MAGNETIC PORE FABRIC ANALYSIS - A RAPID METHOD FOR ESTIMATING PERMEABILITY ANISOTROPY

The average orientation and fabric anisotropy of interconnected pore s paces in sandstones is derived from magnetic pore fabric analysis, a n ew technique which measures the anisotropy of magnetic susceptibility (AMS) of samples impregnated with a magnetic suspension. In magnetic p ore fabric analysis, the permeable part of the porous network, consist ing of pore bodies connected by pore throats, is rendered magnetically susceptible. AMS directly yields the average elongation direction of pore bodies and offers a simple way to investigate the effect of pore- shape anisotropy on petrophysical parameters such as hydraulic and ele ctrical conductivity. AMS-derived pore fabric of sandstones of moderat e diagenetic state is compared to permeability anisotropy measured dir ectly on the same specimens. For any one sample, the orientation of th e two tensors and their representation ellipsoids correlate closely. T he preferred orientation of interconnected pores facilitates fluid flo w parallel to the direction of pore elongation and causes the observed anisotropy of permeability. Axial ratios of the two anisotropy ellips oids correlate less closely but show a trend of proportionality. Compa red to the time-consuming measurement of directional permeabilities, m agnetic pore fabric analysis may, therefore, provide a rapid way to es timate the orientation and, to a lesser extent, the degree of permeabi lity anisotropy in porous sandstones. The analysis of six specimens ex tracted from a large, homogeneous block sample proves the accuracy and reliability of the method. In five fluvial sandstone samples, maximum permeability and pore elongation are roughly parallel to the palaeocu rrent direction. Therefore, magnetic pore fabric analysis can also be used to study the relationship between permeability, pore fabric and s edimentary structures.