Jurassic Walloon Coal Measures Research Articles

Subsurface high-grading potential in Coal Seam Gas in eastern Australia

A case study from eastern Australia is used to demonstrate methods for subsurface high-grading and development-concept optimisation in Coal Seam Gas resource plays. We discuss two aspects of subsurface high-grading: (1) improved prediction of net coal using 2D seismic data, and (2) structure-geological analysis and mapping of the fracture network. Net-coal predictions across the study area focused on the upper Juandah Coal Measures interval of the Jurassic Walloon Coal Measures. There is a positive relationship between extracted and normalised upper Juandah Coal Measures seismic-amplitude strength and net coal in the wells. Areas with considerable amplitude dimming commonly coincide with reflector ‘washout’ areas mapped from seismic data and are interpreted as channel incisions into otherwise continuous coal seams. Integration of seismic and well data can improve the mapping of net coal and help optimise the placement of appraisal wells, reducing the number of unexpected or difficult-to-interpret well finds. It may also guide delineation of resource-classification domains in a more robust manner than mapping from well data alone. Fractures in coal seams of the study area are mostly bed-perpendicular joints and cleats. Three sets of fractures/cleats are identified in the coal beds: a dominant set and two more localised sets. Fracture intensity was mapped per set using well data and then combined. Fracture maps are deemed useful as an indication of permeability anisotropy, and hence the expected shape of well drainage areas, as opposed to indicating a range of permeability values. Based on the fracture interpretations, it may be possible to optimise the development concept of specific areas, i.e. varying well spacing and/or drilling wells slanted perpendicular to dominant fracture strike.

Geoscience Poster G2: The Reids Dome beds and its coal seam gas characteristics – Queensland’s latest coal seam gas target

Poster G2 Queensland’s coal seam gas production industry began in 1996 with production from the Permian Baralaba Coal Measures and Bandanna Formation located in the eastern and western Bowen Basin, respectively. It went to another level when production commenced from the Jurassic Walloon Coal Measures in 2005. Production in 2018/2019 amounted to nearly 1500 PJ with nearly 80% coming from the Walloon Coal Measures. The remaining 20% is sourced from the Late Permian Bandanna Formation, Baralaba Coal Measures and Moranbah Coal Measures. A minor volume comes from the Early Permian Cattle Creek Formation. The Early Permian Reids Dome beds are located in the Denison Trough, a graben complex located in the southwest Bowen Basin. The Reids Dome beds were deposited as the basal unit of variably sized grabens and half-grabens. The unit varies in thickness from 9 to >2770 m but the unit hasn’t been fully intersected where it is thickest. Net coal within the formation is also highly variable (<1 to >65 m). At present there has been no commercial production from the coals of the Reids Dome beds, but a number of small fields have reported commercial production from coals of the Early Permian Cattle Creek Formation. By definition, the Cattle Creek Formation was deposited in a marine environment and is contemporaneous with the upper Reids Dome beds. It is here suggested that coals intersected at these producing locations are located in the Reids Dome beds. The interpreted depositional centres for the Reids Dome beds underlie or are immediately adjacent to major Late Permian field developments with access to infrastructure and transmission. Ongoing exploration and appraisal work is continuing across the Denison Trough to fully understand the potential of these coals. Coals from the Reids Dome beds average >10 m3/tonne, >90% saturation >70% methane and up to 20% ethane. To access the poster click the link on the right. To read the full paper click here

The Reids Dome beds and its coal seam gas characteristics – Queensland’s latest coal seam gas target

