Determination of a More Accurate Porosity and Mineral Composition in Complex Lithologies with the Use Of the Sonic, Neutron and Density Surveys
Authors: Savre Wayland C. 
Published: December 2007

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 which 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 he 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.

Introduction

The Permian system of West Texas and adjacent areas contain large masses of carbonate rocks that often lack readily distinguishable correlative markers and that exhibit subtle diversity of mineralogy.

The neutron survey so widely used in these rocks, in past years, is subject to porosity error when gypsum is present. So also are the sonic and density logs which, in addition, are affected by anhydrite. Field use indicates that the three surveys may be used simultaneously to calculate a rock composition and to obtain a more accurate* value of porosity in formations comprised of dolomite or limestone, anhydrite, and gypsum, when the three major constituents are known.

Log analysis has been difficult in carbonate rocks. It has been complicated by uncertainties in porosity determination and has depended heavily upon "experience factors" in particular areas or fields. The problems of calculating true porosity result from the many variations in rock type, particularly with reference to pore geometry; the changes in matrix composition, which affect to some degree the response of all porosity devices now in use; and the effects of residual oil. The thin-bedded nature of many zones of interest further compounds the problem.

This paper will consider some of these problems in detail and present a solution which has seen practical use with satisfactory results in a large number of applications.

It may appear, to some readers that an excessive amount of space has been devoted to a discussion of rock characteristics in a paper primarily concerned with introducing a method of log analysis. The writer feels that accurate log interpretation can only be accomplished with a sound general knowledge of the rocks in which the surveys are to be used.