Southern Bowen Basin Research Articles

What lies beneath – a review of frontier exploration for deep plays in the Bowen Basin

Recent coal seam gas to liquefied natural gas developments have revitalised the Bowen and Surat basins and provided the impetus to explore further for plays with sufficient scale to target both export and domestic markets. Over the last decade, several wells were drilled through Permian-age sediments in the southern Bowen Basin and stimulated to assess the commerciality of various tight gas formations. Thorough data collection included log, core, injection testing, and observations from hydraulic fracturing operations and diagnostics. These data validated the play while providing a picture of the reservoir and in-situ stress conditions; however, challenges exist that will require further wells and studies. We will summarise and highlight the key data collected, review the fracture treatments that were largely ineffective and discuss the implications for further developments in these low-permeability, tight gas targets.

Petroleum Plays of the Bowen and Surat basins

The plays of the Bowen and Surat basins consist of conventional structural plays along major regional structures and depositional platforms, most of which have some stratigraphic component. These can be subdivided into three main regions: the eastern and western flanks of the Taroom Trough, and the Denison Trough. Coal seam gas is found in fairway zones within the Permian coals of the Bowen Basin, which can be subdivided into two main fairways: structurally associated plays in the southern Bowen Basin, and a tighter play in the northern Bowen Basin. Coal seam gas is also found along a broad fairway in the Walloon Coal Measures of the Surat Basin. Recent exploration for new targets has highlighted potential for tight gas in the deeper sections of the Bowen Basin, though further evaluation is required. More speculative plays within the region include tight gas within potential, but untested targets in the deeper sections of the Denison Trough.

A novel model to estimate the impact of Coal Seam Gas extraction on agro-economic returns

There is an ever growing demand for energy worldwide and the demand for gas alone is predicted to double between 2010 and 2035. This demand together with concurrent advances in drilling technologies caused the production of unconventional natural gas such as shale gas and coal seam gas (CSG), which is in the focus of this paper, to grow rapidly in the last decades. With the gas bearing coal seams extending across vast areas within their respective basins and with CSG production having to follow these seams through a network of production wells, pipelines and access roads, CSG activity affects large areas and therefore interferes with existing land uses, predominantly agriculture. For the eastern Australian Surat Basin and the southern Bowen Basin alone there are projected well numbers in excess of 15,000 to 20,000 between the years 2020 and 2030. The interference of CSG with agriculture on a large scale has raised concerns about the impact of CSG on farmland, food security, water resources and the socio-economic environment within the affected regions and beyond. This paper presents a newly developed spatial model which provides order of magnitude figures of the impact of CSG activity on gross economic returns of current agricultural land uses in a given region over the time of CSG production. The estimated gross figures do not account for any compensation payments received by farmers. The model is capable of accounting for a variation in a variety of parameters including impact frequency of distinct infrastructure elements, differences in soil types and associated varying responses of soil productivity, varying length of the CSG production phase and more. The model is flexible in that it can be transferred and applied in other regions as well. Based upon a literature review and given that CSG is an industry that started operating at larger scales relatively recently, we claim that the presented model is the first of its kind to provide these important agro-economic indicators.

High-precision U-Pb CA-TIMS calibration of Middle Permian to Lower Triassic sequences, mass extinction and extreme climate-change in eastern Australian Gondwana

Twenty-eight new high-precision Chemical Abrasion Isotope Dilution Thermal Ionisation Mass Spectrometry U-Pb zircon dates for tuffs in the Sydney and Bowen Basins are reported. Based on these new dates, the Guadalupian-Lopingian/Capitanian-Wuchiapingian boundary is tentatively placed at the level of the Thirroul Sandstone in the lower part of the Illawarra Coal Measures in the Sydney Basin. The Wuchiapingian-Changhsingian boundary is at or close to the Kembla Sandstone horizon in the Illawarra Coal Measures, southern Sydney Basin, in the middle part of the Newcastle Coal Measures in the northern Sydney Basin, and in the middle of the Black Alley Shale in the southern Bowen Basin. The end-Permian mass extinction is recognised at the base of the Coal Cliff Sandstone in the southern Sydney Basin, at the top of the Newcastle Coal Measures in the northern Sydney Basin, and close to the base of the Rewan Group in the Bowen Basin and is dated at c. 252.2 Ma. The end-Permian mass extinction is interpreted to be synchronous globally in both marine and terrestrial environments, and in high and low latitudes (resolution <0.5 my). The GSSP-defined Permian-Triassic boundary is interpreted to be approximately at the level of the Scarborough Sandstone in the lower Narrabeen Group, Sydney Basin, and in the lower Rewan Group, Bowen Basin. New dates presented here suggest that the P3 and P4 glacial episodes in the Permian of eastern Australia are early Roadian to early Capitanian, and late Capitanian to mid Wuchiapingian in age respectively. The greenhouse crisis in the uppermost Pebbly Beach and Rowan Formations of the Sydney Basin is interpreted as early mid Roadian, a mid-Capitanian age for the crisis at the base of the Illawarra/Whittingham Coal Measures is confirmed. Greenhouse crises in the upper Illawarra/Newcastle Coal Measures and lower Narrabeen Group of the Sydney Basin are dated as upper Changhsingian-Induan, and in the upper Narrabeen Group/lower Hawksbury Sandstone as upper Olenekian.

