Correlation of Fluvio-Lacustrine Strata Using Volcanic Tuffs: New Insights From the Jurassic Walloon Coal Measures, Surat Basin, Australia

Abstract

The Upper Jurassic Walloon Coal Measures of the Surat Basin, eastern Australia hosts the continent's largest and most productive coal-bed methane play with over 32,000PJ of probable and proven (2P) reserves as of 2014. The formation consists of fluvio-lacustrine strata that are up to 500 m thick that were deposited at high latitude (>75°S) during a global greenhouse epoch. Though the coals are thin (<0.4m) they are numerous, with more than 35 coal beds in many wells drilled in the formation. Individual beds are laterally discontinuous and can rarely be correlated for >5 km. The interbedded channel sandstone, overbank mudstone and lacustrine facies are also laterally discontinuous in nature making correlation of strata difficult. Consequently, existing stratigraphic frameworks for the Walloons are riddled with inconsistencies and debate continues on which to utilise for basin modelling, most of which are only useful at a localised scale. This has had a major impact on effective exploration in the basin. The abundance of volcanic tuffs interbedded within coals (up to 10 per well) provides a unique opportunity to examine chronostratigraphic relationships on a regional basis. Zircons from eighteen tuff beds in eight wells across the basin have been successfully dated and correlated between using the CA-TIMS technique to an error margin of <100kyr. These dates substantially modify prior stratigraphic interpretations and provide the basis of a new, rigid sequence stratigraphic framework for the Walloon Coal Measures. Similar high-precision dating of volcanic tuffs may help elucidate the stratigraphy of other continental strata elsewhere in the world.

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