CARR, TIMOTHY R.¹, PEER HOTH², and MICHAEL BAU^2
1Kansas Geological Survey, University of Kansas, Lawrence, KS
²GeoForschungsZentrum, Potsdam, Germany

Abstract: Trace and Rare-Earth Elemental Variation in a Midcontinent Carbonate Sequence -- Understanding Reservoir Development

Subaerial exposure surfaces are common and economically important components of Midcontinent shallow-marine paleozoic sequences. The Swope Limestone, Kansas City Group, was investigated to understand the style of alteration associated with aubaerial exposure, to derive criteria to recognize distinctive signatures in both the surface and subsurface, and to better understand their genesis. Detailed field descriptions, petrographic examination, and spectral gamma-ray logs were integrated with ICP-MS analysis of trace and rare-earth elements.

Increased Zr, Rb, Th, and total REE content indicates that detrital influx generally increased toward the exposure surfaces. The shale normalized REE-patterns have very characteristic features. Important differences between upper and lower parts of the limestone are shown by differences in Ce and Y signatures and the Y/Ho ratio. Ralative Uranium enrichment detected by measurement of gamma radiation (hand-held spectral scintillometer in outcrops, and existing well logs in the subsurface) was confirmed by ICP-MS analysis. Distinctive change in uranium concentration was observed at the contact of the vadose and marine-phreatic zones as indicated by REE-anomalies. Uranium anomalies cut across sedimentary structures, and are caused by selective inclusion of uranium in micrite cements and associated clays concentrated at the contact between vadose and marine-phreatic environments.

Porosity development and preservation in the studied carbonate sequence is related to the position of previously undetected exposure surfaces and related vadose-phreatic contacts. Study of trace and rare earth elemental variations can improve our understanding of reservoir development in shallow-marine carbonate reservoirs.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas