ANALYSIS OF COMPACTION IN THE NIOBRARA FORMATION, DENVER-JULESBURG BASIN, COLORADO

The Cretaceous Niobrara Formation is an unconventional resource play that yields oil and gas from chalks and marls in the Denver-Julesburg Basin, Colorado. Prior workers have noted that porosity is particularly associated with peloids. Thin section analysis reveals there are a number of different types of peloids that can be distinguished based on size, color and susceptibility to compaction. This study asked the question, does compaction of peloids vary by stratigraphic zone, facies, or peloid type? A total of 54 thin sections from the Timbro PC LC 16-17 and the Aristocrat Angus 12-8 well were investigated for peloid shape parameters. Fifty to one hundred peloids per section were analyzed using Image J for their perimeter, circularity, and mean gray scale values, which were chosen as proxies for size, degree of compaction, and color, respectively. The results indicate that one primary peloid population exists with a minor a second, larger, population across all samples. Peloids in all samples show a wide range in circularity (0.3 to 0.9). Correlation between compaction and chalk/marl stratigraphic zones is minimal. Very strong correlation was found at peloid scale as dark peloids are more compacted than light peloids. This difference manifests itself at the microfacies scale depending on the relative abundance of dark and light peloids. The differing response to compaction is presumably related to initial compositional differences such as calcite to clay and/or calcite to disseminated organic matter content.