Lithology Crossplots: Applications In An Evaporite Basin-The Maverick Basin Of Southwest Texas

A combination of two lithology-crossplot techniques, the M-N PLOT and the MID PLOT, may be used for analyzing subsurface zones of complex lithology at the wellsite. This does not supplant later, more accurate machine computations, however. It does help in making field decisions as to whether to make a completion attempt when this must be done in minimum time. Both plots use the three lithology-sensitive logs, Sonic, Density, and Neutron. They differ in the mariner in which these quantities are entered into the cross plots. As a result, the locations of the various mineral points differ on the two plots, and the directions of the gas, salt, and sulfur trends may also differ. By comparing plots of log data on the two cross-plots, and referring to the original logs when necessary, it is possible to identify gas zones even in complex lithology and evaporates, and to detect certain minerals within the matrix. Examples will be shown from the Maverick Basin in Southwest Texas where this approach to well-log analysis helps solve some difficult problems.