A POROSITY AND PERMEABILITY COMPARISON OF THE MANCOS SHALE AND WANAKAH FORMATION OF WESTERN COLORADO

The utilization of technologies such as hydraulic fracking and horizontal drilling has allowed shale gas reservoirs to become attractive targets for the petroleum industry. In order to identify gas reservoirs, estimate their potential, and the potential success of different fracturing strategies, a comprehensive study of the mechanicaland physical properties of the shale formations is required. Two shale units in western Colorado are the Jurassic Wanakah Formation, a terrestrial shale that is not a potential petroleum reservoir, and the Cretaceous Mancos Shale, a marine shale that is an actively producing petroleum reservoir. Provenance and depositional environment of the Wanakah Formation are interpreted in western Colorado to better understand the effects of porosity introduced by paleosol formation and bioturbation as well as secondary changes to porosity and permeability during diagenesis. Porosity and permeability in both formations are also quantified in order to gauge seal efficacy and the success of different fracturing techniques for both terrestrial and marine shale formations.

The hypothesis being tested is that terrestrial shales with paleosol development have different physical and chemical properties than marine shales and these properties may enhance permeability and affect seal efficacy. Terrestrial shales exhibit permeability anisotropies due to bioturbation associated with paleosols that are not present in marine shales. Traditional field methods were used to differentiate facies within the Wanakah Formation and Mancos Shale based. Samples were collected for laboratory analysis such as bulk geochemistry and xray diffraction. These traditional methods are combined with more novel techniques such as QEMSCAN and stable isotope analysis to differentiate chemofacies and in situ mineralogy.

From these data, we are able to document and compare lateral variability of porosity and permeability for the Mancos Shale and Wanakah Formation with application to predicting seal efficacy and to better manage hydrocarbon recovery. This research will ultimately provide the petroleum industry with new information with which to better understand the properties and permeability anisotropies in fine-grained seals and unconventional reservoirs.