A method was developed for predicting lithology from well logs for the Mauddud, Safaniya, and Khafji Members of the Wasia Formation in Safaniya, Zuluf, Marjan and Maharah Fields of offshore Saudi Arabia. A group of electrofacies (i.e. litho-facies or lithofacies groups that exhibit a unique set of well log responses, H.T. Abbott, 1982), were defined using gamma ray, density, sonic, calculated shale volume and caliper-log data. The eight basic rock types represented by the electrofacies are limestone, argillaceous limestone, sandstone, argillaceous sandstone, shale, ironstone, iron-rich sandstone and iron-rich shale.

A FORTRAN program was written to identify these electrofacies. The program output consists of a series of intervals that are defined by a Top and Base depth and an electrofacies code. These numerical data were compiled into a lithology database. A comparison of this information with 6,705 feet of core from 12 wells resulted in an accuracy rate of approximately 97 percent.

Applications for these data are varied and range from use in Saudi Aramco's ZYC0R (STRATVIEW), INTERGRAPH (G.I.P.S.I.E.) and SIERRA cross section display programs to input for three-dimensional geological modeling. The use of electrofacies in reservoir characterization will enable geologists and engineers to better understand and predict the impact of lithologic variation on reservoir performance.

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