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Ritter, Matthew E.1, Robert H. Goldstein1 
(1) University of Kansas, Lawrence, KS

ABSTRACT: Diagenetic and Sea-Level Controls on Porosity Preservation: An Example from Oolitic and Crinoidal Carbonates, Mississippian, Kansas and Missouri

Mississippian (Osagean-Meramecian) oolitic and crinoidal carbonates were deposited on a ramp in Kansas and Missouri. Meniscus and pendant cements provide evidence for an early relative fall in sea level, with subaerial exposure only to the North. This event had only minor impact on porosity. 
Intermediate diagenetic events were most important in controlling porosity. In crinoidal lithologies some porosity (20-30 % of the rock) was reduced by calcite overgrowths after initial compaction, suggesting marginal reservoir potential. In oolitic lithologies, cementation is minor, but because of this, most primary porosity is destroyed by grain-to-grain pressure dissolution, suggesting poor reservoir potential. Syntaxial overgrowths have up to seven nonluminescent-to-luminescent couplets, previously traced up to, but not above, the sub-Pennsylvanian unconformity, an indication that cementation was associated with the unconformity. 18O (-8.20 to -4.55‰ VPDB) and 13C (-0.15 to 3.44‰ VPDB) are inconsistent with simple meteoric cementation. Sr isotopic data suggest an early Pennsylvanian age for some cements. Final melting temperature of ice from primary, all-liquid fluid inclusions of -2.0 and -1.9°C indicate seawater; -1.2° to -0.1°C indicate a meteoric-marine mixing zone; and 0.0°C indicate meteoric waters. This record may reflect a history of climate or sea-level fluctuation during unconformity development. 
This study demonstrates that short-lived, early events of subaerial exposure may exert no control on reservoir potential. Percentage of crinoid fragments and cementation associated with major unconformities may be most important in affecting porosity. The diagenesis from an unconformity may record climate and sea-level change not preserved in the stratigraphic succession.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.