Aquifer injection for CSG water management in the Surat Basin, Queensland

Appropriate water management associated with CSG production is key to regulatory approval and public acceptance of CSG projects. Policy in Queensland makes injection of treated produced water the government's main and preferred option for CSG water management to ensure protection of the Great Artesian Basin as a groundwater resource. In 2010, Australia Pacific LNG started an extensive program of aquifer injection trials across its Surat and southern Bowen Basin CSG fields. The purpose of the trials is to assess the technical and economic feasibility of aquifer injection. The trial program comprises nine different injection targets across four locations and three aquifers. Target depths range from less than 100 m to about 1,500 m. A process of desktop study-exploration-testing has been followed. To date, Australia Pacific LNG has completed all exploration activities. These included about 1,200 m of coring, an extensive suite of geophysical logging and post processing, hydraulic testing and physical, geochemical and mineralogical analyses. This extended abstract describes the results of the exploration activities and the decisions made based on the acquired data. Comparisons are made between the different sites and aquifers. Results of the testing are discussed in the context of technical and economic feasibility of a large-scale injection scheme.

Surface monitoring using radar satellite images to measure ground deformation: applied for coal seam gas production areas to comply with regulatory requirements

This extended abstract introduces radar satellite reservoir monitoring technology (InSAR) and its main applications for monitoring CSG production areas and its surroundings. InSAR technology detects ground motion with millimetric precision: measurements are taken remotely from space. It is a highly efficient tool for ground-motion measurement, even in large and remote areas, where land-based measurement techniques are inconvenient and costly. Altamira Information has developed an advanced differential interferometric chain (Stable Point Network SPN) that can process radar images, achieving millimetric measurements. Ground-motion monitoring with radar images is an efficient technology to comply with regulatory requirements, arising from CSG activities: regulatory requirements specify the obligation for CSG operators to monitor subsidence starting with a baseline then continue with ongoing monitoring to quantify deformation at the land surface within the proponent’s tenures. InSAR is the only technology able to measure ground deformation in the past since archive radar images are available. These measurements in the past allow establishing baselines to determine vulnerable zones affected by subsidence (before CSG production) or subsidence induced by CSG or other activities. Monitoring requirements for the present and future can be covered with high-resolution satellites—which can be programmed—with precise measurement results (up 1 mm). Case studies are presented from historical data processed from the Surat and southern Bowen Basin CSG developments; these cases include the investigation of both CSG and non-CSG induced surface deformations.

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks, Bowen Basin, Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite–smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic ( R≥3) illite–smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic ( R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu) sn ratios show negative correlations with the illite content in illite–smectite and positive correlations with the δ 18O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu) sn ratios and illite content in illite–smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/rock ratio conditions. Increasing HREE enrichment with δ 18O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin. Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins.

Thermal maturation pattern in the southern Bowen, northern Gunnedah and Surat Basins, northern New South Wales, Australia

