A NOVELTY… SINCE 1975. CONSOLIDATED PRACTICES AND PRAGMATIC WORKFLOWS FOR SAND/SHALE THIN BED ANALYSIS.

摘要

The quantitative interpretation of Sand/Shale thin beds isa well-known problem with a multitude of solutions.However, the Thin Bed Analysis (TBA) is still perceivedas overly complicated. The TBA goal is often restrictedto the identification of an additional pay in shalyintervals with low resistivity, but an equally importantobjective is to provide quantitative consistency betweenthe results of the log analysis (shaliness, porosity) andthe core derived relationship (Permeability andSaturation functions).This paper discusses the lessons learned in several fieldswith significant stakes in Sand/Shale thin beds andpresents a pragmatic workflow.The first step is to identify the intervals where the TBAis both applicable and useful. The methodologyeffectively applies to thin beds of Sandstone and Shalewith sharp transition and very distinct petrophysicalproperties. Permeable Sandstone Beds are prone tocontain hydrocarbons while Shale Beds contain nonmovablewater. The identification of Thin Beds Faciesmust be performed using all available data including, butnot limited to, mud gas, borehole images, NMR,electrical and acoustic anisotropy. A preliminarysimplified analysis must outline the potential stakesassociated to sandstone thin beds and evaluate theopportunity to progress with a full study.The second step is the computation of the SandstoneFraction and Sandstone Porosity that constitute the keydeliverables of the analysis. These products can beobtained with different methodologies.Most of the time, Sandstone Fraction and SandstonePorosity are inferred from Porosity and Shaliness usingthe “Thomas and Stieber†Xplot that provides theLaminated Sandstone Fraction along with Dispersed orStructural Shale.A simplified in-house methodology was introduced todeal with the common turbiditic environmentscharacterized by a low amount of clay in Sandstone bedsbut variable properties of both Sandstone and Shale.The third step is the computation of the Sandstone WaterSaturation. Although this is considered the main goal of the TBA it cannot be always performed. In fact, a robustcomputation requires the acquisition of the verticalresistivity. In absence of Triaxial resistivity, the use ofNMR proved crucial in most of the studies and amethodology to use the NMR data as a standalone toolfor providing the three TBA deliverables was introduced.The fourth and fifth step are the validation of the resultsusing core data and the definition of cutoffs to be usedfor summation.