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 log 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 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.
The determination of porosity and lithology from well logs can usually be successfully accomplished using crossplot techniques or algorithms derived from the same crossplot techniques (reference 1 and 2). Such methods often handle only the most common minerals such as calcite (limestone) quartz and dolomite. If more minerals are present in addition to these, serious errors may occur in the resulting porosity and lithology.
In cases where complex lithologies exist, a simultaneous determination of porosity and lithology from a combination of neutron, density and sonic logs porosity and lithology from a combination of neutron, density and sonic logs can be successfully accomplished, especially in heterogeneous carbonate reservoirs. The essence of this method is that porosity and the lithology are determined by solving simultaneous equations describing the responses of the porosity logs. The method is however limited to a maximum of three minerals in any given matrix.
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