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

International Journal of Coal Geology 51 (2002) 145 – 167 www.elsevier.com/locate/ijcoalgeo

Thermal maturation pattern in the southern Bowen, northern Gunnedah and Surat Basins, northern New South Wales, Australia Rushdy Othman*, Colin R. Ward School of Geology, University of New South Wales, Sydney, NSW 2052, Australia Received 22 January 2001; received in revised form 18 December 2001; accepted 18 December 2001

Abstract Comprehensive maximum vitrinite (telocollinite) reflectance data have been obtained for more than 260 polished sections from 28 petroleum exploration wells in the southern Bowen and northern Gunnedah Basins, and the overlying Surat Basin, in an area from north of Boggabri to the New South Wales – Queensland border. The samples studied were coal and dispersed organic matter (DOM) taken from ditch cuttings and drill cores of the Permian, Triassic and Jurassic sedimentary successions. Vitrinite reflectance profiles show that the coals and DOM in the Early Permian Back Creek Group of the southern Bowen Basin have anomalously low (suppressed) reflectance values and a higher reflectance gradient (rate of increase in vitrinite reflectance with depth) relative to the overlying Late Permian, Triassic and Jurassic sequences. Suppression of vitrinite reflectance is recorded in the Early Permian Maules Creek and Goonbri Formations in the northernmost part of the Gunnedah Basin, relative to the overlying Triassic and Jurassic sequences. Dispersed organic matter in the marine sediments of the Watermark and Porcupine Formations of the Gunnedah Basin also shows suppression of vitrinite reflectance. The vitrinite in several wells of the study area, however, displays anomalously high reflectance in parts of the sequence due to the influence of igneous intrusions. After allowing for heat effects due to intrusions and anomalies due to marine influence, the reflectance gradient over equivalent intervals in the southern Bowen Basin is higher towards the west, where the Permian sequence pinches out and the Triassic sequence overlies the basement. Vitrinite reflectance also increases at a higher rate with depth close to the Gil Gil Ridge in the east and close to the Moree High in the south. This may reflect additional heat flow associated with the basement high features. The areas between the Gil Gil Ridge and the Goondiwindi Thrust in the east, and to the west of the Gil Gil Ridge, appear to have lower reflectance gradients, and equivalent or lower reflectance values despite the greater burial depths. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal maturation; Vitrinite reflectance; Coal; Dispersed organic matter; Bowen Basin; Gunnedah Basin; Surat Basin; Permian; Mesozoic; Australia

1. Introduction The reflectance of vitrinite in coal and dispersed organic matter increases during thermal maturation due *

Corresponding author. E-mail address: [email protected] (R. Othman).

to complex, irreversible aromatisation reactions (Peters and Cassa, 1994). In a vertical sequence of strata, the level of maturation is expected to increase steadily with depth according to Hilt’s Law (Stach et al., 1982; Taylor et al., 1998) and, hence, vitrinite reflectance should increase with depth in the strata encountered in an exploration well. However, although thermal matur-

0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 2 ) 0 0 0 8 2 - 4

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ity is the main control on vitrinite reflectance, it has also been shown that depositional factors may affect the reflectance value. Many workers (e.g. Hutton and Cook, 1980; Price and Barker, 1985; Gurba and Ward, 1998) have recognised anomalously low vitrinite reflectance, sometimes described as reflectance suppression, where lower than expected reflectance values are developed in the profile through a stratigraphic sequence. Anomalously low vitrinite reflectance in coal or dispersed organic matter may be caused by a number of factors, including the depositional environment and the type of organic matter involved. In particular, the influence of marine conditions on the organic matter may result in anomalously low vitrinite reflectance values. An abundance of associated liptinite macerals can also give rise to vitrinite reflectance suppression (Stach et al., 1982; Thomas, 1982; Mukhopadhyay and Dow, 1994). Vitrinite reflectance profiles have numerous important applications in both the coal and petroleum industries. In addition to the evaluation of coal rank or organic matter maturation, vitrinite reflectance is an important tool in thermal history studies and kinetic modelling of hydrocarbon generation. Depositional environments and other factors that may influence the reflectance values, however, need to be identified and taken into account as part of such studies. On the other hand, anomalies in vitrinite reflectance due to depositional factors (e.g. marine influence) can also be used as stratigraphic markers, to assist basin analysis and correlation studies (Gurba and Ward, 1998; Othman and Ward, 1999). The present study represents an attempt to investigate the three-dimensional pattern of thermal maturity in two superimposed sedimentary basins, the Permo-Triassic Bowen and Gunnedah Basins and the Jurassic –Cretaceous Surat Basin, in an area of growing significance for petroleum exploration in the northern part of New South Wales. It is based mainly on evaluation of vitrinite reflectance trends in vertical profiles against depth, and among other aspects seeks to use any anomalies in the reflectance trends to assist in stratigraphic correlation and understanding the basins’ depositional history. Since only limited intervals of cored strata are available in the area, the reflectance data have been based mainly on material recovered from well cuttings, and on dispersed organic matter as well as coal seams. The data obtained from

the vertical profiles, after allowance for any anomalies due to depositional or igneous effects, were then used to evaluate vertical and lateral trends in thermal maturity across the basin area, and to relate those trends to the tectonic development of the basins. An ultimate objective of the study was to provide an improved basis for petroleum exploration in the region.

2. Geological setting The Gunnedah and Bowen Basins represent the central and northern parts of the Permo-Triassic Sydney – Bowen Basin system of eastern Australia (Fig. 1). They are overlain in part by the Jurassic –Cretaceous Surat Basin sequence. The Gunnedah and Bowen Basins formed as a rift and succeeding foreland basin system adjacent to the Permo-Triassic orogen of the New England Fold Belt (Tadros, 1993, 1995; Mallett et al., 1995). They constitute a

Fig. 1. Location and major structural elements in the Gunnedah and southern Bowen Basins in northern New South Wales (partly after Tadros, 1995).

