Recognizing Hydrocarbon Signatures
In the 1970s, the realization that hydrocarbons could be directly detected by seismic data caused a major revolution in seismic processing and interpretation. The main drivers were digital recording and digital processing. The digital world led to the preservation of “true” or “relative” amplitude from acquisition through interpretation. The routine application of automatic gain control (AGC) was being reevaluated by all processing centers. Outstanding differences between processing with and without AGC were being displayed around the industry. Figure 4.A.1 contains an early 1970s example. What is surprising about this figure is that the data were acquired with a marine vibro-seis source in the offshore Gulf of Mexico. One can’t miss the gas anomaly at 1.6 s. Thus was bright-spot technology.
Figure 4.A.2 depicts the cry heard throughout the oil industry during the early 1970s: “Hydrocarbon reservoirs can be detected as seismic events that have significantly higher amplitude than surrounding reflections.” This thesis, once realized by an oil company, became the leading-edge technology to be exploited with internal research on new interpretation applications. The economic benefits for a company that had bright-spot technology were staggering when it came to evaluating potential prospects for upcoming lease sales, farm-ins, and the like.
Thirty years after the advent of the bright-spot technology, one has to wonder if there are any other interpretation techniques waiting to be developed that will change the oil industry as drastically as bright spots did. Probably not! However, there have been numerous incremental advances in amplitude interpretation since that time.
Figures & Tables
Seismic Amplitude Interpretation
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.