Organic petrological composition of Triassic source rocks and their clastic depositional environments in some Australian sedimentary basins

International Journal of Coal Geology, 18 ( 1991 ) 165-186 Elsevier Science Publishers B.V., Amsterdam

165

Organic petrological composition of Triassic source rocks and their clastic depositional

environments in some Australian sedimentary basins

Michelle Smyth a and Mar ia Mastalerz b' aCSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde 2113, Australia

blnstitute of Geological Sciences University of Wroclaw C.vbulskiego 30, 50-205 14"roclaw, Poland.

(Received December 3, 1990; accepted in revised form April 25, 1991 )

ABSTRACT

Smyth, M. and Mastalerz, M., 1991. Organic petrological composition of Triassic source rocks and their clastic depositional environments in some Australian sedimentary basins. Int. J. Coal Geol., 18: 165-186.

Kerogen from terrestrial plant debris (type III) has commonly been considered to be a good source for hydrocarbon gas, but not for oil, compared with types I and II kerogen from marine and lacustrine sediments. The Gippsland Basin, Australia, contains giant oil fields producing from organic matter of land plant origin. Clearly some terrestrial paleodepositional environments have produced organic matter of land plant origin with the potential to generate large volumes of oil. An attempt has been made here to identify some environments that contain organic matter of terrestrial origin with the potential to generate oil.

The dispersed organic matter (DOM) in sediments from various paleodepositional environments in the Northern Carnarvon, Clarence-Moreton, Simpson Desert, Bowen and Gunnedah Basins of Australia has been analysed petrographically. To reduce variations in organic matter type due to differences in geological age, examples of Triassic age only have been compared. DOM with relatively high contents of liptinite, which is widely accepted as having a better potential to generate oil than vitrinite and inertinite, was found in the following environments: fluvio-deltaic (Bowen Basin ), proximal lacustrine (Gunnedah Basin) and fluvio-deltaic (Northern Carnarvon ).

Relationships between Triassic DOM types and paleodepositional environments found in one basin did not necessarily hold true for other basins. It is not valid to infer a unique paleodepositional environment from DOM type, but within a given basin, DOM type may be predicted from environment.

INTRODUCTION

Major oil and gas fields in Australia, such as those in the Gippsland and Cooper Basins, produce from coal measure sequences. According to Tissot

Present address: Department of Geological Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, B.C., V6T 2B4, Canada

0166-5162/90/$03.50 © 1990 Elsevier Science Publishers B.V. All rights reserved.

] 6 6 M. SMYTH 4NI) M MAS ['a~LERZ

and Welte (1984) the occurrence of gas versus oil provinces depends on the nature of the parent organic matter and/or the thermal history of the sediments. Kerogen from terrestrial plant debris (type III ) could be a good source rock for hydrocarbon gas, but generates comparatively little oil compared with the types I and II kerogen typical of marine and lacustrine sediments.

Nevertheless, the Gippsland Basin in Australia (Fig. 1 ) contains giant oil fields and the organic matter in the source rocks (Cretaceous to Tertiary) is predominantly of land plant origin and is vitrinite-rich (Smith and Cook, 1984). The dispersed organic matter (DOM) in Tertiary strata of the Niger delta, a large oil producer, is also composed mainly of vitrinite with minor liptinite (Bustin, 1988 ). In both these cases conventional wisdom would indicate that the strata have little oil-generating potential.

DOM in Australian terrestrial sediments consists predominantly of small fragments of plants, generally 5 to 50 #m in size, preserved as vitrinite and inertinite as well as liptinites. Sporinite, cutinite, suberinite, alginite and some resinites commonly have well preserved botanical features and are readily identified. Small fragments and amorphous masses of liptinite are classified as liptodetrinite. Bituminite is rare.

For this study, DOM has been analysed using polished whole rock samples, either solid pieces or granular, if from cuttings. Reflected light, both white light and blue light excitation fluorescence mode) have been used, with oil

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Fig. 1. Locations of Northern Carnarvon, Simpson Desert, Cooper, Gunnedah, Gippsland, Clarence-Moreton and Bowen Basins, Australia.

