IDENTIFYING AUTHIGENIC SILICA ENRICHMENT IN CLAY-RICH MUDROCKS USING X-RAY FLUORESCENCE DERIVED ELEMENTAL CONCENTRATIONS

Concentrations of elements as detected by x-ray fluorescence provide basic data for interpreting the origin of silica (SiO₂) in mudrocks. Concentration profiles of specific elements and ratios of elements are useful in detecting vertical changes in concentration that reflect changing rock composition. In some mudrocks, silica cementation is obvious in the form of silica bands or beds. In others, authigenic silica induced hardness is not readily evident as beds or masses, but is sufficiently abundant to change geomechanical properties and generate reservoir facies that can be fracture stimulated to produce oil and gas. In this study, two very different mudrocks that produce oil and gas in Oklahoma were examined: Woodford Shale and Caney Shale. The studied Woodford Shale contains obvious silica bands and thin beds that are brittle and naturally fracture or can be artificially fractured, whereas the Caney Shale lacks silica banding, but contains intervals of higher authigenic silica concentration that imparts sufficient brittleness to generate frackable reservoirs. Distinguishing brittle zones with authigenic silica enrichment from ductile clay-rich intervals with abundant detrital quartz silt is achieved by comparing concentrations of detrital indicator elements such as titanium (Ti), zirconium (Zr), and aluminum (Al) with the concentration of silicon (Si). Ti and Zr reflect the abundance of rutile and zircon, heavy minerals in siliciclastic sediments. Al is associated with aluminosilicate clay minerals, whereas Si can be of either detrital or authigenic origin. Intervals with authigenic silica enrichment have increasing Si concentration and concurrent decreasing concentrations of Al, Ti and Zr. Detrital clay-mineral-rich intervals with lower silica content show increasing concentrations of Al, Ti and Zr with a concurrent decrease in Si. Intervals rich in detrital clay and quartz with show concurrent increases in Al, Ti, Zr and Si.