Determination of a More Accurate Porosity and Mineral Composition in Complex Lithologies with the Use Of the Sonic, Neutron and Density Surveys

Abstract

The Permian system of West Texas contains large undifferentiated masses of carhonate rocks with admixtures of evaporites. These rocks exhihit marked !y variahle characteristics as observed with radioactivity, acoustic and electrical surveys. All surveys used independently may present different porosity values as a result of mineralogical heterogeneity. inadequate information regarding pore geometry, or variance in residual oil. The widespread, singular lise of gamma ray-neutron surveys has resulted in general misconception regarding the occurrence of porosity and consequently various aspects of reservoir interpretation. Hydrogen bound by water of crystallization in gypsum produces high apparent poros-ities on the neutroll survey. Only recently have analytical methods heen employed which attempt to eliminate these effects in core analysis. Anhy-drite and gypsum may produce poros-ity errors when average velocity or density information is used with the sonic or density logs. With the development and improvement of logging tools provided by more sophisticated electronics and in-strumentation, and the use of parameters which are a fllnction of intrinsic rock properties, more accurate poros-ity determinations may be obtained. The formation density log, the sonic log and the neutron log provide three independent measureillents of physical properties of formations ill situ. Us-ahle relationships hetween these prop