Vitrinite reflectance anomalies in the high-volatile bituminous coals of the Gunnedah Basin, New South Wales, Australia

ELSEVIER

International Journal of Coal Geology 36 (1998) 11I-140

Vitrinite reflectance anomalies in the high-volatile bituminous coals of the Gunnedah Basin, New South Wales, Australia Lila W. Gurba, Colin R. Ward Department

*

of Applied Geology, UniversiQ of New South Wales, Sydney NSW 2052, Australia

Received 27 November 1996; accepted 15 August 1997

Abstract The rank of the Permian coals in the Gunnedah Basin has been analyzed using both petrographic and chemical methods. Apart from the effects of local igneous intrusions, a number of seams in the sequence have vitrinite reflectance values (R,, ,,,= ) that deviate significantly from the trend expected with a steady downward increase in coalification. Correlation of these anomalies with interpreted depositional environments suggests that abnormally low vitrinite reflectance values in the sequence occur in seams either overlain by or intimately associated with marine strata. The three-dimensional distribution of such low reflectance values, in part of the section at least, can be related either to the lithofacies pattern or post-depositional groundwater flow associated with a major fan-delta system. Coals with anomalously high vitrinite reflectance values appear to contain material described elsewhere as pseudovitrinite, a component not previously reported in Australian Permian bituminous coals. Both low-value and high-value anomalies need to be taken into account when interpreting maturation patterns from vitrinite reflectance data. In some cases other rank indicators such as air-dried moisture may be useful to complement vitrinite reflectance in rank studies of high volatile bituminous coals. Abnormally low vitrinite reflectance values due to environmental factors such as marine influence, on the other hand, may be used to identify flooding-surface sequence boundaries in the basin for stratigraphic and sedimentological investigations. 0 1998 Elsevier Science B.V. Keywords:

coal rank; vitrinite; reflectance; depositional environment; Gunnedah Basin; Australia

* Corresponding author. Fax: + 61-2-93855935; e-mail: [email protected]. 0166-5162/98/$19.00 0 1998 El sevier Science B.V. All rights reserved PII SO166-5162(97)00033-5

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1. Introduction

Coal rank has long been known to increase with depth, a phenomenon referred to as Hilt’s Law (Stach et al., 1982). In a sedimentary basin fill this gives rise to a continuously increasing trend when a rank indicator such as vitrinite reflectance is plotted against depth. Such trends, particularly when expressed as profiles of vitrinite reflectance, are the cornerstone of many burial history investigations, and have both coal and petroleum applications. However, it is increasingly being recognized that vitrinite reflectance also depends in part on the coal’s depositional environment; lower than expected values are commonly found, for example, relative to the regional rank trend, where marine influence on the sequence is present. It therefore follows that, in detailed studies of vitrinite reflectance trends, both environmental factors and rank effects need to be taken into full account. The present study has identified several different types of anomalies that affect the three-dimensional distribution of vitrinite reflectance values in high-volatile bituminous coals of Permian age in the Gunnedah Basin, New South Wales, Australia. These coals have vitrinite reflectance values (R v,,,) ranging from 0.6 to around 1.0% (Gurba and Ward, 19951, with air-dried moisture decreasing from more than 8% to less than 2% over this range. In general terms coal rank is higher in the south-western part of the basin than in the east, a feature running counter to the present depth and thickness trends in the coal-bearing Permian strata. The origin of this lateral rank variation is still under investigation; it may be a response to deeper burial and subsequent uplift associated with deposition and erosion of the overlying Surat Basin, or it may be a result of variations in geothermal gradient, perhaps generated by basement features, across the study area. In the vertical sense, however, there are a number of significant departures from a continuous increase in reflectance with depth, and these are the subject of the present discussion. Optical and chemical indices of coal rank have been correlated with information in the sedimentary environments of the seams and associated strata (Beckett et al., 1983; Hamilton and Beckett, 1984; Hamilton, 1985, 1991; Tadros, 1993, 1995b1, partly with the aim of improving the level of confidence in vitrinite reflectance as a rank indicator for the sequence and partly to identify, through the distribution of different types of reflectance anomalies, some of the environmental factors that affected deposition of the coal-bearing sequence. This is important if the three-dimensional pattern of rank distribution within the basin is to be fully delineated.

2. Types of anomalies in vitrinite reflectance profiles

Vitrinite reflectance, measured either on vi&mite-group macerals in coal or on dispersed vitrinite particles in sediments, has been used as the basis for many coal rank (or basin maturation) studies. However, some limitations have been noted on the value of this parameter as a rank indicator (Price and Barker, 1985; Teichmiiller, 1987; Gentzis and Goodarzi, 1994; Mukhopadhyay and Dow, 1994). Different factors may

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affect vitrinite reflectance values, and modify occurs during the process of coalification.

the degree of reflectance

increase

113

that

2.1. Associated maceral and mineral materials Among factors contributing to these effects are type of the organic matter (maceral type) and the mineral matrix with which the vitrinite is associated. Variations in vitrinite reflectance with matrix lithology have been reported by Jones et al. (1971) Bostick and Foster (1975) Goodarzi et al. (1988, 1993) Pearson and Murchison (1990) and Gentzis et al. (1993). Such studies have shown, for example, that the reflectance of vitrinite in shale is lower than that of equivalent-rank material in coal. By contrast, vitrinite in carbonates typically has a higher reflectance than in coal at an equivalent rank level. The effect of abnormal maceral compositions has also been cited as evidence for anomalous reflectance values. Studies by Hutton and Cook (1980) Kalkreuth (1982) Kalkreuth and McMechan (1984), Price and Barker (198.5) Snowdon et al. (1986) Raymond and Murchison (1991) and Mastalerz et al. (1993) show that vitrinite reflectance in coal or sediments may be anomalously low in the presence of abundant liptinitic material. 2.2. Marine influence on coal seams The best-known effects on vitrinite reflectance, however, other than those due to rank alone (or to related effects such as igneous intrusions), are the anomalously low reflectance values associated with marine influence on the coal deposit (e.g., Stach et al., 1982). Marine-influenced coals include those formed in swamps that were in close proximity to the sea (and were possibly affected by periodic marine inundation) during peat accumulation, and/or coals that were affected by marine transgression over the peat accumulation after a non-marine influenced depositional process (Rathbone and Davis, 1993). The vitrinite in such coals commonly displays a lower reflectance and higher fluorescence intensity, as indeed do practically all humic degradation products, relative to coals not influenced by these environmental conditions (Diessel, 1990). The high fluorescence intensity and lower reflectance associated with such effects are not always evenly distributed throughout the seam profile, but are commonly concentrated closest to the source of the marine influence. In thick seams with marine roof strata, for example, the reflectance and fluorescence anomalies typically occur mainly in the upper part of the seam section (Stevenson, 1991). The same relationship is also evident with sulphur content. Sulphur abundance has been found to increase up-section within the seam, for example, where the Lower Kittanning coal seam of the USA is overlain by marine or restricted marine strata (Williams and Keith, 1963). Rathbone and Davis (1993) showed that there is a general increase in fluorescence intensity with increasing sulphur content. This relationship suggests that a coal-forming environment favoring sulphur enrichment (e.g. one with a marine influence) also imparts a higher fluorescence intensity (and lower reflectance) to telocollinite. Diessel (1992a) suggests that the basic cause of the relatively high fluorescence in

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marine-influenced coals is an increase in pH of the initial peat under the marine influence, which allows an increase in bacterial activity resulting in biodegradation and addition of bacteria-derived lipids to the residual humic degradation products. The net result of these processes on a sub-microscopic to molecular scale is the production of a vitrinite with a bulk perhydrous chemical composition and distinctive fluorescence (and reflectance) properties. Newman and Newman (1982) were among the first to invoke environment of deposition and plant type as factors contributing to anomalous (both low and elevated) vitrinite reflectance in studies of New Zealand coals. Rathbone and Davis (1993) investigated the effect of depositional environment on fluorescence intensity under varying degrees of marine influence. The effect of marine influence on coal seams, and the value of vitrinite fluorescence as an environmental indicator in coal deposits, have also been discussed by Diessel (1988, 1990, 1992a,b).

