Calculating Lithology from Wireline Logs
Several techniques have been proposed for determining lithology from wireline log data, such as principal component and cluster analyses and discriminant analysis. Both techniques have not been widely used because their calculations are expert intensive, and results are subjective and semi-nonreproducible.
The Delfiner et al technique is embodied in the Geocolumn program. This program has a prebuilt data base of lithofacies and corresponding log responses. Typically, density, neutron, sonic transit time, gamma ray, photoelectric factor, thorium, uranium, and potassium can be used. A generalized data base of 182 facies and 8 logs thus requires 10,192 crossplots. The facies occur as ellipses on the crossplots, and the user must determine placement, orientation, size, shape, and degree of overlap of the ellipses. Overlap is needed to prevent undefined or unidentified zones, but it can introduce instability when data fall in the overlap. Because provenance, diagenesis, and tectonic history vary, the general data base in many geologic basins was modified. The Geocolumn program can now produc objective repeatable calculations and lend itself to use at the well site. A well-site lithology derived from logs allows a synergism with mud logs that provides information previously unavailable.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.