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single depositional basin system with a prominent structural subdivision. The Bowen Basin comprises two major depocentres, the Denison Trough in the northwest and the Taroom Trough in the east. The Taroom Trough is the main part of the basin beneath the Surat Basin succession, and extends southwards from Queensland into northeastern New South Wales. The trough is an elongated, north – south trending half graben, plunging steadily to the north. The Taroom Trough represents an asymmetric depocentre, with the deepest part located in Queensland at a latitude of approximately 25jS (O’Brien, 1984). Exon (1976) and Totterdell et al. (1995) have estimated 10,000 m of sediment in this area. Totterdell and Krassay (1995), however, indicate that the equivalent succession in the Bowen Basin of northern New South Wales is thinner and less complete. The Gil Gil Ridge (Fig. 1; also shown in Fig. 1 of Etheridge, 1987) divides the Bowen Basin in New South Wales into two mainly elongated north south trending structural units. The ridge probably continues, over the Moree High, to the Boggabri Ridge in the adjoining Gunnedah Basin to the south. The present eastern margin of the Bowen Basin in New South Wales is the Moonie– Goondiwindi Thrust, and the Permo-Triassic succession thickens towards this feature. The Permian and Triassic sequence wedges out to the west, where the successive units overlap the basement. An angular unconformity is recognised between the Bowen Basin and the overlying Surat Basin succession (Elliott, 1993). The Gunnedah Basin is a structural trough with an elongated appearance, trending north – northwest in northeastern New South Wales. It is separated from the Bowen Basin by the Moree High. Tadros (1988, 1993, 1995) has divided the Gunnedah Basin to three subbasins, the Gilgandra, Mullaley and Maules Creek Subbasins, separated by the Rocky Glen and Boggabri Ridges. The northern part of the Mullaley Subbasin, which includes the Bellata and Bohena Troughs (Fig. 1) was examined as part of the present study. Russell and Middleton (1981), Gurba (1998) and Gurba and Ward (1998, 1999) have described variations in vitrinite reflectance in the Gunnedah Basin. The present study extends this work for a distance of some 200 km to the north, linking the Gunnedah Basin evaluated in the previous work with the Bowen Basin in Queensland. It also embraces part of the

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Surat Basin succession, overlying both basins, which has not yet been extensively studied in New South Wales with respect to vitrinite reflectance.

3. Depositional history 3.1. Bowen Basin A total of up to 4000 m of Permian strata are preserved in the southern Taroom Trough in the Queensland sector of the Bowen Basin (Thomas et al., 1982; Hawkins et al., 1992). The Permian sequence consists of terrestrial and shallow marine, largely clastic sediments, along with substantial deposits of bituminous coal (Mallett et al., 1995). During the Early Permian, up to 3000 m of volcaniclastic sediment was deposited, mainly under marine conditions but with a brief episode of deltaic sedimentation, to form the Back Creek Group. A major regression during the Late Permian reversed the marine conditions, however, and resulted in peat swamp sediments that formed the Blackwater Group on an extensive coastal plain (Hawkins et al., 1992). The Permian sequence in the New South Wales portion of the basin (Fig. 2) consists of the Kuttung Volcanics, the Back Creek Group and the Kianga Formation (Othman and Ward, 1999). The Back Creek Group and its equivalents have been divided into several formations in the northern Bowen Basin, but not in the NSW portion in the south. The sequence is equivalent in age to the Watermark and Porcupine Formations and the marine-influenced lower part of the Black Jack Group in the adjacent Gunnedah Basin (Morton et al., 1993). The Kianga Formation, an equivalent to the Late Permian Baralaba Coal Measures of the Blackwater Group in southern Queensland (Beeston and Green, 1995; Mallett et al., 1995), is almost completely confined to the eastern and northern parts of the study area. The Permian coal-forming environments in the Bowen Basin gave way in the Early Triassic to deposition of the Rewan Group, a coal-barren terrestrial red-bed sequence. During the early Middle Triassic, uplift of the craton to the west, and possibly also of the orogen to the east, brought large quantities of quartzose sediment into the basin. The Clematis Group was deposited during this episode in large,

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Fig. 2. Representative stratigraphic section for the southern Bowen and lower part of the overlying Surat Basin sequences, northern New South Wales (modified from Othman and Ward, 1999).

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poorly confined alluvial channels (Fielding et al., 1990). The Showgrounds Sandstone, a quartzose interval that forms a significant petroleum reservoir unit in the southern Bowen Basin, is probably a correlative of the upper Clematis Group (Hawkins et al., 1992). Fluvial deposition was terminated by a lacustrine transgression, probably from the Gunnedah Basin in the south (Jian and Ward, 1993, 1996). This affected the southern and western parts of the Bowen Basin, and resulted in accumulation of the fine-grained lacustrine sediments of the Snake Creek Mudstone Member within the lower Moolayember Formation. The Middle Triassic Moolayember Formation reflects a return to a dominantly fluvial environment across the basin, and represents the youngest preserved remnant of the Bowen Basin sequence (Fielding et al., 1990). The Rewan Group is not present in the New South Wales portion of the basin, except in the northernmost part close to the Queensland –NSW border (Shaw, 1995). The Showgrounds Sandstone and the Moolayember Formation, including the Snake Creek Mudstone Member, represent the Triassic sequence in the New South Wales portion of the basin (Othman and Ward, 1999). 3.2. Gunnedah Basin The Permian sequence in the Gunnedah Basin consists of the Goonbri, Maules Creek, Porcupine and Watermark Formations, and the overlying Black Jack Group (Tadros, 1993, 1995). The Triassic sequence includes the Digby, Napperby and Deriah Formations (Jian and Ward, 1993, 1996). A summary of the sequence is given in Fig. 3. The Permian sequence is characterised by three major constructive depositional episodes, each consisting of fluvial and deltaic successions, separated by two significant marine transgressive events (Hamilton and Beckett, 1984). The basin fill in the Early Permian was localised in small rapidly subsiding troughs, separated by ridges, and in some cases highlands, consisting of silicic and mafic volcanics. Fine-grained lacustrine sediments, represented by the Goonbri Formation (Thomson, 1986), accumulated in the most rapidly subsiding parts of the troughs; fluvial sedimentation ultimately filled the lakes, and covered the