Queensland’s coal seam gas production industry began in 1996 with production from the Permian Baralaba Coal Measures and Bandanna Formation located in the eastern and western Bowen Basin, respectively. It went to another level when production commenced from the Jurassic Walloon Coal Measures in 2005. Production in 2018/2019 amounted to nearly 1500 PJ with nearly 80% coming from the Walloon Coal Measures. The remaining 20% is sourced from the Late Permian Bandanna Formation, Baralaba Coal Measures and Moranbah Coal Measures. A minor volume comes from the Early Permian Cattle Creek Formation. The Early Permian Reids Dome beds are located in the Denison Trough, a graben complex located in the southwest Bowen Basin. The Reids Dome beds were deposited as the basal unit of variably sized grabens and half-grabens. The unit varies in thickness from 9 to >2770 m but the unit hasn’t been fully intersected where it is thickest. Net coal within the formation is also highly variable (<1 to >65 m). At present there has been no commercial production from the coals of the Reids Dome beds, but a number of small fields have reported commercial production from coals of the Early Permian Cattle Creek Formation. By definition, the Cattle Creek Formation was deposited in a marine environment and is contemporaneous with the upper Reids Dome beds. It is here suggested that coals intersected at these producing locations are located in the Reids Dome beds. The interpreted depositional centres for the Reids Dome beds underlie or are immediately adjacent to major Late Permian field developments with access to infrastructure and transmission. Ongoing exploration and appraisal work is continuing across the Denison Trough to fully understand the potential of these coals. Coals from the Reids Dome beds average >10 m3/tonne, >90% saturation >70% methane and up to 20% ethane.

Identification of Breakout Behind Casing: Methodology to Obtain Openhole-Equivalent Caliper Measurements Through Slotted Liner Using the Density Tool

Growth in the coal seam gas industry in Queensland, Australia, has been rapid over the past 15 years, with greater than USD 70 billion invested in three liquified natural gas export projects supplied by produced coal seam gas. Annual production is of the order of 40 Bscm or 1,500 PJ, with approximately 80% of this coming from the Jurassic Walloon Coal Measures of the Surat Basin and 20% from the Permian Coal Measures of the Bowen Basin. The Walloon Coal Measures are characterized by multiple thin coal seams making up approximately 10% of the total thickness of the unit. A typical well intersects 10 to 20 m of net coal over a 200- to 300-m interval, interbedded with lithic-rich sandstones, siltstones, and carbonaceous mudstones. The presence of such a significant section of lithic interburden within the primary production section has led to a somewhat unusual completion strategy. To maximize connection to the gas-bearing coals, uncemented slotted liners are used; however, this leaves fluid-sensitive interburden exposed to drilling, completion, and produced formation fluids over the life of a well. External swellable packers and blank joints are therefore used to isolate larger intervals of interburden and hence minimize fines production. Despite these efforts, significant fines production still occurs, which leads to the failure of artificial lift systems and the need for expensive workovers or lost wells. Fines production has major economic implications, with anecdotal reports suggesting up to 40% of progressive cavity pump artificial lift systems in Walloon Coal Measures producers may be down at any one time. The first step in solving this problem is to identify the extent and distribution of fines production. The wellbore completion strategy above, however, precludes the use of mechanical calipers to identify fines-production-related wellbore enlargement. A new caliper-behind-liner technique has therefore been developed using a multiple-detector density tool. Data from the shorter-spacing detectors are used to characterize the properties of the liner as well as the density of the annular material. This is particularly important to evaluate as the annulus fill varies between gas, formation water, drilling and completion fluids, and accumulated fines. The longer-spacing detector measurements are then used in conjunction with pre-existing openhole formation density measurements to determine the thickness of the annulus, and hence hole size, compensating for liner and annulus properties.

Estimating porosity and permeability in the Springbok Sandstone, Surat Basin (Queensland), using new wireline log-based workflow

The Upper Jurassic Walloon Coal Measures of the Surat Basin (Queensland) host some of the most prominent coal seam gas (CSG) resources in Australia. The Walloon Coal Measures are directly overlain by the Springbok Sandstone formation, historically referred to as a regional aquifer. An increasing number of studies and industry models suggest relatively limited hydraulic connectivity within the formation and between it and the underlying coal measures, due to extreme lithological heterogeneity. Accurate evaluation of the permeability, as well as lateral and vertical continuity of the lithological units within the Springbok Sandstone, is critical in reservoir models that form the basis of reasonable aquifer protection practices and impact prediction. This study presents a wireline log-based workflow applied to identify permeable zones within the Springbok Sandstone in 31 CSG wells across the Surat Basin that allows robust estimations of porosities and Klinkenberg permeabilities. The workflow primarily utilises spontaneous potential, density, neutron and resistivity logs, and was developed by integrating current industry practices implemented by operators on a local scale to identify risk (permeable) zones in the vicinity of targeted coal seams. The results of this case study indicate that permeable zones within the interval are volumetrically minor (on average 25% N/G) and likely isolated, with Klinkenberg permeabilities rarely exceeding 10–20mD. This evidence for low hydraulic connectivity, as well as significant local variations in the character of the Springbok Sandstone, suggests that the definition of the formation as a regional, continuous aquifer and the way it is modelled needs to be revised.

