Integrating coal petrology, geochemistry and geochronology to understand spatial variation in the Walloon Subgroup

Abstract

The Jurassic Walloon Subgroup in the Surat Basin is a prolific source of coal seam gas (CSG) in Queensland. Production of CSG commenced from the fairway along the north-eastern basin margin, but despite decreasing unit- and net coal thickness of the Walloon Subgroup, exploration and production rapidly extended across to the Roma Shelf towards the Nebine Ridge in the west. This ridge separates the Surat and Eromanga basins, across which the Walloon Subgroup and the overlying Springbok Sandstone are correlated to transition into the Birkhead Formation and Adori Sandstone respectively. A regional lithostratigraphic model of the Walloon Subgroup based on wireline data, highlights westward thinning of the unit through pinching of the lower (Taroom) coal measures, and thinning or erosion of the upper (Juandah) coal measures by the overlying Springbok Sandstone. The dis- or unconformable surface represents a change in gradient within the basin, between the rising to stable base levels (the coal measures) and falling base levels, the timing and mechanisms of which are debated. This thesis tests the lithostratigraphic framework model of the Walloon Subgroup by providing alternative markers and trends within the coal maceral and carbon isotopic composition, coupled with vertical and lateral variation in the mineralogy and zircon provenance of the interburden strata. A petrographic and stable carbon isotope study of eight wells from east to west of the basin, records a transition from a wet, herbaceous marsh or fen environment during the deposition of the Taroom and Lower Juandah Coal Measures to a more arid system, as indicated by a characteristic oxidized material-rich (inertinite) signature of the Upper Juandah Coal Measures. In each well across the basin, the organic stable carbon isotopes show a positive excursion from the Lower to Upper Juandah Coal Measures that sets in well before the increased inertinite content, followed by a shift to more negative compositions in coals interpreted to occur within the Springbok Sandstone. The positive excursion in organic stable carbon isotope composition represents a global trend, and in the Surat Basin, a shift to drier conditions, which ultimately could have caused a drop in regional base level and the creation of exposed surfaces, resulting in the Springbok unconformity. The d13C trend including the positive excursion was applied as a tool to test the current lithostratigraphic framework of the Walloon Subgroup, and in particular, its relationship with the overlying Springbok Sandstone. On the Roma Shelf in the west, this positive excursion, as well as its turning point, occur within the Juandah Coal Measures, rather than across the Springbok Unconformity as observed in the east. U-Pb geochronology of detrital zircon of the interburden sediments of the Walloon Subgroup corroborates south-oriented main drainage fed by westward flowing tributaries, as interpreted fromnsubsurface image log studies and unit isopach maps. While the Walloon Subgroup records extensive syndepositional volcanism from a potentially easterly lying source, it also inherited older material from northerly lying surface exposures of the Thomson Orogen, as well as the easterly lying New England Orogen. Youngest detrital zircon ages approximate recently published depositional ages derived from dating of tuffs, and show that preserved strata in the west is significantly younger. Detrital zircon age spectra of these younger strata are different and show additional input from the Mossman Orogen. The source of volcanic lithics and tuffs within this unit remains unknown, but mineralogical analysis through hyperspectral scanning of core (Hyloggert) and x-ray diffraction analysis shows an overall decrease in volcanic influence to the west of the Surat Basin, corroborating an easterly or northeasterly lying source. An abundance of swelling clays, which impact on CSG production behaviour, is characteristic for the Upper Juandah Coal Measures in the eastern Surat Basin, whereas the Tangalooma Sandstone and Taroom Coal Measures are typically kaolinite-rich, corroborating increased drainage during their time of deposition. On the Roma Shelf, the swelling-clay rich zone associated with the Upper Juandah Coal Measures appears to be absent. Trends in maceral and isotopic composition of the coals, as well as U-Pb geochronology of detrital zircon and hyperspectral analysis of the interburden sediments of the Walloon Subgroup proved helpful in testing the current lithostratigraphic framework, and showed that a thinning of the unit across the Roma Shelf and towards the Nebine Ridge is likely attributed to erosion of the Juandah Coal Measures. In this interpretation, preserved (younger) strata in the west would be better correlated with the Springbok Sandstone. However, on the Roma Shelf, near the present-day subcrop line of the Walloon Subgroup, no clear evidence for more pronounced incision, such as angular unconformity or thick, channelized sandstone was observed in wireline data. Therefore, the findings of this study could alternatively be interpreted as depositional thinning and gentle peneplanation of the Juandah Coal Measures on the Roma Shelf, causing the decrease in net coal thickness of the Walloon Subgroup.