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trough areas with thick sequences of sandstone and conglomerate, along with coal seams, to form the Maules Creek Formation (Tadros, 1993). Regional subsidence and widespread marine incursion resulted in deposition of the Porcupine – Lower Watermark Marine Shelf system (Hamilton, 1991; Tadros, 1993), with the maximum extent of the Late Permian marine transgression into the Gunnedah Basin being represented by deposition of the lower Watermark Formation. Marine shelf conditions ended when a large delta system prograded southwesterly across the basin. Prodelta and delta front facies of this system are generally assigned to the upper part of the Watermark Formation. Regression continued, and fluvial-dominated deltas deposited the lower part of the Black Jack Group (Hamilton et al., 1988). Renewed thrusting in the New England Fold Belt induced basin subsidence, and subsequent infilling from the east and northeast by volcano-lithic detritus in a shallow marine setting deposited the Arkarula Formation (Hamilton, 1993). The shallow marine transgression associated with the Arkarula Formation was accompanied by deposition, along the western margin, of bed-load fluvial sediments of the Brigalow Formation, sourced from the Lachlan Fold Belt in the west (Hamilton, 1985). Late Permian plutonism and associated felsic volcanism in the New England Fold Belt coincided with rapid subsidence of the foreland basin, and a return to terrestrial sedimentation that terminated marine conditions and established basin wide swamps in which peat accumulated. This process formed the Hoskissons Coal Member in the lower Black Jack Group (Tadros, 1995). A rejuvenation of sediment supply in the New England Fold Belt then deposited a thick sequence of coarse volcano-lithic and tuffaceous sediments with intercalated coals in a major alluvial system to form the upper part of the Black Jack Group (Hamilton et al., 1988). In the Early Triassic, renewed tectonism in the New England Fold Belt, followed by uplift of the craton in the west, resulted in widespread deposition of coarse alluvial clastics in the Gunnedah Basin. Deposition in this episode formed the Digby Formation (Jian and Ward, 1993), which is separated by an angular unconformity from the underlying Permian Black Jack Group (Tadros, 1986, 1995). Renewed basin subsidence in the Middle Triassic resulted in

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Fig. 3. Stratigraphy of the northern Gunnedah Basin and the overlying Surat Basin, northern New South Wales (modified from McMinn, 1993; Tadros, 1995).

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deposition of the Napperby Formation, containing well-developed upward-coarsening sequences derived mainly from the New England Fold Belt that represent several prograding lacustrine delta systems (Jian and Ward, 1993). The upper part of the Triassic sequence, referred to as the Deriah Formation (Jian and Ward, 1993; Tadros, 1995), consists of an irregularly interbedded fluvial sandstone and siltstone succession. 3.3. Surat Basin Exon (1974) has recognised five major depositional cycles in the Jurassic of the Surat Basin, each

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representing successive deposition in braided streams, meandering streams, and finally swamps, lakes and deltas. This phase of sedimentation was followed by an Early Cretaceous transgression, that produced mainly shallow marine deposits (Hawkins et al., 1992). The first cycle produced the Lower Jurassic Precipice Sandstone and the overlying Evergreen Formation (Fig. 2). The Precipice Sandstone is mainly composed of medium to coarse grained braided stream sandstone, introduced from the northwest over a peneplained basal Jurassic unconformity surface. The Evergreen Formation consists of interbedded

Fig. 4. Borehole locations in the study area (source of data New South Wales Department of Mineral Resources).

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shale, siltstone and sandstone with minor coal beds, considered to have been deposited in a complex pattern of near-shore lacustrine and deltaic environments (Shaw, 1995). This succession was followed by a second fining upward cycle, represented by the Early to Middle Jurassic Hutton Sandstone and the overlying Middle Jurassic Walloon Coal Measures. The Hutton Sandstone consists of medium to coarsegrained sandstone deposited in an essentially fluvial environment (Hawke and Burke, 1984). The Walloon Coal Measures consist of carbonaceous shale, siltstone, sandstone and coal, deposited in fluvio-lacustrine to lower delta plain conditions (Shaw, 1995). In the area south of the Moree High, the Jurassic Purlawaugh Formation (Fig. 3) represents fluvial and lacustrine sediments that are chronostratigraphically equivalent to the upper Evergreen Formation and lower Hutton Sandstone of the area to the north. The Purlawaugh Formation, however, is younger in the Bellata area, where the upper Purlawaugh shales are time-equivalent to the Walloon Coal Measures succession (Morgan, 1984; Hamilton et al., 1988).

4. Experimental procedure Coal, and shale and siltstone containing coaly material were sampled where available from the Permian, Triassic and Jurassic sequences encountered in coal and petroleum exploration wells throughout the study area (Fig. 4). Samples were taken from cores wherever possible, but in the absence of cores at suitable horizons samples were based on well cuttings. Fully cored boreholes were only available at two locations, the Bellata-1 and Coonarah-1 (and 1A) wells. Cores taken from other wells were mainly intended to evaluate hydrocarbon reservoir sequences; the materials cored usually contained little if any coaly material, and were of limited use for vitrinite reflectance studies. The intervals sampled were selected from depths at which the presence of the relevant lithologies was indicated by geophysical logs. Care was taken to minimise inclusion of contaminants from overlying strata due to borehole caving. Vitrinite and other coal particles were hand-picked from the cuttings over

Fig. 5. Histogram plots of reflectance distribution for two selected samples in Limebon-1 showing two populations: one from the target interval and one from lower-rank caved debris.

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intervals where the well logs showed coal seams to be present. Otherwise, shale and siltstone intervals indicated by geophysical logs were sampled to evaluate the contained dispersed organic matter. Polished sections were prepared from selected coal and other rock samples for petrographic study. The sections were examined in reflected light using a Ziess Axioskop system with oil-immersion objectives. Measurements were made of the percentage of the incident light reflected from the vitrinite (telocollinite) particles in the samples using a wavelength of 546 nm. Mean maximum vitrinite reflectance determinations (Appendix A) were carried out, based on at least 30 individual measurements for each sample studied. 4.1. Contamination effects due to caving Contamination of cuttings samples may arise due to caving of overlying strata around the drill hole. The process results in mixing of cuttings from shallower intervals with cuttings from the deeper intervals indicated by the drilling depth. The vitrinite reflectance measured on such samples may, therefore, represent a combination of both the value for the deeper (target) samples and the reflectance of any vitrinite in the caved debris and, thus, does not always embrace material only from the interval intended. Depth profiles involving caved materials would be expected to show abnormally low mean vitrinite reflectance values. The presence of low-reflecting vitrinite due to caving from beds higher in the sequence could be identified from reflectance histograms for some of the individual samples involved. The two samples in Fig. 5, for example, show vitrinite with a reflectance of 0.45 –0.49% occurring as a separate population from the remaining vitrinite in each case (Rv max = 0.55– 0.59% and 0.65 –0.74%, respectively). Removal of the low-reflecting caved material from consideration shows a regular increase in reflectance with depth (Fig. 6) for the uncontaminated vitrinite component. Hand picking of vitrinite particles from selected intervals indicated by wireline log data as containing coal seams or shaley materials helped to reduce problems with such contamination in the present study. The contrast in reflectance between the (lower-reflecting) vitrinite from shallow depths and the material from the target horizon, identified from

