Abstract

A common practice in log interpretation is to cross-plot various porosity log readings in order to determine formation lithology and compute porosity accurately. Crossplots of Sonic versus Density logs are widely used in the interpretation of shaly sands. For carbonates, Density versus Neutron cross-plots are commonly employed. These plots and the calculations based on them are extremely plots and the calculations based on them are extremely useful, but, when the lithology is a complex mixture of minerals, interpretation of the data often becomes ambiguous.

The "Litho-Porosity" cross-plot is introduced for interpretation in formations of complex lithology. It presents simultaneously the data from all three of the standard porosity tools: the Sidewall Neutron Porosity log, or the porosity tools: the Sidewall Neutron Porosity log, or the GNT; the Formation Density Compensated log, and the Borehole Compensated Sonic log. From the readings of these logs two porosity-independent parameters. "M" and "N", are derived-M from the Sonic and Density, and N from the Neutron and Density.

In the plot of M versus N, each pure rock mineral is represented by a unique point regardless of porosity. For a formation of complex lithology, the position of the log data points on the M-N plot relative to the pure mineral points points on the M-N plot relative to the pure mineral points is of great assistance in identifying various minerals in the formation. Lithological information so derived is then used to calculate accurate values of porosity.

The computer can be programmed to produce cross-plots of M versus N from logging data recorded on magnetic tape, or on punched cards. The method allows detailed studies of individual formations and comparisons with other wells in a fraction of the time required using manual methods.

The Litho-Porosity technique has many applications in formation evaluation and interpretation. Examples are shown in the paper.

Introduction

The Sonic-Density-Neutron logging suite and the use of the computer for log data processing have opened a whole new branch in the field of well log evaluation. Computations of solutions to the simultaneous equations describing responses of the three porosity logs began in the Permian Basin in 1962. This work provided the first accurate porosity values to be routinely computed from logs in the porosity values to be routinely computed from logs in the complex San Andres carbonate-evaporite section. A by-product of these "Tri-Porosity" calculations was fractional mineral percentages presented as a lithology log. Subsequent technical developments have provided equipment to record log data digitally on magnetic tape at the wellsite and transmit it via dataphone to the computer for rapid analysis.

Today, variations of the tri-porosity calculations are being used worldwide to compute rock characteristics which include:

  1. Porosity computation in complex carbonates and shaly sands, including the detection of secondary porosity and unflushed gas.

  2. Lithology determination for stratigraphic and environmental studies.

  3. Detection and evaluation of mineral deposits such as sulfur, potash, coal, oil shale, and certain metallic minerals.

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