One fundamental characteristic of sedimentary rocks is layering, which is caused by sediment deposited by alternating conditions of environmental energy influenced by the gravitational field. Gravity causes bedding to be close to horizontal (with notable exceptions). On a finer scale, alternating depositional energy results in sediment laminations, which are layers at the cm to mm scale. In important hydrocarbon-bearing basins, laminations are a common phenomenon in source rock mudstones. Laminations create a profound anisotropy in layered sediments that governs many of the rock properties, including permeability and hydraulic stimulation response. In a drilled borehole, the best way to identify the degree and nature of laminations is by collecting core. Core collection, however, is costly and not possible for an entire well. That’s where borehole images step in.
Borehole images map laminations at a fine scale, something that is beyond the ability of other borehole logging methods. Under optimal conditions, it is possible to map every bedding lamination and these data can be used to build lamination density curves, quantifying bedding thickness changes with depth. In many cases, alternating zones of finer and thicker laminations are potential indicators of cyclic sedimentation, directly reflecting depositional conditions. The resulting graphical displays provide a visual indication of the intensity and density of lamination and this information can be compared both vertically and laterally.