Group) is exposed over a 220 km² area in eastern Kansas and western Missouri providing a three-dimensional sample. The basal unit is ~1 m-thick diverse fossiliferous packstone, consisting of coated phylloid algal grains, gastropods, brachiopods, and crinoids. On the basis of contained ooids, it is interpreted to represent subtidal, normal marine conditions marginal to ooid shoals higher up slope. The unit’s upper surface is erosional, generated by a relative fall in sea level responsible for 0.7 m of paleotopographic relief. This subtle paleotopography had a profound effect on overlying grainy facies. Initial deposits are shaley, only 0-0.15m thick, but upper units consist of oolitic and fossil fragment grainstone, of variable thickness (0-4.5 m). Some thick shoal deposits developed on relatively flat surfaces in response to low sea level. Other oolitic deposits are concentrated where currents were focused into erosional paleolows, but these deposits build relief locally and focused successive deposits into adjacent paleolows. The grainstones are cross-cut by a paleosol, indicating deposition during an overall relative fall. This complex distribution of grainy facies illustrates how in some situations, oolitic systems are relief builders and in others they are relief fillers, but during a relative fall in sea level, eventual focus of currents and sediment lead to filling of relief and more extensive lateral distribution of reservoir facies.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas