Mississippian Chert Reservoirs in Kansas, 2: Silicon Isotope Geochemistry and Mechanism of Silicification

Cao, Hongsheng¹, S. J. Mazzullo¹ (1) Wichita State University, Wichita, KS

Multi-generation replacive cherts in Mississippian bedded spiculites-shales and massive breccias, which are gas reservoirs in Kansas, were studied for their Si isotopic composition. In some of these rocks silicification occluded primary porosity, whereas in other rocks porosity was either preserved or created during silicification. Hence, the origin of silicification and its timing are critical in understanding reservoir origin and distribution in these rocks. To address silica origin, two cores of Mississippian age from Rhodes Field in Barber County, KS were selected for study of chert Si isotopic composition.

Samples from collapse breccias along the Tippecanoe sequence-bounding unconformity have three distinct chert generations. In the 1st generation (or primary) cherts the highest δ³⁰Si values (0.5 to 0.3‰) are in samples immediately beneath the unconformity while lower δ³⁰Si values (0.1 to -0.2‰) are found in the 2nd and 3rd generation cherts of these top-most breccias. In contrast, the 2nd and/or 3rd generation cherts in underlying bedded spiculites and shales have progressively lower δ³⁰Si values with increasing depth below the unconformity -- from 0.1 to -0.3‰ at mid-depth ranges to 0 to -0.9‰ at greater depth. Chert formation and diagenesis are believed to have resulted largely from meteoric alteration during latest Mississippian to pre-Desmoinesian time. Progressive depletion in δ³⁰Si composition with depth below the unconformity is interpreted to reflect the evolution of groundwater Si geochemistry and possibly progressive leaching of cherts in the breccias over time.