3. Geological setting of the Gunnedah Basin The Gunnedah Basin is a part of the Permo-Triassic Sydney-Bowen foreiand basin system of eastern Australia (Fig. 1). The Bowen, Gunnedah, and Sydney Basins are three interconnected sedimentary basins (Tadros, 1993, 1995a; Mallett et al., 1995), which together extend north-south for about 2000 km from central Queensland to southern New South Wales. The Gunnedah Basin contains up to 1200 m of marine and non-marine Permian and Triassic sediments (Tadros, 1995b) that rest unconformably upon an effective basement of Permian (and possibly Late Carboniferous) silicic and mafic volcanics. In the western and northern parts of the basin the Permo-Triassic strata are overlain by rocks of the Jurassic and Cretaceous Surat Basin sequence. The study area for the present paper is located in the Mullaley Sub-basin (Tadros, 1993, 1995b), which makes up the central part of the Gunnedah Basin. This sub-basin is bounded by the Rocky Glen Ridge in the west and Boggabri Ridge in the east (Fig. 1). Around 120 fully cored boreholes have been drilled in this area by the New South Wales Department of Mineral Resources, all of which have penetrated at least the upper part of the Permian succession.

3.1. Stratigraphic sequence The stratigraphy of the Gunnedah Basin is discussed in some detail by Tadros (1993, 1995b). The Permo-Triassic sequence and that of the Jurassic beds in the overlying Surat Basin are shown in Fig. 2. The Gunnedah Basin contains two coal measure sequences of Permian age: a lower sequence embracing the Leard and Maules Creek Formations (Bellata Group of Tadros, 1995b), which is up to 180 m thick (in the Mullaley Sub-Basin), and an upper sequence, the Black Jack Group, which is up to 470 m thick and occurs at the top of the Permian succession. These two coal-bearing intervals are separated by a sequence of marine sandstones, conglomerates, and shales

L.W. Gurba, C.R. Ward/International

Journal of Coal Geology 36 (1998) 111-140

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referred to as the Porcupine and Watermark Formations. The Permian strata are overlain with local unconformity by an upward-fining Triassic fluvial to lacustrine sequence up to 200 m thick (Jian and Ward, 19931, referred to as the Digby, Napperby and Deriah Formations.

3.2. Depositional

environments

The Permian sequence of the Gunnedah tially terrestrial depositional episodes

Basin was deposited in three major essen(Fig. 21, separated by two marine

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Journal of Coal Geology 36 (1998) 111-140

Purlawaugh Formation

Purlawaugh fluvialllacustrine

Napperby lacustrine system

Digby Formation

Digby alluvial system

Upper Black Jack alluvialllacustrine system

Pamboola Formation

Watermark Formation Porcupine-lower Watermark Porcupine Formation marine-shelf system

Goonbri Formation Leard Formation Basal volcanic units

Fig. 2. Stratigraphic sequence of the Gunnedah Basin (Permian to Triassic) and part of the overlying Basin (Jurassic). After Tadros (1995b). Lithofacies shading patterns are explained in Fig. 3.

Swat

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Journal of Coal Geology 36 (1998) 111-140

I17

transgressive/regressive events (Hamilton and Beckett, 1984; Tadros, 1993, 1995b). The first terrestrial episode was dominated by colluvial and alluvial environments and formed the Leard and Maules Creek Formations. The first marine transgression, which probably came from the south (Beckett et al., 19831, changed this alluvial sedimentation to coastal fan development and formed the lower part of the Porcupine Formation. The upper part of the Porcupine Formation and the lower part of the overlying Watermark Formation were deposited with continuation of this transgression in a marine shelf environment (Hamilton and Beckett, 1984). The top of the lower Watermark Formation represents the maximum extent of the first marine transgression. A large deltaic system then prograded across the basin from the north-east, depositing the thick prodelta and delta-front sequences of the upper Watermark Formation. The second terrestrial depositional episode resulting from this progradation is represented by the delta plain deposits which form the lower part of the Black Jack Group (Fig. 2). The second marine transgression inundated the lower Black Jack delta system and resulted in widespread, shallow marine conditions. It led to deposition of the Arkarula Sandstone, a distinctive marker horizon within the shallow marine lithofacies succession (Beckett et al., 1983). The third terrestrial depositional episode followed deposition of the Arkarula Sandstone. Termination of the marine conditions allowed the development of a vast peat swamp system, which is represented by the Hoskissons Coal Member. The peat swamps that formed this seam were terminated by a lacustrine system in the east and a westerly-sourced fluvial sandstone system in the west (Tadros, 1993, 1995b). These deposits were then overlain by coal-barren alluvial fan and braidplain deposits, sourced mainly from the east, to form the Triassic Digby Formation (Jian and Ward, 1993).

4. Experimental Approximately 200 samples of core were collected for the present study, and for related more general work on rank distribution, from numerous boreholes in the Gunnedah Basin. Samples were taken mainly from coal seams, with others from carbonaceous shales and coaly fragments in non-coal rocks. They were prepared for microscopical examination as polished coal grain mounts or as epoxy-impregnated blocks according to Australian Standards. The polished samples were examined using a Zeiss Axioskop reflected-light microscope, fitted with both white (100 W halogen) and blue-violet (HBO) light sources. Maximum reflectance in oil of vitrinite (telocollinite) was measured on the particulate samples. The number of reflectance measurements per sample ranged from 25 to 100, with standard deviations from 0.02 to 0.09. Data were obtained mainly from composite coal samples, representative in each case of the whole-seam section. In some seams, however, seam subsection samples were studied, representing individual plies within the various coal beds. In other cases, small samples of whole-coal were examined, rather than crushed grain mounts, to elucidate relations among the individual maceral components.

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Complementary chemical analysis data on many of the coals (proximate and ultimate analyses and sulphur content) were provided for the project by the New South Wales Department of Mineral Resources.

