A steady-state neutron source technique for measuring the thermal neutron absorption cross section of small formation/injection brines, oils, and rock samples (w, o, and ma) is presented. The apparatus consists of a water moderating tank, a Cf-252 neutron source (~ 106 n/sec), two slim k-3 detectors, and approximately 350 cm3 of sample material in a cylindrical plexiglas container. Special design features are incorporated in the system for placing the source at the center of the measured sample and for precise sample position reproducibility and for countering buoyancy of oil samples. System calibration curves are obtained by using petrophysics laboratory brines, boric acid solutions of various concentrations, and sedimentary rocks saturated with boric acid solutions. A large number of formation and injection brines and a smaller number of geological oil and rock samples from oil fields in the U. S., Canada, and other countries have been measured in the apparatus. The measured cross section values for rocks have been compared with values measured by other methods and computed on the basis of chemical analysis and published cross sections. The uncertainty of the present measurements is comparable to that of other methods.

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