Bowen Basin Coals Research Articles

Biogenic methane generation in the degradation of eastern Australian Permian coals

Hydrocarbons extracted from one Sydney Basin coal have a distribution of aromatic and aliphatic hydrocarbons consistent with a high degree of biodegradation as previously demonstrated for Bowen Basin coals. On the same basis, hydrocarbons from two other coals from the Sydney Basin are less biodegraded and display an additional minor series of alkylaromatic degradation products. All these degradation reactions generate CO 2 which on biogenic reduction to CH 4 is reservoired in association with the Permian coal-seams of the Sydney and Bowen Basins, Australia. Variations in access to oxygenated waters, nutrients and micro-organisms are held to be largely responsible for the extent of biodegradation of coal hydrocarbons and for the reduction of CO 2.

Combustion behavior of Bowen Basin coals, Australia

Combustion behavior of Bowen Basin coals, Australia

Comment on Boreham et al. (1998), “Factors controlling the origin of gas in Australian Bowen Basin coals” (Organic Geochemistry 29, 347–376)

Comment on Boreham et al. (1998), “Factors controlling the origin of gas in Australian Bowen Basin coals” (Organic Geochemistry 29, 347–376)

Reply to Comment of Smith and Pallasser on “Factors controlling the origin of gas in Australian Bowen Basin coals”

Firstly, we wish to acknowledge the leadership and contribution that Smith and his various coworkers have made over the last three decades to the understanding of the origin of natural gas and coal seam gas, particularly through the use of stable isotopes. Our intention was in no way to criticise their views. Rather, as expressed in Boreham et al. (1998) our view was and remains ``that the origin of coal seam gases especially methane, is not as straightforward as previously thought''. Indeed, Smith (1999) has recently o€ered an alternative abiogenic origin for methane and CO2 via decomposition of acetic acid (Smith et al., 1998). The essence of our paper was that laboratory studies, using open system pyrolysis methods with extrapolation to geological heating rates, demonstrate that a carbon isotope composition heavier than y50% for methane is consistent with a thermogenic origin. Secondly, we acknowledge the incorrectness of a statement in Boreham et al. (1998) which credited Smith and Pallasser (1996) as reporting that they have ``argued for a predominantly microbiological origin for methane based on reduction of CO2 (sourced from a magmatic origin) by methanogenic bacteria''. We inadvertently included the phrase ``sourced from magmatic origin'', which has lead to this rebu€ on the part of Smith and Pallasser. We acknowledge the proposition in Smith and Pallasser (1996), and re-stated in Smith (1999) and Ahmed and Smith (1999), that the origin of the isotopically light methane in the coal seam gases is from the biogenic reduction of CO2 (dC y23%) where the source of the CO2 is organic, derived from decarboxylation of the coal. The view of Smith and Pallasser (1996, p. 891) is that the magmatic-derived CO2 (dC 7 2% and >10 mol%) is inert to attack by methanogens. However, we have diculty with this model since it is hard to see how a microorganism can distinguish the source of CO2, although Smith (1999) has o€ered some possible reasons. Another weakness of the Smith and Pallasser (1996) model is that they attribute the downhole increase in the C content of methane to an increasing mix of heavy methane (dC y5%) relative to the isotopically light biogenic methane. However, they o€er no source for this unusually heavy methane. The main concern expressed in the Comment of Smith and Pallasser is our handling of data appearing in Smith and Pallasser (1996 and references therein) and reproduced by Boreham et al. (1998) in their Fig. 2 and Table 1. This ®gure presented a Rayleigh model ®t to the data to show that the data in Smith and Pallasser (1996) are inconsistent with a `closed system' approach. With this model, the small systematic isotopic changes in the carbon isotopic compositions of the product methane and the residual CO2 cannot be reconciled with the large kinetic isotope e€ect generally associated with the methanogenic process. As noted above, Smith and Pallasser (1996, p. 891) have argued that there is no genetic relationship between CO2 of magmatic origin and isotopically light methane. Speci®cally, they state ``where it (CO2) occurs in seam gas in highly variable proportions, the relatively constant isotopic composition of the CH4 shows that neither reduction of CO2 to CH4, nor isotopic equilibration between CO2 and CH4 has occurred''. However, this observation is also consistent with microbial reduction of magmatic CO2 where the kinetic isotope e€ect is 55% and the CO2 availability is non-limiting; i.e. an open system. Additionally, coal seam gases in the Sydney Basin (Fig. 1) with CO2 contents <10 mol% show

