Class I AVO Analysis in a North Texas Reservoir

By

Pankhurst, Duane A.

University of Houston, Houston, TX

The objective of this study was to identify by-passed hydrocarbons through amplitude variation with offset (AVO) modeling. A 20 square mile portion of high-resolution (10-90 Hz), high fold (40) 3D seismic data set was the focus of this research. At the producing horizon, high impedance sands encased in lower impedance shales exhibited Class I (dim out) AVO behavior. Well data included 53 well logs, with 36 in digital format. Sonic measurements were made at 10 well locations. Also, core and well fluid samples were analyzed and calibrated to well log and AVO responses. Source-receiver offsets were two times the objective depth, providing an opportunity to evaluate prestack common depth point (CDP) gathers at very long offsets. At large offsets, positive (+) reflectivities associated with Class I AVO anomalies reduced in amplitude (dimmed), passed the zero crossing and reversed polarity. At further offsets, amplitudes tended to increase in negative (-) amplitude and brightened with offset. This condition is analogous to Class IIp (phase reversal) behavior. On stacked sections, dimming amplitude anomalies are hard to recognize and validate. However, by evaluating pre-stack gathers at far offsets, Class IIp brightening behavior may be exhibited. Since brightening events are more readily observed than dimming events, this novel technique was an alternative methodology when working within the Class I AVO environment.