Abstract

A combination of two lithology-crossplot techniques, the M-N Plot¹ and the MID Plot¹, 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 the analysis must be done in minimum time.

Both plots use the three lithology-sensitive logs, sonic, density, and neutron. They differ in the manner in which these quantities are entered into the crossplots. 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 crossplots, and referring to the original logs when necessary, it is possible to identify gas zones even in complex lithology and evaporites, 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.