Abstract

A multilog-evaluation package using linear programming techniques has been developed to evaluate porosity and lithology in hydrocarbon bearing, complex lithologies. Conventional log evaluation techniques, using crossplots or iterative analytical solutions derived from two porosity logs, tend to give unsatisfactory results in hydrocarbon bearing, complex lithologies.

Linear log-response equations can be defined for several logs including the neutron and sonic logs, lithodensity log, (micro-) resistivity log and natural gamma ray spectroscopy log. By introducing an error term in the log-response equations, the linear programming technique calculates porosity, hydrocarbon saturation in the invaded zone and mineral fractions. This is accomplished by minimizing the sum of the error terms. The linear programming technique has no limitations for the number of logs used and the number of variables calculated. However, if the number of variables is significantly greater than the number of logs used, the solution calculated may not be unique and the results must be treated with caution.

The method has been successfully applied to evaluate porosity and lithology in the Central Luconia gasbearing carbonates offshore Sarawak, East Malaysia. These carbonates consist of limestones and dolomites with porosities ranging from 0 to 40 percent. The porosity and mineral fractions calculated agree very closely with core data and sidewall sample information.