1963 Paper (K)

DETERMINATION OF TRUE POROSITY AND MINERAL COMPOSITION IN COMPLEX LITHOLOGIES WITH THE USE OF THE SONIC, NEUTRON, AND DENSITY SURVEYS

by Wayland C. Savre, Gulf Oil Corporation Odessa, Texas and Jack A. Burke, Schlumberger Well Surveying Corporation Midland, Texas

ABSTRACT

The Permian System of West Texas contains large undifferentiated masses of carbonate rocks with admixtures of evaporites. These rocks exhibit markedly variable 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 use 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 porosities on the neutron survey. Only recently have analytical methods been employed with attempt to eliminate these effects in core analysis. Anhydrite and gypsum may produce porosity 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 instrumentation, and the use of parameters which are a function of intrinsic rock properties, more accurate porosity determinations may be obtained.

The formation density log, the sonic log, and the neutron log provide three independent measurements of physical properties of formations in situ. Usable relationships between these properties and porosity are well known. The inaccuracies of computing porosity from average matrix parameters (grain density, matrix velocity, and neutron response) are minimized by considering the matrix parameters to be limited variables and making simultaneous solutions of three equations. This method of analysis permits determination of more accurate porosity values, and the percentage of each of three minerals comprising the rock matrix, dolomite or limestone, gypsum and anhydrite. In certain cases gas may be detected. Production tests, known reservoir characteristics and core information point out interesting correlative and corroborative results.