00/03022 Combustion behavior of Bowen Basin coals, Australia

Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms for gasification chemistry of char with CO2 gas. The molecular models used were based on experimental representations of coal chars derived from the vitrinite- and inertinite-rich South African coals at 1000 °C. The HRTEM and XRD techniques were used to construct parallelogram-shaped PAH stacks of highest frequency in the vitrinite-rich (7 × 7) and intertinite-rich (11 × 11) char structures. Computations were executed to get the nucleophilic Fukui functions, at DFT–DNP level, to elucidate the nature and proportions of carbon active sites and quantify their reactivity. The DFT–DNP-computed reaction pathways and transition states, to obtain the energy of reaction and activation energies for the gasification reactions of CO2 with active carbon sites were examined. These results were compared with TGA experimental results at 900–980 °C. The mean nucleophilic Fukui function of the H-terminated char models and active sites located at similar edge positions decreased with increasing size of char molecules and followed the sequence: zigzag > armchair > tip active sites. The mean DFT–DNP values for the activation energy of 233 kJ mol−1 at the reactive carbon edge was in agreement with the experimental 191 ± 25 kJ mol−1 and 210 ± 8 kJ mol−1 for the respective chars.

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.

Combustion behaviour of Bowen Basin coals

Tightly constrained experimental thermogravimetry proves to be effective for characterising the effects of rank and maceral variations on Bowen Basin coal combustion behaviour. These coals show an increase in char burnout temperature with rank for both dull, inertinite-rich and bright, vitrinite-rich coals. Most dull coals have higher char burnout and peak combustion temperatures than their bright rank equivalents. The maximum rates of combustion for dull coals are lower than their bright counterparts, with the difference between the two varying with rank.

Instantaneous outbursts in underground coal mines: An overview and association with coal type

Instantaneous outbursts in underground coal mines have occurred in at least 16 countries, involving both methane (CH 4) and carbon dioxide (CO 2). The precise mechanisms of an instantaneous outburst are still unresolved but must consider the effects of stress, gas content and physico-mechanical properties of the coal. Other factors such as mining methods (e.g., development heading into the coal seam) and geological features (e.g., coal seam disruptions from faulting) can combine to exacerbate the problem. Prediction techniques continue to be unreliable and unexpected outburst incidents resulting in fatalities are a major concern for underground coal operations. Gas content thresholds of 9 m 3/t for CH 4 and 6 m 3/t for CO 2 are used in the Sydney Basin, to indicate outburst-prone conditions, but are reviewed on an individual mine basis and in mixed gas situations. Data on the sorption behaviour of Bowen Basin coals from Australia have provided an explanation for the conflicting results obtained by coal face desorption indices used for outburst-proneness assessment. A key factor appears to be different desorption rates displayed by banded coals, which is supported by both laboratory and mine-site investigations. Dull coal bands with high fusinite and semifusinite contents tend to display rapid desorption from solid coal, for a given pressure drop. The opposite is true for bright coal bands with high vitrinite contents and dull coal bands with high inertodetrinite contents. Consequently, when face samples of dull, fusinite- or semifusinite-rich coal of small particle size are taken for desorption testing, much gas has already escaped and low readings result. The converse applies for samples taken from coal bands with high vitrinite and/or inertodetrinite contents. In terms of outburst potential, it is the bright, vitrinite-rich and the dull, inertodetrinite-rich sections of a coal seam that appear to be more outburst-prone. This is due to the ability of the solid coal to retain gas, even after pressure reduction, creating a gas content gradient across the coal face sufficient to initiate an outburst. Once the particle size of the coal is reduced, rapid gas desorption can then take place.

