Computer determination and comparison of volume of clay derived from petrophysical and laboratory analysis

Volume of clay (Vcl) is the most critical parameter to determine in shaly sand analysis. Yet in many areas it is very difficult to determine volume of clay accurately. An overestimation of clay volume can result in effective water saturations (Swe) that are too low, thus making the reservoir look productive. Under-estimation of clay volume can result in effective water saturations that are too high, which can result in the bypassing of a productive zone. The underestimation and overestimation of volume of clay will also effect the calculation of effective porosities used to determine both effective water saturations and net pay.

The FORTRAN program SHALE is designed to calculate volume of clay from 14 different petrophysical methods using up to 7 petrophysical logs. In the program, corrections are made for lithology, pressure, mud weight, borehole variations, and bed thickness. In addition, corrections can be made to convert old gamma ray logs to the corresponding API scale. Laboratory determined volumes of clay can be input into the program and each petrophysical volume of clay raethod can be statistically compared so that the user can select the petrophysical volume of clay method that best estimates the true volume of clay. Using the most accurate petrophysical method chosen, SHALE will then derive a linear transform equation that can be used to correct petrophysically derived volumes of clay.

Laboratory determined volume of clay data from 7 samples from a lower Miocene deltaic sand in the Malay basin indicates that the gamma ray unconsolidated method was the most accurate in determining volume of clay. Laboratory determined volume of clay from 30 samples from the Cretaceous Olmos shelf sands in Webb County, Texas indicates that the gamma ray unconsolidated method was the most accurate in calculating volume of clay. However, in both the lower Miocene and Olmos sands, all petrophysical methods overestimated the volume of clay and required transforms to better estimate the volume of clay. The overestimation of volume of clay in both the lower Miocene and Cretaceous Olmos sands indicates that it is critical to calibrate petrophysically derived clay volumes to laboratory analysis.