5. Reflectance anomalies in the Gunnedah Basin Evaluation of the vitrinite reflectance data, in relation to other petrographic and chemical parameters, has indicated three different types of reflectance anomalies in the coals of the Gunnedah Basin. Such anomalies represent laterally or vertically defined areas within the sequence where the reflectance values depart in some way (higher or lower) from the expected regional trend or from a steady increase with depth. They must be allowed for when using reflectance in rank evaluation and burial history studies. The overall increase of reflectance with depth is relatively low, especially in the eastern part of the basin, and in some cases negative rank gradients may appear to be present if these effects are not taken into account. As outlined briefly by Gurba and Ward (1996) the different types of anomalies are: (a) The occurrence of higher vitrinite reflectance arising from proximity to igneous intrusions, which as indicated by Martin (1993) are relatively common in the coal-bearing succession of the Gunnedah Basin. (b) The occurrence of lower than expected vitrinite reflectance due to marine influence on the coal seam, or in some cases to an abundance of liptinite in the coal itself. As discussed above, such lower reflectance values are also reported from studies of a number of other sequences, including the work of Hutton and Cook (1980) Newman and Newman (1982) Kalkreuth (1982) Price and Barker (1985) and Diessel (1992a,b). (c) The occurrence of higher than normal reflectance, noted during the present study in vitrinite which has a characteristic pattern of slits in one or two directions. This material resembles a component first described in American coals by Benedict et al. (1968) as ‘pseudovitrinite’. Similar high reflectance is also noted in some vitrinites that have an abundance of mineral (typically quartz) inclusions, but do not display the slit pattern effects. Cook and Taylor (1963) observed a greater spread of vitrinite reflectance in coals of the Triassic Ipswich Coal Measures in south-east Queensland having R,,,, values of 0.9 to 0.96%, relative to those in higher-rank seams CR,,,, > l%), and in the same paper noted that vitrinite with an arcuate slit pattern is also present. However, pseudovitrinite has not previously been identified by name in Australian coals, nor have its effects on rank studies been specifically evaluated. Except for the higher reflectance and the slit pattern, the material described as pseudovitrinite resembles telocollinite sufficiently closely to be grouped with that component in most maceral classifications. Its reflectance is therefore measured along with that of more normal telocollinite if standard procedures are followed. Indeed, the material is grouped with collotelinite in the telovitrinite sub-group by ICCP (1995). As discussed elsewhere in the present paper, however, mean vitrinite reflectance values are higher, for coals of equivalent rank, where significant proportions of pseudovitrinite are present. This in turn provides some difficulty in using vitrinite reflectance data as the

L.W. Gurba, C.R. Ward/InternationalJournal of Coal Geology36 (1998) III-140

basis for precise rank evaluation, range. 5.1. Igneous

at least for coals in the high-volatile

bituminous

119

rank

intrusive effects

Igneous intrusions are common in the Gunnedah Basin. They typically invade and preferentially replace coal seams in the Permian section, forming sills which may be up to 86 m thick when encountered in drill cores (Martin, 1993). Anomalies due to igneous intrusions are marked by sharp increases in vitrinite reflectance in the coal above and below the intrusive body, with a corresponding decrease further from the intrusive heat source. Effects are generally not detected, with single intrusions, more than 20 m above the upper contact of the intrusive body. The reflectance associated with such intrusions is typically very high, and reaches up to 7% near some contacts with intrusive material. As an example, the profile of DM Bando DDH 1 (Fig. 3) shows vitrinite reflectance values of 4.03 and 3.68% in the coal seams at depths of 457 and 546 m respectively. These are situated on each side of a thick igneous intrusion (Fig. 3b), and clearly represent a response to the heat flow from the intrusive body. The results of drilling programs in the Gunnedah Basin show the distribution of intrusive bodies in the Permian sequence to be concentrated in the south-east (Fig. 1). Anomalous vitrinite reflectance values associated with intrusions are therefore most common in this area. The area of highest overall coal rank, however (indicated by both moisture and vitrinite reflectance), is located near the Rocky Glen Ridge to the south-west (Gurba and Ward, 1995), away from the most heavily intruded portion of the basin. Although locally abundant, the intrusive bodies do not therefore appear to be responsible for any regional reflectance effects beyond their immediate influence zone. Anomalies in vitrinite reflectance due to intrusions, although significant, are not discussed further in the present paper. 5.2. &jects of marine influence Evidence for lower vitrinite reflectance values due to marine influence on the coal seams can be identified in two major intervals of the Gunnedah Basin sequence: the Maules Creek Formation in the lower part of the Permian section and the lower Black Jack Group in the upper part of the coal-bearing succession. 5.2. I. Ma&es Creek Formation The Early Permian Maules Creek Formation (encompassed in the Maules Creek alluvial/lacustrine system of Tadros, 1993) reaches a maximum thickness of slightly more than 120 m in the central part of the Mullaley Sub-basin (Fig. 4). Coal seams within this formation are generally interbedded with conglomerate and sandstone channel deposits and do not usually have economic thicknesses. At the top of the formation they are commonly pyritic, as a result of marine conditions associated with deposition of the overlying Porcupine Formation (Hamilton et al., 1991). Two patterns of marine influence on vitrinite reflectance in coal seams can be recognized in the Maules Creek and underlying Leard Formations:

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Journal of Coal Geology 36 (1998) 111-140

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L. W. Gurba. C.R. Ward/International

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and

The first pattern is one in which only the coals at the top of the Maules Creek Leard sequence have a lower vitrinite reflectance, more intense vitrinite fluorescence, higher volatile matter, higher sulphur content and an abundance of pyrite (Fig. 5a). Some of these coals are enriched in liptinite, especially cutinite (Fig. 5b). Such features have been observed, for example, in hand-picked samples from coal seams over an interval of 20 m at the top of the Maules Creek Formation in DM Bando DDH 1 (Fig. 3). They have also been observed in coal samples from the top 20 m of the Maules Creek Formation in DM Walla Walla DDHl (Fig. 6). The underlying coal seams in both boreholes, however, and in several others, do not show any such lower reflectance. Vitrinite reflectance in the seams affected by marine influence may be up to 0.1 or even 0.2% below that of the adjacent unaffected coals in the sequence. Volatile matter

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123

(dry ash-free) in the samples with anomalously low reflectance is higher than in either the underlying unaffected Maules Creek and Leard Formation seams (Fig. 3), a feature consistent with the observed reflectance trends. Air-dried moisture, however, continues to follow a decreasing trend across the interval with lower reflectance values (Fig. 3~); this parameter is not significantly different for these samples from that of the unaffected and therefore higher reflecting coals in the underlying parts of the Maules Creek and Leard succession. In DM Bando DDH 1, for example (Fig. 3c), the coal seam at a depth of 865.00 m has a higher percentage of volatile matter (42.3% dry, ash-free), whereas air-dried moisture for the same seam is 1.2%. Each point plotted in Fig. 3c represents a calculated composite for the respective coal seam, based on individual determinations of moisture (air-dried) and volatile matter (dry, ash-free) for the various plies or seam sub-sections. Numerous other boreholes in the basin show similar trends. The second pattern is one in which all of the coal seams in the Maules Creek Formation have reflectance values which are significantly lower than the trend suggested by the profile projected from the overlying upper Black Jack succession. An example of this is seen in the Maules Creek Formation in Denison DDHl (Fig. 7). If expressed by reflectance, rank would appear to decrease rather than increase with depth in this particular borehole, and the overall trend could be interpreted as a reversed rank profile. Extrapolation of reflectance data from the upper part of the Black Jack Group, however, where values of around 0.8% are noted, suggests that the reflectance (R,,,,) should be at least 0.85% in the Maules Creek Formation in DM Denison DDH 1 rather than the values of 0.63, 0.69 and 0.71% indicated by the actual analytical results. The overall increase of vitrinite reflectance with depth in the Gunnedah Basin is relatively low, especially in the east. If not considered in context, use of the anomalously low values in broad-scale thermal modelling could easily give rise to incorrect conclusions about palaeogeothermal gradients and maturation trends. In extreme cases, as with the example in Fig. 7, coal rank may appear to decrease, rather than increase. with depth. The top coal seam in the Maules Creek Formation, being closest to the overlying marine succession, would be expected to reflect the greatest marine influence derived from the Porcupine Formation. Nevertheless, in some boreholes all coal seams in the Maules Creek Formation, covering a stratigraphic interval typically some 40 m in thickness, show similar marine-influenced features. Although the actual origin of the anomalies is not clearly understood, the distribution of reflectance suppression in coals of the Maules Creek Formation appears to be related to the overall depositional setting of the unit. The unit is dominated by a thick (up to 125

Fig. 5. Photomicrographs of marine-influenced coals from the Gunnedah Basin succession (reflected light, oil immersion; width of field = 0.22 mm): (a) Small pyrite inclusions in telocollinite from coal of the Ma&s Creek Formation (DM Denison DDH 1, 354.60 m). R,,,,, = 0.69%; (b) Typical occurrence of cutinite in coals of the Maules Creek Formation under blue light excitation (DM Turrawan DDH 1, 424.40 m). (c) Framboidal pyrite in desmocollinite from coal of lower Black Jack Group, below the Arkarula marine transgression (ACM Yannergee 1, 913.99 m). R,,,, = 0.67%; (d) Telocollinite (left) and desmocollinite (right) in coal of the Hoskissons seam of the upper Black Jack Group, above the Arkarula marine transgression (ACM Yannergee 1, 888.40 m). R,,,, = 0.76%.