Fig. 6. Vitrinite reflectance profile for contamination due to caving (open circles) and target intervals (dark circles) for Limebon-1. W = Walloon Coal Measures; H = Hutton Sandstone; E = Evergreen Fm.; P = Precipice Sandstone; M = Moolayember Fm.; S = Showgrounds Sandstone; K = Kianga Fm.; BC = Back Creek Group.

individual reflectance histograms (e.g. Fig. 5), also helped to separate any contaminating caved particles from the remaining organic matter where the target interval and the caved debris have similar lithologies. Low reflectance values identified as representing caved materials were omitted from consideration in evaluating the results from samples where such contamination was identified by reflectance histogram data. 4.2. Oxidation with sample storage Oxidation effects were noted in some vitrinite particles within the cuttings as a result of exposure during storage. The polished surfaces of oxidised vitrinite particles had a noticeably higher reflectance than the remainder or, in some cases, the outlines of the particles were visibly brighter than the central parts. Care was taken to avoid such visibly oxidised vitrinite particles in the reflectance measuring process.

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Fig. 7. Effect of oxidised sample (open circle) on reflectance gradient in Pearl-1. Reflectance gradient in A = 0.047% per 100 m and in B = 0.054% per 100 m. KV = Kuttung Volcanics. See Fig. 6 for identification of other units.

Measurement of reflectance in oxidised vitrinite particles without recognising the oxidation effects would give anomalously high reflectance values relative to those obtained from equivalent fresh material. As indicated in Fig. 7, inclusion of data from oxidised vitrinite may significantly affect the regression line representing reflectance increase with depth. Recognition of anomalously high vitrinite reflectance values in depth profiles, in cases where the high reflectance could not be attributed to the presence of ‘‘pseudovitrinite’’ (Gurba and Ward, 1998) or to igneous intrusions (see below), provided another basis for identifying oxidised materials. Where oxidation could be seen to have occurred with storage, the relevant data were eliminated from consideration in rank profiles and regional studies.

5. Reflectance profiles and reflectance anomalies Reflectance of coal macerals has long been used to evaluate coal rank (Tissot and Welte, 1984). Vitrinite is the maceral most often used for this purpose

because its optical properties alter more uniformly during rank advance than do those of the other maceral groups (Dow, 1977). Due to contrasts in thermal conductivity, vitrinite particles in coal generally have a slightly higher reflectance than vitrinite phytoclasts buried under equivalent conditions dispersed in associated mineral-rich noncoal lithologies (Heraux et al., 1979). Vitrinite reflectance may also increase with the thickness of the host coal bed (Heraux et al., 1979). In the present study slightly lower reflectance values were recorded in a few DOM samples than from the adjacent coal samples. The overall effect on the resulting profiles, however, does not seem to have been significant, compared to other sources of anomalous reflectance values. Mukhopadhyay (1994) has discussed in detail the reasons for anomalously low vitrinite reflectance, in comparison to more normal values in overlying or underlying intervals. The causes of such anomalies may include marine influence on the vitrinite during or shortly after deposition, as well as the presence of abundant liptinite macerals in close association with

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The reflectance – depth profile for Bellata-1, in the Bellata Trough of the northern Gunnedah Basin, shows a steady increase with depth through the Jurassic and the upper part of the Triassic strata. Although interrupted by locally elevated values due to igneous intrusions near the base of the Triassic, the profile shows a lower than expected reflectance in the immediately underlying Early Permian Maules Creek and Goonbri Formations (Fig. 8). The Late Permian Black Jack Group, as well as the Watermark and most of the Porcupine Formations (Fig. 3), have been removed from the sequence in this area, due to an Early Triassic contraction event. The sedimentary sequence in the adjoining part of the Bohena Trough is also significantly affected by local heat flow from igneous intrusions, and similar suppression of vitrinite reflectance within the Permian sequence cannot be identified in some of the borehole profiles (Figs. 9 and 10). As indicated by Gurba and Ward (1998), suppression of vitrinite reflectance in the Permian Maules Fig. 8. Vitrinite reflectance profile for Bellata-1. Samples at 829.60, 871.55 and 1104.16 m are highly affected by local igneous intrusions. Samples below 900 m (except for heat-affected sample) have suppressed reflectance. Pi = Pilliga Sandstone; Pu = Purlawaugh Fm.; N = Napperby Fm.; D = Digby Fm.; Po = Porcupine Fm.; MC = Maules Creek Fm.; G = Goonbri Fm.; B = Basement; Major igneous intrusions ( ).

the vitrinite component. Anomalously high reflectance values may arise due to additional heat flow from igneous intrusions, or due to the presence of highly reflecting and commonly slitted ‘‘pseudovitrinite’’ particles (Gurba and Ward, 1998). 5.1. Vitrinite reflectance suppression Anomalously low vitrinite reflectance values due to marine influence have been described in coal seams of the Gunnedah Basin south of the study area by Gurba and Ward (1998). These occur in the upper part of the Early Permian Maules Creek Formation, and in the lower part of the Late Permian Black Jack Group. The same phenomenon has also been documented in the marine-influenced Early Permian Greta seam of the Sydney Basin (George et al., 1994).

Fig. 9. Reflectance profile for Wilga Park-1. High reflectance values due to igneous intrusions occur throughout the section, even though only one actual intrusion is encountered at 621.18 m. W/P = Watermark/Porcupine Fm. See Fig. 8 for identification of other units.

