THE LITHO-DENSITY* LOG

Abstract

The Litho-Density Log (LDT) measures a new formation parameter, Pe, the effective photoelectron cross-section. This measurement is made at the same time as the measurement of bulk density, ρb. The equipment is com bined with Gamma Ray and Compensated Neutron Loqqinq (CNL*) devices.

Pe is a function of Z. the effective atomic number. The Z of a formation determines its chemical content and therefore its lithology.

The effect of gas on the Pe measurement is almost neglible so it becomes a good lithology Indicator in non-shaly gas-bearing reservoirs. As a result, it provides a means for confirming porosity determinations made with corrected bulk density and CNL porosity data.

The radioactive logging parameters for a number of sedimentary minerals have recently been computed. With this knowledge and the addition of the Pe measurement it is possible to discriminate between tight streaks and heavy minerals such as siderite.

A number of clays, such as glauconite, may be identifiable with cross-plot methods. Knowledge of the presence of glauconite, which is thought to be formed in shallow marine waters, can be useful in establishing depositional environments.

If a Natural Gamma Ray Spectroscopy (NGS*) Log is available, additional cross-plots are proposed which will enhance the identification of clay minerals

Several of the proposed interpretation techniques are illustrated with data from Australian and other well logs.

*Mark of Schlumberger