An assessment of basin development and coal rank in the Kaitangata coalfield, South Otago, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Kaitangata Coalfield, South Otago is located on the western boundary of the Great South Basin. It is comprised of conglomerate, sandstone, mudstone and coal belonging to the Late Cretaceous to Early Paleocene aged Taratu Formation. The Taratu Formation contains in excess of 16 coal seams (many of which are economically significant). The deposits were analysed to evaluate the relative influence facies relationships, burial depth, syn and post depositional faulting and the intrusion of the Miocene Dunedin Volcanics on coal rank.
A basin-specific lithological coding system allowed the stratigraphic reconstruction of the Kaitangata Coalfields architecture via cross-sections and coal thickness isopach maps. This showed the basin development could be divided into three phases. The Lower Taratu Members are comprised of greywacke conglomerate and coal which was deposited in an alluvial fan setting by the basin bounding Castle Hill Fault scarp. The Middle Members are composed of quartz conglomerates and coal and were deposited in a lower delta plain to marginal marine environment. Localised paloehighs still provided greywacke clasts in some areas, with the Castle Hill Fault still the main bounding fault. The Upper Taratu Members were deposited in a lower delta plain but across a much broader basin overtopping the Castle Hill Fault onto the previous adjacent paleohigh.
The Kaitangata Coalfield is best described as a Late Cretaceous rift basin in which the deposition of the basal Henley Breccia and Lower Taratu Formation were the result of synrift basin controls from the Castle Hill Fault. The Middle Taratu Formation represents the transition between syn and post rift basin development, whereas, the Upper Taratu Formation represents post-rift thermal subsidence. Other allogenic controls such as climate and eustacy also influenced on basin development.
A study of coal rank study utilised a total of 31 samples for vitrinite reflectance (VR), 8 for vitrinite and inertinite reflectance, and 8 for coal chemistry. An additional 336 coal chemical analyses were compiled from previous drilling reports and corrected to a dry, mineral matter, sulphur free basis (dmmsf). Coal rank was assessed using VR, Calorific Value (CV) and Volatile Matter (VM). VR showed an overall increase in rank with depth in the Kaitangata Sector of 0.037/l00m, whereas the Benhar showed a negligible Downhole VR increase over 300m. However, CV showed an average linear downhole rank increase in the Benhar Sector of 469Btu/lb/1 00m, and a trend of 505Btu/lb/1 00m in the Kaitangata Sector. Lateral rank trends showed that coal rank increased towards the Castle Hill Fault in the Benhar Sector, whereas rank in the Kaitangata Sector showed localised rank increases. Rank trends across the coalfield are best explained primarily as the result of depth of burial processes; however, fluid flow along faults and volcanic intrusions may have also had local effects on coal rank.