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No significant decrease in vitrinite reflectance is noticed between the Permian and the overlying Triassic sequence, and the strata overlying the coal measures display a reflectance profile continuous with that of the Kianga Formation. This indicates that there was no major development of coalification in the Permian sequence during the interval represented by the angular unconformity between the Permian and Triassic successions. The Triassic sequence includes several coarsening upwards deltaic successions, interpreted by Jian and Ward (1993, 1996) as representing freshwater lake-fill fan-delta deposits rather than marine delta sequences. The absence of reflectance suppression in the Triassic sediments suggests that no marine influence occurred on this part of the basin fill, and further confirms the essentially terrestrial nature of the environment into which the fan-deltas prograded during Napperby deposition. 5.2. Igneous intrusion effects Fig. 10. Reflectance profile for Coonarah-1A. High reflectance values occur throughout the section due to igneous intrusions. BJ = Black Jack Group; Wa = Watermark Fm. See Fig. 8 for identification of other units.

Creek Formation and the lower part of the Black Jack Group in the Gunnedah Basin is mostly due to marine influence. In the Goonbri Formation, however, it is generally attributed to the liptinite rich organic matter content (Othman and Ward, 1999). Similar suppression of vitrinite reflectance due to associated high liptinite percentages is well documented by various workers, including Hutton and Cook (1980), Gentzis and Goodarzi (1994) and Goodarzi et al. (1994). Anomalously low (suppressed) vitrinite reflectance is also observed in dispersed vitrinite from the Permian Back Creek Group of the New South Wales Bowen Basin sequence. Reflectance profiles through boreholes that penetrate this sequence show steady increases through Jurassic, Triassic, and where present the uppermost Permian Kianga Formation, but decreases in the Permian Back Creek Group (Fig. 11). It is well documented that the Back Creek Group was deposited under predominantly marine conditions.

Contact metamorphism from igneous intrusions, where present, may affect the organic matter maturity in the surrounding strata (e.g. Dow, 1977; Beeston, 1978). Studies in the Gunnedah Basin (Gurba, 1998; Gurba and Weber, 2001) have shown that intrusive effects on vitrinite reflectance may extend over a maximum vertical distance of about twice the thickness of the intrusive body. Due to preferential movement of hydrothermal solutions with convection and pressure, the effects also often extend further above the intrusion than below (Fig. 12). The actual extent of the metamorphic aureole depends on the temperature difference between the intrusion and the invaded rock, the depth at which the intrusion was emplaced, and the rate of cooling. The maturation profile in the vicinity of the intrusion is not a straight line because the thermal gradient that caused it was continuously changing (Dow, 1977). Some of the boreholes in the study area have penetrated igneous intrusions, especially wells in the southern Bowen Basin close to the Moree High. Further south, in the northern Gunnedah Basin, igneous intrusions were also encountered in both the Bohena and Bellata Troughs. The intersected thicknesses of the individual intrusions are variable, ranging up to a maximum of 192 m for an intrusion

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Fig. 11. Reflectance profiles in McIntyre-1 and Goondiwindi-1. Low values can be seen in the Back Creek Group in both sections. The reflectance gradient of the upper and lower sequences in Goondiwindi-1 are 0.045% and 0.059% per 100 m and in McIntyre-1 are 0.028% and 0.054% per 100 m, respectively. BC = Back Creek Group, KV = Kuttung Volcanics; Major volcanics ( ). See Fig. 6 for other units.

in the Back Creek Group penetrated by the Lantern-1 well. Two types of vitrinite reflectance profiles affected by igneous intrusions can be identified from the samples studied. In the simplest case, with discrete, relatively thin intrusive bodies, the elevated reflectance values above and below the intrusion can be readily distinguished from the general downward increase in vitrinite reflectance with depth (Fig. 12). Shifts in the reflectance profile due to marine influence can still be seen, despite the intrusions, in many such successions. In other cases, despite there being only small intruded intervals, the entire sedimentary sequence in some boreholes appears to reflect a high local heat flow (e.g. Figs. 9 and 10), obliterating the rank trends due to burial as well as any reflectance suppression effects. The high heat flows associated with igneous intrusions in the area may give rise to local development of hydrocarbon generation. Natural gas was discovered in 1985, for example, in Wilga Park-1. The gas

appears to have been generated a result of local heat effects in conjunction with the more usual reflectance – depth profile. Vitrinite reflectance in the samples studied from Wilga Park-1 (Fig. 9) ranges between 1.97% and 5.51%. As indicated above, the lower part of the Triassic Napperby Formation in Bellata-1 is also affected by igneous intrusions (Fig. 8). Geochemical studies by Othman et al. (2001) have shown that oil stains in the overlying Jurassic Pilliga Sandstone were generated from relatively high-temperature metamorphism of the organic matter in this lower Triassic succession.

6. Reflectance gradient Present-day geothermal gradients in the boreholes studied, based on bottom hole temperatures, range from 1.84 jC per 100 m in McIntyre-1 to 5.51 jC per 100 m in Wilga Park-1. The reflectance gradient for the nonsuppressed sequences in the boreholes studied,

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Where the Permian sequence is present, the different boreholes may also show different reflectance gradients. The profile in McIntyre-1 (Fig. 11) shows a reflectance gradient of 0.028% per 100 m in the upper part of the sequence, above the interval with suppressed vitrinite reflectance values (i.e. above a depth of 2206 m). The same but shallower section in Goondiwindi-1, however, shows a reflectance gradient of 0.045% per 100 m. The gradient in the section with anomalously low (suppressed) reflectance has a higher gradient in both wells at a little over 0.05% per 100 m. Fig. 14 shows the reflectance gradient, expressed as the increase in vitrinite reflectance per 100 m depth, for the wells between the Moree High and the New South Wales – Queensland border. The data represent

Table 1 Reflectance gradient (excluding lower suppressed sequence) and present-day geothermal gradient in the boreholes studied Fig. 12. Localised increase in vitrinite reflectance around intrusive body in Mt Pleasant-1. Despite this effect, the Back Creek Group (BC) clearly has a lower reflectance profile than the upper part of the sequence. Major igneous intrusion ( ). See Fig. 6 for other units.

based on vitrinite reflectance profiles excluding data from any localised heat-affected samples, ranges between 0.021% per 100 m in Limebon-1 and 0.763% per 100 m in Coonarah-1A (Table 1). If allowance is made for any intrusive effects, the reflectance –depth profiles and reflectance gradients in the different sections studied provide useful information on the overall temperature history of the Permian and Mesozoic succession. Fig. 13 shows that the sedimentary section in Glencoe-1 has a lower reflectance gradient than the section in Pearl-1, despite the fact that the relevant sediments in Glencoe-1 are currently more deeply buried. The reflectance gradients in these two boreholes are also lower than the gradient in Werrina-1 to the west. The Permian sequence is thin or absent in these and other nearby boreholes. The Triassic sequence itself is absent from Werrina-1, and in that borehole the Jurassic sediments directly overlie the basement rocks (Fig. 13).

