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ABSTRACT: Silurian Laurel Formation, Central Kentucky: Lithofacies, Porosity, and Dolomitization

HENDRICKS, TODD, University of Kentucky, Lexington, KY, and STEPHEN O. MOSHIER, Wheaton College, Wheaton, IL

The Laurel Dolomite (Silurian, Wenlockian) is exposed in central Kentucky and forms petroleum reservoirs in western and southern Kentucky. The basal Lower Vuggy (4-30 ft, the productive subsurface "Blue Sand") includes muddy lithofacies with coarse dolomite textures and high intercrystalline porosity. The Quarrystone (9-22 ft) includes dolomicrites featuring horizons with stromatolites, thin laminations with short desiccation cracks, and small vugs containing quartz nodules. Silica phases probably replaced nodular anhydrite within the sediment, for they contain abundant anhydrite inclusions. The Upper Vuggy (0-9 ft) includes muddy lithofacies, but with finer dolomite textures than the Lower Vuggy. The Oolite (0-6 ft) features high intergranular porosity.

Evidence supports dolomitization by hypersaline fluids during early burial. Low faunal diversity suggests hypersaline conditions during deposition. Quarrystone beds resemble shallow subtidal to supratidal deposits in the Persian Gulf. The d34S of inclusion-anhydrite corresponds to reported values of Silurian marine sulfates. The d18O of dolomites ranges from -1.5 o/oo to -4.5 o/oo PDB. The isotope {13}C of the dolomites are lithofacies specific: about 3 o/oo for the Oolite, becoming lighter down-section to about 0.5 o/oo for the Quarrystone and Lower Vuggy. Heavier d18O values (-1.5 o/oo ) and mid-range isotope {13}C values (1.5 o/oo ) may represent the isotopic composition of dolomite derived from Silurian seawater, being of similar isotope {13}C and enriched in isotope {13}C relativ to reported values of Silurian marine calcite. The stratigraphic isotope {13}C distribution may reflect separate initial phases of dolomitization or changes in the composition of the dolomitizing fluids along their flow path.

The trend toward lighter (-) d18O probably reflects reequilibration of the dolomite with subsurface fluids at higher temperatures during burial, corresponding with the d18O of late-stage dolomite cements. Trace element and Sr-isotope compositions of dolomite and sulfate phases should provide information on fluid origins and patterns of subsurface flow during dolomitization.

 

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)