Nature and origin of tuff beds in Jurassic strata of the Surat Basin, Australia: Implications on the evolution of the eastern margin of Gondwana during the Mesozoic

Volcanogenic rocks in the Great Australian Superbasin provide one of the principal records of contemporaneous volcanism in eastern Australia during the Mesozoic. However, the paucity of primary Jurassic to Early Cretaceous-age extrusive or intrusive igneous bodies on the Australian continent makes it particularly challenging to deduce their source and character. This, in turn, makes it difficult to ascertain how the eastern margin of Gondwana evolved during this timeframe. Despite some studies of this enigmatic volcanism, there have been little or no detailed analyses of these Jurassic to Early Cretaceous-aged sediments. Based on the multidisciplinary analyses of age-constrained air-fall tuffs (168 to 148 Ma) from the Jurassic Walloon Coal Measures of the Surat Basin, we suggest from zircon grains of a similar age that the tuffs were erupted from volcanoes fed by intermediate to felsic magmas supported by their quartz and feldspars-rich composition, and from zircon grains with low to moderate Nb values (0.5 to 100 ppm) and high U and Th values (30 to 1000 ppm). The mapping of tuff isopachs and a mean zircon crystal size of 170 μm supports the source being from volcanoes approximately 280 to 1000 km from the palaeoeast-southeast with a volcanic explosivity index (VEI) of 8. Our results indicate the tuffs originated from a continental arc setting associated with the Whitsunday Igneous Association, and the long-lived (late Palaeozoic to Cretaceous) westward subduction of the palaeo-Pacific oceanic crust beneath eastern Australia. Such a tectono-magmatic environment would help constrain the timing of the transition of eastern Gondwana from a convergent to a divergent margin.

Sedimentary and drainage evolution of the Condamine Valley Transition Zone (eastern Australia)

The basal succession of the Condamine Valley, which overlays the boundary between the Surat and Clarence-Moreton basins (eastern Australia), contains a clay-rich horizon ‘the Transition Zone’ that marks a pronounced unconformity between the Jurassic Walloon Coal Measures and the Quaternary Condamine Alluvium. This paper provides insights into the tectonic and drainage evolution of the Condamine Valley through integrated analysis of U–Pb ages of detrital zircon from three samples (494 concordant analyses), stable oxygen isotope analysis on eight authigenic clay samples, X-ray fluorescence of primary and trace elements, and hyperspectral mineral analysis from two drill cores (Lone Pine 17 and Daandine 164). The Transition Zone is interpreted to consist of both weathered Jurassic sediments and Cenozoic clay deposits. Two sequential cycles of erosion, deposition and pedogenesis, related to geomorphological and climatic conditions are recognised. Distinctive oxygen isotope signatures of the two weathering fronts demonstrated an initial Early Cretaceous phase (δ18O = 11.9–15.7‰ VSMOW) associated with laterisation and possible uplift, followed by Paleogene (δ18O = 16.4–17.3‰ VSMOW) silcretisation of reworked Jurassic sediments. Detrital zircon geochronology yielded Jurassic maximum constraints for the age of deposition of three samples that are indistinguishable within error, the oldest of which (163 ± 8 Ma from the lateritic profile) corresponds to the age of the underlying Walloon Coal Measures. The two overlaying samples from a silcrete profile and granular alluvium yielded overlapping yet younger ages of 150 ± 6 Ma and 156 ± 9 Ma, respectively. Vitrinite reflectance used as a proxy for the thermal condition of the coal strata enabled an estimated 2–3 km of burial and subsequent (likely Early Cretaceous) uplift. Geochemical insights from the Condamine Valley correspond to broad-scale climatic and tectonic conditions, suggesting that ‘transition zone equivalents’ and corresponding groundwater dynamics may occur in similar sedimentary settings throughout eastern Australia.

