ELECTRON MICROPROBE ANALYSIS OF FRACTURE FILLING CEMENTS IN THE WOODFORD SHALE, SE OKLAHOMA
The Woodford Shale is a late Devonian/early Mississippian unit that is located below carbonates and shale of the "Mississippi Limestone" and above limestone/dolostone of the Hunton Group. This shale is typically comprised of three separate sections. The beds in these sections all have low permeability, and porosity is seemingly dependent on fractures. Core samples of over one hundred feet of highly fractured shale were taken from a recently drilled well in southeastern Oklahoma. Fractures within the shale are mineralized. As part of a course at OSU, we used an Electron Microprobe to analyze the fracture fill and adjacent host shale. An Energy Dispersive Spectrometer obtained qualitative compositional information and a Wavelength Dispersive Spectrometer was used for element mapping.
Fractures in the uppermost section of the Woodford Shale are filled with calcite. However, the middle section fractures contain dolomite with minor amounts of pyrite. Many fractures in the lower section are filled with pyrite and minor amounts of dolomite. The upper and middle sections contain pyrite in the matrix of the rock, however, fracture-filling pyrite is rare.
We suggest that the adjacent carbonates may be one source for calcite present in fractures in the uppermost and lowermost Woodford Shale. Another source is hydrothermal fluids, which could precipitate calcite throughout the formation. Hydrothermal fluids rich in magnesium entered the middle section, leading to dolomitization. The lowermost section contains fracture-filling pyrite that may be the result of depositional geochemisty or diagenesis.