CHARACTERIZATION OF OUTBURST CHANNEL SANDSTONES IN THE PHALEN COLLIERY, CAPE BRETON, NOV A SCOTIA

Abstract

The Phalen Colliery of New Waterford, Cape Breton, Nova Scotia is currently mining coal from the Phalen Seam of the Sydney Mines Formation, part of the offshore Carboniferous Sydney Basin. Since opening in 1984, the colliery has experienced one rock outburst (September 28, 1994) within the channel sandstone that overlies the Phalen coal seam. An outburst is a violent and extremely dangerous explosion of rock and gas, the occurrence and intensity of which depend on the presence of high litho static pressures, high gas pressure, high modulus of rigidity (brittleness), and very low permeability. Samples of the channel sandstone have been obtained from two cores drilled into the roof of drivage tunnels distal (PH-102) and proximal (PH-250) to the site of a recent outburst, and from the outburst sites. Sandstones from PH-1 02 are fme-grained, and have evenly distributed intergranular porosity (range 6.9 to 12.1 %, mean 8.5%) and variable horizontal permeability (range 0.03 to 3.02 millidarcies, mean 0.88). Sandstones from core PH-250 are very fme-grained to very coarsegrained, and show generally large amounts of compacted lithic fragments. Isolated porosity occurs within corroded potassium feldspars and pore-filling kaolinite and ranges from 4.5 to 7.7% (mean 6.5%). Horizontal permeability is low throughout the core, ranging from less than 0.01 to 0.19 millidarcies (mean 0.05). The sandstone in PH-102 is inferred to represent a lower outburst risk. The evenly distributed intergranular porosity and higher permeability allow for a more controlled release of gas, decreasing the potential energy of the system. Under similar depth conditions, sandstone from PH-250 is inferred to have a much higher risk for outbursts. The restricted porosity within rock from PH-250 allows for the storage of methane (energy), but the very low permeability greatly restricts the escape of gas when pressure on the rocks is reduced during mining, spawning an outburst. Samples of outburst sandstones from the Phalen Colliery, #26 Colliery, and Merlebach Colliery, France, show the same characteristics of corroded potassium feldspar, kaolinite pore filling, low permeability, and predominantly intragranular porosity as rock from the base of PH-250.

Key words: Phalen colliery, channel sandstone, outburst, corroded feldspar, kaolinite pore filling, porosity, permeability