Revisions to the chronostratigraphic framework of the Upper Jurassic Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures (WCM) in the Surat Basin host the largest coal seam gas (CSG) resource in Australia. Despite this, a poorly defined lithostratigraphic framework hinders the development of reservoir models and groundwater flow simulations. Correlations in the WCM are challenging, owing to the complex arrangement of facies over short distances and the absence of a reliable regional stratigraphic datum. To better correlate the strata, 26 tuff beds were dated using the U–Pb chemical abrasion thermal ionisation mass spectrometry methodology across the Surat Basin CSG fairway. This initially suggested that coal-bearing strata in the basin were diachronous. However, the acquisition of a new date from the Surat Basin has identified a five million year time gap between dated tuffs ~20 m apart. This suggests the presence of an unconformity and that there were two independent episodes of coal accumulation in the basin. Above the unconformity, there are incised valleys with a sedimentary infill that transitions from fluvial- to tidal-influenced facies, as indicated by dinoflagellate cysts and tidal sedimentary structures, including double mud drapes. The cause of the unconformity is likely to be tectonic, as eustatic sea-level was rising during the Kimmeridigian. The marine incursion into the basin is the consequence of a highstand of sea-level during the early Tithonian. The application of the new chronostratigraphic framework should elucidate the evolution of fluviolacustrine systems in the basin and aid in resource prediction. Further dating of tuffs in the basin could refine the stratigraphic framework.

Evolution of the depositional environments of the Jurassic Walloon Coal Measures, Surat Basin, Queensland, Australia

AbstractThe Jurassic Walloon Coal Measures of the Surat Basin in eastern Australia host the continent's most significant coal bed methane resources. Previous studies have interpreted the Walloon Coal Measures within a single depositional facies model encompassing a wholly terrestrial setting. Using a multidisciplinary approach (facies analysis, palynology and wireline logs), the evolution of the Walloon Coal Measures is described within a new chronostratigraphic framework defined by accurate and precise U–Pb tuff dates. Analysis of sedimentary facies indicates that the majority of the Walloon Coal Measures was deposited by relatively small (<300 m wide), low gradient rivers on a poorly‐drained floodplain with numerous small lakes and mires. However, this study also identified some marine‐influenced facies with brackish palynomorphs (notably dinoflagellate cysts) and tidal sedimentary structures. These facies appear to have been deposited in estuaries during times of transgression. The evidence for base level shifts suggests that the coals may not have coevally accumulated with at least some of the thicker sandstones. Palaeogeographic maps for eleven time intervals suggest that rivers drained towards to the south/south‐west and south‐east, as indicated by sandstone percentage and gross unit isopach maps, presumably into proximal estuarine complexes. Marine incursions into the continent probably came from the north and east during times of high eustatic sea level and as precursors to those of the more persistent and extensive transgressions of the Early Cretaceous. A similar multidisciplinary approach should help to elucidate the evolution of other fluviolacustrine systems in other basins and aid in resource prediction.

Using precise CA-TIMS ages of volcanic air-fall tuff beds in correlating the Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures of the Surat Basin is one of Australia’s largest and most productive gas provinces. Despite the drilling of over 8500 wells and numerous publications, the stratigraphic framework is poorly defined. The laterally discontinuous nature of the sedimentary facies, including coals and fluvial channel sandstones, makes correlation difficult. The abundance of volcanic air-fall tuff beds within strata across the basin provides a unique opportunity to independently verify existing stratigraphic frameworks. Using the high-precision chemical abrasion thermal ionisation mass spectrometry technique, zircon grains from 28 tuff beds have been successfully dated within an error margin of less than 100 kyr. These dates substantially revise biostratigraphic and lithostratigraphic frameworks. Lithostratigraphic units are diachronous across the basin. In addition, the sparsity of key spore–pollen taxa limits the application of biostratigraphy. The complex interplay of climate and subsidence on facies distributions can now be documented over a time frame of ~4 Ma. Syntectonism played an important role in variable palaeodrainage patterns across the basin, the frequency of fluvial avulsions and preferential sites of peat accumulation through time. The new stratigraphic framework should aid in future exploration for coal seam gas in the area. Dating tuff beds using high-precision dating techniques should also assist in correlation of non-marine strata elsewhere in the world.

