Skip to Main Content
Book Chapter

Lithology Determination from the Overlay of Density and Neutron Logs

By
Published:
January 01, 1994

Abstract

Up to this point, the explicit identification of lithology from logs has been almost entirely restricted to the differentiation of shales from non-shales. In some cases, lithologies or minerals with unusual properties can be recognized, such as those with anomalously high or low densities, including anhydrite, halite or coal. Otherwise, the distinction of sandstones from either limestones or dolomites on logs has been inferred from porosity character, stratigraphic correlation or drill-cuttings information. The nature of the matrix mineral composition is important to the log analyst, because he or she needs this knowledge in order to come up with accurate estimates of true porosity. The calculation of porosity from either the neutron, density or sonic logs is keyed to the log response of the matrix. The identification problem becomes even more complex, if the matrix is a mixture of several minerals, such as in a cherty, dolomitic limestone, further aggravated by compositional changes with depth. Log overlays and crossplots of porosity logs were introduced to resolve this problem, primarily as a means to derive true porosities. However, the lithological information from these methods are invaluable byproducts.

The three porosity logs (neutron, sonic and density) are recorded in radically different units of equivalent porosity units, microseconds per foot, and grams per cubic centimeter. If the logs are to be overlaid in a meaningful way, then they must be related to a common reference framework. A scale of equivalent limestone percentage porosity is the most commonly used reference. Because this is a common scale for neutron logs, they generally do not require to be rescaled. Bulk density readings may be converted to this scale by equating the grain density of calcite (2.71 gm/cc) with zero porosity, the porosity fluid density (approximately 1.00 for fresh water mud-filtrate) to 100% porosity, and interpolating other values between these two extremes (see Fig. 30).

You do not currently have access to this article.
Don't already have an account? Register

Figures & Tables

Contents

SEPM Short Course Notes

Geological Log Interpretation

John H. Doveton
John H. Doveton
Kansas Geological Survey, University of Kansas
Search for other works by this author on:
SEPM Society for Sedimentary Geology
Volume
29
ISBN electronic:
9781565761094
Publication date:
January 01, 1994

GeoRef

References

Related

A comprehensive resource of eBooks for researchers in the Earth Sciences

This Feature Is Available To Subscribers Only

Sign In or Create an Account

This PDF is available to Subscribers Only

View Article Abstract & Purchase Options

For full access to this pdf, sign in to an existing account, or purchase an annual subscription.

Subscribe Now