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Unconventional Gas Reservoir Productivity in Australian Proterozoic Rocks — Studies From the McArthur, Beetaloo, Mount Isa and Amadeus
Red-bed basins with rich hydrocarbon source rocks are widely distributed in south China, such as Banshi Basin in southern Jiangxi, which may have very good prospects for oil and gas exploration. However, due to poor ground conditions, and complex geological structures, seismic exploration and conventional electromagnetic methods cannot provide useful information for hydrocarbon evaluation. This study uses the wide field electromagnetic (WFEM) method to investigate the distribution and geoelectric characteristics of the target stratum of a red-bed basin in Jiangxi province, China. The inversion results demonstrate that the WFEM method could quickly delineate the favorable area and determine the location of the parametric well, confirming that the WFEM method is an effective geophysical exploration method for evaluating hydrocarbon resources in red-bed basins.
The Mount Isa Basin is a new concept to describe the area of Palaeo- to Mesoproterozoic rocks south of the Murphy Inlier (not the Murphy Tectonic Ridge) and inappropriately described as the Mount Isa Inlier. The new basin concept presented in this paper allows the characterisation of basin-wide structural deformation and the recognition of areas with petroleum exploration potential.The northern depositional margin of the Mount Isa Basin is the metamorphic, intrusive and volcanic complex referred to as the Murphy Inlier. The eastern, southern and western boundaries of the basin are obscured by younger basins (Carpentaria, Eromanga and Georgina Basins). The Murphy Inlier rocks comprise the seismic basement to the Mount Isa Basin sequence. Evidence for the continuity of the Mount Isa Basin with the McArthur Basin to the northwest and the Willyama Block (Basin) at Broken Hill to the south is presented. These areas combined with several other areas of similar age are believed to have comprised the Carpentarian Superbasin.The application of seismic exploration within Authority to Prospect (ATP) 423P at the northern margin of the basin was critical to the recognition and definition of the Mount Isa Basin. The northern Mount Isa Basin is structurally analogous to the Palaeozoic Arkoma Basin of Oklahoma and Arkansas in the southern USA but as with all basins it contains unique characteristics, a function of its individual development history. The northern Mount Isa Basin is defined as the basin area northwest of the Mount Gordon Fault.
Recent improvements to Australian apparent polar wander paths for the different geological periods allow palaeomagnetic analysis to contribute increasingly to reconstruction of mineralisation environments and the study of ore formation processes. This paper looks at the application of palaeomagnetism to the timing of regional chemical alteration, mapping of hydrothermal fluid flow paths , identification and timing of intraplate tectonism (which controls the evolution of sedimentary basins that host mineralisation as well as the circulation of mineralising fluids in the upper crust), location of displaced metallogenic provinces of formerly single tectonic units, and dating of strata.
Numerous examples are available of the application of palaeomagnetism to mineralisation studies, especially in North America. An Australian example is the discovery in the southeastern McArthur Basin of magnetic overprint signatures that suggest wide• spread circulation of fluid s during the general period when the HYC Pb-Zn deposit formed. Another Australian example is the dating of the Au-bearing Quamby Conglomerate in the Cloncurry region: the date is substantially older than the earlier, tentatively estimated ages, opening the possibility that the mineralisation occurred within a large fluid circulation system that existed at the time of intrusion of late Palaeoproterozoic granites. Palaeomagnetism has also been used to establish a geological framework that integrates tectonic, magmatic and mineralisation events in northern Australia. A recent palaeomagnetic reconstruction suggests the proximity during the Palaeoproterozoic of the Pacific margins of the Australian and No rth American cratons, with implications for mineral exploration in both continents.
