ABSTRACT
Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques with the objective of better visualizing interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate reliability and potential pitfalls in subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck to log vertical cliff or quarry faces. The second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops.
Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. We have found that by using these examples, the following common wireline log correlation problems can be clearly explained to engineers and geoscientists, and understood, because the strata from which the logs were measured can be visualized and discussed at the outcrop: (1) the reliability of subsurface log correlations can be greatly improved by understanding expected subsurface depositional geometries and lateral fades changes, (2) wireline log correlations in strati-graphically 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, and (3) correlations and stratigraphic and reservoir quality 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.
