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Abstract: Seismic Techniques in the Cotton Valley Reef Play in East Texas

MEYER, JOERG H., Schlumberger Wireline & Testing and WAYNE TITTLE, Sonat Exploration


The Cotton Valley Reef Trend of East Texas has undergone a tremendous increase of interest and activity rarely seen in mature domestic onshore basins. It is one of the closest watched natural gas plays in the United States. The reserves and opportunities for high-volume wells are enormous. Drilling targets in the trend are small pinnacle reefs of Jurassic age with an average size between 20 and 60 acres located at depths that often exceed 15,000 ft. Cost and risk are high as a result of the hostile conditions including high temperature, pressure, sour gas content, and dense rocks. The high success rates in the trend are possible due to the technological and economical evolution of 3-D seismic methods within the last few years. Discoveries typically have recoverable reserve estimates between 6 and 60 BCF. Although 3-D seismic is the driving exploration tool to locate and pinpoint productive reef development the technical requirements for 3-D to work are complex. Minor residual errors in determining correct processing parameters like migration velocities can skew the true position of a reef resulting in a dry hole. It is possible to lower the exploration risk by integrating surface and borehole seismic methods. Velocities measured in the wellbore at intermediate casing point can be used to remigrate a small volume of surface data around the target. This can happen in a time frame short enough to allow the well to be steered toward the crest of the reef which is now more crisply imaged thanks to the new migration velocities. The resulting interpretation is enhanced and confirmed by the acquisition and utilization of multiple look ahead offset VSPs. The integration of all three data sets results in a more precise image of the reef crest and can help lower the exploration risk.