Detection of gas in sandstone reservoirs using AVO Equation, the Geostack technique
Speaker: SU, SHIH-YU
Abstract
Amplitude versus offset concepts can be used to generate weighted stacking schemes (called Geostack technique) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties. One of the outputs of the analysis is a set of direct hydrocarbon indicator traces called ˇ§fluid factorˇ¨ traces. The fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the ˇ§mudrock line.ˇ¨ The mudrock line is the line on a cross-plot of P-wave velocity against S-wave velocity on which water-saturated sandstones, shales, and siltstones lie. Some of the rock types that lie off the mudrock line are gas-saturated sandstones, carbonates, and igneous rocks. In the absence of carbonates and igneous rocks, high amplitude reflections on fluid factor traces would be expected to represent gas-saturated sandstones. Of course, this relationship does not apply exactly in nature, and the extent to which the mudrock line model applies varies from area to area. Geostack processing has been done on a 2-D and 3-D seismic data set. We found that anomalously high amplitude fluid factor reflections occurred at the top and base of the gas-reservoir sandstone. it was found that the high amplitude values were restricted mainly to the gas field area as determined by drilling. The highest amplitudes were found to be located roughly in the areas of best reservoir quality (i.e., highest porosity). This processing also fix the problem that is hard to locate gas sandstone with higher acoustic impedance than the encasing shale by amplitude gradient method.
Reference
Smith G. C., and Gidlow P. M., 1987, Weighted stacking for rock property estimation and detection of gas. Geophysics 35,993-1014.
- Fatti J.L, Smith G.C, Vail P.J, Strauss P.J, Levitt PR, 1994, Detection of gas in sandstone? reservoirs using AVO analysis : A 3D seismic case history using the Geostack technique.? Geophysics 59, 1362-1376.