A COMPARISON OF QUANTITATIVE TECHNIQUES FOR DETERMINING THE 3D ANISOTROPY OF SHALE SAMPLES: APPLICATION TO HORN RIVER BASIN SHALES,BRITISH COLUMBIA,CANADA

摘要

With increasing interest in shale gas and oil plays,an understanding of the anisotropic properties of shales is becoming important for modelling their potential response to stress and their 3D petrophysical properties(such as permeability anisotropy),and thus for exploiting them via hydraulic fracturing. Various studies have mentioned that shales are anisotropic,but a quantitative 3D comparison of different anisotropy parameters is lacking,and details of the factors controlling anisotropy are limited. Part of the reason for this is the fissile nature of many shales,which makes it difficult to cut consolidated core plugs or obtain plugs that remain intact whilst undertaking the anisotropic measurements. We therefore first describe a sample preparation technique which maintains the integrity of shale samples and allows anisotropic measurements(primarily magnetic,but also potentially electrical and acoustic)to be taken on cylindrical or cubic core samples. We then detail quantitative 3D anisotropy measurements on several shale samples from the Horn River Basin in Canada(NW British Columbia,primarily in the Muskwa,Otter Park and Evie formations)by comparing the anisotropy of magnetic susceptibility(AMS),the anisotropy of magnetic remanence(AMR),the anisotropy of electrical resistance(AER),and the observed petrofabric from thin section and polished section analyses. A key result is that several samples exhibit”inverse AMS fabrics”where the maximum magnetic susceptibility axis is perpendicular to the bedding plane and not within it as one would normally expect. This can be explained by the presence of uniaxial stable single-domain(SSD)ferrimagnetic particles,which have a maximum magnetic susceptibility perpendicular to their long axes. Larger multidomain(MD)ferrimagnetic particles,in contrast,have a maximum magnetic susceptibility parallel to their long axes. This effect has been documented in igneous rocks(Potter and Stephenson,1988),but never before in shales. We demonstrate,therefore,the importance of undertaking AMR measurements(rarely performed on shales before),because the maximum remanence of SSD or MD particles is always along their long axes so the maximum AMR axis will represent the true particle alignment axis. Studies that merely use the more conventional AMS technique could incorrectly infer a preferred particle alignment axis that is 90 degrees from the true orientation if a significant proportion of SSD particles are present. This may have important consequences for optimizing hydraulic fracturing procedures. Our results are helping us distinguish different elements of shale anisotropy due to: mineralogy,mineral alignment,organic matter etc. Integration of AMS,AMR,AER,and thin section and polished section analysis suggest that pyrite is a significant control on anisotropy in the Otter Park formation,whereas organic matter appears to be responsible for the high anisotropy in the Evie formation.