ORGANIC PETROLOGICAL COMPOSITION OF TRIASSIC SOURCE ROCKS 16 7

immersion, and quantitative results have been obtained by the point-count- ing technique.

There is a great diversity of terrestrial environments from high energy alluvial fans to lower energy delta plains and lacustrine depositional environments, in which kerogen, or DOM, and coal accumulate. The probability that coals are good source rocks for oil (Cook, 1986; Bertrand, 1989), is not dis- puted, but only DOM has been considered here for comparison of organic matter type in a number of basins and environments. Work on Permian sediments in Australian basins has shown that various paleodepositional environments have produced different types of coals and DOM (Hunt and Hob- day, 1984; Smyth, 1984; Hunt et al., 1986; Smyth, 1989 ). In the Cooper Basin the source of oil is plant remains from non-marine Permian coal measure sediments. In the Gunnedah Basin (Fig. 1 ) the maceral composition of DOM of both Permian and Triassic age has been related to depositional environment (Hamilton et al., 1988).

If it could be established that some terrestrial environments are more likely to produce organic matter that is relatively more "oil-prone" than others, then explorationists could preferentially target those areas. At least, such knowl- edge would provide an indication of whether to expect profilic oil, or not, in a sparsely explored area.

P R E V I O U S W O R K

The data available on the relationships between paleodepositional environments and the types of DOM they contain have been summarized in Smyth (1989). In Australia, relationships established in one sedimentary basin do not necessarily hold true for other basins. The bulk of the DOM studied was Permian and the maceral compositions typical of several environments are listed in Table 1. Each composition represents the center of the area enclosed by the highest density contour for the samples analysed. These points, re- ferred to as "centers of concentration" (Smyth, 1984 ), are used in preference to average compositions, which can lend undue weight to atypical samples.

All DOM compositions in Table 1 show high inertinite values (50-87%); those for the Cooper Basin (Fig. 1 ) are particularly high. At the basinal level, not the individual depositional environment level, significant differences in maceral content of the DOM are apparent. DOM in the Gunnedah Basin has a higher liptinite content than that in the Cooper Basin. As liptinite has been considered to be the best source material for oil, this suggests that, irrespective of maturity, Gunnedah Basin DOM has a better potential to generate oil than DOM in the Cooper Basin. [However, sediments in the Gunnedah Basin are only marginally mature to mature in the Lower Permian (Hamilton et al., 1988), whereas the Permian sequences in the Cooper Basin are mature to overmature ].

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Maccral Compos i t ions of Centers of (7oncentration of Permian and Triassic DOM m the ('o()pcr and G u n n c d a h Basins, Austral ia

Basin Deposi t ional env i ronmen t Compos i t ion (V-L-I) Age

Cooper

G u n n e d a h

Upper coastal plain 17- 6-77 Permian Lower coastal plain 10-10-80 "" Restr icted sea or large lake 3-11-86 "" Overbank 12- 1-87 "" Coal swamps 13- 4-83 " "

Channe l deposi ts 13- 9-78 +' Channe l belt 20-15-65 " "

Shallow mar ine shelf 7-28-65 " '

Distal lake bo t tom 14-20-66 "' Marginal lake bo t tom 30-20-50 " Proximal lake bo t tom 19-39-42 Triassic Fluvial 42-36-22 " '

Fluvio-deltaic 22-30-48 "

V = vitrinite, L =l ip t in i te , I = inert ini te

Triassic DOM in the Gunnedah Basin from fluvial, fluvio-deltaic and proximal lake bottom paleoenvironments had concentration centers with much higher liptinite contents than the Permian DOM (Table 1 ). The Triassic sediments appear to have a better potential to generate oil than the Permian ones on the basis of type (maturity being excluded from this part of the assessment). Within the Triassic, the proximal lake bottom environment appears to have the best potential.

P E T R O G R A P H Y O F T R I A S S I C D O M IN A U S T R A L I A N BASINS

To reduce variations in organic matter type due to differences in geological age, DOM in sediments of Triassic age has been compared.