Factors controlling the origin of gas in Australian Bowen Basin coals

Open system pyrolysis (heating rate 10°C/min) of coal maturity (vitrinite reflectance, VR) sequence (0.5%, 0.8% and 1.4% VR) demonstrates that there are two stages of thermogenic methane generation from Bowen Basin coals. The first and major stage shows a steady increase in methane generation maximising at 570°C, corresponding to a VR of 2 – 2.5%. This is followed by a less intense methane generation which has not as yet maximised by 800°C (equivalent to VR of 5%). Heavier (C 2+) hydrocarbons are generated up to 570°C after which only the C 1 (CH 4, CO and CO 2) gases are produced. The main phase of heavy hydrocarbon generation occurs between 420 and 510°C. Over this temperature range, methane generation accounts for only a minor component, whereas the wet gases (C 2–C 5) are either in equal abundance or are more abundant by a factor of two than the liquid hydrocarbons. The yields of non-hydrocarbon gases CO 2 and CO are greater then methane during the early stages of gas generation from an immature coal, subordinate to methane during the main phase of methane generation after which they are again dominant. Compositional data for desorbed and produced coal seam gases from the Bowen show that CO 2 and wet gases are a minor component. This discrepancy between the proportion of wet gas components produced during open system pyrolysis and that observed in naturally matured coals may be the result of preferential migration of wet gas components, by dilution of methane generated during secondary cracking of bitumen, or kinetic effects associated with different activations for production of individual hydrocarbon gases. Extrapolation of results of artificial pyrolysis of the main organic components in coal to geological significant heating rates suggests that isotopically light methane to δ 13C of −50‰ can be generated. Carbon isotope depletions in 13C are further enhanced, however, as a result of trapping of gases over selected rank levels (instantaneous generation) which is a probable explanation for the range of δ 13C values we have recorded in methane desorbed from Bowen Basin coals (−51 ± 9‰). Pervasive carbonate-rich veins in Bowen Basin coals are the product of magmatism-related hydrothermal activity. Furthermore, the pyrolysis results suggest an additional organic carbon source from CO 2 released at any stage during the maturation history could “mix” in varying proportions with CO 2 from the other sources. This interpretation is supported by C and O isotopic ratios of carbonates that indicate mixing between magmatic and meteoric fluids. Also, the steep slope of the C and O isotope correlation trend suggests that the carbonates were deposited over a very narrow temperature interval basin-wide, or at relatively high temperatures (i.e., greater than 150°C) where mineral-fluid oxygen isotope fractionations are small. These temperatures are high enough for catagenic production of methane and higher hydrocarbons from the coal and coal-derived bitumen. The results suggests that a combination of thermogenic generation of methane and thermodynamic processes associated withCH 4/CO 2 equilibria are the two most important factors that control the primary isotope and molecular composition of coal seam gases in the Bowen Basin. Biological process are regionally subordinate but may be locally significant.

98/00256 Methane capacities of Bowen Basin coals related to coal properties

98/00256 Methane capacities of Bowen Basin coals related to coal properties

Atomic force microscopy studies of Bowen Basin coal macerals

The porous structure of Bowen Basin coal macerals is very important in the storage and transport of coal seam gases. As part of a full characterization of this porosity, several coal macerals were examined by atomic force microscopy. Mesopores of ≥ 8 nm deriving from different types of porosity were imaged. The pores were either formed by contact points between spherical clusters of molecules or were defined channels that had been created by some physical process. The potential of this technique for characterizing the porous structure in greater depth was demonstrated.

97/03736 Methane capacities of Bowen Basin coals related to coal properties

Wellbore stability in shale is a recurring crisis during oil and gas well drilling. The adsorption of water and ions from drilling fluid by shale, which causes clay swelling, is the primary cause of wellbore instability. Nanomaterials have been a subject of interest in recent years to be an effective shale inhibitor in drilling fluid, intending to minimize clay swelling. This article presents a comprehensive review of the current progress of nanoparticle role in water-based drilling fluid with regards to wellbore stability, reviewing the experimental methods, the effect of nanoparticles in drilling fluid, the mechanism of shale stability and the outlook for future research. This paper employed a systematic review methodology to highlight the progress of nanoparticle water-based drilling fluids in recent years. Previous studies indicated the current trend for drilling fluid additives was nanoparticles modified with surfactants and polymers, which minimize colloidal stability issues and enhance shale stability. A review of experimental methods showed that the pressure transmission test benefits shale stability assessment under reservoir conditions. Parametric analysis of nanoparticles showed that parameters such as concentration and size directly affected the shale stability even in high salinity solution. However, there is a lack of studies on nanoparticle types, with silica nanoparticles being the most popular among researchers. Nanoparticles enhance shale stability via physical plugging, chemical inhibition, and electrostatic interactions between surface charges. To better comprehend the influence of nanoparticles on shale stabilization, it is necessary to evaluate a wider range of nanoparticle types using the proper experimental techniques.