No. Borehole name

TD (m)

BHT (jC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1704.13 1128 1650.18 1595.29 882.39 2135.73 650.00 2237.00 1600.80 1604.77 1958.03 2222.60 1538.43 2076.90 2010.15 2535.63 1147.73 1221.33

73.33 3.13 60.00 57.78 47.78 65.56 37.00 73.00 60.00 62.22 62.22 68.33 60.00 60.00 66.67 66.67 48.89 55.56

2.42 2.37 3.15 2.13 2.62 2.37 2.50 2.63 2.16 2.17 2.60 1.93 2.32 1.84 2.52 2.91

2266.18 811.37 1575.81 1670.30 824.78 1519.45 1566.74 795.83

66.67 44.50 58.33 60.00 45.00 54.44 56.67 63.89

2.06 3.02 2.43 2.39 3.03 2.27 2.34 5.51

Barb-1 Bellata-1 Bohena-1 Boomi-1 Camurra-1 Chester-1 Coonarah-1A Edendale-1 Garah-1 Gil Gil-1 Glencoe-1 Goondiwindi-1 Kinnimo-1 Lantern-1 Limebon-1 McIntyre-1 Moree-1 Moree-2 Moree-3 Mt Pleasant-1 Nyora-1 Pearl-1 Quack-1 Wee Waa-1 Werrina-1 Werrina-2 Wilga Park-1

Geothermal Reflectance gradient gradient (jC/100 m) (%/100 m)

TD = total depth; BHT = bottom hole temperature.

0.030 0.149 0.032 0.033 0.763 0.037 0.022 0.055 0.027 0.045 0.023 0.023 0.021 0.028 0.025 0.080 0.110 0.024 0.047 0.049 0.087 0.055

R. Othman, C.R. Ward / International Journal of Coal Geology 51 (2002) 145–167

159

Fig. 13. Variation in reflectance gradient for selected boreholes through Triassic and Jurassic sequences in the northwestern part of the study area. R = Rewan Group. See Figs. 6, 7 and 8 for other rock unit abbreviations.

the gradients after elimination of anomalies due to contamination and oxidation of well cuttings, and to the effects of any localised igneous intrusions. Reflectance gradients in the Permian Back Creek Group, where the vitrinite reflectance is anomalously low due to marine influence, are shown separately as the lower figure against the respective boreholes in the diagram. The gradients in the Early Permian marine interval are often somewhat higher than in the overlying Kianga Formation (where present) and Mesozoic sequence. The Back Creek Group is not present in the west, and the overlying strata rest directly on the basement materials. Fig. 14 also shows that the reflectance gradient in the New South Wales portion of the Bowen Basin,

especially in the upper Permian and Mesozoic succession, increases generally towards the basin margin to the west. This is apparently related to depth to basement, as the increase takes place in conjunction with a thinning sedimentary succession. The Permian sequence pinches out to the west and the overlying sequences thin and progressively overlap the basement (Fig. 13). The reflectance gradient also increases close to the Goondiwindi Thrust in the east, over the Gil Gil Ridge in the centre of the area, and towards the Moree High in the south (Fig. 14). A similarly high reflectance gradient has been reported on the margins and ridges of the Bowen Basin portion further to the north in Queensland (Beeston, 1981).

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Fig. 14. Lateral variation in reflectance gradient for the upper (normal) and lower (suppressed) parts of the sequence in the southern Bowen Basin.

It is well documented that organic matter maturation is a result of time and temperature (e.g. Tissot and Welte, 1984; Taylor et al., 1998). The time factor may be eliminated, however, if comparison is based on the same geological sequence in different areas in the basin. The variation in organic maturation at such horizons can possibly be attributed to differences in burial depth or to variations in heat flow. Higher heat flow might be expected with ascending magmas or hydrothermal fluids associated with igneous processes, or with increasing proximity to radiogenic heat sources (e.g. potassium-bearing granites or acid volcanics) in the basement materials. The present-day depth of the Permian section in McIntyre-1 is greater than the present-day depth of the same section in Goondiwindi-1. However, the maximum vitrinite reflectance for the same geological unit and the reflectance gradients for both suppressed and nonsuppressed sequences are lower in McIntyre-1 than in Goondiwindi-1 (Fig. 11). The presence of

similar levels of organic maturation at shallower depths in Goondiwindi-1 may, therefore, be attributed to a local heat-flow effect, particularly as the well is close to the Goondiwindi Thrust. More normal heat flows in McIntyre-1 produced similar reflectances at greater burial depths. Such a conclusion is in a good agreement with the previous results of Raza et al. (1995) based on apatite fission track analysis (AFTA), which suggested that the maximum temperature achieved in McIntyre-1 is a reflection of the burial depth. The higher reflectance gradient in the Back Creek Group could also in part be an expression of the different chemical composition of the vitrinite deposited under marine conditions. Newman (1997) suggested that depositional and diagenetic influences on vitrinite compositions result in variable hydrogen content, with a consequent variability in vitrinite reflectance. Reflectance suppression effects disappear at higher maturation levels (Figs. 9 and 10), thus,

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implying an ultimate convergence of reflectance profiles for suppressed and nonsuppressed materials. The higher gradient in the lower part of the section, however, may also be due to higher heat flow in the graben stage of the basin’s tectonic development, and lower heat flow in the subsequent foreland phase.