Maceral compositions of Triassic DOM from the Northern Carnarvon, Clarence-Moreton, Simpson Desert, Bowen and Gunnedah Basins, are plotted in Figs. 2 to 6, respectively.

These suites of DOM samples all formed parts of other studies; detailed data can be found in Cook et al. (1985); Smyth (1991); Smyth and Saxby ( 1981 ); Smyth ( 1985a,b,c, 1986, 1988 ); Smyth and Russell ( 1986); and Hamilton et al. (1988).

In the Northern Carnarvon Basin (Fig. 2 ) the Triassic DOM composition covers a wide range, from 0 to 95% vitrinite, 0 to 70% liptinite and 4 to 90% inertinite (Table 2 ). DOM compositions in the other basins are less variable,

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TABLE 2

Maceral Compositions and Average Volumes of Triassic DOM in Five Australian Basins

Basin Formations Range of Centers of N umber of Average compositions concentration samples vol. DOM(%) f % ) ( V-L-I ) ( range )

Northern Mungaroo \:: 0-95 Carnarvon l_ 0-70 54-16-30 148 2.8 ( 1-8 )

1 4-90 Clarence- (a) Woogaroo Subgroup V: 0-81 (a)3.2 t 1-9) Moreton (b) lpswich Coal L: 0-50 50-20-30 47 (b)l .7 (I-4)

Measures i: 0-95 Simpson Peera Peera V: 0-44 Desert Walkandi L: 0-50 13-10-77 31 3 (1-8)

1: 25-90 Bowen (a) Moolayember V: 0-62 30-53-17 (a) 3 (1-5)

L: 11-76 (b) Snake Creek I: 0-70 20-30-50 (b) 3 (1-5)

Gunnedah (a) Deriah v: 0-72 (a) 6.0 (2-10) (b) Napperby l_: 0-70 20-27-53 114 (b) 4.0 (1-14) (c) Digby I: 1-76 (c) 4.8 (1- 8)

V = vitrinite, L = liptinite, I = inertinite

compositions being most restricted in the Simpson Desert Basin (Table 2, Fig. 4 ). These differences in variability may be due to the numbers of samples used: 148 and 31, respectively (Table 2).

Density contours, which are an estimate of the trivariate probability density and join areas where data points are equally dense, have been drawn for the Triassic DOM data points for each basin. Bounding lines and centers of concentration derived from the density contours are shown in Fig. 7. Maceral compositions of the centers of concentration (centers of hatched areas in Fig. 7 ) are given in Table 2.

The DOM falls into four categories as follows: (a) the vitrinite-rich North- ern Carnarvon and Clarence-Moreton Basins DOM (Figs. 7a,b); (b) the very inertinite-rich Simpson Desert Basin DOM (Fig. 7c); (c) the less inertinite- rich Bowen and Gunnedah Basins DOM (Fig. 7d,e); and (d) the liptinite- rich Bowen Basin DOM (Fig. 7d).

The above DOM compositions were plotted in Figs. 2 to 6 irrespective of clastic depositional environments and precise age, though all are Triassic. At this basinal level, differences in composition are apparent, as was found for Permian DOM, and may represent variations in the tectonic settings of the basins in which the sediments were deposited. For example, the Northern Carnarvon Basin was a Late Paleozoic-Early Mesozoic intracratonic down- warp (Barber, 1982 ); the Bowen and Gunnedah Basins were foreland basins

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along the eastern margin of the continent (Hamilton et al., 1988; Elliott, 1989). The occurrence of two centers of concentration for the Bowen Basin is no doubt due to smaller scale effects of varying paleodepositional environments (Fig. 7d).

PETROGRAPHY OF TRIASSIC DOM WITH RESPECT TO CLASTIC DEPOSITIONAL

ENVIRONMENTS

In the Bowen Basin two Triassic environments are represented and each is clearly defined (Fig. 8 ). These environments are fluvio-deltaic (Moolayem- ber Formation ) and lacustrine (Snake Creek Formation ) (Golin and Smyth, 1986). The high density contours for these two paleoenvironments overlap very little. (Low density contours do overlap. ) The centers of concentration are given in Table 3. The fluvio-deltaic DOM is liptinite-rich and the lacustrine DOM is inertinite-rich.