Comprehensive maximum vitrinite (telocollinite) reflectance data have been obtained for more than 260 polished sections from 28 petroleum exploration wells in the southern Bowen and northern Gunnedah Basins, and the overlying Surat Basin, in an area from north of Boggabri to the New South Wales–Queensland border. The samples studied were coal and dispersed organic matter (DOM) taken from ditch cuttings and drill cores of the Permian, Triassic and Jurassic sedimentary successions. Vitrinite reflectance profiles show that the coals and DOM in the Early Permian Back Creek Group of the southern Bowen Basin have anomalously low (suppressed) reflectance values and a higher reflectance gradient (rate of increase in vitrinite reflectance with depth) relative to the overlying Late Permian, Triassic and Jurassic sequences. Suppression of vitrinite reflectance is recorded in the Early Permian Maules Creek and Goonbri Formations in the northernmost part of the Gunnedah Basin, relative to the overlying Triassic and Jurassic sequences. Dispersed organic matter in the marine sediments of the Watermark and Porcupine Formations of the Gunnedah Basin also shows suppression of vitrinite reflectance. The vitrinite in several wells of the study area, however, displays anomalously high reflectance in parts of the sequence due to the influence of igneous intrusions. After allowing for heat effects due to intrusions and anomalies due to marine influence, the reflectance gradient over equivalent intervals in the southern Bowen Basin is higher towards the west, where the Permian sequence pinches out and the Triassic sequence overlies the basement. Vitrinite reflectance also increases at a higher rate with depth close to the Gil Gil Ridge in the east and close to the Moree High in the south. This may reflect additional heat flow associated with the basement high features. The areas between the Gil Gil Ridge and the Goondiwindi Thrust in the east, and to the west of the Gil Gil Ridge, appear to have lower reflectance gradients, and equivalent or lower reflectance values despite the greater burial depths.

K–Ar and Rb–Sr dating of authigenic illite–smectite in Late Permian coal measures, Queensland, Australia: implication for thermal history

K–Ar and Rb–Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), in order to determine the timing of maximum paleotemperatures, which were responsible for coal maturation and coal seam gas generation. The results indicate two major thermal events affected the Bowen Basin region, at 205–215 and 140–155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived thermal events, rather than gradual temperature increase due to progressive burial as previously believed. The earlier, thermal event in the latest Triassic postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205–215 Ma correspond with the published evidence for regional Late Triassic extension in eastern Australia. The younger (140–155 Ma), thermal phase during the Late Jurassic–Early Cretaceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K–Ar dates in conjunction with vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier one, except in the shallow part of the southern Bowen Basin. This event is recorded exclusively in less illitic, R=0 illite–smectite (I–S) mixed-layer samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures).

Petrographic and isotope constraints on the origin of authigenic carbonate minerals and the associated fluid evolution in Late Permian coal measures, Bowen Basin (Queensland), Australia

Authigenic carbonate minerals are ubiquitous throughout the Late Permian coal measures of the Bowen Basin, Queensland, Australia. In the northern Bowen Basin, carbonates include the following assemblages: siderite I (delta O-18(SMOW) = +11.4 to + 17%, delta C-13(PDB) = - 5.3 to + 120), Fe-Mg calcite-ankerite-siderite II mineral association (delta O-18(SMOW) = +7.2 to + 10.20, delta C-13(PDB) = 10.9 to - 1.80 for ankerite) and a later calcite (delta O-18(SMOW) = +5.9 to + 14.60, delta C-13(PDB) = -11.4 to + 4.40). In the southern Bowen Basin, the carbonate phase consists only of calcite (delta O-18(SMOW) = +12.5 to + 14.80, delta C-13(PDB) = -19.4 to + 0.80), where it occurs extensively throughout all stratigraphic levels. Siderite I occurs in mudrocks and sandstones and predates all other carbonate minerals. This carbonate phase is interpreted to have formed as an early diagenetic mineral from meteoric waters under cold climate and reducing conditions. Fe-Mg calcite-ankerite-siderite Il occur in sandstones as replacement of volcanic rock fragments. Clay minerals (illite-smectite, chlorite and kaolinite) postdate Ca-Fe-Mg carbonates, and precipitation of the later calcite is associated with clay mineral formation. The Ca-Fe-Mg carbonates and later calcite of the northern Bowen Basin are regarded as having formed as a result of hydrothermal activity during the latest Triassic extensional tectonic event which affected this part of the basin, rather than deep burial diagenesis during the Middle to Late Triassic as previously reported. This hypothesis is based on the timing relationships of the authigenic mineral phases and the low delta O-18 values of ankerite and calcite, together with radiometric dating of illitic clays and recently published regional geological evidence. Following the precipitation of the Ca-Fe-Mg carbonates from strongly O-18-depleted meteoric-hydrothermal fluids, continuing fluid circulation and water-rock interaction resulted in dissolution of these carbonate phases as well as labile fragments of volcaniclastic rocks. Subsequently, the later calcite and day minerals precipitated from relatively evolved (O-18-enriched) fluids. The nearly uniform delta O-18 values of the southern Bowen Basin calcite have been attributed to very low water/rock ratio in the system, where the fluid isotropic composition was buffered by the delta O-18 values of rocks. (C) 2000 Elsevier Science B.V. All rights reserved.