Methane capacities of Bowen Basin coals related to coal properties

To help in assessing coalbed methane resources, methane adsorption capacities of Permian coals from the Bowen Basin of Queensland were investigated and related to other coal properties. Maximum methane capacities of moisture-equilibrated coals, normalized to a pressure of 5 MPa and 30°C, showed a continuously increasing (and reasonably linear) trend with increasing rank over the range 80–92 wt% total carbon. Such a linear trend was not observed for methane adsorption calculated on a dry basis. Methane adsorption capacity decreased with increasing temperature by ∼0.12 mL g −1K −1. Methane capacity also decreased as moisture content increased, by ∼4.2 mL g −1 coal for each 1 wt% increase in moisture. Coal surface area (CO 2, Dubinin-Radushkevich) showed reasonable correspondence with methane capacity, although not precise enough to provide a reliable estimate of capacity. Comparison of Langmuir adsorption isotherms for nitrogen and carbon dioxide on Bowen Basin coals with the corresponding methane isotherms showed that, with a knowledge of the methane isotherm alone, nitrogen and carbon dioxide isotherms could be reliably constructed. Volumetric and gravimetric methane isotherms measured on the same coal were identical, confirming the accuracy of the procedures and calculations used.

Coal microstructure and secondary mineralization: their effect on methane recovery

Abstract Methane production from coal seams rather than porous sandstone reservoirs is now recognized as a valuable and recoverable energy source in Australia. The Bowen Basin of Australia possesses well defined coal seams that contain major methane resources. However, commercial gas production to date has been hampered by the low permeabilities of the coal seams. Recovery of this valuable resource will be assisted by a fundamental understanding of coal microstructures and presence of mineralization, and their influence on the gas flow behaviour through coal. This paper examines the relationships between coal type, microstructure, secondary mineralization and gas flow behaviour. The study demonstrates that Bowen Basin coals should not be viewed as simply a dual porosity system of micropores which are surrounded by cleats. Instead, studies using scanning electron microscopy show the Bowen Basin coals have a third porosity system comprising a hierarchy of micron-sized fractures and micronsized cavities at a level between the micropores and the cleat/macropore system, which vary according to coal type. To determine the influence such microstructures have on the flow of gas through the coal matrix, sorption experiments were carried out on small solid blocks of coal, using a new gravimetric technique. The results demonstrate that a clear distinction exists between diffusivity of dull and bright coals in response to coal microstructure. The gas sorption data suggests that both dull and bright coals can be divided into two categories: coal which have a rapid sorption behaviour and coals which have a slow sorption behaviour. The results of the sorption experiments indicate that size, continuity, connectivity of the microstructures, and the extent of minerals infilling the fractures and cavities, play a significant contribution to overall permeability, and are likely to play a major rate-limiting factor in the flow of methane through coal at a level between diffusion at the micropore level and laminar flow at the cleat level. The studies indicate that the flow behaviour of gas through coal seams in the Bowen Basin is unlikely to be solely dependent on the cleat system but rather a combination of the cleat, microstructure and secondary mineralization in coal.