7. Lateral reflectance trends Vitrinite reflectance was estimated from the individual borehole profiles for a horizon at the contact of the Moolayember Formation with the Showgrounds Sandstone. This contact was chosen as the datum for an isoreflectance map because it is widely distributed in the study area, no suppression of reflectance has been recorded above this level, and the contact is easy to recognize on wireline logs. The Showgrounds

161

Sandstone is also a unit with good petroleum reservoir characteristics. The reflectance at this level in the sequence (Fig. 15) increases towards the west. It also increases over the Gil Gil Ridge, close to the Goondiwindi Thrust in the east, and over the Moore High in the south. In addition, the values of the isoreflectance lines increase towards the north, over the Queensland –New South Wales border, where the Bowen and Surat Basins sequences are thicker. The presence of higher reflectance values at stratigraphically equivalent horizons in areas above the basement ridges may be due to continued uplift of the ridges after the main period of rank advance. Thinner stratigraphic sections over the ridges, combined with anticlinal folding in the basin-fill succession, suggest that the ridges were active as positive tectonic elements both during and after deposition. The presence of higher reflectance gradients on the

Fig. 15. Isoreflectance map for a horizon at the contact of the Moolayember Formation with the Showgrounds Sandstone in the southern Bowen Basin.

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ridges, discussed above, may also be an expression of higher heat flow from the shallower-buried basement materials. Due to extensive intrusion effects and limited numbers of boreholes, it was not possible to extend the isoreflectance study across the Moree High and into the northern part of the Gunnedah Basin, linking to similar work further to the south by Gurba (1998) and Gurba and Ward (1999). The Gunnedah Basin has had a similar tectonic history to the Bowen Basin. However, the total amount of tectonic subsidence in the Gunnedah Basin is lower than that in the Taroom Trough, the northern continuation of the Bowen Basin system in Queensland (Korsch et al., 1993). Although some high vitrinite reflectance values occur in the northern Gunnedah Basin, they are mainly a result of igneous intrusive activity in the area rather than greater burial depths.

8. Conclusions Although subject to difficulties with sample contamination due to caving, and also in the present study due to some oxidation of organic material with storage, reliable data on vitrinite reflectance variations with depth can be obtained from noncored wells, where only cuttings samples are available. Selection of material based on the lithologies indicated by down-hole geophysical logs at the target depth (e.g. hand-picking of vitrain from intervals where coal is indicated) can help to reduce problems associated with contamination due to caving. Plotting of reflectance histograms can also be used to identify and eliminate values due to contaminating organic matter from shallower borehole depths. High-reflecting oxidised vitrinite particles can be identified and avoided in the reflectance measurement process. Samples affected by oxidation that are not detected in this way, however, can be identified from their anomalous positions on the reflectance –depth profile of the borehole in question. Reflectance – depth profiles for the Permian Back Creek Group of the Bowen Basin show anomalously low reflectance values due to marine influence, relative to the overlying Kianga Formation and Mesozoic sequence. This can be used to aid stratigraphic correlation within the Bowen Basin sequence of northern

New South Wales. Continuity of the reflectance – depth profile above the Kianga Formation suggests that little significant removal of strata took place in this area, despite the presence of angular unconformities at the base of the Triassic and the base of the Surat Basin succession. Igneous intrusions have produced local intervals of anomalously high vitrinite reflectance in different parts of the Gunnedah –Bowen Basin sequence. The extent of these effects is not always related to the intersected thickness of intrusive material. Intrusive effects are particularly extensive in the Bellata area, where independent geochemical study (Othman et al., 2001) suggests that oil has been generated in response to the high heat flow. The reflectance gradient in the Early Permian Back Creek Group, after due allowance for igneous effects, is significantly higher than for the overlying Late Permian and Triassic succession. This may be due to a higher heat flow associated with graben tectonics in the early stages of basin development, relative to the foreland basin phase of its later tectonic history, or it may be due to differences in the vitrinite composition and the response of the suppressed vitrinite to rank advance, relative to the other vitrinite types. Reflectance gradients, especially in the upper part of the sequence, tend to be greater over the basement highs (Gil Gil Ridge; Goondiwindi Thrust), compared to the troughs that occupy the remainder of the basin. Reflectance gradients also increase from east to west. Both features may be due to higher radiogenic heat flow from the basement rocks, which would have been closer to the sediment succession in these areas during the basin’s burial history. The presence of high reflectance gradients across the basement ridges, combined with postdepositional uplift on the highs relative to the troughs, has resulted in strata at equivalent stratigraphic levels (e.g. Showgrounds – Moolayember contact) having higher vitrinite reflectance levels on the ridge features. This may be of significance in the generation of hydrocarbons.

Acknowledgements The study was completed under an Australian Postgraduate Award (Industry) provided by the

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163

Well: Goondiwindi-1 TD: 2222.60 m No. of samples: 19; core: 3; cuttings: 16

Appendix A. Vitrinite reflectance data for samples studied Well: Bellata-1 TD: 1128 m No. of samples: 22; core: 22; cuttings: 0

No.

No.

Formation

Type

Depth (m)

Rv max

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Pilliga Fm. Pilliga Fm. Purlawaugh Fm. Up. Napperby Fm. Up. Napperby Fm. L. Napperby Fm. L. Napperby Fm. L. Napperby Fm. L. Napperby Fm. L. Napperby Fm. Maules Creek Fm. Maules Creek Fm. Maules Creek Fm. Maules Creek Fm. Maules Creek Fm. Goonbri Fm. Goonbri Fm. Goonbri Fm. Goonbri Fm. Goonbri Fm. Goonbri Fm. Goonbri Fm.

Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core Core

566.73 585.75 613.60 642.40 665.77 781.80 805.18 813.89 829.60 871.55 939.93 959.20 985.70 987.55 1005.90 1019.22 1049.49 1054.74 1073.86 1104.16 1104.95 1112.68

0.42 0.52 0.55 0.60 0.63 0.84 0.99 1.01 2.21 2.43 0.57 0.66 0.67 0.69 0.75 0.65 0.66 0.67 0.73 2.16 0.61 0.61

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Formation

Type

Depth (m)

Rv max

Walloon Coal Measures Walloon Coal Measures Walloon Coal Measures Walloon Coal Measures Evergreen Fm. Evergreen Fm. Evergreen Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Kianga Fm. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp.

Cutting

996.70

0.54

Cutting

1045.46

0.54

Cutting

1112.52

0.60

Cutting

1158.24

0.59

Cutting Cutting Cutting Core Cutting Core Cutting Cutting Cutting Cutting Cutting Cutting Core Cutting Cutting

1264.92 1307.59 1356.36 1459.69 1530.10 1554.18 1584.96 1776.98 1886.71 1941.58 1987.30 2017.78 2119.30 2170.18 2209.80

0.60 0.67 0.72 0.76 0.74 0.77 0.75 0.58 0.61 0.61 0.63 0.70 0.73 0.77 0.80

Well: McIntyre-1 TD: 253.56 m No. of samples: 16; core: 0; cuttings: 16 No.

