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Chapter 119: Synthetic Seismic Modeling of Turbidite Outcrops
Seismic forward models of Turbidite outcrop sections are used to illustrate how various reservoir architecture details may be expressed with the scale of resolution of commonly available marine seismic data. Six outcrop sections having sufficient length and thickness were digitized at a bed scale to generate models. These cover a variety of reservoir-architecture types from different parts of the world, and include sheet-sand architecture, channelsand architecture, and mixtures. Testing the seismic response of a single geometry, assuming different acoustic contrasts, velocities, seismic wavelets, and frequency contents illustrates the scale of heterogeneity resolvable by common marine seismic data.
With sufficient frequency and rock-property contrast between sand and mud, channel vs. sheet sands clearly have different character, with channels being more chaotic with rapid amplitude and dip changes. However, as acoustic contrast between sand and shale diminishes and frequency diminishes, it can become very difficult to distinguish channel- from sheet-sand internal architectures. Even where frequency content is good, the internal details of potentially significant scour surfaces rarely can be identified with confidence, and individual bed geometry is usually below resolution. Three-dimensional (3-D) seismic data can help enormously. Oftentimes, seemingly minor features in cross section, such as subtle dip or amplitude changes, can map into clearly recognizable channelform features in map or volume view. However, even where such features are mapped in 3-D, the nature of the critical connections between adjacent channels, or between channels and nonchannel sands, is almost always below seismic resolution. A common observation in the channel sections is that the number of scours would tend to be underestimated from seismic, and their depth overestimated. Outcrops tend to show more numerous, shallower scours than can be typically interpreted on seismic data. Seismic data can typically indicate where the channel fairways lie, but not their detailed internal architecture or stacking pattern. Outcrop studies are important for filling in the details of potential architectural scenarios where seismic resolution and sparse well control leave significant room for alternative interpretations.
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