At Big Sky, Montana, in 1992, the SEG/EAEG Joint Research Committee held the summer workshop, “How useful is amplitude-versus-offset (AVO) analysis?” From the worldwide exploration examples shown during this six-day conference, the answer could clearly be stated that AVO was an integral part of recognizing and high-grading prospects and well locations.
During the Big Sky Conference, Revoir et al. (1992) and Russell et al. (1992) presented evaluations of AVO through seismic processing, modeling, and inversion utilizing the same data set from a Pliocene-trend gas field in the Gulf of Mexico. The field is located offshore in a fault basin bounded by major growth faults, and is near several salt and shale diapirs. The structure is broken up into several reservoirs by small (50–150-ft) (15-50-m) faults (Fig. 7.A.1). A 2-D amplitude analysis was performed from a 3-D seismic data sail line. A directional well that was parallel to the 2-D line provided borehole control.
The expanded sedimentary section consists of regressive sequences of deep marine deposits (including slope fans) overlain by shallower deltaic deposits (shelf, delta front and lower delta-plain deposits). The major productive sands are between 5000 and 12,000 ft (1500 and 3700 m) in depth and range in thickness from 20 to 180 ft (6 to 50 m) (Fig. 7.A.2). The high amplitudes in the figure characterize the major sand pays. In addition, flat spots and phase reversals were dominant HCI’s for recognizing the gas/water contacts. Overall, the amplitude anomalies from the three productive sands showed excellent fit to structure.
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During the last 30 years, seismic interpreters have routinely applied bright spot and AVO technology for recognizing prospects and predicting lithology. New amplitude attributes were added to this technology as new exploration problems were defined. R&D continues in the field of amplitude interpretation, especially when E&P costs escalate as more severe environments are explored, such as the ultra-deepwater plays. With the high interest in reducing exploration risk, this course addresses the methodology of an amplitude interpretation and the subsequent benefits and limitations that one can expect in various rock-property settings. This book, originally produced for use with the fourth SEG∕EAGE Distinguished Instructor Short Course, begins with a review of relationships between rock properties and geophysical observations. Practical problems illustrate the assumptions and limitations of commonly used empirical transforms, and procedures for conducting and verifying fluid-substitution techniques are presented. The book identifies components of the seismic response best suited for differentiating pore fluid from lithologic effects. Field examples emphasize what combination of seismic signatures should be expected for different rock-property environments. To help select the best seismic attribute for calibrating amplitude to rock properties, rules of thumb are provided for predicting AVO responses and interpreting lithology from observed responses. A case history is also provided. The last part examines the numerous amplitude attributes that can be extracted from seismic data to quantify an interpretation. Benefits and limitations of these attributes in soft- to hard-rock environments are discussed with model data and in case histories.