Well: Coonarah-1A TD: 650 m No. of samples: 14; core: 14; cuttings: 0

1

No.

Formation

Type

Depth (m)

Rv max

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Napperby Fm. Napperby Fm. Napperby Fm. Napperby Fm. Napperby Fm. Napperby Fm. Black Jack Gp. Black Jack Gp. Black Jack Gp. Black Jack Gp. Black Jack Gp. W/P Fm. W/P Fm. W/P Fm.

Core Core Core Core Core Core Core Core Core Core Core Core Core Core

393.35 421.96 438.51 442.23 449.18 453.35 512.36 522.60 550.98 572.09 604.74 613.24 635.35 646.54

0.53 0.56 0.79 0.88 0.83 0.87 2.74 1.80 1.69 1.99 1.91 1.72 1.92 1.87

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Formation

Type

Depth (m)

Rv max

Walloon Coal Measures Walloon Coal Measures Hutton Fm. Hutton Fm. Evergreen Fm. Evergreen Fm. Moolayember Fm. Moolayember Fm. Kianga Fm. Kianga Fm. Kianga Fm. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp.

Cutting

1450.85

0.57

Cutting

1548.38

0.60

Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting

1624.58 1722.12 1810.51 1901.95 1965.96 2087.88 2139.70 2151.89 2200.66 2225.04 2286.00 2313.43 2353.06 2441.45

0.62 0.65 0.70 0.65 0.67 0.74 0.76 0.77 0.72 0.65 0.67 0.68 0.73 0.75

164

R. Othman, C.R. Ward / International Journal of Coal Geology 51 (2002) 145–167 Well: Mt. Pleasant-1 TD: 2266.18 m No. of samples: 22; core: 0; cuttings: 22

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Formation

Type

Walloon Coal Measures Walloon Coal Measures Hutton Ss. Moolayember Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Snake Creek Mb. Kianga Fm. Kianga Fm. Kianga Fm. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp.

cutting

Well: Glencoe-1 TD: 1958.03 m No. of samples: 7; core: 0; cuttings: 7 Depth (m)

Rv max

No.

Formation

Type

Depth (m)

Rv max

871.73

0.51

1

Cutting

1377.70

0.59

cutting

914.40

0.54

2

Cutting

1469.14

0.64

cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting

957.07 978.41 1021.08 1066.80 1143.00 1188.72 1240.54 1310.64 1371.60 1423.42 1469.14 1511.81 1566.67 1609.34 1655.06 1737.36 1810.51 1871.47 1929.38 1987.30

0.55 0.60 0.64 2.06 1.36 0.64 0.61 0.61 0.63 0.65 0.68 0.68 0.62 0.64 0.65 0.67 0.68 0.70 0.73 1.34

3 4 5 6 7

Walloon Coal Measures Walloon Coal Measures Hutton Ss. Hutton Ss. Evergreen Fm. Moolayember Fm. Moolayember Fm.

Cutting Cutting Cutting Cutting Cutting

1502.66 1578.86 1661.16 1740.41 1783.08

0.67 0.78 0.77 0.69 0.70

Well: Wilga Park-1 TD: 795.83 m No. of samples: 6; core: 1; cuttings: 5 No.

Formation

Type

Depth (m)

Rv max

1 2 3

Black Jack Gp. Black Jack Gp. Watermark/ Porcupine Fm. Maules Creek Fm. Maules Creek Fm. Goonbri Fm.

Cutting Cutting Core

423.67 472.44 639.11

1.97 2.14 2.29

Cutting Cutting Cutting

691.90 716.28 737.62

2.17 4.31 5.52

4 5 6

Well: Pearl-1 TD: 1575.81 m No. of samples: 7; core: 0; cuttings: 7

Well: Limebon-1 TD: 2010.15 m No. of samples: 14; core: 0; cuttings: 14 No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

No.

Formation

Type

Depth (m)

Rv max

1

Walloon Coal Measures Walloon Coal Measures Hutton Ss. Hutton Ss. Moolayember Fm. Moolayember Fm. Snake Creek Mb.

Cutting

1249.02

0.53

Cutting

1307.616

0.55

Cutting Cutting Cutting Cutting Cutting

1369.296 1400.136 1458.732 1501.908 1557.42

0.64 0.62 0.62 0.66 0.66

2

Formation

Type

Depth (m)

Rv max

Walloon Coal Measures Walloon Coal Measures Hutton Ss. Evergreen Fm. Evergreen Fm. Moolayember Fm. Moolayember Fm. Moolayember Fm. Snake Creek Mb. Kianga Fm. Back Creek Gp. Back Creek Gp. Back Creek Gp. Back Creek Gp.

Cutting

1118.61

0.55

Cutting

1222.25

0.59

Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting Cutting

1283.21 1328.93 1374.65 1463.04 1557.53 1633.73 1679.45 1712.98 1798.32 1880.62 1941.58 2002.54

0.60 0.63 0.63 0.63 0.65 0.66 0.67 0.69 0.60 0.64 0.66 1.29

3 4 5 6 7

Well: Werrina-1 TD: 1519.45 m No. of samples: 4; core: 0; cuttings: 4 No.

Formation

Type

Depth (m)

Rv max

1

Walloon Coal Measures Walloon Coal Measures Hutton Ss. Evergreen Fm.

Cutting

1219.20

0.55

Cutting

1289.30

0.58

Cutting Cutting

1341.12 1380.74

0.63 0.69

2 3 4

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Australian Research Council and the New South Wales Department of Mineral Resources. Thanks are expressed to David Alder and Vic Tadros of the Department for considerable advice and assistance with the work. Thanks are also expressed to Lila Gurba of UNSW and Mohinudeen Faiz of CSIRO for advice on reflectance measurement and other aspects of the work program. Thanks also to Rad Flossman of UNSW for preparation of polished sections, to Chris Boreham for access to complementary data held by the Australian Geological Survey Organisation and to Jim Beeston and Vic Tadros for constructive comments on the manuscript.

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