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Fluvbo deltaic : outer line encloses 14/18 points = 78%

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ORGANIC PETROLOGICAL COMPOSITION OF TRIASSIC SOURCE ROCKS 177

TABLE 3

Maceral Compositions for Centers of Concentration of Triassic DOM Based on Clastic Depositional Environments in the Bowen and Gunnedah Basins

Basin Palaeodepositional Center of concentration No. of Average vol. environment (V-L-I) samples D O M (%)

(%) ( range)

Bowen Fluvio-deltaic 30-53-17 * 18 3 ( 1-5 ) (below Surat) Lacustrine 20-30-50 38 3 (1-5)

Gunnedah Lacustrine 30-37-33 * 26 3.6 (1-14) Proximal lacustrine 18-40-42 * 5 4.4 (4.-5) Distal lacustr ine 10-29-61 + 13 3.9 ( 1-6 ) Fluvial n.s.c. Fluvio-deltaic n.s.c. Delta front n.s.c. Prodelta 15-27-58 + 13 3.4 (2-5) Interdistributary bay 30-25-45 13 4.3 (2-8)

+ Both very low water energy *Theoretically the better source rocks for oil V = vitrinite, L = liptinite, I = inertinite n.s.c. = not sufficiently concentrated

These results suggest that in passing from the highly energetic fluvio-deltaic conditions towards a low-energy lacustrine regime, the proportion of inertin- ire increases at the expense of vitrinite and also, to some extent, liptinite. Inertinite, because of its already oxidized and fragmentary nature, can be transported further without alteration than other macerals and can remain in suspension in deeper, less turbulent environments. Liptinite is known to be readily deposited in overbank zones (Bustin, 1988; Hamilton et al., 1988). However, some liptinite, because of its low density, can reach lacustrine zones, being deposited in their proximal parts. This could explain the presence of minor amounts of liptinite-rich DOM within the lacustrine environments of the Bowen Basin (Fig. 8 ).

The DOM in the Gunnedah Basin has been deposited in 16 lacustrine clastic depositional environments, as interpreted by three sedimentologists (Jian, 1987; Hamilton et al., 1988; see also Acknowledgements). Despite discrep- ancies and overlap in the terms used to describe the paleoenvironments, they are reproduced here strictly from the literature.

With the density contouring method used here to characterize DOM type, clustering of points is more important than the number of points plotted. Those environments for which density contouring of maceral analyses shows some concentration of DOM compositions are: lacustrine, proximal lacustrine and distal lacustrine (Fig. 9); and fluvial, fluvio-deltaic, delta front,

I 7'~ ,kl S M Y T I t -~.NI) M M~SI,~,I.I:RZ

prodelta and interdistributary bay (Fig. 10). The compositional centers for the above groupings are given in Table 3.

Assuming that the water energy regime is a principal factor controlling the sedimentation of organic matter in clastic depositional environments, some of the above subenvironments can be grouped together: prodelta and distal lacustrine environments are characterized by very low water energy. A proximal lacustrine environment is similar to a delta front one with respect to the water-energy regime and the interdistributary bay belongs to the fluvio-deltaic environment. "Fluvio-deltaic" is a very broad term and therefore a whole range of organic matter compositions could be expected. Nevertheless, this environment and the fluvial one represent the highest water energy.

The compositions of DOM in the environments grouped as above are presented in Fig. 11. The composition of DOM in the Gunnedah Basin is similar to that in the Bowen Basin: moving from the highest-energy environment

V i t r i n i t e

Lacustr ine : outer line enc loses 20/26 points = 77%

Proximal lacustr ine : outer line enc loses 4/5 points = 8 0 %

Distal lacustr ine : outer line enc loses 11/13 points = 8 5 % ,

5 0 /q- f- -- - - \ -~50

/ / \ \ X

( \ " ' ~ I I " \ \ X ,'~-~---. __ \ \ \

Liptinite 50 Inertinite

Fig. 9. Density contours and centers of concentration for maceral compositions of Triassic DOM from lacustrine, proximal lacustrine and distal lacustrine paleodepositional environments, Gunnedah Basin.


