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Sequence Stratigraphic Control on Distribution and Porosity Evolution in Cherts in the Mississippian of the Mid-Continent


Intervals of chert in the Mississippian carbonates of the Mid-Continent are primary reservoir targets, yet little is known about the origin of the chert or the controls on porosity development within them. The primary source of silica is generally thought to be from the dissolution of siliceous sponges, interpreted due to the presence of abundant spicules and spiculitic molds in many of the cherts. Further influx of silica into the system (volcanic ash or detrital quartz) may accelerate sponge productivity and thus help account for the abundance of chert found in these units. Chertification of carbonate was initiated by dissolution of biogenic amorphous silica and the subsequent precipitation of opal-CT and quartz. Carbonate replacement was achieved by a force of crystallization controlled replacement, where dissolution of carbonate material was driven by the precipitation of various silica phases. Dissolution of carbonate and silica, as well as the volumetric change from amorphous silica to quartz, created pore space in many of the cherts. Porosity abundance in the cherts is controlled by the relative rate of burial during deposition, as it controls the initial ratio of carbonate to spicules. Extremely slow burial resulted in minimal carbonate input, resulting in the dissolution and re-precipitation of spicules as nearly pure, non-porous cherts. Slow burial rates resulted in a higher percentage of spicules in the carbonate, allowing for nearly complete replacement of limestone by silica and forming abundant porosity in the rock. Faster burial increased the overall ratio of carbonate to spicules, effectively decreasing the volume of carbonate that could be replaced, resulting in a decrease in the total porosity. In outcrop, the variety of chert exhibits a strong correlation to the sequence stratigraphic framework. Overall, gradational changes from pure, to highly porous, to less porous cherts are observed vertically at multiple frequencies due to varying orders of sea level cyclicity. The link between chert variety, relative sea level fluctuation, and the observed sequence stratigraphic framework aids in explaining the controls on porosity distribution at both the 3rd and 4th order scales.