Nested approaches to modelling swamp and fluvial channel distribution in the Upper Juandah Member of the Walloon Coal Measures, Surat Basin

Laterally discontinuous coal measures are common in alluvial settings due to interaction with fluvial systems. Under these conditions it is difficult to accurately represent coalbeds and interburden sandstone bodies in static and dynamic models at a regional scale. These challenges are compounded in the Walloon Coal Measures by non-uniform drill spacing, which varies from clustered to sparse and insufficient outcrop exposures available to constrain the correlations. To address these issues, this study investigates a nested approach to facies modelling of the Upper Juandah Member of the Jurassic Walloon Coal Measures in the Surat Basin, Queensland, which contains some 3,600 wells, of which half were analysed for lithofacies distributions. This approach contrasts the application of truncated Gaussian simulation, object modelling and multiple-point geostatistical simulation. First, a regional scale structural model was developed based on the correlation of sub units within the basin and the lithofacies were then interpreted from normalised wireline logs. Then geometries of individual facies were defined from two local scale models (~6 × 6 km2) where dense drilling, 3D seismic and paleocurrent analysis data were available to constrain the models. Three training images, generated by object modelling, an analogue of one part of the Ob River, and an interactive method were subsequently used to model primary channels, channels and crevasse splays, respectively. Truncated Gaussian simulation was used in modelling the distribution of marginal and coal swamp. The final model is a combination of the model with primary channels and channels, and the model with marginal and coal swamp. This approach is the first trial in modelling swamp and channel distributions at a regional scale by integrating data from local models, depositional analogues and paleo-flow interpretation in the Surat Basin.

U–Pb zircon age of the Walloon Coal Measures in the Surat Basin, southeast Queensland: implications for paleogeography and basin subsidence

The Jurassic Walloon Coal Measures of the Surat Basin were previously estimated to be of Middle Jurassic age, ranging from Aalenian to Callovian, based on an uncalibrated eastern Australian biostratigraphic framework. New U–Pb dates of 162.55 ± 0.05 Ma and 158.86 ± 0.04 Ma obtained from zircons in ash-fall volcanic tuffs now place the Walloon Coal Measures of the Surat Basin in the Upper Jurassic Oxfordian. The new dates have several implications for the interpretation of the Jurassic strata in the Surat Basin. First-order subsidence rates of 61 m/Myr for the Walloon Coal Measures are more akin to those of foreland basins than the previously assumed intracratonic setting. The dates also imply deposition of the Walloon coals in substantially higher latitudes than previously assumed and that they accumulated as peats in mires that experienced more than three months’ continual darkness each winter. Zircon dating of tuffs and associated geochemistry should assist with the correlation of the laterally impersistent coals, fluvial sandstone and mudstone of the Walloon Coal Measures, which are currently difficult to correlate over distances of more than a few kilometres. Dating of the palynostratigraphic zones APJ4.2 to APJ5 (Aequitriradites norrisii Association Zone to Murospora florida Association Zone) will also need to be recalibrated.

DISCOVERY AND DEVELOPMENT OF THE KOGAN NORTH AND TIPTON WEST COAL SEAM GAS (CSG) FIELDS, SURAT BASIN, SOUTHEAST QUEENSLAND

