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Identification and assessment of depressions within the Ochoan evaporites through the integrated interpretation of 3D seismic and Full Tensor Gravity Gradiometry (FTG) in the Delaware Basin of Texas
Authors : Alan Morgan*, James Brewster - Bell Geospace, David Paddock, Vasudhaven Sudhakar, Jie Jenny Fang - Schlumberger Western Geco
FTG DATA APPLIED - SHALLOW VELOCITY MODELING
Summary
A series of NW-SE trending linear anomalies visible in the near-surface on 3D seismic data flanking the southwestern edge of the Delaware Basin in Reeves County, Texas are interpreted as filled depressions formed by the dissolution of Ochoan evaporite. These depressions have been associated with fractures and faults at the Lamar limestone level and have produced an excess of water and H2S by horizontally drilled wells crossing beneath them. High Resolution Airborne Full Tensor Gradiometry (FTG) and 3D seismic data are interpreted to map the near surface extent of such depressions. Source body migration (SBM) is used to estimate velocity changes within the depressions in order to provide better imagery between the near-surface dissolution and the underlying fault and fracture network. A method is presented that integrates FTG data into the depth migration workflow in order to pave the way towards a better 3D seismic image which is not contaminated by push downs produced by unresolved shallow velocity values.
Video : SBM results reveal density variation at 1200’ above sea level that strongly correlate to the NE-SW trends in the 3D seismic data. FTG Tzz (colour) and -245’ slice (b&w) show excellent correlation, however the SBM indicates that the source is much shallower.
Courtesy of Schlumberger
Key Takeaways
- The Delaware Basin in west Texas is characterized by near surface challenges which include layered anhydrites with sand/silt deposits sitting atop a thick layer of salt. Differing amounts of dissolution and other weathering factors create a complex near surface
- Seismic in the Permian has inconsistent control of the shallow section and therefore a seismic-derived shallow velocity model is not very detailed
- FTG sees the shallow in much more detail because FTG data are sensitive to the shallow density contrast that is used to constrain 3D seismic depth migration efforts
- FTG data are useful for constraining velocity models where shallow log data and coherent reflections necessary to perform tomographic estimation of velocity values are absent