Castagna, John P.2019-12-17December 22019-12December 2Portions of this document appear in: Omovie, Sheyore John, and John P. Castagna. "P-to-S-wave velocity ratio in organic shales." Geophysics 84, no. 6 (2019): MR205-MR222. And in: Omovie, Sheyore, and John Castagna. "Acoustic response to fluid properties in hydrocarbon-rich shales." In SEG Technical Program Expanded Abstracts 2017, pp. 3675-3679. Society of Exploration Geophysicists, 2017.https://hdl.handle.net/10657/5601In situ P-wave and S-wave velocity measurements in a variety of organic-rich shales exhibit compressional-to-shear-wave velocity ratios that are significantly lower than lithologically similar fully brine-saturated shales having low organic content. It has been hypothesized that this drop could be explained by the direct influence of kerogen on the rock frame and/or by the presence of free hydrocarbons in the pore space. Theoretical bounding equations, using pure kerogen as an end-member component without associated gas, indicate that kerogen reduces both the P-wave and S-wave velocities but does not in general reduce their ratio. The theoretical modeling is consistent with ultrasonic measurements on organic-shale core samples that show no dependence of velocity ratios on kerogen volume alone. Sonic-log measurements of compressional and shear-wave velocities in seven organic-rich shale formations deviate significantly from the Greenberg-Castagna empirical brine-saturated shale trend towards lower velocity ratios. In these formations, and on core measurements, Gassmann fluid substitution to 100% brine saturation yields velocity ratios consistent with the Greenberg-Castagna velocity trend for fully brine-saturated shales, despite the high organic content. These measurements, as well as theoretical modeling, all suggest that the velocity-ratio reduction in organic shales is best explained by the presence of free hydrocarbons. The limitation of the Greenberg-Castagna shear-wave velocity prediction method when applied to organic-rich shales has been resolved, by modifying the original Greenberg-Castagna algorithm. The modified workflow accurately predicts shear-wave velocity for seven organic-shale formations with appreciable solid organic matter to within ±1% percent mean error. For a number of low-permeability well-lithified shales, utilizing laboratory measurements on dry and fully brine-saturated samples as well as comparing to log data and theoretical modeling, we find no statistically significant intrinsic dispersion from seismic to sonic and laboratory-measurement frequencies due to fluid effects. At in situ stress conditions, the Gassmann zero-frequency P-wave velocity prediction for a Permian-basin sample was within 0.2% to 2.2% of the measured velocity on the brine-saturated sample at ultrasonic frequency. Based on the Biot-Gassmann model, the characteristic frequency occurs at about 10^10 Hz. Applying a squirt-flow model also predicts a transition to the high-frequency regime occurring at about 10^9 Hz.application/pdfengThe author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. UH Libraries has secured permission to reproduce any and all previously published materials contained in the work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).Velocity-ratioShales2019-12-17Thesisborn digital