The thermal history of the Bowen Basin, Queensland, Australia: vitrinite reflectance and clay mineralogy of Late Permian coal measures

The thermal history of the Bowen Basin (Queensland, Australia) has been investigated using vitrinite reflectance data and clay mineralogy. Vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the Bowen Basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest Middle Triassic–earliest Late Triassic as previously believed. The results of the present study indicate that the development of a zone of high heat flow in the latest Late Triassic had a major control on the thermal history of the Bowen Basin. High palaeogeothermal gradients estimated in the northern Bowen Basin are interpreted to result from convective heat transfer during a hydrothermal event. Variable heat distributions due to localised fracture-enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures. The estimated palaeogeothermal gradients in the southern Bowen Basin also indicate high heat flow in the lower sections of the stratigraphy. Sections in the southern Bowen Basin, however, are believed to reflect a rock dominated semi-closed system with low water/rock ratio, where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction. The correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite/smectite (I/S) mixed-layer clays in the northern Bowen Basin. This phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks. The relationship between I/S expandability and vitrinite reflectance for the Bowen Basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the Bowen Basin. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects.

Stable isotope geochemistry of authigenic clay minerals from Late Permian coal measures, Queensland, Australia: implications for the evolution of the Bowen Basin

Oxygen and hydrogen isotope analyses were carried out on authigenic clay minerals from Late Permian coal measures of the Bowen Basin (Australia). In the northern Bowen Basin, the oxygen isotope compositions of the mixed-layer illite/smectite show significant irregular variations with respect to depth, which parallel the changes in the extent of the illitisation reaction and are interpreted as reflecting changes in water/rock ratio in turn related to permeability. The δ 18O and δD values of illite–smectite and kaolinite in the northern Bowen Basin and the calculated fluid isotopic composition ( δ 18O=−3‰ to +1‰; δD=−70‰ to −90‰) in equilibrium with these clays are considerably lower than those typically reported for deeply buried sedimentary basins. These stable isotope data, together with relatively high inferred palaeotemperatures (up to 235°C) and abnormally high geothermal gradients are consistent with a hydrothermal origin for clay mineral formation in the northern Bowen Basin. The hydrothermal system is interpreted to be a result of the Late Triassic extensional tectonic regime, which developed in large parts of eastern Australia and affected the northern part of the Bowen Basin. In the southern Bowen Basin, by contrast, clays are more enriched in 18O and deuterium, which is explained by lower temperatures (in the shallow coal measures) and a significant enrichment in the fluid isotopic composition ( δ 18O=−3.6‰ to +5.6‰, δD=−66‰ to −35‰) under low water/rock ratio conditions, especially in deeper pelitic rocks.

Clay-Mineral Authigenesis in the Late Permian Coal Measures, Bowen Basin, Queensland, Australia

AbstractMineralogical studies were performed on authigenic clay minerals of mudrocks, sandstones, and bentonites from 38 boreholes in the Late Permian coal measures of the Bowen Basin. Clay-mineral separations of samples from the northern Bowen Basin consist mainly of (Reichweite, R) R = 1 and R ≥ 3 interstratified illite-smectite (I-S), chlorite, and kaolinite. In the southern Bowen Basin, samples from higher stratigraphie sections are characterized by randomly ordered (R = 0) I-S mixed layers, and kaolinite and chlorite in smaller amounts. Samples from the lower sections consist of (R ≥ 3) I-S, chlorite, chlorite-rich chlorite-smectite (C-S), and laumontite.Examination of the mineralogy and distribution of authigenic clay minerals from the Late Permian coal measures in the northern part of the Bowen Basin indicated that the presence of clay minerals is not systematically related to depth and clay occurrences do not occur regularly. These mineralogical variations of clay in volcaniclastic sediments are incompatible with thermal control. Variations in the rate of fluid flow and potassium supply owing to permeability exert major influences on clay-mineral paragenesis and the reaction of illitization. In more permeable zones (possibly faults or fracture zones), highly illitic clays with lath-shaped morphologies may have precipitated directly from potassium-rich fluids migrating from deeper parts of the basin. In addition, abundant chlorite precipitated contemporaneously with illitic clays, which may have resulted from sufficient magnesium and iron occurring in the fluids as a result of dissolution of intermediate or mafic-rock fragments. At the same time, clay paragenesis with less illitic I-S, kaolinite, and minor chlorite occurs outside the channelized zones of high fluid flow, where a diffusive-flow regime may have predominated with lower ratios of the activities of K+ and H+ (i.e., αK+/αH+) in the solutions.In the southern Bowen Basin, depth-related changes in the distribution of clay minerals are evident and may be indicative of thermal control on clay-mineral reactions. Zeolites are present locally in the Late Permian volcaniclastic rocks in the southern Bowen Basin, but not in the north. This is attributed to a low ratio of αCO2/αH2O (where α = activity) and/or more saline and alkaline solutions.

