No abstract

The Surat Basin’s Middle Jurassic Walloon Subgroup is a productive coal seam gas source in Queensland, Australia. The Walloon Subgroup can be subdivided into the Upper and Lower Juandah coal measures, the Tangalooma Sandstone, the Taroom Coal Measures, and the Eurombah/Durabilla Formation, from top to bottom. Correlation across the basin is challenging due to high lateral variability and lack of extensive stratigraphic markers. The Walloon Subgroup is also, in places, incised by the overlying Springbok Sandstone, sometimes interpreted as far down as the Tangalooma Sandstone. New age dates suggest that the Walloon Coal Measures are Oxfordian in age and mark a period of high rates of Corg production and burial, and an intermittent decrease of atmospheric pCO2. The un- or dis-conformable base of the Springbok Sandstone coincides with a turning point of this supposedly global phenomenon. This study uses organic stable carbon isotope trends as a correlation tool within the Surat Basin’s Walloon Subgroup and its overlying Springbok Sandstone. Analysis of a stratigraphic suite of coal samples from several wells across the Surat Basin shows a gradual enrichment in 13C up section from the Taroom to the Lower Juandah Coal Measures, with the most positive δ13C values within the Upper Juandah Coal Measures. Thereafter there is a rapid reversal to more negative δ13C values for coal samples of the Springbok Sandstone. The upward enrichment occurs well before the shift in maceral composition to increased inertinite content in the coals, suggesting more global allogenic processes are controlling the carbon isotopic trend. The consistency of these trends lends a more confident correlation for sub-units within the Walloon Subgroup, and assists in determining the level of incision disconformity of the Springbok Sandstone.

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 CoalMeasures 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 δ 13 C 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.

The Late Jurassic Walloon Subgroup (recently dated as Oxfordian) is a productive, subbituminous coal seam gas source in the Surat Basin and can be subdivided from bottom to top into the Taroom Coal Measures, the Tangalooma Sandstone, the Lower and Upper Juandah Coal Measures, which have different coal character. The lower Taroom coals are commonly thick, associated with sandstones and interpreted to form as base level is rising, creating sodden anoxic conditions for peat accumulation. The middle Tangalooma to Lower Juandah contains fewer and thinner coals, and transitions upwards from a sandstone to siltstone dominated sequence responding to inundation with the development of floodplain lakes. The strata then coarsen upward in both grain size and coal thickness in the Upper Juandah Coal Measures, which may be eroded by an overlying unit, the Springbok Sandstone. This unconformable surface is basin wide and depending on age, can be tied into global changes in climate and base level. Existing models for peat growth under changing base level and the variability in terms of the conditions of peat formation through time, as well as throughout the basin, are tested. Environment of peat deposition and changes therein, are investigated by petrographic analysis of the Walloon coals, coupled with high resolution lithotype logging of core and organic stable carbon isotope analysis. Fine microlayering and abundance of root suberinite, telo-and detrovitrinite indicate precursory peat formation in a mostly herbaceous marsh to fen environment, in which bigger trees are either infrequent or absent, except for the lower seams of the Taroom Coal Measures and the upper seams of the Lower Juandah Coal Measures, where bright bands are thicker (>=10mm) and more frequent. No extended periods of dehydration-oxidation (less than 1 vol. % mmf inertinite group macerals) are indicated until the deposition of the Upper Juandah Coal Measures that contain greater amounts (5 to 15 vol. % mmf with rare 68 vol. %) of inertinite group macerals. Suberinite is interpreted to reflect dense root mats that are resistant to decay by microbial activity. They leave behind their suberinised exoderms, which originally helped wetland plants to protect themselves from deleterious