In 2001, Arrow Energy NL, a fledgling coal seam gas (CSG) explorer, drilled the first wells of a multi-well exploration program in two Authorities To Prospect (ATP) permits—ATPs 683P and 676P—that covered an area totalling 13,817 km2 of the Jurassic Walloon Coal Measures in the eastern Surat Basin. The objective was to discover significant CSG resources and, if successful, to commercialise to reserve status. Early exploration success in 2002 saw the discovery of the Kogan North and Tipton West CSG fields. This paper reviews the discovery and subsequent appraisal and development work that Arrow Energy has completed to establish production from these fields.By 2004, Arrow Energy had independently certified Probablereserves in the Kogan North field of 85 PJ, and Possible reserves of 157 PJ. Results from a five-well CSG pilot operation demonstrated the feasibility of commercial gas flow rates sufficiently to justify commercialising CSG from the Walloon Coal Measures in the Kogan North field. Under the terms of a staged development agreement, CS Energy Ltd—Queensland’s largest electricity generator—farmed into the Kogan North Project to earn a 50% interest in PL194 and an adjoining portion of ATP 676P by funding A$13.1 million of the project’s development and appraisalcosts. The funds provided by CS Energy covered the majority of the development costs required for Arrow’s Kogan North development project. The initial gas sales contract from Kogan North will supply sales gas of 4 PJ/a for 15 years to CS Energy from March 2006. Arrow Energy retains the remaining 50% interest and operates the project.With 25 PJ Probable, 90 PJ Probable and 1,980 PJ Possiblegas reserves certified independently, the Tipton West field could potentially be one of the largest onshore gas fields in eastern Australia. Final appraisal of the Tipton West field is currently underway with financial close on the development expected in late 2005. Beach Petroleum Ltd has entered into an agreement to fund the A$35 million required for upstream developmentto supply the initial 10 PJ/a sales gas from the field in 2007, in exchange for 40% interest in th Dalby block of ATP683P. Arrow Energy retains the remaining 60% interest and operates the project.Diligent environmental and land management systems are required with the development of any CSG field. For example, formation water produced from CSG activities needs to be managed effectively. To deal with this water Arrow Energy is developing and implementing several innovative strategies, including forced evaporation dams, water supply to local coal-washing plants and trialling desalination plants to provide drinking water for nearby towns, aquaculture and stock watering.Arrow Energy has also implemented a Cultural Heritage Management Plan within the development areas in cooperation with the local indigenous claimant groups, the Western Wakka Wakka and the Barunggam peoples. The plan was designed to minimise risk of any disturbance to indigenous artefacts and areas of significance during the exploration, construction and ongoing operations associated with the development of both gas fields.The discovery and future development of the Kogan North and Tipton West fields has been achieved by using an appropriate mix of geological evaluation, efficient drilling techniques, innovative well completion methods and successful marketing strategies, integrated with cooperative environmental and cultural heritage management systems.

Variation in pyrolysate composition of sediments from the Jurassic Walloon Coal Measures, eastern Australia as a function of thermal maturation

Examination of a natural maturity sequence from the Jurassic Walloon Coal Measures by quantitative flash pyrolysis-gas chromatography (Py-GC) and low temperature sealed tube pyrolysis reveals systematic changes in pyrolysate composition. In particular, gas to oil ratios, composition of gas and liquid normal hydrocarbons, aromatic and heteroatom content are examined for this immature oil prone rock as it is taken artificially and naturally through the oil window. Liquid hydrocarbon generation, giving a product with a high wax content, is shown to occur over a relatively narrow range of maturation. Significant quantities of gas are generated and the gas to oil ratio remains relatively constant during the main phase of oil generation. This reflects a uniform distribution of oil and gas precursors in the original labile kerogen. At higher rank, where increasing generation from the residual refractory kerogen occurs there is a progressive decrease in the yield of liquid hydrocarbons concomitant with an increase in the gas to oil ratio.

PETROLEUM GENERATION AND SOURCE ROCK ASSESSMENT IN TERRIGENOUS SEQUENCES: AN UPDATE

Analytical pyrolysis and sealed tube pyrolysis at low temperatures have been used to study the timing and petroleum generating capacity of selected Permian through Tertiary coals and carbonaceous shales in relation to their petrographic and elemental composition. The results show that judicious application of flash pyrolysis techniques in conjunction with more conventional procedures are essential for effective source rock assessment in terrigenous source rocks, particularly in those of lower quality.Although the petroleum potential of the samples follows the broad trends in petrographic composition established for Australian coals, that is, relative proportions of vitrinite, inertinite and liptinite, there is much variation which cannot be explained petrographically at the maceral group level. Furthermore, there is no simple relationship between pyrolytic hydrocarbon yield from terrigenous kerogens and overall elemental composition. The yield and composition of pyrolysable normal hydrocarbons varies widely depending on the nature and amount of liptinite macerals, particularly for samples with Hydrogen Indices below 300. Liptinite-poor (Mass balance calculations based on Rock-Eval analyses of samples from the Jurassic Walloon Coal Measures show that the maximum oil formation occurs over a very narrow maturation window from 0.8 to 1.0 per cent Ro, although small amounts of oil may be generated at lower maturation levels. The gas to oil ratio of the generated hydrocarbons is constant up to a reflectance level of 1.0 per cent Ro, where upon the proportion of gas increases rapidly. The low quality Permian source rocks from the Cooper Basin have a lower ratio of labile to refractory kerogen than the Jurassic and Tertiary examples. As a result, the gas to oil ratio of hydrocarbons formed in the oil window is higher and the oil potential appears to be exhausted at an earlier stage of maturation. Efficient migration of hydrocarbons from Permian sediments in the Cooper Basin also appears to occur at a relatively early stage of maturation compared with the Jurassic Walloon Coal Measures.