Petrography and clay mineralogy of volcanoclastic sandstones in the Triassic Rewan Group, Bowen Basin, Australia

The sandstones of the Early Triassic non-marine Rewan Group in the southern Bowen Basin represent a potential reservoir for hydrocarbons. They are mainly lithic in nature, with a large proportion of volcanic rock fragments and contain quartz grains of variable size and an authigenic clay matrix. The results of petrographic analysis show a dual provenance basin-fill pattern (cratonic–volcanic). Reservoir qualities vary considerably, but are best developed in the Quartzose and Conglomerate Intervals where partial quartz cement retarded compaction and secondary porosity developed by dissolution of labile material and flushing of clay minerals. The presence of different clay minerals in pore networks may have a significant impact on petrophysical properties of the reservoir and hence affect reservoir productivity.

Green ferric clay in non-marine sandstones of the Rewan Group, southern Bowen Basin, Eastern Australia

AbstractAccessory green ferric clay occurs in fluvial litharenites of the Early Triassic Rewan Group. Although resembling glauconitic minerals in thin-section, electron microprobe analyses indicate that the green ferric clay is mainly ferric illite. The ferric illite may have formed in a small hypersaline lake or well drained, flood-plain soil, and its presence in the Rewan Group cannot be used to support a notion that the unit is partly marine influenced. Identification of the ferric illite by electron microprobe analysis shows that some green ferric clays, particularly those that resemble glauconitic minerals optically, may require precise quantitative elemental analysis before they can be used as a basis for environmental interpretation. Green ferric clay in the Rewan Group also includes detrital celadonite that most likely originated in a volcanic arc setting.

Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, southern Bowen Basin, eastern Australia

ABSTRACTEarly concretionary and non‐concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO3, and are commonly enriched in manganese. δ13C (PDB) values are highly variable, ranging from ‐ 18·4 to +2·9‰, whereas δ18O (PDB) values are very consistent, ranging from ‐ 14·0 to ‐ 10·2‰ (mean= ‐ 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non‐marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ13C values demonstrate that, contrary to current views, highly 13C‐depleted siderite can be produced at shallow burial depths in anoxic non‐marine sediments.

A tide- and wave-moulded shelf sequence from the Permian of the southwest Bowen Basin, Queensland, Australia

The Early Permian Staircase Sandstone Member of the Cattle Creek Formation forms part of the Permo-Triassic basin fill in the Denison Trough (southwest Bowen Basin). The interval is interpreted as the deposits of a wave- and tide-influenced delta because of its interbedded lithological character, alternation of marine and non-marine strata, bipolar palaeocurrent distribution and assemblage of preserved sedimentary structures. The Staircase Sandstone delta prograded eastward into the basin from the northwestern margin, passing distally into progressively more offshore facies. Three component facies are recognized within the inner shelf facies association; facies 1 — pebble conglomerate, facies 2 — sandstone, and facies 3 — siltstone/very fine grained sandstone. The presence of amalgamated hummocky cross-stratification and winnowed, top-surface pebble lags attests to the influence of storms on sediment deposition, while the palaeocurrent distribution demonstrates the operation of tides. Palaeohydraulic calculations on wave-generated structures suggest that during deposition of the Staircase Sandstone Member, the southern Bowen Basin was 250–300 km wide. Despite this narrow, elongate and hence restricted geometry, the basin was subjected to both modest (?micro) tidal circulation and the influence of storm-generated waves and currents.

