An important application of borehole image data is the validation of image features by comparing them with core from the same wellbore interval. This comparison of depth-resolved core data with image logs provides a “ground truth” for the observed image features. Since coring is very expensive, image logs serve as a “core-like” proxy in areas of the field where core samples are not available. Additionally, cores are rarely taken from the entire drilled interval of a wellbore, while image logs can be acquired over larger intervals, offering more comprehensive information. Additionally, most core collection is unoriented. The comparison of inclined features mapped in the oriented image log permits establishing the orientation of the core cylinder.
In the example provided, 180 feet of core is compared to both image log data and gamma data. The lighter sections of the core, made up of sandstone, correspond to a lower gamma response and exhibit a lighter (more resistive) color on the image log. Conversely, the darker rock in the core, which consists of mudstone and shale, aligns with higher gamma responses and darker (more conductive) colors in the image log.
The image above shows a close-up of the core and image log, highlighting a bioturbated sandy interval situated between layers of shale. This stacking pattern is prevalent in the cored section and complicates petrophysical evaluations, as the presence of shale reduces formation resistivity, making these areas appear water-saturated. By combining the high resolution of the image log with nuclear magnetic resonance (NMR) and dielectric data, we can achieve accurate calculations of water saturation (SW) in laminated rock formations. This type of analysis, particularly in a vertical well, can improve the accuracy of estimated ultimate recovery (EUR) calculations and help identify potential horizontal target zones.