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Abstract

Although originally used for structural interpretation, borehole image and dipmeter logs are finding progressively more application to identifying sedimentary features in both exploration and development wells. The general consensus has been that to gain the most sedimentary information from image logs, they should be calibrated to rocks, particularly to corresponding cores. We describe five case studies that calibrate image logs not only to cores, but also to outcrops of analogous facies, using behind-outcrop wells. In particular, the behind-outcrop logging has proven very successful in the calibration process, particularly as it pertains to identifying features that provide indications of sedimentary features laterally away from the wellbore. The five case studies are from three deep-water (turbidite) rock sequences: Pennsylvanian Jackfork Group (Arkansas), Miocene Mt. Messenger Formation (New Zealand), and Cretaceous Dad Sandstone Member of the Lewis Shale (Wyoming). Behind-outcrop wells have been drilled, logged, and cored for each area.

The results demonstrate the following: (1) Although calibrating image logs of deep-water strata to corresponding core is strongly recommended, it is not always essential because features illustrated in this chapter and other publications provide a partial catalog for characterization. (2) Interpretation of sedimentary environments and facies from an image log requires the identification of a group of features, which can be related to sedimentary processes, instead of a single feature. (3) Laminae-scale stratification features are commonly more readily seen on the image log than in corresponding core; however, some small-scale sedimentary structures may be difficult or even impossible to identify. (4) Dipmeter logs can be used to differentiate certain deep-water facies, even when from old wells, provided the structural dip is deleted from the data set.

These findings provide a variety of applications to both exploration and development, for example, in the areas of volumetric and net sand calculation, prediction of reservoir trend and geometry, and the overall value of obtaining borehole image logs, which are less expensive to obtain than core.

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