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Abstract: Cyclic Attributes on Seismic Data and Sequence Stratigraphy-New Criteria for Exploration,
New Interpretation Styles
Integrated Geophysics Corporation, Houston, TX
The application of sequence stratigraphy to seismic data has long centered on the process of interpretation of seismic reflection geometries of onlap and downlap, and the tying of well data to seismic. But in many basins, and especially in deepwater areas, well data may be rare or nonexistent. Tying shelf sequences to basin sequences is often impossible because of long distances, gaps in seismic data, and complex structures. The concept of aggradation cycles within sequence architectures offers new criteria for exploration and new ways to interpret seismic 2D and 3D datasets with modern visualization tools. Manipulation of seismic voxels and attributes become tools to study stratigraphy. The focus of seismic interpretation shifts from finding reflection geometries to finding cyclic vertical stacking patterns even if geometries are absent or subtle. This framework can give insight into the sediment delivery system of margins and to the aggradation of sediments in deep water in areas of sparse or no geologic control. These criteria have been applied for almost a decade to the Offshore Nigeria exploration areas and key discoveries have been made using these techniques. Other areas of application include Gulf of Mexico, NW Shelf Australia, offshore Brunei, and Bangladesh.
The key criterion that guides the interpreter in these settings is the repetitive cycles of seismic reflection attributes and seismic facies patterns. The most useful attribute cycles consist of changes in seismic instantaneous amplitude and frequency. Vertical stacking patterns of seismic attributes can be utilized in much the way that well log curve stacking patterns are used to guide sequence stratigraphic analysis. Cyclic seismic facies patterns often change upward from laterally continuous reflections to
Unnumbered Figure. TRIO attribute display for offshore Nigeria shows amplitude and frequency cycles build the deep water seismic sequence architecture. An example of a slope well is tied to the seismic TRIO attribute display from offshore Nigeria. Only the highest amplitudes are visible. The high frequency (hot pink) and the low frequency (red and yellow) voxels show cycles on the 3rd-order sequence scale and build the architecture for this slope setting. The sequence boundaries are marked in orange with the deep-water aggradation cycles in bold white curves. The gamma ray curve is displayed in the color-coded lathe display and the thickest sands (yellow) match to the lowest frequency responses. Note that the cycles are not confined to the syncline, but continue into deeper water.
subtle mounded patterns or chaotic patterns. The attribute cycles and succession of seismic facies most often correlate to the 3rdorder sequence and the different depositional energy and styles that predominate as sea level falls and then rises. The key parameters that change through this cycle are bed thickness, lithology and facies assemblages, and depositional styles such as sheet-forms or sinuous channel-forms. In deep-water settings, these cycles are often a very prominent feature of the seismic data. Full analysis of the seismic data from these areas typically reveals the framework on three scales; the mega-architecture basin scale of 2nd-order sea level change and tectonic subsidence, the 3rd-order "building block" sequence scale of many sea level falls and rises, and the parasequence scale suitable for well prediction and reserve calculation. The repetitive nature of the cycles implies a time of balance for important parameters like sedimentation rate, subsidence, sea level, and the development of a matured, efficient sediment delivery system. These patterns also imply a high potential for recycled sediments stored in an intermediate position ready to be efficiently delivered to the basin at each lowstand of sea level. Thus, the more repetitive the cycles, the better the potential for good-quality reservoir sands occurring in the deepwater facies.
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