GEOMETRY AND CONNECTIVITY OF DISTRIBUTARY CHANNEL SANDSTONES IN THE LATE PERMIAN MORANBAH COAL MEASURES, BOWEN BASIN, AUSTRALIA

Abstract

The fluvio-deltaic Moranbah Coal Measures are an extensively mined Late Permian coal-bearing sequence that crops out along the western limb of the Bowen Basin, Queensland. Data from operating coal mines provide high resolution datasets for detailed petroleum reservoir analogue studies. In this study, data from subsurface drilling and high wall exposures were used to produce a three-dimensional sub surface model of coal and sandstone geometry in a 200 m stratigraphic section over an area of 184 km2. Detailed analysis of the interval between the extensive Goonyella Middle seam and the 'P'-Tuff identified: 1) thin (<5 m) widespread basal sandstones interpreted as amalgamated splay deposits; and 2) major southerly-trending, laterally-offset sandstone belts (3 km wide by 25–60 m thick and >10 km long) interpreted as trunk distributary channel-fill deposits. These are flanked by finer grained sediments containing a series of heterolithic units that display large siltstone-draped accretionary cross-stratification reflecting channel/splay deposition. Reservoir continuity in the Moranbah Coal Measures is controlled by coal seam splitting and the degree of amalgamation between sandstone bodies. Analogues for potential reservoirs within these systems include 1) very thick (>40 m), vertically-stacked amalgamated sandstone sequences with good connectivity; 2) thick (>25 m) stacked sandstone units separated by carbonaceous mudstone horizons that reduce vertical connectivity; and 3) thick sing le-sequence sandstone units with good lateral connectivity Marginal zones are fine grained and often faulted, acting as barriers to connectivity between potential reservoir sandstones.