The Middle Jurassic Walloon Coal Measures (CM) is a prolific, low-rank coal seam gas (CSG) resource in the Surat Basin, Australia. Gas content is one of the main factors that control the resource abundance. Continued successful exploration and production, and possible future microbial regeneration of the Walloon CSG resource require an improved understanding of the controls on gas content distribution across the core region of production in the eastern Surat Basin. The interplay of four geological factors has been identified as determining coal seam gas content: (1) depositional setting; (2) tectonic and structural setting; (3) coal petrology and quality which could be related to gas content, such as coal rank, coal type, mineral matter content, moisture content, volatile matter, ash content, macerals; (4) hydro-geological properties including water geochemical analyses, drainage systems, gas isotope analyses, reservoir temperature and pressure, burial and charge history, fracturing, hydraulic gradient and overburden sealing capacity. Most of the controls above were examined in eastern Surat Basin. Based on the gas content versus depth profiles of 58 wells, the gas content trends of different geological regions and the whole basin were obtained. Within individual wells, gas content either increases; increases, then decreases; or decreases with depth as the result of the variable coring intervals. While the total dataset shows a general increase-decrease trend in gas content (dry-ash-free; d.a.f.) with increasing depth, although there is considerable scatter in the distribution. The gas content maxima coincides with the Tangalooma Sandstone, regardless of depth. It won't cross-cut the stratigraphy in the majority of wells. The datasets analyses suggest a variety of mechanisms influence present-day gas content and relative saturation. Gas migration from underlying, higher rank coals, meteoric and biogenic recharge from above are possible interpretation for high gas content in the Tangalooma Sandstone. Alternatively, this distribution may be the result of alternating adsorption–desorption cycles linked to burial and uplift. Preferable gas content coincides with meteoric recharge areas, such as the Undulla Nose and Cecil Plains Anticline recharged by Condamine River and its tributaries. The results form the basis for the calibration of gas content-depth fitting formula used in geomodelling, thus a more reliable prediction of gas content.
AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016