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Journal of Coal Geology 36 (1998) 11 l-140

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seam; MS = Melvilles seam; BV = basal volcanics.

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sediment near Narrabri, north of DM Walla Walla DDH 1, and deposition appears to have been controlled by a less-active easterly-flowing fluvial system. It also thins in the south and east of the basin, where slow subsidence and more persistent swampy conditions apparently prevailed. Anomalously low vitrinite reflectance occurs only in the topmost seam of the Maules Creek Formation in the areas of alluvial fan deposition, including the subsidiary area of thick sediment near Walla Walla DDH 1. However, it occurs throughout the whole of

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the unit in the areas outside this lithofacies succession. Such a distribution may simply reflect the fact that the Maules Creek Formation is thicker in the alluvial fan area, and that marine influence associated with deposition of the Porcupine Formation did not penetrate through the full section of the unit in the central part of the Mullaley Sub-basin. However, reflectance suppression occurs over a greater thickness of strata outside the fan area than reflectance suppression within the fan area (where only the upper 20 m or so are affected), and hence a simple post-depositional invasion is unlikely as the main cause of the lower reflectance values. Another explanation is that the more active river systems associated with alluvial fan deposition maintained a freshwater environment for the peat in the vicinity of the fan, while the peat outside the fan were subjected, perhaps during minor but frequent transgressions, to a greater degree of syn-depositional marine influence. It is also possible that freshwater flow continued, in the form of groundwater, through the fan sediments after deposition. This would have helped to prevent post-depositional marine influence from affecting the lower coal seams in the fan area of the Maules Creek succession when the transgression associated with the Porcupine Formation brought marine conditions to the whole of the Mullaley Sub-basin. 5.2.2. Lower Black Jack Group The sediments of the Lower Black Jack Group were deposited by a combination of fluvial-dominated and wave-dominated deltas, with distributary-channel, crevasse-splay and splay-subdelta, interdistributary bay and marsh deposits recognized from drill cores (Hamilton, 1985; Tadros, 1995b). Most coal seams in this sequence are less than one meter thick; they appear to represent marshes that flanked the distributary channels and were capped by interdistributary bay deposits. Some seams (such as the Melvilles Coal Member) are thicker, however, and may represent blanket peat formed on more extensive abandoned delta lobes (Hamilton, 1985). The coals are therefore mostly influenced by marine conditions, except along the western and north-western margins of the Mullaley Sub-basin where fluvial input apparently continued to predominate. Deposition of the lower delta plain facies was followed by a short-lived transgression which resulted in widespread, shallow-marine shelf conditions. In the main study area of the Mullaley Sub-basin this transgression is represented by the sandy beds of the Arkarula Formation, which is an important time-marker horizon (Beckett et al., 1983; Hamilton et al., 1991). Hamilton (1985) has divided the sediments associated with the Arkarula Formation into a tidal shelf depositional system in the north and a wave-dominated delta system in the southern part of the basin. Sedimentation along the western margin of the basin, however, was dominated by a westerly-sourced fluvial wedge which formed the Brigalow Formation within the Black Jack Group (Fig. 2). This fluvial system interfingers basinwards with and ultimately overlies the shallow-marine deposits of the Arkarula succession. With the exception of the south-east of the basin, where the Arkarula Formation is absent, the vitrinite reflectance levels in the Black Jack Group coals below the Arkarula Formation are significantly lower than those of the overlying more terrestrial-dominated coals in the upper part of the Black Jack sequence. In Nea DDH 2, for example (Fig. 81,

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127

Hydrogen daf % 0.60

0.80

0

50

W

50

150

i=

la -I

!-

i(arula trE

b __

200

i

-L

-

1

250

Fig. 8. Profile of mean maximum vitrinite (telocollinite) reflectance (left) and hydrogen content (right) against depth for the Black Jack Group in DM Nea DDH 2. Changes in both parameters are clearly seen below the Arkarula transgression. For reference see Fig. 3. HS = Hoskissons seam; MS = Melvilles seam.

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the mean maximum reflectance drops from 0.75% a short distance above the Hoskissons seam to 0.70% a short distance below the Arkarula Formation. As with the coals with lower reflectance in the Maules Creek Formation, the vitrinite in the lower part of the Black Jack section also shows an increased fluorescence intensity and contains some pyritic material (Fig. 5~). A map showing the contrast between the vitrinite reflectance in the seam sampled immediately above the Arkarula Formation (typically the Hoskissons seam, Fig. 5d) and that in the seam sampled immediately below the Arkarula is given in Fig. 9. Except for a single borehole in the extreme south-east (where the Arkarula Formation is absent), the reflectance of the vitrinite in the lower Black Jack coal is always lower than that of the overlying upper Black Jack material. The difference, as shown on the map, may be up to 0.16%. The difference is greatest in the north-east, where the Arkarula Formation is thickest, and are lowest in the south, where that sequence thins out.

r \

“~&

‘, \ ‘5

NEW

\ \“m I ‘p

58 / /”

ENGLAND

‘,z ,-

FOLD

\oO \z

BELT

LACHLAN FOLD BELT

6 10

Sample

location

/---

0.71

20km

Fig. 9. Map showing contrast between mean maximum vitrinite (telocollinite) reflectance for Black Jack Group seams immediately above (upper value) and below (lower value) the Arkarula transgression horizon for selected boreholes, showing the contrast between them. Contours represent the total thickness of the Arkarula Formation.

nd = not determined

nd 52.5 67.5 54.4

Total vitrinite

Lower Black Jack Group 205.41 nd nd 234.12 17.5 35.0 238.72 24.3 43.2 250.19 34.1 20.3

Desmocollinite

nd 24.9 53.3 21.1 27.0 14.9 35.8 33.5 4.3 26.7

Telocollinite

Upper Black Jack Group 60.38 nd nd 86.69 9.7 15.2 88.59 14.6 38.7 95.70 6.3 14.8 99.08 9.5 17.5 100.74 1.7 13.2 104.34 16.3 19.5 146.68 18.6 14.9 150.69 0.6 3.7 162.03 8.3 18.4

Cm)