THE PETROLEUM PROSPECTIVITY OF THE CLARENCE-MORETON BASIN IN NEW SOUTH WALES

The Clarence-Moreton Basin covers an area of some 28 000 km2 in north-eastern New South Wales and south-eastern Queensland. The basin is relatively unexplored, with a well density in New South Wales of one per 1600 km2. Since 1980, Endeavour Resources and its co-venturers have pursued an active exploration programme which has resulted in the recognition of significant petroleum potential in the New South Wales portion of the basin.Previous studies indicated that the Upper Triassic to Lower Cretaceous Clarence-Moreton Basin sequence in general, lacked suitable reservoirs and had poor source- rock potential. While exinite rich, oil-prone source rocks were recognised in the Middle Jurassic Walloon Coal Measures, they were considered immature for oil generation. Moreover, during the 1960's the basin acquired a reputation as an area where seismic records were of poor quality.These ideas are now challenged following the results of a new round of exploration which commenced in the New South Wales portion of the basin in 1980. This exploration has involved the acquisition of over 1000 km of multifold seismic data, the reprocessing of some 200 km of existing single fold data, and the drilling of one wildcat well. Over twenty large structural leads have been identified, involving trapping mechanisms ranging from simple drape to antithetic and synthetic fault blocks associated with normal and reverse fault dependent and independent closures.The primary exploration targets in the Clarence- Moreton Basin sequence are Lower Jurassic sediments comprising a thick, porous and permeable sandstone unit in the Bundamba Group, and channel and point-bar sands in the Marburg Formation. Source rocks in these and the underlying Triassic coal measures are gas-prone and lie at maturity levels compatible with gas generation. In contrast, it was established from the results of Shannon 1 that the Walloon Coal Measures are mature for oil generation and this maturity regime is now considered to be applicable to most of the basin in New South Wales.A consideration of reservoir and source rock distribution, together with structural trends across the basin in Petroleum Exploration Licences 258 and 259, has led to the identification of three prospective fairways, two of which involve shallow oil plays. Exploration of these fairways is currently the focus of an ongoing programme of further seismic data acquisition and drilling.

HYDROCARBON HABITAT OF THE SURAT/BOWEN BASIN

Ever since the early discoveries at Cabawin (1960) and Moonie (1961), the origin of oil and gas in the Surat/Bowen Basin has been a subject of speculation. Hydrocarbons have been found in reservoirs ranging in age from Permian to Early Jurassic; even fractured pre-Permian 'basement' rocks have occasionally recorded shows.Recent geochemical studies have identified rich source rocks within the Jurassic, Triassic and Permian sequences. The Middle Jurassic Walloon Coal Measures are thermally immature throughout the Surat Basin and are unlikely to have generated significant amounts of hydrocarbons. Lower Jurassic Evergreen Formation source rocks have reached 'nominal early maturity' (VR = 0.6) in parts of the basin. The Middle Triassic Moolayember Formation lies within the oil generation zone in the northern Taroom Trough. However, no oil has yet been confidently correlated with either a Jurassic or a Triassic source. On geochemical and geological grounds it is likely that most, if not all, of the hydrocarbons discovered to date were generated from Permian source rocks.The probability of finding gas as well as oil in Permian, Triassic or Jurassic reservoirs increases from south to north, in accord with organic maturity trends in the Permian of the Taroom Trough. On the narrow thrust-bounded eastern flank, vertical migration has occurred, resulting in oilfields at Moonie and Bennett. In contrast, extensive lateral migration of hydrocarbons across the gentle western flank of the basin is indicated by numerous small oil and gas fields on the Roma Shelf and Wunger Ridge.

Geology and sub-surface waters of the Jurassic Walloon coal measures in the eastern portion of the Coonamble Basin, N.S.W

Geology and sub-surface waters of the Jurassic Walloon coal measures in the eastern portion of the Coonamble Basin, N.S.W