Final Comments and Future Expectations
As noted earlier in Fig. 1.B.5, the beginning of the second era of amplitude interpretation is keyed to Ostrander’s 1982 verification of AVO with prestack field data tied to drilling results. Since that time, the field of amplitude interpretation using AVO has been a fertile area of research, discoveries, and practical applications. The interpretation procedure in Fig. 8.A.1 outlines the AVO philosophy on validation that was followed in these notes. The sections in these notes on petrophysics, modeling, and amplitude attributes were key items that were discussed to reach the goal of validation through “Compute and Compare,” as defined in Fig. 1.A.3. However, this amplitude procedure is not new—it is almost identical to Pan’s amplitude procedure of the 1970s (Fig. 1.B.4). Little has changed in our validation philosophy. Only the available amplitude attributes have changed. Figure 8.A.2 summarizes some of the amplitude attributes that were developed or expanded in this second era of amplitude interpretation. Starting with the CDP gathers, there are two paths that can be followed in the figure. The path on the right of the figure provides the more robust attribute estimates. As one proceeds downward in either path more information to distinguish pore-fluid and/or lithology is derived from the amplitude attributes. At the same time, though, more interpretative bias is added to the amplitude interpretation in the downward path. This is not necessarily an unwanted result if petrophysical constraints are available. However, the interpreter must always be cognizant of the assumptions and interpretative bias that the amplitude attributes contain.
Figures & Tables
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.