Utilizing the Delta Log R method for Determining Total Organic Carbon of the Niobrara Formation, B Bench, Denver-Julesburg Basin, Colorado
Madeline K. Beitz, Robert Cunningham, and Lisa E. LaChance
The Niobrara Formation was deposited in the Western Interior Seaway during the late Cretaceous Coniacian-Santonian stages which coincided with high eustatic sea-level. In the Denver-Julesburg Basin study area, the Niobrara Fm occurs as a mixed shale/marl/carbonate sequence comprised of parasequences interbedded with organic-rich shale/marl units. These parasequences are developed as the three primary carbonate (chalk) benches. Total thickness of the Niobrara Fm ranges from two hundred feet to four hundred feet. Weight percent Total Organic Carbon (TOC) was calculated for the B Bench of the Niobrara Fm using the Delta Log R method of Passey et al (1990) within third-party well interpretation software. The study includes 129 wells that had both deep resistivity and sonic logs available digitally. The B Bench of the Niobrara formation was the focus of the study because recent drilling has indicated this zone to be a primary target for unconventional resource development. The methodology for Delta Log R calculation of TOC involved overlay and base-lining of the resistivity and sonic logs and consideration of Niobrara thermal maturity patterns. Zonal statistics (maximum, minimum, mean) for Delta Log R predicted TOC were calculated for the B Bench and the results were gridded over the study area using the top and bottom of the B Bench as a zone. The resulting mean values range from 2 % to 8 % in the study area and reflect both thermal maturity and deposition controls. This work aligns with and extends the published source characteristics for the Niobrara Fm.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013