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Interactive interpretation of 3-D seismic data is now an effective tool for mapping complex stratigraphic targets on land. Slicing a 3-D seismic cube through a single reflector provides an insight into complex stratigraphic environments.
A case study from Grayson County, Texas, is used as an example. Production from the lower Davis sands of the Atokan series was established in the Southmaud area in 1978. Further development of the field led to a success ratio of only 50%. Stratigraphic complexity of the fluvial to deltaic environment made reservoir prediction and placement of offset wells difficult. A 6-mi2 3-D survey was shot for the purpose of mapping the sand distribution. Acquisition and processing costs were less than the cost of one dry hole. The producing sands were a difficult seismic target. Resolution of these sand bodies was in question due to their vertical and horizontal extent as well as their small reflection coefficients.
The interpretation was completed in eighteen hours using Gulf's Interactive Seismic Interpretation System (ISIS). Only one well was used initially to identify seismic horizons. Structural mapping was completed using dual polarity, random vertical lines, and isotime slices. Stratigraphic interpretation was done with discrete amplitude coloring to take advantage of seismic tuning effects. Stratigraphic slices revealed the distribution and thickness of the reservoir sand. Geological features interpreted include a meandering channel with point-bar buildups, a distributary complex, and erosional or nondepositional areas.
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