"Macroscopic Anisotropy" Approach to Analysis of Thinly Laminated Sand/Shale Sequences: Sensitivity Analysis of Sand Resistivity Estimate and Environmental Corrections

Abstract

Abstract Thinly laminated sand/shale sequences are often mistaken as low-resistivity wet sands in spite of their high hydrocarbon saturations, because conventional electric logs and induction logs read the low average resistivity of the formations. Imaging tools and other high-resolution logging tools may be needed to identify such laminated formations when cores are not available. Such formations appear as homogeneous and anisotropic formations to conventional resistivity tools when the typical lamina thickness is much finer than the tool's vertical resolution. The anisotropic formation resistivity is determined by the sand- and shale-laminae resistivities, and the sand fraction (or sand/shale ratio). Conversely, the sand-laminae resistivity can be estimated from the anisotropic resistivity data if the shale-laminae resistivity, the sand/shale ratio, and the relative dip (deviation) angle are all known. We call this approach to laminated-sand analysis the "macroscopic anisotropy" approach. When multiple resistivity values are measured, as in the case of using LWD 2-MHz resistivity devices in highly deviated boreholes, the sand-laminae resistivity and the sand/shale ratio can be estimated simultaneously from the anisotropic resistivity measurements. We examine the sensitivity of sand-laminae resistivity determination to each parameter and review how these parameters are estimated. The two most sensitive parameters are the shale-laminae resistivity and the sand/shale ratio, especially at smaller dip angles. When the total thickness of such a laminated sequence is not thick and/or sand laminae are invaded with mud formation evaluation becomes complicated, because the logging tools cannot read the macroscopic resistivity correctly. To estimate the correct macroscopic resistivity, special shoulder-bed (or thin-bed) corrections and/or invasion corrections must be applied. We examine special shoulder-bed corrections for anisotropic formations at higher dip angles. In homogeneous anisotropic formations, significant borehole effect is observed for laterologs and spherically focused logs (SFLs), but not for induction logs. We found that difference between induction-log resistivity and laterolog- or DFL/SFL-log resistivity is logarithmically proportional to the formation anisotropy, but is almost independent of borehole size. The difference may be used to estimate the formation anisotropy. Shale formations are often anisotropic. The effect of shale anisotropy is examined and a method is provided to correct such shale anisotropy in laminated-sand analysis. Introduction: Macroscopic Anisotropy Approach to Thin Bed Problems Consider a thinly laminated sand/shale (or any binary) sequence. Even fine-resolution resistivity tools, such as DFL/SFL and high-frequency dielectric tools (HFDTs, EPTs) fail to differentiate sand and shale laminae. Neither induction tools nor laterolog tools can even detect lamination when the lamination is much thinner than the tools' vertical resolution. Instead, these tools respond to a thinly laminated sand/shale sequence as though it were a homogeneous low-resistivity formation even though the sand lamina may be very resistive. Any logging tool has its own thin-bed problem when bed thickness is comparable to or thinner than its vertical resolution. When beds are thinly laminated and any resolution improvement is not available, an alternative "macroscopic anisotropy" approach becomes useful. We ignore the detail of individual laminae in this approach. and throughout a sequence we assume that all sand laminae are identical and all shale laminae are identical. P. 275^