Study of the organic content and mineralogy effects on the acoustic properties of the Niobrara Formation, Denver Basin

Wattenberg Field has been producing hydrocarbons since 1970. Over the past 20 years, evolution of horizontal drilling and hydraulic fracturing allowed for the expansion of unconventional gas and oil production from Niobrara chalk and marl intervals across the Greater Wattenberg Area (GWA). The Niobrara carbonate system in the Denver Basin contains a TOC range of 1--8 wt.%. Additionally, GWA is associated with a geothermal anomaly. This study investigates the effects of TOC, clay content and kerogen maturity levels on the acoustic properties of Niobrara chalk and marl intervals. Core samples from Gill Land Associates 2 and Berthoud State 3 wells were analyzed for total organic content richness and maturity. Compressional and shear ultrasonic velocities were measured under variable confining pressures up to 4000 psi. Lab measurements were integrated with source rock analysis and XRD mineralogy to establish significant correlations between acoustic properties and organic maturity. A "blind test" was performed in which the correlation models were used to predict organic carbon richness and source rock maturity indices using wireline logs from Aristocrat PC H11-07 well with measured mineralogy and organic carbon richness. Correlations with statistical significance are presented in this study. A significant correlation was established between core-calibrated porosity and VP0/VS0 ratio. Results show that B and C intervals within the Smoky Hill Member generally have higher stiffness than A and D intervals and Ft. Hays limestone. Higher illite clay content decreases the 45° and 90° velocities significantly. In contrast higher carbonate volume shows a reverse effect. Samples rich in TOC had noticeably lower C33 and C12 stiffness coefficients. Existing hydrocarbons in the sample increase the rigidity of the sample. Higher S2 content seem to increase the shear wave velocity in the 45° and 90° directions while slowing down the overall compressional wave velocity. The previously introduced uranium spectral gamma ray method for TOC prediction by (ElGhonimy, 2015) show closer pattern to the core measured TOC. In contrast, the method presented in this study is a function of shear and compressional wireline acoustic log responses. TOC prediction from C12 provides qualitative TOC richness assessment. The introduced model results show moderate accuracy in clay-rich zones. Accuracy of estimated TOC richness was observed to be higher specifically in the C marl interval compared to a higher accuracy in the B marl when using the uranium method.