Stratigraphic architecture and coal character of Late Permian Fort Cooper Coal Measures, Bowen Basin, Queensland

Ayaz, Syeda (2016). Stratigraphic architecture and coal character of Late Permian Fort Cooper Coal Measures, Bowen Basin, Queensland PhD Thesis, School of Earth Sciences, The University of Queensland. doi:10.14264/uql.2016.381

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Author Ayaz, Syeda
Thesis Title Stratigraphic architecture and coal character of Late Permian Fort Cooper Coal Measures, Bowen Basin, Queensland
School, Centre or Institute School of Earth Sciences
Institution The University of Queensland
DOI 10.14264/uql.2016.381
Publication date 2016-07-01
Thesis type PhD Thesis
Supervisor Joan Esterle
Sue Golding
Total pages 280
Language eng
Subjects 0402 Geochemistry
0499 Other Earth Sciences
0403 Geology
Formatted abstract
This research investigates the tuffaceous Late Permian Fort Cooper Coal Measures (FCCM) and their equivalents of the Bowen Basin. The interbedded and high ash nature of the coals has reduced their economic attractiveness but during the course of this thesis, exploration drilling for metallurgical coal and coal seam gas escalated, providing an opportunity to revisit the FCCM stratigraphy, sedimentary environments and coal character across the basin. This study establishes a regional stratigraphic architecture across the basin using 500 open file wireline logs and examines the controls on coal seam geometry and the sedimentary palaeoenvironment of coal and clastic strata. Four marker tuff horizons were recognised regionally, including the Yarrabee Tuff that acts as the upper stratigraphic boundary of the FCCM from the Rangal Coal Measures.

Regionally, the FCCM passes from north to south into the coal-bearing upper Burngrove and lower Fair Hill formations, separated by the marine-derived Black Alley Shale in the southern Bowen Basin. In this study, a sequence of coals, named the Middle Main Seams was recognised as the northern equivalent of the Black Alley Shale. Internal coal seam architecture follows the morphotectonic zones of the basin where thick, coalesced seams are associated with areas of low accommodation, while seam splitting and thinning are associated with high accommodation.

The tuff horizons were dated using the CA-IDTIMS technique (Chemical Abrasion-Isotope Dilution Thermal Ionization Mass Spectrometry) to validate the correlations and calculate sedimentation rates in different parts of the basin. The Yarrabee Tuff dated at a range of 252.69 ± 0.16 Ma - 253.07± 0.22 Ma. Three accessary tuffs within the Burngrove Formation have an upper range of 252.85 ± 0.16 Ma – 253.12 ± 0.12 Ma and a lower range of 253.57± 0.18 Ma – 253.77 ± 0.17 Ma. This places them into the upper Changhsingian Stage. One of the accessary tuff from the Fair Hill Formation has an age of 254.03 ± 0.03 Ma and belongs to the lower Changhsingian Stage.

The tuff dates were used to calculate sediment accumulation rates in different parts of the basin, taking into account differential compaction of sediment type. The Burngrove Formation had consistently higher rates of 234.5 - 224.5 m/Ma in the Taroom Trough and lower rates of 112 m/Ma in the Roma Shelf, corroborating the variable accommodation setting across the basin. Stratigraphically, the lower Fair Hill showed much higher decompacted sediment accumulation rates at 902m/Ma than the Burngrove Formation at 234.5 m/Ma, representing an upward decrease in accommodation space. This resulted in the southerly progradation of these and the overlying alluvial Rangal Coal Measures in response to shedding from the rising Hunter-Bowen Orogeny that was active during the evolution of the Bowen Basin.

The rank of the FCCM coals in the southern part of the basin is found to increase eastwards coincident with increasing depth of burial. Unusually high ranks at present-day shallow depths are interpreted to have resulted from deeper thrust loading during the Triassic closure of the basin, followed by hydrothermal activity and magmatism. Tuff frequency declines but the thickness of successive ash falls increases upwards through the Burngrove Formation. This possibly represents intensified volcanism, else more open mire landscapes that could preserve the volcanic ash.

Although the coals appear dull and relatively thinly banded in hand sample, compositionally, they are high vitrinite (80-90% mmf) in the Fair Hill Formation and Middle Main Seams (the landward equivalent of the Black Alley Shale) with increasing inertinite content (30% mmf) in the Burngrove Formation. Although subtle in some wells, this may be interpreted to represent increasing aridity in the atmosphere, else follow increased fluctuation in water table as the coal measures prograde southward. Stable carbon isotope (δ13C) analysis of coal shows an overall 13C-enrichment from the Fair Hill through the Middle Main Seams and lower part of the Burngrove that reverts to negative in the top seams of the Burngrove Formation and overlying Rangal Coal Measures. The 13C-enrichment is not coupled to the increased inertinite and in fact shows an inverse relationship at the end of the coal measures. This supports previous findings that atmospheric δ13C values responded to global climatic changes, specifically due to the release of methane hydrates before the Permo – Triassic boundary events.

A small part of the thesis investigates gas content variation in the FCCM. From available data, gas content increases directly with coal rank, up to 40m3/t on a dry ash free basis. Gas isotope analysis shows a thermogenic origin of methane, confirming the coal measure as a self-sourcing reservoir. Thinly banded, vitrinite-rich and gassy seams of the FCCM can be considered as a potential CSG target but the great depths and structurally complex areas may reduce permeability and target areas for successful gas production.
Keyword Fort Cooper Coal Measures
Yarrabee Tuff
Burngrove Formation
Fair Hill Formation
Seam splitting
Tuff age dating
Hunter-Bowen Orogeny
Maceral composition
Stable carbon isotopes

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Created: Wed, 22 Jun 2016, 18:35:39 EST by Syeda Ayaz on behalf of Learning and Research Services (UQ Library)