A STATISTICAL-METHOD FOR LITHO-FACIES IDENTIFICATION

In this paper a procedure for the estimation of the lithological description of the stratigraphical formation by wells is described; in this procedure the results of wireline measurements and a litho-facies database are combined. The litho-facies are defined from log values and histograms and are translated into gaussian distributions. In order to optimise type lithologies we have used the discriminant function referred to as the Bayes rule. In addition, several logic rules have been applied in order to discriminate between different litho-facies (test of shale volume and bore hole conditions). The wireline logs used are density, sonic transit time, gamma ray, spontaneous potential and resistivity. The litho-facies groups represented in this database are sandstone, calcareous sand, shaly sand and shale. The procedure has been applied to two wells in a fluvial environment (sand-shales series). The results have been compared with drilling rates, cutting descriptions and Dipmeter information. For each level a lithological column can be produced with a computed probability. The examples are used for the study of sequences and facies in the Tertiary formations in the foreland Chaco basin of Bolivia (latitude 19-20-degrees-S).