By Nam Nguyen
Product Manager – Seismic Interpretation and Analysis, Halliburton Landmark
Part of the Spotlight Series
Fluid and lithology predictions from seismic data.
To determine the reservoir makeup and identify the best potential reservoirs, an understanding of the seismic response to changes in fluid and lithology is essential. This can be achieved by examining seismic amplitude data. Near-mid-far angle/offset stacks are a very common dataset available to the interpreter. These are key inputs for calculating AVO attributes where pre-stack gathers are not available or are in areas of noisy data.
By combining AVO intercept and gradient attributes along an angle of rotation, an optimal seismic stack can be designed to maximize the discrimination between either fluids (fluid stack) or lithologies (lithology stack). The fluid stack is used to highlight hydrocarbon reservoirs, and the lithology stack can help enhance lithology variations where fluid effects are removed or reduced. The lithology stack is used for mapping instances of sand.
Using Landmark’s DecisionSpace® Seismic Analysis® software, AVO intercept and gradient attributes are computed from a combination of near-mid-far angle/offset stacks to produce the fluid and lithology stacks. This requires:
The calculation of the background/ shale trend from a crossplot of intercept and gradient: Allows the interpreter to classify how lithological and/or pore fluids differ
The calculation of the rotation angle: Angle between the background trend and the vertical axis of a cross plot
The rotation angle can then be used to generate fluid and lithology stacks, which can help verify the fluid contacts and lithology/facies changes in the reservoir across the entire 3D seismic survey.