Outcrop gamma-ray logging to improve understanding of subsurface well log correlations

Abstract

Abstract Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques which aim to improve the visualization of interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate the reliability and potential pitfalls of subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck in order to log vertical cliff or quarry faces. A second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops. Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas, USA, of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. Because the strata from which the logs were measured can be visualized and discussed at the outcrop, these examples can clearly illustrate the following aspects of wireline log correlation: (1) reliability can be greatly improved by understanding expected subsurface depositional geometries and lateral facies changes; (2) wireline log correlations in stratigraphically and structurally complex settings, such as many oil or gas fields, may not be reliable without sufficient coring and special logging in addition to well testing; (3) erroneous correlations can result from the common practice of inferring three-dimensional rock geometries from two-dimensional well log data. Additional examples demonstrate that correlations and interpretations of subsurface wireline logs in both oil/gas fields and in exploration areas can be improved by comparing the subsurface logs with outcrop gamma-ray logs of nearby analogous strata.