Coal petrology and coal seam gas contents of the Walloon Subgroup — Surat Basin, Queensland, Australia

Abstract

Core, exploration and appraisal drilling over the last four years have targeted the Juandah (upper) and Taroom (lower) Coal Measures of the Middle Jurassic Walloon Subgroup of the Injune Creek Group. These wells have shown that the high-volatile bituminous, perhydrous coals of the Walloon Subgroup have gas contents of between 1 and 14 m3/t and some wells have encountered gas flows at rates of over 30,000 m3/day. This recent work has confirmed the initial supposition that the coals within the Walloon Subgroup contain sufficient quantities of gas to be economically viable as a coal seam gas source. Coal volumes, depth and structure have also been confirmed from the initial work. What has become apparent during this work is the variability in gas content across each of the seams in the Juandah and Taroom Coal Measures and the variability in coal petrographic composition of these two units are controlled by many factors. What has also become apparent is that the Taroom Coal Measures have a lower average gas content than the overlying Juandah Coal Measures. The obvious conclusion is that the difference in gas content is related to a difference in coal petrology. Not only do the gas contents of these coal intervals differ, their gas adsorption capacities also differ with the Taroom Coal Measures coals displaying, on average, lower gas content and lower gas adsorption capabilities. Again the obvious cause would be coal petrological differences, but the Juandah and Taroom Coal Measures coals have similar maceral percentages. An understanding of the relationship between gas desorption, gas adsorption and coal composition is vital in determining which areas in the basin offers the most economically viable targets for the commercialisation of coal seam gas in the Surat Basin.