Sedimentology and Stratigraphy of the Walloon Subgroup, Surat Basin, Australia

Abstract

The Jurassic Surat Basin of eastern Australia is one of the world's major coal seam gas (CSG) basins. It currently (2013-14) produces 4441 Mm³ gas (157 Bcf) per year and holds 2P reserves of 32795 PJ (∼27.4 Tcf). An extensive sedimentological and stratigraphic data set is available from fully cored boreholes over 400 m in length, wireline log suites including image logs, chemostratigraphy, coal composition analyses and 3D seismic surveys. These data show the Walloon Subgroup, which contains the Juandah and Taroom Coal Measures, to consist of single storey to amalgamated sands deposited in a terrestrial setting, in 1-2 km wide channel belts flanked by floodplains, peat mires and lakes. Image log analysis of paleocurrent patterns indicates that rivers in the Miles area of the basin flowed primarily in a SW to SE direction. Individual channel fills ranged between 3-8 m thick, are often amalgamated and frequently show a meandering channel style with a fining upward grainsize trend, organized into 3 broad fining upward units. Channels of the Walloon Coal Measures consist of mostly fine sandstone, with frequent evidence of soft sediment deformation including sandstone dykes, suggesting an active tectonic environment. The overlying Springbok Sandstone is coarser grained, more quartzose with paleocurrent patterns dominantly to the SE, and separated from the Walloon Coal Measures by a regional unconformity. Coals in the Walloon Subgroup are of relatively low rank (VR of 0.36-0.6) producing from 300-600 m depth. These coals are relatively thin, and individual coal groups range in continuity from less than 3 km to over 15 km. The coals often have high permeability dominantly derived from fractures, while intervening sandstones are immature and of low porosity and permeability, allowing effective coal dewatering and gas production. Seismic data including attribute volumes have assisted in the delineation of stratigraphic and structural features, showing stratigraphic growth on the hanging wall of faults and erosion on the elevated footwall. Volcanic activity was continuous throughout deposition of the coal measures, and geochemistry of low silica tuffs suggests a nearby Jurassic subduction zone and magmatic arc source to the east. Understanding the stratigraphy has proved important for creating detailed geocellular models necessary for accurate determination of reservoir continuity, well location planning and reserves prediction.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016