On the development of a non-linear calibration relationship for the purpose of clay content estimation from the natural gamma ray log

Abstract

The Shurijeh Formation of Early Cretaceous age is an important clastic sandstone reservoir rock in the Gonbadli field, NE Iran. The quantitative determination of Shurijeh clay minerals has been considered to be a very important and challenging task due to the heterogeneity of formation. The calibrated gamma ray log has been traditionally used as the primary estimator of volume of clay in the subsurface formations. The comparison between the core-measured values and the resulting estimates from the old calibration methods in Shurijeh Formation revealed that despite of applying old calibrations, the Shurijeh clay contents suffers from overestimations. Thus, the most accurate calibration for natural gamma ray log was developed, using the integration of quantitative mineralogical analyses from 76 core samples to estimate the volume of clay in Shurijeh Formation. The least percent relative error, i.e., 11.4%, was achieved in developing a new regression relationship in the form of a rational function acting as the most accurate non-linear clay indicator. The reliability of it was verified by the excellent correlation between the laboratory-measured clay contents in the core samples from another wells drilled in the mentioned Formation and the estimated values. The spectral log ratios have also been used to estimate the Formation depositional environment and different clay minerals types in it. It could truly be inferred that the Shurijeh was deposited in a marine environment with most of data having a thorium to uranium (Th/U) ratio in the range of 1–10. However, the indirect clay typing accomplished by several cross-plots, did not provide consistent results with the laboratory-determined types from the XRD test.