Crystallite Size of Diagenetic Chert in Carbonate Rocks--Probable Indicator of Prelithification Porosity

Deba P. Bhattacharyya, Mark R. Seely

Silicification and dolomitization in seven constituent lithofacies of the Meramecian carbonate sequence in central eastern Missouri are genetically related and are facies dependent. In the arenite facies with variable lime-mud matrix (0-30%), both were probably caused by phreatic freshwater and seawater mixing. Silicification is characterized by microcrystalline quartz-replaced (MCQ-replaced) bioclasts and pore-occluding fibrous chalcedony (CHY); the latter is common as micronodules. In these facies-specific, silica-replaced bioclasts, MCQ has abnormally large crystallite size in parts of the profile characterized by blocky low-matrix cement. In contrast, the matrix-rich facies, characterized by extensively dolomitized matrix, have smaller crystallite size in the MCQ-repl ced bioclasts.

The conventional model of thermal annealing does not adequately explain the observed, intermittently anomalous, large crystallite size of the diagenetic chert in the Meramecian arenite facies, considering the regional thermal and burial history of the rocks. Also, the chert crystallite size inversely correlates with the matrix mud content in the facies. Therefore, we propose that the observed variation in the size of the chert crystallites reflects the degree of silica saturation in the pore water. The degree of silica saturation is a function of permeability controlled by the effective prelithification porosity in these carbonates. Preliminary data suggest that chert crystallite size can be used to map the distribution of prelithification porosity in carbonate rocks that were substan ially modified in texture during diagenesis. Oxygen isotopic composition of these cherts may help delineate the fluid-migration paths in these rocks. This information is useful in exploring for energy resources.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.