_ _ Fluvial : outer line enc loses 3/6 points - 50%

Fluvio-del ta ic : outer line enc loses 5/8 points = 6 3 %

_ _ _ _ Del ta front : outer line enc loses 5/8 points = 63%

Prodel ta : outer line enc loses 11/13 points = 8 5 %

. . . . . Interdistr ibutary bay : oute I line enc loses / 11/13 points = 8 5 %

V i t r i n i t e

L i p t i n i t e 5 0 I n e r t i n i t e

Fig. 10. Density contours and centers of concentration for maceral compositions of Triassic DOM from fluvial, fluvio-deltaic, delta front, prodelta and interdistributary bay paleodepositional environments, Gunnedah Basin.

(fluvial) to the lowest (prodelta and distal lacustrine), the amount ofinertin- ite increases. In the environments presented in Fig. I l there is not much difference in the amount of liptinite in particular environments. The lacustrine environment (Fig. 9) has not been taken into account in Fig. I l, where environments have been grouped with respect to water-energy regime. "Lacus- trine environment" is also a very broad term and, if particular zones are not distinguished, a wide range of DOM compositions can be expected. This is shown, experimentally, to be the case in both the Bowen and Gunnedah Bas- ins (Figs. 8 and 9 ).

The composit ion of DOM in the lacustrine environment of the Bowen and Gunnedah Basins shows a great similarity (Figs. 8 and 9). However, a considerable difference exists in DOM composition from the fluvio-deltaic environments; in the Bowen Basin there is much more liptinite and much less inertinite than in the Gunnedah Basin (Figs. 8 and 11 ). The vitrinite content

1 8 0 M. S M Y T t t ,',.ND M. MAS] ,kLERZ

Vitrinite

Fluvlo.del ta lc + mterdistr ibutary Day

Delta front + Proximal lacustr ine

Prodet la , delta lacustrine

. . . . . . 1 50

°°.~°° " "° °

": ... "-~_.t .. "

'"...,2 ' : J ........ ':

kiptinite 50 Inertinite

Fig. 11. Density contours and centers of concentration for maceral compositions of Triassic DOM grouped with respect to water energy regimes - - high energy (fluvial) to low energy (prodelta) and distal lacustrine - - Gunnedah Basin.

is comparable in both basins, especially when interdistributary bay is grouped with fluvio-deltaic (Fig. 11 ).

PETROGRAPHY OF TRIASSIC DOM WITH RESPECT TO AGE IN NORTHERN

CARNARVON BASIN

The non-marine Triassic sediments in the Northern Carnarvon Basin have been described generally as fluvio-deltaic, with some alluvial plain sediments of Rhaetian-Norian age (Cook et al., 1985). DOM compositions for this basin have been plotted on the basis of age m Carnian, Norian and Rhaetian-No- rian - - and density contours are shown in Fig. 12. The centers of concentration are listed in Table 4.

There is some difference in the composition of the DOM with respect to age, the Norian DOM having the highest liptinite content, for example. How-


Vitrinite

Rhaetian-Norian : outer line encloses 43/52 points = 83%

Lesser centre of concentration (alluvial plain)

Norian : outer line encloses 35/38 points = 9 2 % t

Carnian : outer line encloses / 39/43 points = 91% f

I

.. . f "%..

' I " -

, -

\ ~ "'" ~.. .~.~

50

flu v io- delta ic

~ . . . ~ t s ~ . . . . . . . vial plain

(.. \.. i \ • \... \

v ~... ,," \ Liptinite 50 Inerlinite

Fig. 12. Density contours and centers of concentration for maceral compositions of Triassic DOM, grouped with respect to age and fluvio-deltaic and alluvial plain paleodepositional environments, Northern Carnarvon Basin.

