True pay thickness determination of laminated sand and shale sequences using borehole resistivity image logs
Reid, R.R.; Enderlin, M.B.
SPWLA 39th Annual Logging Symposium, May 26-29,1998
Once an exploration well has been logged, economic evaluation begins. True pay thickness from logs is an important input to that economic evaluation. Knowledge of the true pay thickness is of particular importance in sands and shales laminated at a scale below the resolution of the standard logging suit. Resistivity image logs provide information about the spatial distribution of shallow resistivity. Since resistivity is a function of both the rocks and included fluids, resistivity images can provide information about the spatial nature of the rocks and fluids. With proper processing (which includes data transformation from depth to the time domain, correction for tool acceleration, transformation back to the depth domain, and static normalization) the resistivity images can provide a quantitative measure of the shallow resistivity at a resolution of a few centimeters. A resistivity-to-pay sand cutoff operator is selected by the optical application of petrophysical reasoning. The resistivity-to-pay sand operator determines which sand layers are pay and their apparent thickness. Further processing can resolve the true dip of individual pay sand layers. By combining the local structural dip interpreted from true dip of the pay sand layers with borehole orientation data, the apparent thickness of each pay sand layer can be converted into a true pay sand layer thickness. Summing over all the true pay sand layer thickness yields the true pay thickness. Resistivity images from a Gulf of Mexico exploration well are used to illustrate a processing technique to achieve an understanding of the true pay thickness.