Investigation of cm-scale heterogeneity and relationships among organic carbon, mineralogy and rock fabric in mudstone and siltstone of the Permian Phosphoria Formation

The purpose of this project is to investigate, document, and correlate physical, mineralogical, and geochemical heterogeneity in organic-rich rocks of the Permian Phosphoria Formation. The hypothesis that I seek to test is that changes in organic carbon and mineralogy vary with macroscopically distinct variations in rock fabric at the centimeter scale. Forty-nine samples of organic rich mudstones and siltstones from the Meade Peak and Retort members of the Permian Phosphoria formation were collected from over 25 locations in southwest Montana, western Idaho, eastern Wyoming, northeastern Utah and northwestern Colorado. These samples are to be cut, polished, and then described in terms of physical rock fabric. Macroscopically distinct changes in rock fabric will be then sub-sampled and analyzed for total organic carbon content and mineralogy. Through comparison of these parameters in the lamina of the rocks, I expect to test my hypothesis that a correlation exists between macroscopic physical rock fabric, organic carbon, and mineralogy. This project is important because total organic carbon and mineralogical analyses commonly are reported as a single number with no accompanying context regarding physical rock fabric. If cm-scale variations in these parameters are found to exist, that would suggest that bias may be introduced when organic carbon and mineralogy are reported by laboratories without accompanying information regarding sedimentary fabric. Preliminary analysis of Phosphoria samples suggests that mudrocks fall into two broad categories: 1) massive samples with no apparent heterogeneity on the cm-scale; and 2) samples that do contain cm-scale heterogeneity. In the latter case, heterogeneity at the cm-scale appears to be defined by alternations between phosphatic peloid-bearing microfacies and peloid-free laminae microfacies