TABLE4

Maceral Compositions for Centers of Concentration of Triassic DOM Based on Age in the Northern Carnarvon Basin

Age Depositional Center of concentration No. of environment (V-L-I) samples

( % )

Rhaetian-Norian Fluvio-deltaic 60-15-73 52 Alluvial plain 10-17-73

Norian Fluvio-deltaic 41-29-30 38

Carnian Fluvio-deltaic 48-20-32 43


I g 2 M. SMYTH AND M. MASTALERZ

ever, the outstanding difference in DOM types is in the Rhaetian-Norian, where the fluvio-deltaic DOM is vitrinite-rich and alluvial plain DOM is inertinite-rich. That is, the fluvio-deltaic DOM for the three ages is similar, but for the Rhaetian-Norian samples paleodepositional environment appears to have had influence on DOM compositions.

PETROGRAPHY OF TRIASSIC DOM: GENERAL DISCUSSION

In overall type, DOM in the Clarence-Moreton Basin is most similar to that from the Northern Carnarvon Basin. Triassic DOM from two formations in the Clarence-Moreton Basin (Fig. 13 ) is similar to the fluvio-deltaic Carnian and Rhaetian-Norian DOM of the Northern Carnarvon Basin (Fig. 12, Ta- ble 5). This may be evidence that the Clarence-Moreton Basin DOM is of fluvio-deltaic origin.

The Triassic DOM from the Simpson Desert Basin (Fig. 7c) is most simi-

Vitrinite

Woogaroo Formation : outer line encloses 12/19 points = 63%

Ipswich Coat Measures : outer line encloses 22/28 points = 79% /

/ / - - /

-

I L'" / / /

O

1 V Lip t in i te 5 0 I ne~ in i t e

Fig. 13. Density contours and centers of concentration for maceral compositions of Triassic DOM from the Ipswich Coal Measures and Woogaroo Subgroup, Clarence-Moreton Basin.


TABLE 5

Maceral Compositions for Centers of Concentration of Triassic DOM in the Clarence-Moreton and Simpson Desert Basins

Basin Formations Centers of concentration No. of (V-L-I) samples (%)

Clarence- Woogaroo Subgroup 55-10-35 19 Moreton Ipswich Coal Measures 50-20-30 18

Simpson Desert Peera Peera 13-10-77 31 Walkandi


lar in type to the alluvial plain DOM of the Northern Carnarvon Basin (Ta- bles 4 and 5 ). The DOM composition and the palaeogeographic setting may indicate that it also has an alluvial plain origin.

It is not, however, valid to derive paleodepositional environments from DOM types because of the considerable overlap of types found for Triassic DOM. DOM composition depends on many factors, such as type of organic matter, availability of organic matter, the transport distance, tectonic stabil- ity of the area, oxidizing potential of the environment, and climate. These factors can differ considerably even for one particular type of depositional environment in different sedimentary basins or, instead, within a single sedimentary basin. Thus, a particular DOM type cannot be related back to a unique depositional environment. However DOM type may be predictable from paleoenvironment, given the restriction that it will be specific to the nominated sedimentary basins, and it will not be a unique type.

TRIASSIC D O M MOST FAVOURABLE FOR OIL SOURCE ROCKS IN AUSTRALIAN

BASINS

The question of oil source rocks in the basins studied must be looked at in a relative sense: none of the Triassic DOM may be suitable for oil generation, as most of it contains less than 40% liptinite, much of it less than 30%, predominantly sporinite and cutinite (Type II ). The cutinite could be expected to yield waxy oils and may have originally been associated with suberinite. The suberinite is prone to physical degradation during transport and is diffi- cult to recognise in DOM, but it too is likely to yield a waxy oil (Powell et al., 1991 ). Powell et al. (1991) have shown that gross liptinite content is not a good indicator of the yield of paraffin compounds during pyrolysis. It is nec- essary for predictive work to divide the liptinite group into the individual macerals. Their work also appears to show that vitrinite has a higher specific

184 M SMYTft X, ND M. M.~SIA[,ERZ

yield of n-alkanes than some liptinites which are dominantly naphthenic (sporinite and resinite).

