Mississippian Chert Reservoirs in Kansas: Oxygen and Silicon Isotope Geochemistry and Timing of Silicification and Porosity Formation
Mississippian cherts associated with bedded spiculite and shale, and also subunconformity breccias, are prolific gas (and locally oil) reservoirs in Kansas. Cores from Rhodes field in Barber County generally have three chert generations. The 1st and 2nd generations replaced spiculite and shale, respectively, in bedded deposits, and clasts and matrix shale, respectively, in breccias. Later dissolution resulted in porosity formation in 2nd generation chert and enhanced micro-porosity in unaltered spiculites. The 3rd chert generation partly replaced earlier cherts and occluded some porosity. 1st and 2nd generation cherts in bedded deposits are mechanically compacted, there is soft-sediment deformation of surrounding sediments, and micro-stalactites in breccias suggest vadose diagenetic overprint. These cherts formed early Ð that is, soon after deposition. The 3rd chert generation obviously formed later, although evidence suggests that all silicification occurred after deposition and before Middle Pennsylvanian transgression. Hence, it appears that meteoric fluids attending Tippecanoe emergence affected both silicification and porosity formation, an interpretation that is confirmed by oxygen and silica isotopes. d18OSMOW values of the 1st generation cherts are the most enriched, with means of 29.7ä in breccias and 28.4ä in bedded deposits. d 30Si values in 1st generation chert in breccias are highest close to the unconformity (0.5-0.3ä) and decrease in later cherts (0.1 to -0.2ä). In contrast, 2nd and 3rd generation cherts in bedded deposits have progressively lower d 30Si values with increasing depth below the unconformity (from 0.1 to -0.3ä to 0 to -0.9ä with increasing depth). Mean d 18O values of 2nd and 3rd generation cherts are progressively more depleted (26.7-27ä in 2nd generation cherts, and 25.1-24.1ä in 3rd generation cherts). Meteoric silicification and porosity formation occurred during latest Mississippian to pre-Desmoinesian subaerial exposure, and at least some silica likely was derived from exposed lower Paleozoic cherty carbonates to the north. Progressive d 18O and d 30Si depletion suggest early rock-dominated alteration and/or initial silicification in mixed marine-meteoric fluids, followed by continued silica formation in meteoric fluid-dominated systems attending sea-level fall with attending changes in ground-water chemistry.
AAPG Search and Discover Article #90067©2007 AAPG Mid-Continent Section Meeting, Wichita, Kansas