Lamination Frequency
A fundamental characteristic of sedimentary rocks is their layering, which results from the deposition of sediment under varying conditions of environmental energy influenced by gravity. Typically, gravity causes bedding to be nearly horizontal, although there are notable exceptions. On a finer scale, Variations in depositional energy create sediment laminations, which are layers that range from centimeters to millimeters in thickness. In significant hydrocarbon-bearing basins, these laminations are commonly found in source rock mudstones.
These laminations create distinct anisotropy in layered sediments, which affects various rock properties, including permeability and response to hydraulic stimulation. The most effective way to identify the degree and nature of these laminations in a drilled borehole is through core collection. However, core collection can be expensive and is not feasible for an entire well, making borehole imaging a valuable alternative.
Borehole images can map laminations at a fine scale, surpassing the capabilities of other borehole logging methods. Under optimal conditions, it is possible to map each bedding lamination, allowing for the creation of lamination density curves that quantify changes in bedding thickness with depth. Often, zones with alternating finer and thicker laminations may indicate cyclic sedimentation, which directly reflects past depositional conditions. The graphical representations produced from this data provide a visual overview of the intensity and density of laminations, enabling both vertical and lateral comparisons.