On the basis of overall liptinite content, at a basinal level (Table 2 1 DOM in the Bowen Basin is best suited for oil generation and that in the Simpson Desert Basin is least suited.

Within basins, the fluvio-deltaic (Bowen and Norian age, Northern Car- narvon) and lacustrine (Gunnedah) paleoenvironments have the most liptinite-rich DOM (Tables 3 and 4). The most liptinite-poor environments are interdistributary bay and prodelta (Gunnedah), lacustrine (Bowen) and fluvio-deltaic/alluvial plain (Rhaetian-Norian age, Northern Carnarvon ).

CONCLUSIONS

In absolute terms, very little of the Triassic DOM in the five basins studied here is liptinite-rich, so none of it may be very "oil-prone". In a relative sense, DOM in these basins can be classified as liptinite-rich or "oil-prone" in the following order, from most to least: Bowen, Gunnedah, Clarence-Moreton, Northern Carnarvon and Simpson Desert.

Comparison of available results for five Australian basins leads to the con- clusion that relationships between paleodepositional environments and Triassic DOM types found for one basin may not hold true in another. For example, DOM in the fluvio-deltaic sediments of the Northern Carnarvon Basin is vitrinite-rich, quite different from DOM in similar sediments of the Gunnedah and Bowen Basins (Tables l, 3 and 4 ), which is inertinite-rich and liptinite-rich, respectively. Even within a single basin an environment may produce a variety of distinctly different DOM types.

Relationships established between paleodepositional environments and DOM types are basin-specific. The environments within basins with the highest liptinite contents are: fluvio-deltaic (Bowen), proximal lacustrine (Gun- nedah) and fluvio-deltaic (Northern Carnarvon).

It is not valid to infer a unique paleodepositional environment from DOM type, but DOM type may be predicted from environment, within a specific basin.

Much more information on DOM types in Triassic sediments is needed before reliable correlations can be established between depositional environments and oil prospectivity. Standardized sedimentological terms are also es- sential for such comparative analyses. However, despite the confusion in ter- minology and the fact that the samples were selected for purposes other than this study, the present results indicate that environments and DOM types are related within a specific basin.

ORGANIC PETROLOGICAL COMPOSITION OF TRIASSIC SOURCE ROCKS 18 5

ACKNOWLEDGEMENTS

Sedimentological interpretations were provided by D.S. Hamilton, L.H. Etheridge and Jian Feng Xu. Thanks are due to K. Mastalerz and W. Sliwinski of Wroclaw University and J. Hamilton, CSIRO, for reading the paper and their constructive criticisms.

REFERENCES

Barber, P.M., 1982. Palaeotectonic evolution and hydrocarbon genesis of the Central Exmouth Plateau. Aust. Pet. Explor. Assoc. J., 22( 1 ): 131-144.

Bertrand, P.R., 1989. Microfacies and petroleum properties of coals as revealed by a study of North Sea Jurassic coals. Int. J. Coal Geol., 13: 575-595.

Bustin, R.M., 1988. Sedimentology and characteristics of dispersed organic matter in Tertiary Niger Delta: origin of source rocks in a deltaic environment. AAPG Bull., 72 (3): 277-298.

Cook, A.C., 1986. The nature and significance of the organic facies in the Eromanga Basin. In: D.I. Gravestock, P.S. Moore and G.M. Pitt (Editors), Contribution to the geology and hydrocarbon potential of the Eromanga Basin. Geol. Soc. Aust. Spec. Publ., 12: 203-219.

Cook, A.C., Smyth, M. and Vos, R.G., 1985. Source potential of Upper Triassic fluvio-deltaic systems of the Exmouth Plateau. Aust. Pet. Explor. Assoc. J., 25 ( 1 ): 204-215.