Depth

nd 21.7 12.9 15.9

nd 23.6 16.7 33.4 51.5 41.8 31.4 34.9 1.5 32.4

Semifusinite

Table 1 Maceral analysis data for coals from DM Nea DDH 2

nd 2.4 1.9 1.9

nd 1.8 2.6 2.4 1.6 6.6 5.6 4.0 0.3 3.3

Fusinite

nd 12.0 6.3 16.7

nd 18.5 11.1 25.3 4.7 26.4 11.0 19.0 51.6 21.4

Inertodetrinite

nd 36.1 21.1 34.5

nd 43.9 30.4 61.1 57.8 74.8 48.0 57.9 53.4 57.1

Total inertinite

nd 7.1 6.2 5.0

nd 0.9 5.1 2.8 2.4 0.9 2.1 3.7 16.4 6.4

Sporinite

nd 1.5 2.6 1.1

nd 1.2 2.6 0.0 0.3 1.1 1.5 2.0 8.8 2.6

Other liptinite

nd 8.6 8.8 6.1

nd 2.1 7.7 2.8 2.7 2.0 3.6 5.7 25.2 9.0

Total liptinite

nd 2.2 2.2 4.4

nd 28.6 8.0 11.4 12.0 7.5 12.2 2.4 16.4 6.6

Minerals

0.70 0.70 0.70 0.73

0.89 0.70 0.70 0.71 0.79 0.71 0.75 0.74 0.74 0.75

R Ymar

Vitrinite reflectance

0.04 0.04 0.03 0.03

0.07 0.06 0.04 0.06 0.07 0.04 0.05 0.05 0.05 0.06

SD

20 50 45 50

85 24 30 50 25 15 30 50 8 50

n

130

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The majority of the coals studied from the lower Black Jack Group, which represent lower delta-plain deposits, are rich in vitrinite compared to those of the more fluvialdominated upper Black Jack succession (Table 1). With a few minor exceptions, however, liptinite makes up less than 10% of the total macerals present in both the upper and lower Black Jack successions, although it is slightly more abundant in the lower Black Jack seams. The lower delta-plain coals of the lower Black Jack Group are generally perhydrous in character. Fig. 8 shows a clear increase in hydrogen content (dry, ash-free basis) accompanying the decrease in vitrinite reflectance values below the Hoskissons seam. More complete chemical data for the Black Jack Group coals from this particular borehole (Table 2) also show a decrease in oxygen and an increase in H/C ratio and nitrogen over the same depth interval. The higher hydrogen and lower oxygen contents of the lower Black Jack coals point to initial deposition of the organic matter in an environment where anaerobic conditions prevailed, resulting in an enrichment of hydrogen in the vitrinite. Reflectance of vitrinite is related to its hydrogen content (Gentzis and Goodarzi, 1994), and hence the enrichment in hydrogen is linked to both the lower reflectance and the increased fluorescence intensity. Possible controlling factors for the high hydrogen and related optical properties include differences in the original plant material, the nature of the depositional environment associated with its preservation, incorporation of lipid material and the behavior of the resulting macerals during diagenetic and coalification processes.

Table 2 Chemical analysis data (dry, ash-free basis) for coals from DM Nea DDH 2 (supplied by NSW Department Mineral Resources)

Depth (m)

Carbon %

Upper Black Jack Group 60.8 80.5 88.1 82.0 91.9 81.5 95.8 83.0 99.2 79.1 102.6 83.2 104.8 82.0 146.9 81.8 150.9 80.5 151.9 82.4 162.5 82.5 181.9 82.2

Lower Black Jack Group 205.9 80.4 234.1 82.8 240.3 82.4 250.4 83.0

Nitrogen %

Sulphur %

Oxygen %

H:C ratio

4.7 5.2 5.4 4.4 4.1 4.6 4.9 5.0 5.1 4.9 4.9 4.2

1.2 1.9 1.8 1.8 1.6 1.8 1.8 1.7 1.7 1.7 1.8 1.9

0.76 0.99 0.86 0.57 0.71 0.53 0.52 0.61 0.69 0.59 0.54 0.50

12.8 9.9 10.5 10.2 13.2 9.8 10.8 10.8 12.1 10.4 10.3 11.1

0.70 0.76 0.79 0.64 0.71 0.66 0.71 0.73 0.75 0.71 0.71 0.62

5.8 5.4 5.5 5.6

2.2 2.0 2.1 2.2

0.74 0.64 0.64 0.60

10.9 9.1 9.3 8.6

0.86 0.78 0.80 0.80

Hydrogen

%

of

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131

Air-dried moisture, by contrast, shows a small but significant decrease between the Hoskissons seam and the coal immediately beneath the underlying Arkarula Formation (Table 3). This is consistent with an increase in rank with depth, despite the decrease rather than increase in vitrinite reflectance and the changes in vitrinite properties over the same stratigraphic interval. 5.3. High reflectance

due to pseudovitrinite

and related materials

Anomalously high vitrinite reflectance values, in relation to the expected trend of reflectance increase with depth, were noted in a number of different seams in the Gunnedah Basin sequence, particularly in the upper Black Jack Group and the lower part of the Maules Creek Formation. The sample from a depth of 421.7 m in Fig. 3a, for example, represents such an anomaly on high side of the reflectance profile for DM Bando DDH 1; the samples from 60.38 and 99.08 m in Fig. 8 and Table 2 represent a similar anomaly in DM Nea DDH 2. Close inspection of the vitrinite in these and other coal samples shows the presence of a distinctive type of otherwise homogeneous, moderately reflecting material along with the more normal telocollinite and desmocollinite components. This material, examples of which are illustrated in Fig. 10, has a characteristic slit pattern on the polished surface, with the individual slits typically being sigmoidal in form. The outer edges of broken particles of the material in grain mounts also tend to be serrated due to breaks along the slit traces. This material has a significantly higher reflectance than the more normal telocollinite and desmocollinite in the same coal sample. In a coal from the upper Black Jack Group in DM Nombi DDH 1, for example (Fig. 11 - see separate discussion below), its mean maximum reflectance is 0.92%, compared to a value of 0.80% for the other telocollinite

Table 3 Moisture content (air-dried) for coals immediately NSW Department of Mineral Resources)

above and below the Arkarula Formation

(data supplied by

Borehole

Moisture in Hoskissons seam of upper delta plain facies %

Moisture in top coal of lower delta plain facies %

DM DM DM DM DM DM DM DM DM DM DM DM DM

2.6 3.8 2.2 2.7 3.3 4.4 2.4 2.1 3.4 3.4 1.2 5.3 3.3

1.7 (heat affectedja 3.4 2.1 2.7 2.9 4.3 2.0 2.8 2.9 3.2a 1.1 4.Y 2.8

Benalabri DDH 1 Breeza DDH 1 Brigalow DDH 2 Brown DDH 2 Caroona DDH 1 Caroona DDH 3 Coogal DDH 1 Denison DDH 1 Goran DDH 1 Nea DDH 2 Terawinda DDH I Turrawan DDH 1 Wallala DDH 1

“Indicates

seam is the Melvilles Coal Member.

132

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Journal of Coal Geology 36 (1998) 111-140

Fig. 10. Photomicrographs of pseudovitrinite in coals of the upper Black Jack Group (DM Nombi DDH 468 .30 m; reflected light, oil immersion; width of field 0.22 mm): (a) Pseudovitrinite showing characteri: stic sert ,ated grain boundary, slit-like openings and the presence of faint cell structure. Note small grains of py rite = 0.90%. (b) Contrast in reflectance between pseudovitrinite cRymax = (P) in some slit openings. R,,,, 0.8! )%-left) and other telocollinite CR.,,,, = 0.78%) and desmocollinite (right) in the same coal sample.

