Log Analysis in Formations with Complex Lithologies

Abstract

Here is a method of interpreting well logs for formations having lithologies that are mixtures of silica, limestone, dolomite, anhydrite, and clay, or that are mixtures of any two specified minerals and clay. Analysis is made for clay content, matrix density, porosity, hydrocarbon saturation, and secondary-porosity index. Introduction During the past several years interpretation methods have been developed to obtain porosity and lithology from combinations of neutron, density, and sonic logs. One of these is the Dual Mineral Method which uses a crossplot of the neutron- and density-log data to arrive at values of porosity and apparent matrix density of the formation. The sonic log is used to indicate zones of secondary porosity and to help define the lithology. The Dual Mineral Method has been in use for a number of years. It gives good results in clean, liquid-saturated formations or clean, gas-bearing formations of known lithology. However, its use is restricted to these cases. This paper describes a more general method, based on the Dual Mineral technique, which has been developed for the interpretation of formations having lithologies that are mixtures of silica, limestone, dolomite, anhydrite, and clay, or that are mixtures of any two specified minerals plus clay. This new method takes account of both formation shaliness and hydrocarbon effects. In a preinterpretation procedure, log corrections are made, and certain parameters are evaluated. By study of crossplots of the log readings over intervals within lithologic units, the main mineral constituents of the rock matrix are identified and the corresponding properties are evaluated. properties are evaluated. Level-by-level computations are then made. Using the values of clay properties determined during preinterpretation, clay content is evaluated at each preinterpretation, clay content is evaluated at each level, and corrections are made for clay content and hydrocarbon effects. For the hydrocarbon correction, a value of hydrocarbon density is used, based on field information or crossplot study. Values of porosity, apparent matrix density, and water saturation are computed. Safeguards are included to minimize the possible effect of adverse borehole conditions on possible effect of adverse borehole conditions on some log readings. A moved-hydrocarbon index can also be calculated. The logs used for this method include a density log, a gamma ray-neutron log, a sonic log, and an induction log or a Laterolog, preferably with an SP curve. The addition of an Rxo log (Microlaterolog or Proximity log) and the value of the hydrocarbon Proximity log) and the value of the hydrocarbon density provides a reliable evaluation of hydrocarbon effect. The sonic log is used to determine lithology in the M-N plot, to evaluate secondary porosity, and to provide a limiting value of porosity in clay-free formations that are caved. The method can still be used without an Rxo log or sonic log, but the reliability of some of the results is diminished. The method is used in a computer program called CORIBAND. A preinterpretation pass is made in the computer, and depth plots and frequency crossplots are produced to determine parameters needed for the final computations. JPT P. 995

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