Inorganic nitrogen in Australian semi-anthracites; implications for determining organic nitrogen functionality in bituminous coals by X-ray photoelectron spectroscopy

Inorganic nitrogen in the form of ammonium-bearing clay material has been identified in an Australian (Bowen Basin) semi-anthracite by means of chemical analysis and X-ray photoelectron spectroscopy (XPS). Such an occurrence is similar to the previously reported presence of ammonium-bearing illite in anthracite from eastern Pennsylvania, and consistent with the recently presented X-ray diffraction evidence for ammonium-bearing illite in another Bowen Basin coal of similar rank. The bulk concentration of inorganic nitrogen in the semi-anthracite was ca. 0.1% (5% of the total nitrogen), however the ammonium-bearing clay material was not uniformly distributed throughout the coal; some surfaces prepared for XPS analysis contained no ammonium nitrogen whereas other surfaces contained up to 40% of the total nitrogen in ammonium form. No evidence was obtained for inorganic nitrogen in Australian bituminous coals, including those from the Bowen Basin, nevertheless the presence of very low concentrations could not be excluded. The implications of low concentrations of ammonium nitrogen in bituminous coals for the assignment of the component near 401.5 eV observed in the N(1s) photoelectron spectra of these coals have been considered. It was concluded that any ammonium nitrogen present in bituminous coals would be expected to have a N(1s) binding energy of at least 402.0 cV, and therefore would not be the source of the component near 401.5 eV.

Drill Core Orientation using Palaeomagnetism

Palaeomagnetic orientation of drill core from the Sydney-Bowen Basin is feasible. A consistent magnetisation is observed in coal measures in the basin enabling the remanence of an unoriented sample to be utilised to orient the sample. Palaeomagnetic orientation of drill core for fracture analysis has been used successfully in the Sydney-Bowen Basin. The direction of magnetic remanence in the sediments of the southern Sydney Basin is consistent across a wide area, this direction being north and up. Similarly, the remanence direction observed in Bowen Basin coal measures is consistent from the south (Moura mine) to the north (Goonyella mine) of the basin, and this direction is also north and up. The consistency of the remanence direction is lithology dependent. Fine sandstone and siltstone/mudstone units gave the most reliable results. Coarse sandstones gave inconsistent remanence directions and should not routinely be used for drill core orientation using palaeomagnetism. A longcore magnetometer constructed by the CSIRO Rock Magnetism Group enables measurement of the remanence of HQ drill core without the need to subsample. The measurement of the remanence of HQ drill core using a longcore magnetometer shows similar results to the remanence measured using laboratory magnetometers indicating that drilling does not alter the NRM. Thus, the noninvasive orientation of drill core by measurement of remanence using a longcore magnetometer is feasible for samples from the Permian coal measures of the Sydney-Bowen Basin. As well, storage of the drill core has no or limited effect on the remanence of the drill core. A small component is sometimes present but is easily removed by AF demagnetisation. Weathering has a severe effect on the remanence of samples, inhibiting orientation by palaeomagnetism. Weathering can produce hematite/goethite which retains a strong chemical remanent magnetisation and thus dominates the NRM of the sample.

Sediment Body Quantification: Examples from a Late Permian Mixed Influence Deltaic System, Bowen Basin, Australia: ABSTRACT

The Bowen basin open-cut coal mines of central Queensland, Australia, provide some of the best exposures of alluvial and fluviodeltaic sequences in the world. Within the Bowen basin coal is mined extensively along a strike length of 5-600 km exposing various parts of an Upper Permian terrestrial to shallow-marine sequence. Individual mines cover 10-30 km of strike and may work several different seams that are correlatable between mines. The project described here is aimed at providing a detailed database on facies geometry within coal-bearing alluvial and fluviodeltaic sequences of the Bowen basin. Data has been collected from the profusion of very large open-cut coal mine exposures in eastern Queensland, Australia, and rationalized into a form useful for hydrocarbon reservoir description. High-quality geometrical data is obtained from controlled photomosaics of highwalls supplemented by information from close-spaced borehole networks. Detailed information from core and accessible exposures allows accurate sedimentological interpretation. Interpretation of the German Creek Formation, one of the coalbearing units of the Bowen basin, suggests that accumulation occurred on an extensive lower delta plain, within a mixed (i.e., wave-tide-fluvial influenced) delta. Four facies have been identified, two of which are possible reservoirs: facies A, major channelized sandstone bodies (4-11 m thickmore » and 0.5-5 km wide, elongate perpendicular to basin edge), interpreted as deltaic distributary channel fills; and facies B, tabular sandstone bodies (9-20 m thick tens of km in areal extent, elongate parallel to basin margin) internally dominated by hummocky cross-stratification, interpreted as proximal mouth bar deposits.« less