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133

50 DESMOCOLLINITE R max = 0.75% SD

25

r-l

0 0.6

=0.04

I--, 0.7

0.8

0.9

1

1.1 &,,%

50 TELOCOLLINITE

25

&,

= 0.80%

SD

= 0.05

0 0.6

0.7

0.8

1

0.9

1.1 R,,,

%

50 PSEUDOVITRINITE

-

&,,

= 0.92%

SD

= 0.03

25

rl

rl

0 0.6

0.7

0.8

0.9

1

1.1

R,.%

R,,,= SD

50

1.25% = 0.28

25

0.6

Fig. 11. Histograms nite), pseudovitrinite m).

0.7

0.8

0.9

1

‘.I

R,,,

%

showing reflectance distribution for desmocollinite, telocollinite (excluding pseudovitriand semifusinite for an upper Black Jack Group coal sample (DM Nombi DDH I. 468.3

in the same coal sample. It also tends to occur in thicker bands than the other vitrinite types in the same coal seam. Small crystals of pyrite are commonly found within the slits, suggesting that the slits themselves were open and able to receive such mineralization at some stage during coal formation, and that they are not an artifact of exposure or sample preparation. A similar vitrinite material with a higher reflectance and a characteristic slit pattern was described in Carboniferous coals from the Appalachian region of the USA by

134

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Benedict et al. (1968) who along with others have referred to it as pseudovitrinite. Such material has also been described, for example, in the Pennsylvanian coals of southern Illinois by Kravits and Crelling (1981) in the Lower Cretaceous coals of the Bickford Formation in Canada by Kalkreuth (1982) and in bituminous coals of the Upper Cretaceous Pike River coalfield, New Zealand by Newman and Newman (1982). It is reported in Permian coals from southern Africa by Falcon and Snyman (1986). Vitrinite with a fine arcuate fracture pattern was reported in an Australian Triassic coal by Cook and Taylor (1963) but pseudovitrinite as such has not previously been recognized separately in Australian coal deposits. Brown et al. (1964) introduced the concept of vitrinite A and B for Australian Gondwana coals; these have since become recognized as telocollinite and desmocollinite. Bennett (1968) also described as ‘granular vi&mite’ material in Gondwana coals which has higher than normal bireflectance and a granular optical texture. However none of these materials resemble the material described in the present study and elsewhere as pseudovitrinite. Three populations of vitrinite can be identified from statistical study of reflectance data in samples of Gunnedah Basin coal containing this type of material. Fig. 11 provides separate histograms for the individual maximum reflectance values of desmocollinite, telocollinite and pseudovitrinite, together with a histogram of semifusinite reflectance, for a coal sample containing abundant pseudovitrinite material. The pseudovitrinite clearly represents a separate population in reflectance terms from either the telocollinite on the one hand or the semifusinite on the other. Many coal samples from the Gunnedah Basin studied for the present project contain more than 10% of material with the features of pseudovitrinite, and a few coals contain more than 20%. Since the coals of this area tend to have relatively low overall vitrinite percentages (especially in the upper Black Jack Group), the incorporation of data from this material, if not recognized and measured separately, can significantly affect routinely-determined mean vitrinite reflectance values. In areas where it is recognized, pseudovitrinite is generally thought to have formed by primary oxidation, drying or desiccation during peat formation (Benedict et al., 1968). Newman and Newman (1982) therefore suggest that the presence of pseudovitrinite indicates relatively dry conditions in the early stages of peat accumulation. Kaegi (1985) however, has suggested that slitted material resembling pseudovitrinite can also be produced by a post-coalification oxidation process, as air or oxygen-bearing water passes through relatively permeable coal beds. Koch (1970) on the other hand, suggests that the difference in reflectance can be partly attributed to differences in the vegetable matter of the parent peat deposit. Detailed investigations into the chemical character and origin of pseudovitrinite in these coals are currently in progress. Preliminary data from an electron microprobe study of different macerals in two Gunnedah Basin coal samples, following the techniques outlined by Mastalerz and Bustin (1993) are given in Table 4. Although the overall oxygen values are slightly high for a coal of this rank, the data clearly show that the material identified as pseudovitrinite has an affinity with the other vitrinite macerals, and not with the semifusinite of these particular coal samples. Despite its higher reflectance, it is distinctly different from the inertinite macerals such as semifusinite in the same coal

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135

Table 4 Average chemical composition of vitrinite and semifusinite Basin coal samples, as determined by electron microprobe DM Texas DDH 1,272.9

Semifusinite Pseudovitrinite Telocollinite Desmocollinite

m

components analysis

(wt%, dry, ash-free) in two Gunnedah

DM Nombi DDH 1,468.3

m

carbon

oxygen

sulphur

carbon

oxygen

sulphur

83.89 80.30 80.09 80.49

11.31 14.50 15.69 14.79

0.22 0.43 0.41 0.39

80.46 76.50 17.21 77.41

1I .96

0.45 0.95 0.86 0.85

15.11 14.93 13.96

samples. The name was originally applied by Benedict et al. (1968) to represent the different behavior of the material on coking, but in a sense also implies that the material is not strictly vitrinite at all. A revision of the name may be therefore appropriate, but this is beyond the scope of the present discussion. From the point of view of the present study, the reflectance value of the pseudovitrinite material, if added to that of more normal (slit-free and lower-reflecting) telocollinite, can provide a misleading indication of the coal’s rank level in maturation studies. Allowance should be made in identifying rank changes with depth from vitrinite reflectance profiles in samples where such a component is present.

6. Overall trends in the Gunnedah Basin A generalized profile showing the vertical pattern of vitrinite reflectance and air-dried moisture through the Permian succession of the Gunnedah Basin is given in Fig. 12. Although the actual gradient varies across the basin, vitrinite reflectance shows an overall increase with depth from the top of the Black Jack Group to the base, where present, of the Leard Formation. There is, however, a marked departure from this trend in the middle part of the Permian section, with anomalously low values for R,,,, (up to 0.2% below the upper Black Jack to Leard regression line) in the lower Black Jack (Upper Watermark - Lower Black Jack delta systems) and uppermost Maules Creek (Leard - Maules Creek alluvial lacustrine system) successions. The reflectance-depth regression line is shifted to the left (lower reflectance values) in these parts of the sequence. The lower reflectance values are not, moreover, a localized feature; they can be recognized through entire sections of strata and at the same horizons over wide areas of the basin. Air-dried moisture, on the other hand, decreases from around 8% to less than 2% over the same depth interval (Fig. 12). Unlike vitrinite reflectance, it does not depart from this trend in the marine-influenced parts of the coal-bearing sequence. Air-dried moisture is not in itself a highly reproducible parameter in analytical terms, due mainly to variations in the air-drying process, but the trend in moisture content suggests that, when large numbers of samples are evaluated from within the same program (3,000 samples with air-dried moisture data were included in the present study), air-dried moisture may provide a more consistent basis for detailed rank evaluation where marine

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Depositional Systems

Journal of Coal Geology 36 (1998) 1 I1 -140

1

Characteristics

,

low vitrinite content normal or elevated values for R,, suppression mainly due to high liptinite (rare) moisture (ad) decreases with depth reflectance increases with depth from about 0.55 to above 0.8 % R,, normal reflectance-depth profile some elevated values due to pseudovitrinite

Upper Black Jack alluvialllacustrine system

Hoskissons peat swamp system Western bed-load fluvial system Arkarula shallow-marine system

??

Upper Watermark-lower Black Jack delta systems

??

Porcupine-lower Watermark marine shelf system

??

no coal seams high hydrogen content lower reflectance due to marine influence high sulphur and nitrogen contents moisture decreases with depth reflectance-depth profile displaced to low-rank side

no coal seams lower reflectance in dispersed organic matter

lower (usually at the top) or normal R,, . high vitrinite content . air-dried moisture decreases with depth 9 elevated R,, due to pseudovitrinite . mostly normal reflectancedepth profile ??

