Tutorial: Petrophysics of Thinly Bedded Formations

This paper was written in response to a personal request from the Editor to contribute a “tutorial” on thin-bed petrophysics. The paper is not intended to be an exhaustive review of all the work that has been done regarding thin-bed petrophysics. Instead, it is an introduction to the different types of methods available, with some discussion on the strengths and weaknesses of each. Thin beds have posed problems for petrophysicists for many decades, mostly because conventional interpretation techniques in thin beds tend to overestimate water saturation, owing to the very low resistivities often seen in hydrocarbon-bearing intervals. There are a number of different techniques available to the petrophysicist for evaluating thin beds, including conventional “bulk volume” techniques, various types of high-resolution modeling, and different low-resolution modeling techniques, both with and without triaxial resistivity measurements. There is no single best technique to use. Instead, the most appropriate method is dependent on the formation complexity and the types of data available. Generally, it is very useful to have triaxial resistivity measurements to understand the formation anisotropy better. However, in some cases, simple “triple-combo” logging suites may be sufficient, provided the interpretation can be improved and verified with core data. In other cases, complex formation modeling is required to provide the best results with low uncertainty. If using either low- or high-resolution modeling, the reporting of evaluation results is not straightforward. Net reservoir and net pay cutoffs require special consideration, and the results are specific to the formation components rather than the bulk volume formation, as they are with conventional reservoir summaries. Furthermore, the quantification of petrophysical uncertainties in formation modeling of thinly bedded formations is better done using scenario modeling rather than Monte Carlo processing, which is commonly applied with bulk volume techniques.