Use of Thermal and Epithermal thermal Neutron Measurements in Shaly Sand Evaluation

John Puffer, Gerrit d'Ablaing IV, Tim Toth, Harold Darling

The effect of shaliness on sand quality is of great importance in the evaluation of Gulf Coast and offshore reservoirs. Shaliness needs to be determined so that proper hydrocarbon producibility potentials can be assessed for economic and completion decisions. The use of thermal and epithermal neutron measurements, in conjunction with standard methods, increases the accurate measurement of shaliness in Gulf Coast formations.

Thermal neutron measurements are most commonly used in well logging. The thermal measurement characterizes the rate of decrease in the thermal neutron population as a function of source-to-detector distance by "migration length," which can be related to porosity but has many environmental effects to consider. The epithermal measurement characterizes the rate of decrease in the epithermal neutron population as a function of source-to-detector distance by "slow-down length," which is principally related to hydrogen content of the formation. Comparison of the two measurements leads to more accurate porosity and shaliness determination by taking advantage of thermal neutron absorption associated with clays and other minerals and by avoiding the thermal diffusion phase of the thermal measu ement that is not associated with hydrogen content of the formation. The calibration of a gamma-ray, SP, and thermal neutron-density crossplot to the additional parameters resulting from thermal-epithermal comparison leads to more accurate and consistent results. Neutron data are processed by "ratio" and "count-rate" methods. Commonly, the ratio of count-rates of "near" and "far" detectors are used for neutron porosity computation. Environmental corrections are applied to this computed porosity.

A newer method of count-rate processing applies the environmental corrections to the count-rates to computation of porosity values. An additional benefit to count-rate processing of individual detector measurements lies in "thin-bed enhancement" of the computed porosity data. Wireline logging techniques are revised to provide maximum data quality. Log examples from the coastal Wilcox and the offshore Flexure trend show good improvement in shaliness evaluation.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.