Leard - Maules Creek alluvial/lacustrtne system

7

Fig. 12. Generalized vertical profiles of mean maximum vitrinite (telocollinite) reflectance and air-dried moisture for the Gunnedah Basin Permian succession. Key characteristics of the different depositional systems are also tabulated.

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137

influence is involved. Similar results have been observed by Newman and Newman (1982) in a study of the Pike River coalfield in New Zealand. Dulhunty et al. (1950) used air-dried moisture as a rank indicator in an early study of the Sydney Basin, New South Wales, and Damberger (1971) used ash-free hygroscopic moisture to indicate coal rank in the Illinois Basin of the USA. More recently, Rajarathnam et al. (1996) have used moisture data as a rank indicator for an Indian coal deposit. 6.1. Reflectance

anomalies

as environmental

indicators

The horizons at which reflectance values below the overall trend occur are in both cases (upper Maules Creek and lower Black Jack successions) at horizons where the coal-bearing interval is overlain by marine strata (Porcupine and Arkarula Formations respectively). The coals in these intervals also have other distinctive features, such as a perhydrous composition and an increased fluorescence intensity, that make them distinctive in petrographic studies. Maximum vitrinite reflectance in the seams of Gunnedah Basin is up to 0.2% lower than expected from regional trends. This is believed to be due to a marine influence on the depositional environment rather than to an abundance of liptinite material. The occurrence of such coals in the succession, although hindering what might be an otherwise simple rank and maturation study, have an alternative use as an environmental indicator. In the present study they occur beneath horizons caused by marine transgressions, and hence underlie flooding surfaces of basin-wide significance. Correlation and use of such anomalous intervals may also be of value in other basins, especially where more direct evidence of marine incursion is lacking, to assist in sequence-stratigraphy interpretations. 6.2. Significance

in rank and burial history studies

Where marine-influenced coals are present in the lower part of the sequence and pseudovitrinite-bearing coals in the upper part, as is the case with the Black Jack Group, the resulting reflectance profile, if not carefully analyzed, may give the appearance of an abnormally low rank increase with depth, with resulting errors in interpreting geothermal gradients or burial history. A decrease in rank with depth may even appear to be present, as in parts of the Gunnedah Basin, where low base-line reflectance gradients are involved. The vitrinite in some of the coals from the upper Black Jack sequence, and to a lesser extent from coals in the Leard and lower Maules Creek Formations, includes a significant proportion of material with the petrographic features of (so-called) pseudovitrinite. Although this material has a higher reflectance than the more normal telocollinite in the same coals, it would be expected to be counted along with that of other telocollinite varieties if Standard procedures (e.g. Standards Australia, 1986; ICCP, 1995) are followed. If present in sufficient proportions it may therefore give rise to abnormally high overall R v max values for any samples in which it occurs. Such samples would appear as points somewhat to the right of the regression line in an R, profile, such as is noted for several samples from the Maules Creek Formation

138

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and the upper part of the Black Jack Group. Although it represents a departure from accepted practice, omission of the pseudovitrinite material from consideration in the reflectance measuring process has been found for the Gunnedah Basin to eliminate these anomalies. In the absence of such selectivity, however, care must be taken in interpreting the resulting reflectance profile, at least in high-volatile bituminous coals, if detailed rank trends are required.

Acknowledgements Thanks are expressed to the New South Wales Department of Mineral Resources and to Pacific Power for assistance in the provision of samples and other information used in the project. The personal assistance of Carl Weber, Jeff Beckett, Julie Moloney, Vie Tadros, Adrian Hutton and Harold Read, among others, is gratefully acknowledged. Thanks are also expressed to Alan Cook, Alan Davis and Maria Mastalerz for constructive comment on the paper. The study was funded in part under the Small Grants Scheme of the Australian Research Council.

References Beckett, .I., Hamilton, D.S., Weber, C.R., 1983. Permian and Triassic stratigraphy and sedimentation in the Gunnedal-Narrabri-Coonabarabran Region. Geol. Surv. New South Wales, Q. Notes 51, 1-16. Benedict, L.G., Thompson, R.R., Shigo, J.J., Aikman, R.P., 1968. Pseudovitrinite in Appalachian coking coals. Fuel 47, 125-143. Bennett, A.J.R., 1968. The reflectance and coking behavior of vitrinite-semifusinite transition material. Fuel 47, 51-62. Bostick, N.H., Foster, J.N., 1975. Comparison of vitrinite reflectance in coal seams and in kerogen of sandstones, shales and limestones in the same part of sedimentary section, In: B. Alpem (Ed.), Colloque International, Petrographie de la Matiere Organique des Sediments, Paris, pp. 13-25. Brown, H.R., Cook, A.C., Taylor, G.H., 1964. Variations in the properties of vitrinite in isometamorphic coal. Fuel 43, 111-124. Cook, A.C., Taylor, G.H., 1963. The petrography of some Triassic Ipswich coals. Australasian Inst. Mining Metall. Proc. 205, 35-55. Damberger, H.H., 1971. Coalification patterns of the Illinois Basin. Econ. Geol. 66, 488-494. Diessel, C.F.K., 1988. A vertical sedimentological profile through the Upper Carboniferous coal measures of the Ruhr Basin, Germany, Proceedings of 22nd Symposium on Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 17-25. Diessel, C.F.K., 1990. Marine influence on coal seams, Proceedings of 24th Symposium on Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 33-40. Diessel, C.F.K., 1992a. The problem of syn- versus post-depositional marine influence on coal composition. Proceedings of 26th Symposium on Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 154-163. Diessel, C.F.K., 1992b. Coal-bearing Depositional Systems, Springer Verlag, Berlin, 721 pp. Dulhunty, J.A., Hinder, N., Penrose, R., 1950. Rank variation in the central eastern coalfields of New South Wales, J. Proc. R. Sot. New South Wales 82, 99-106. Falcon, R.M.S., Snyman, C.P., 1986. An introduction to coal petrography: atlas of petrographic constituents in the bituminous coals of southern Africa, Geol. Sot. S. Afr., Review Paper 2.

