Geologic Considerations for Stratigraphic Modeling and Interpretation1
Seismic modeling techniques are attempts to mathematically and geometrically represent subsurface geology and to depict the seismic response of that geology to a propagating wavefront. In this context, modeling has become an important exploration tool (1) to test the mappability of a geologic concept, (2) to analyze the impact of expected geologic variability (porosity, thickness, etc.) on the seismic response, and (3) to evaluate the significance of event reflectivity changes, or anomalies, on uncalibrated seismic data.
Traditionally seismic data were used to identify events to map subsurface structure (faults, folds, noses) or large scale depositional geometries (pinnacle reefs, unconformities). To accomplish this, it was important to strengthen weak seismic events during processing of the recorded signals. Therefore, variations in signal strength (true amplitude) were purposely eliminated to accentuate event visibility.
Today one realizes that valuable geologic information is encoded into the shape, polarity, and true amplitude of the reflection. Where calibrated, it is possible to deduce important rock-fluid information from true amplitude seismic data. The information may be lithology changes indicating the reservoir boundary (and thus the trap), or fluid changes directly indicating the actual hydrocarbon accumulation. For the explorationist, this becomes another measurement tool and one with significant predictive value. The predictive value is vastly improved where subsurface response can be calibrated to rock-fluid data, preferably logs and rock samples. Modeling becomes an important vehicle to establish this calibration.
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Papers from a research symposium at the 1975 American Association of Petroleum Geologists and supplemented by later reports became “Seismic Stratigraphy Applications to Hydrocarbon Exploration”, one of AAPG’s best-selling book publications. Dramatic improvements in seismic imaging were demonstrated, a result of developments in seismic data quality and the processing capability of electronic technology. Twenty-eight articles are grouped into three sections. The first describes principles that both permit and also limit interpretations. The second section presents sixteen articles that describe the qualitative approach to stratigraphic interpretations of reflection records, and the final section presents techniques and examples of modeling. Of particular interest are a series of eleven papers in the second section under the subject heading of “Seismic stratigraphy and global changes of sea level”. Prepared by P. R. Vail, R. M. Mitchum and others from Exxon, they describe the regional unconformities and stratigraphic changes resulting from sea level fluctuations, and the manner in which these changes can be interpreted from seismic surveys. For many individuals within the oil industry who purchased this book, it was their first introduction to the modern concept of sequence stratigraphy that would have a major impact on the methodology of petroleum exploration.