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Sonic Properties as a Signature of Overpressure in the Marcellus Gas Shale of the Appalachian Basin


The Marcellus Formation, a Devonian gas shale in the Appalachian Basin, is a heterogeneous rock as the result of a complex depositional, diagenetic, and deformational history, and it is overpressured over a large portion of its economic area. Here we use sonic properties of the Marcellus to understand the origin and distribution of pore pressure within the gas shale. First, vertical P- and S-wave velocities throughout the overburden section were analyzed using a calibration well. For the entire section below the intermediate casing, the sonic velocities generally decrease with gamma-ray API and increases with density in all mechanical units, except in the Upper Devonian where sonic velocities decrease with density because of the increased volume of high density clay minerals. Sonic velocity within the Marcellus is influenced by the same properties except a reduction in effective stress causes a concomitant reduction in velocity. Laboratory results from seven Marcellus sidewall core samples show that the ultrasonic velocities the variation in mineral composition causes a larger variation between samples than the change in confining pressure. Log-based velocities are close to the core based ultrasonic velocities at the same effective stress. The basin-wide variation of sonic velocity is then analyzed in the Marcellus for a suite of 53 wells in the Appalachian Basin of Pennsylvania and West Virginia, focusing on the influence of burial depth, effective stress, total organic carbonate, and density. The sonic velocities generally increase with burial depth (or equivalent confining stress), decrease with pore pressure and increase with effective stress. They also decrease with TOC and increase with density. Abnormal pore pressure is verified by the observation that the correlation between sonic properties and effective stress is strengthened relative to the correlation between sonic properties and depth. The best-fit between sonic velocity and effective stress was achieved using an effective stress coefficient of about 0.7. This confirms that pore pressure is generated by a natural gas, a ‘soft’ fluid which is known to reduce the effective stress coefficient relative to the Biot-Willis coefficient of volumetric pressure.