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Gentzis, T., Goodarzi, F., 1994. Reflectance suppression in some Cretaceous coals from Alberta, Canada, In: Mukhopadhyay, P.K., Dow, W.G. (Eds.), Vitrinite Reflectance as a Maturity Parameter: applications and limitations, vol. 570, American Chemical Society Symposium Series, pp. 93-l 10. Gentzis, T., Goodarzi, F., Snowdon, L., 1993. Variation of maturity indicators (optical and Rock-Eval) with respect to organic matter type and matrix lithology: an example from Melville Island, Canadian Arctic Archipelago. Marine Pet. Geol. 10, 507-513. Goodarzi, F., Gentzis, T., Feinstein, S., Snowdon, L., 1988. Effect of maceral subtypes and mineral matrix on measured reflectance of subbituminous coals and dispersed organic matter. Int. J. Coal Geol. 10, 383-398. Goodarzi, F., Gentzis, T., Snowdon, L.R., Bustin, R.M., Feinstein, S., Labonte, M., 1993. Effect of mineral matrix and seam thickness on reflectance of vitrinite in high to low volatile bituminous coals: an enigma. Marine Pet. Geol. 10, 162-171. Gurba, L.W., Ward, CR., 1995. Coal rank variation in the Gunnedah Basin, In: Boyd, R.L., Mackenzie, G.A. (Eds.), Proceedings of the 29th Symposium, Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 180-187. Gurba, L.W., Ward, C.R., 1996. Reflectance anomalies in Permian coals of the Gunnedah Basin implications for maturation studies, In: Boyd, R.L., Mackenzie, G.A. (Eds.), Proceedings of the 30th Symposium, Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 69-76. Hamilton, D.S., 1985. Deltaic depositional systems, coal distribution and quality, and petroleum potential. Permian Gunnedah Basin, New South Wales, Australia. Sediment. Geol. 45, 35-75. Hamilton, D.S., 1991. Genetic stratigraphy of the Gunnedah Basin, New South Wales. Aust. J. Earth Sci. 38. 95-l 13. Hamilton, D.S., Beckett, J., 1984. Permian depositional systems in the Gunnedah region. Geological Survey of New South Wales. Q. Notes 5.5, 1-19. Hamilton, D.S., Beckett, J., Weber, C.R., 1991. Gunnedah Basin, In: Harrington, H.J. (Ed.), Permian Coals of Eastern Australia, Bureau of Mineral Resources Bulletin, Bull. 231, pp. 43-58. Hutton, A.C., Cook, A.C., 1980. Influence of alginite on the reflectance of vitrinite from Joadja. New South Wales, and some other coals and oil shales containing alginite. Fuel 59, 71 l-714. International Committee for Coal and Organic Petrology, 1995. Vitrinite Classification, ICCP System, 1994. International Committee for Coal and Organic Petrology, Aachen, 24 pp. Jian, F.X., Ward, C.R., 1993. Triassic depositional episode, In: Tadros, N.Z. (Ed.), The Gunnedah Basin. New South Wales, Geological Survey of New South Wales, Memoir Geology 12, pp. 297-326. Jones, J.M., Murchison, D.G., Saleh, S.A., 1971. Variation of vitrinite reflectivity in relation to lithology, In: Gartner, H.W., Wehner, H. (Eds.), Advances in Organic Geochemistry. Pergamon Press, New York. pp. 601-612. Kaegi, D.D., 1985. On the identification and origin of pseudovitrinite. Int. J. Coal Geol. 4, 309-319. Kalkreuth, W.D., 1982. Rank and petrographic composition of selected Jurassic - Lower Cretaceous coals of British Columbia, Canada. Bull. Can. Pet. Geol. 30 (21, 112-139. Kalkreuth, W., McMechan, M., 1984. Regional pattern of thermal maturation as determined from coal-rank studies, Rocky Mountain Foothills and Front Ranges north of Grande Cache, Alberta - implications for petroleum exploration. Bull. Can. Pet. Geol. 32 (3). 249-271. Koch, J., 1970. Haufigkeitsverteilung von Vitrinitreflexionswerten und reflexionsmassig unterscheidbare Vitrinite. Erdol Kohle 1, 2-6. Kravits, C.M.. Crelling, J.C., 1981. Effects of overbank deposition on the quality and maceral composition of the Herrin (No. 6) coal (Pennsylvanian) of southern Illinois, Int. J. Coal Geol. 1, 195-212. Mallett. C.W., Pattison, C., McLennan, T., Balfe, P., O’Sullivan. D., 1995. Bowen Basin, In: Ward, CR.. Harrington, H.J., Mallett, C.W., Beeston, J.W. (Eds.), vol. I, Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group, Special Publication 1. pp. 299-339. Martin, D.J., 1993. Igneous intrusions, In: Tadros, N.Z. (Ed.), vol. 12, The Gunnedah Basin, New South Wales. Geological Survey of New South Wales, Memoir Geology 12, pp. 349-366. Mastalerz, M., Bustin, R.M., 1993. Variation in elemental composition of macerals: an example of the application of the electron microprobe to coal studies. Int. J. Coal Geol. 22, 83-99. Mastalerz, M., Wilks, K.R., Bustin, R.M., 1993. Variation in vitrinite chemistry as a function of associated liptinite content; a microprobe and Fl-IR investigation. Org. Geochem. 20 (51, 555-562.

140

L. W. Gurba, C.R. Ward /International

Journal of Coal Geology 36 (1998) 11 I-140

Mukhopadhyay, P.K., Dow, W.G. (Eds.), 1994. Vitrinite Reflectance as a Maturity Parameter: applications and limitations, vol. 570, American Chemical Society Symposium Series, 294 pp. Newman, J., Newman, N.A., 1982. Reflectance anomalies in Pike River coals: evidence of variability in vitrinite type, with implications for maturation studies and Suggate rank. New Zealand J. Geol. Geophys. 25, 233-243. Pearson, J., Murchison, D.G., 1990. Influence of sandstone washout on the properties of an underlying coal seam. Fuel 69, 251-253. Price, L.C., Barker, C.E., 1985. Suppression of vitrinite reflectance in amorphous rich kerogen - a major unrecognised problem. J. Pet. Geol. 8 (I), 59-84. Rajarathnam, S., Chandra, D., Handique, G.K., 1996. An overview of chemical properties of marine-influenced Oligocene coal from the northeastern part of the Assam-A&an basin, India. Int. J. Coal Geol. 29 (4), 337-361. Rathbone, R.F., Davis, A., 1993. The effects of depositional environment on vitrinite fluorescence intensity. Org. Geochem. 20, 177-186. Raymond, A.C., Murchison, D.G., 1991. Influence of exinitic mace& on the reflectance of vitrinite in Carboniferous sediments of the Midland Valley of Scotland. Fuel 70, 155- 161. Snowdon, L.R., Brooks, P.W., Goodarzi, F., 1986. Chemical and petrological properties of some liptinite-rich coals from British Columbia. Fuel 65, 460-472. Stach, E., Mackowsky, M.Th., Teichmiiller, M., Taylor, G.H., Chandra, D., Teichmliller, R., 1982. Stach’s Textbook of Coal Petrology, Gebriider Bomtrager, Berlin, 53513. Standards Australia, 1986. Coal - maceral analysis, Australian Standard 2856, 24~. Stevenson, D., 1991. The extent and timing of marine influence on the Greta and Pelton seams in the Ellalong-Pelton-Quabolong area, New South Wales, BSc. (Honours) Thesis, University of Newcastle, New South Wales, unpublished. Tadros, N.Z. (Ed.), 1993. The Gunnedah Basin, New South Wales, Geological Survey of New South Wales, Memoir Geology 12, 649~. Tadros, N.Z., 1995a. Sydney-Gunnedah Basin overview, In: Ward, C.R., Harrington, H.J., Mallett, C.W., Beeston, J.W. (Eds.), Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group, Special Publication 1, pp. 163-175. Tadros, N.Z., 1995b. Gunnedah Basin, In: Ward, C.R., Harrington, H.J., Mallett, C.W., Beeston, J.W. (Eds.), Geology of Australian Coal Basins, Geological Society of Australia Coal Geology Group, Special Publication 1, pp. 247-298. Teichmbller, M., 1987. Recent advances in coalification studies and their application to geology, In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata - recent advances, Geological Society Special Publication 32, pp. 127-169. Thompson, S., 1986. Early Permian sedimentation and provenance studies in the Gunnedah Basin, New South Wales, Proceedings 20th Symposium on Advances in the Study of the Sydney Basin, Department of Geology, University of Newcastle, New South Wales, pp. 15-18. Williams, E.G., Keith, M.L., 1963. Relationship between sulphur in coals and occurrence of marine roof beds. Econ. Geol. 58, 720-729.