Elliott, L., 1989. The Surat and Bowen Basins. Aust. Pet. Explor. Assoc. J., 29( 1 ): 398-416. Golin, V. and Smyth, M., 1986. Depositional environments and hydrocarbon potential of the

Evergreen Formation, ATP 145P, Surat Basin, Queensland. Aust. Pet. Explor. Assoc. J., 26(1): 156-171.

Hamilton, D.S., Newton, C.B., Smyth, M., Gilbert, T.D., Russell, N., McMinn, A. and Ether- idge, L.T., 1988. The petroleum potential of the Gunnedah Basin and overlying Surat Basin sequence, New South Wales. Aust. Pet. Explor. Assoc. J., 28( 1 ): 218-241.

Hunt, J.W., Brakel,. A.T. and Smyth, M., 1986. Origin and distribution of the Bayswater seam and its correlatives in the Permian Sydney and Gunnedah Basins, Australia. Aust. Coal Geol., 6: 59-75.

Hunt, J.W. and Hobday, D.K., 1984. Petrographic composition and content of coals associated with alluvial fans in the Sydney and Gunnedah Basins, Eastern Australia. Spec. Publ. Int. Assoc. Sedimentol., 7: 43-60.

Jian, Feng, Xu, 1987. Triassic stratigraphy and sedimentary environments in the Gunnedah Basin: a summary. CSIRO/UNSW Res. Grant Scheme 1986-87, End-of-Grant Rep., 12 pp.

Powell, T.G., Boreham, C.J., Smyth, M., Russell, N. and Cook, A.C., 1991. Petroleum source rock assessment in non-marine sequences: pyrolysis and petrographic analysis of Australian coals and carbonaceous shales. Org. Geochem., 17 (3): 375-394.

Smith, G.C. and Cook, A.C., t 984. Petroleum occurrence in the Gippsland Basin and its rela- tionship to rank and organic matter type. Aust. Pet. Explor. Assoc. J., 24 ( 1 ): 196-216.

Smyth, M., 1984. Coal microlithotypes related to sedimentary environments in the Cooper Basin, Australia. Spec. Publ. Int. Assoc. Sedimentol., 7: 333-347.

Smyth, M., 1985a. Organic petrology of sediments from Renlim 2 well, Surat Basin. CSIRO unpubl, rep., 12 pp.

Smyth, M., 1985b. The organic petrology of sedimentary rocks from Lincoln 1 well, Surat Basin. CSIRO unpubl, rep., 12 pp.

Smyth, M., i 985c. The organic petrology of sediments from Roswin 1 well, Surat Basin. CSIRO unpubl, rep., 19 pp.

Smyth, M., 1986. Organic petrology of sediments from Springrove 1 well, Surat Basin. CSIRO unpubl, rep., 18 pp.

]8(3 M SM~711 , \N I )M ',, I~SFAIER/

Smyth, M.. 1988. Organic petrolog~ of the Triassic Snake {reek Member m ATt' 145P, Sural Basra, Queensland. CSIRO unpubl, rep., 18 pp.

Smyth, M., 1989. Organic petrology and clastic depositional environments with special refer., ence to Australian coal basins. Int. J. Coal Geol., 12: 635-656.

Smyth, M., 1991. Organic petrolog~ of sediments in the Clarence-Mormon Basin. In: A.T. Wells and P.E. O'Brien (Editors), The geology and petroleum potential of the Clarence-Moreton Basin, New South Wales and Queensland. BMR Bull., 241, in press.

Smyth, M. and Russell, N.J., 1986. Organic petrology of sediments from Sirrah # 4 well, ATP 145P, Surat Basin, Queensland. CSIRO unpubl, rep., 20 pp.

Smyth, M. and Saxby, J.D., 1981. Organic petrology and geochemistry of source rocks in the Pedirka-Simpson Desert Basins, Central Australia. Aust. Petrol. Assoc. J., 21 ( 1 ): 187-199.

Tissot, B.P. and Welte, D.H., 1984. Petroleum formation and occurrence, 2nd Ed. Springer, Berlin, 699 pp.

Documents

Organic petrological composition of Triassic source rocks and their clastic depositional environments in some Australian sedimentary basins