The Term, Lawn, Wide and Doom Supersequences represent tectonically driven, second-order sedimentary accommodation sequences in the Isa Superbasin. The four supersequences are stacked to form two major depositional wedges or packages extending south from the Murphy Inlier onto the central Lawn Hill Platform. A major intrabasin structure, the Elizabeth Creek Fault Zone separates the two depositional wedges. The Term and Lawn Supersequences each form a thick, crudely fining-upward sedimentary succession. The basal part of each supersequence comprises sand-dominated facies, deposited under lowstand conditions. The overlying transgressive deposits comprise thick successions of carbonaceous, shale-prone sediment that represents times of increased accommodation. Synsedimentary fault activity along the northwest-trending Termite Range Fault and major northeast-trending faults including the Elizabeth Creek Fault Zone resulted in overthickened sections of parts of the Term and Lawn Supersequences in regional depocentres. A regional extensional event occurred during Wide Supersequence time, and resulted in strike-slip deformation, uplift and tilting of fault blocks and erosion of underlying Lawn sequences. This tectonic event created small, fault-bounded depocentres, where basal silty turbidites of the Wide Supersequence are locally thickened. Denudation of fault blocks in the hinterland provided increasing coarse clastic sediment-supply forming thick, sand-dominated, lowstand deposits of the upper Wide Supersequence. Overall, the Wide Supersequence exhibits a coarsening-upwards facies trend. Tectonic quiescence resulted in the accumulation of siltstone-dominated transgressive and highstand turbidite deposits in mid-Wide time. The base of the Doom Supersequence comprises thick, feldspathic, debris-flow sandstones signalling a new provenance. Decreasing accommodation is reflected by coarsening- and shallowing-upwards facies trends in late Doom time. Declining accommodation and the end of sedimentation in the Isa Superbasin were most likely initiated by deformation at the start of the Isan Orogeny.
The River Supersequence represents a 2nd-order accommodation cycle of approximately 15 million years duration in the Isa Superbasin. The River Supersequence comprises eight 3rd-order sequences that are well exposed on the central Lawn Hill Platform. They are intersected in drillholes and imaged by reflection seismic on the northern Lawn Hill Platform and crop out in the McArthur Basin of the Northern Territory. South of the Murphy Inlier the supersequence forms two south-thickening depositional wedges on the Lawn Hill Platform. The northern wedge extends from the Murphy Inlier to the Elizabeth Creek Fault Zone and the southern wedge extends from Mt Caroline to the area south of Riversleigh Station. On the central Lawn Hill Platform the River Supersequence attains a maximum thickness of 3300 m. Facies are dominantly fine-grained siliciclastics, but the lower part comprises a mixed carbonate–siliciclastic succession. Interspersed within fine-grained facies are sharp-based sandstone and conglomeratic intervals interpreted as lowstand deposits. Such lowstand deposits represent a wide range of depositional systems and palaeoenvironments including fluvial channels, shallow-marine shoreface settings, and deeper marine turbidites and sand-rich submarine fans. Associated transgressive and highstand deposits comprise siltstone and shale deposited below storm wave-base in relatively quiet, deep-water settings similar to those found in a mid- to outer-shelf setting. Seismic analysis shows significant fault offsets and thickness changes within the overall wedge geometry. Abrupt thickness changes across faults over small horizontal distances are documented at both the seismic- and outcrop-scales. Synsedimentary fault movement, particularly along steeply north-dipping, largely northeast-trending normal faults, partitioned the depositional system into local sub-basins. On the central Lawn Hill Platform, the nature of facies and their thickness change markedly within small fault blocks. Tilting and uplift of fault blocks affected accommodation cycles in these areas. Erosion and growth of fine-grained parts of the section is localised within fault-bounded depocentres. There are at least three stratigraphic levels within the River Supersequence associated with base-metal mineralisation. Of the seven supersequences in the Isa Superbasin, the River Supersequence encompasses arguably the most dynamic period of basin partitioning, syndepositional faulting, facies change and associated Zn–Pb–Ag mineralisation.
Middle Proterozoic organic-rich sediments from the McArthur Basin in northern Australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence, are representative of the organic remains of microorganisms living at that time. The major classes of hydrocarbons identified were n-alkanes, monomethyl branched alkanes, cyclohexyl alkanes and acyclic isoprenoids. There were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes. Low concentrations of steranes were present in most samples, but like the triterpanes, they were only easily detected in the least thermally altered sediments, and hence were of limited use in detailed assessments of thermal maturity. The presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 Ma, although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes.
Structural architecture, 3D modelling and target generation in the Lawn Hill Platform, Queensland: Project G14, Final report, 37 p. Website accessed
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Murphy, B., L. Ailleres, B. Jupp, L. Leader, T. Lees, and I. Roy, 2007, Structural architecture, 3D modelling and target
generation in the Lawn Hill Platform, Queensland: Project G14, Final report, 37 p. Website accessed November 30, 2015,
http://www.ga.gov.au/corporate_data/69791/69791.pdf.