Paleotopography and Sea-Level Controls on Facies Distribution and Stratal Architecture in the Westerville Limestone (Upper Pennsylvanian), NE Kansas, NW Missouri
Oolitic grainstones form important reservoirs. The Westerville Limestone (Pennsylvanian) is a 2-6 m-thick oolitic grainstone-rich reservoir analog that was deposited in a mid ramp location during glacioeustatic changes. Outcrops in NE Kansas and NW Missouri (510 km2), hand samples, and thin sections were used to define 8 lithofacies and demonstrate how even subtle paleotopography can result in significant heterogeneities of potential reservoir and non-reservoir facies. Facies are bioclastic packstone, bioclastic grainstone, oolitic grainstone, oncolitic packstone, fossiliferous siliciclastic-mudstone, peloidal packstone, microbial boundstone, and coarse-grained packstone. The studied unit is divided into three intervals, W1, W2, and W3, each separated by a marine or subaerial erosion surface. W1, composed mostly of bioclastic packstone of relatively uniform thickness (~1 m), was deposited on and drapes the underlying Wea Shale. A marine erosion surface truncated W1 deposits and created subtle (cm-scale) local relief. W2 (0-4 m thick) is composed mostly of oolitic grainstones that fill paleolows and overly bioclastic grainstones locally. W2 is capped by a subaerial exposure surface with m-scale erosional relief indicating the grainstone facies were deposited during a relative fall in sea level. Initial intertidal facies of W3 (0-1 m thick) were deposited during transgression, starting with a thin layer of microbial boundstone that drapes paleotopography followed by onlapping oncolitic packstone and fossiliferous siliciclastic-mudstone that fill paleolows. Overlying peloidal packstone (0-3 m thick) drapes deposits throughout most of the field area and intertongues with oolitic grainstone (0-1 m thick) that both fills paleolows and builds constructional relief on paleotopographic highs during highstand to falling sea level. A subaerial exposure surface truncates W3 strata. Oolitic grainstone is typically thought to form constructional relief on high areas in shallow water, but this study indicates that oolitic grainstones can accumulate in lows, especially during falling stages of sea level. Deposition of other facies, including a mix of siliciclastic and carbonate facies during a sea-level cycle can result in vertical and lateral heterogeneity that also interact with paleotopographic highs and lows in predictabie ways. Understanding the interaction of sea level and even subtle paleotopography can lead to better prediction of oolitic facies in the subsurface.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California