A tide- and wave-moulded shelf sequence from the Permian of the southwest Bowen Basin, Queensland, Australia

The Early Permian Staircase Sandstone Member of the Cattle Creek Formation forms part of the Permo-Triassic basin fill in the Denison Trough (southwest Bowen Basin). The interval is interpreted as the deposits of a wave- and tide-influenced delta because of its interbedded lithological character, alternation of marine and non-marine strata, bipolar palaeocurrent distribution and assemblage of preserved sedimentary structures. The Staircase Sandstone delta prograded eastward into the basin from the northwestern margin, passing distally into progressively more offshore facies. Three component facies are recognized within the inner shelf facies association; facies 1 — pebble conglomerate, facies 2 — sandstone, and facies 3 — siltstone/very fine grained sandstone. The presence of amalgamated hummocky cross-stratification and winnowed, top-surface pebble lags attests to the influence of storms on sediment deposition, while the palaeocurrent distribution demonstrates the operation of tides. Palaeohydraulic calculations on wave-generated structures suggest that during deposition of the Staircase Sandstone Member, the southern Bowen Basin was 250–300 km wide. Despite this narrow, elongate and hence restricted geometry, the basin was subjected to both modest (?micro) tidal circulation and the influence of storm-generated waves and currents.

METHANE DRAINAGE POTENTIAL OF THE NORTHERN BOWEN BASIN

Large volumes of methane plus some other gases are generated during the coalification process. Under suitable conditions some of this gas is adsorbed within the microporosity of coals. The rate at which the gas can desorb is a function of the permeability, degree of fracturing or cleating, moisture content, geochemistry of the coals, and the pressure differential. Flow rates from coals are generally low but can be dramatically improved by artificial stimulation and techniques such as lateral drilling.Methane drainage or coal de-methanisation has been carried out for many years, primarily for safety reasons. The resource value of methane in coal seams is now being recognised and considerable research is being undertaken both overseas and in Australia.In the Northern Bowen Basin, several million tonnes of coal are mined each year. The main seams of the Permian Collinsville, Moranbah, German Creek, and Rangal Coal Measures are generally thick and laterally extensive. The area north of Blackwater probably contains more than 100 billion tonnes of coal from which several hundred billion m3 (several Bcf) methane could conceivably be recovered in those areas where the coals are too deep for commercial exploitation.The coals of the Northern Bowen Basin are considered to have better physical parameters for the commercial development of methane drainage projects than those of the central and southern Bowen Basin where methane drainage projects were undertaken several years ago. It is estimated that more than 85 million m3 (3 Bcf) of recoverable gas per square km could be present in some areas. This gas can probably be produced for less than $1.50/GJ (1 Mcft, a figure which compares favourably with many conventional natural gas sources.The Northern Bowen Basin is well-situated with respect to potential gas markets at Townsville and Gladstone. The gas could also be used as a chemical feedstock for products such as ammonia, fertilisers, explosives or synfuels, with the plants located close to the producing wells, thus significantly reducing gas transport costs.

PETROLEUM RESOURCES OF QUEENSLAND

Queensland produces oil mainly from the small Moonie and Alton fields in the Surat Basin. Proven reserves of these two fields were 3262 m3 × 103 initial recoverable. Production began in 1964, after completion of a pipeline, 300 km long, from Moonie to Brisbane. The rate peaked at about 1600 m3 per day, but has since declined to about 200 m3 per day. With the exception of the fields presently supplying the pipeline, virtually no other appreciable reserves of oil or condensate are known in the State.Gas is produced from a group of small fields near Roma in the Surat Basin and underlying southern Bowen Basin. Proven reserves were 4573 m3 × 106 initial recoverable. Production for other than local use in town lighting (1906), condensation for motor fuel (1928–1931) and electricity generation (1961 –) did not begin until 1969, when a pipeline 450 km long was completed to Brisbane. The rate is currently about 0.83 m3 × 106 per day. In addition to those at Roma, reserves of gas are also known in small fields: in the Surat and southern Bowen Basin near Surat (about 80 km south of Roma); in the Bowen Basin near Injune and Springsure (respectively about 80 km and 280 km north-northwest of Roma); in the Adavale Basin; in the Queensland portion of the Cooper Basin. Exploration has been intensive only in parts of the Surat and southern Bowen Basins. Accordingly, persistent efforts in selected areas are likely to lead to success. The more prospective basins in Queensland are considered to be the Adavale, Bowen, Cooper, Galilee, Georgina, and Surat.