Hydrocarbons and fatty acids in the Evergreen Shale, Surat Basin, Queensland, Australia

Qeochimicaet Casmochimica Acta,1972,Vol. 86,pp.885to 896. Pergamon Press.Printedin Northern Ireland

Hy~oc~bo~ and Mty acids in the Evergreen Shale, Surat Basin, Queering Australia R. T. MATHEWS Department of Geology, University of Melbourne, Melbourne, Austrelis

and X. P. I~uAL*, K. S. JACKS~K and R. B. JOHKS Depsrtment of Organic Chemistry, University of Melbourne, Melbourne, Australia (Received

29 December

1970; accepted in revised form 20 March

1972)

Abstr&--Alkane hydrocarbon and n-fatty acid distributions have been examined in cores taken over 8 550 ft thickness through the lower Jurassic, largely non-marine Evergreen Shale, Surat Basin, Queensland, Australia. No depth trends in compound abundances or carbon preference indices are discernible. There is no evidence for significant generation of n-alkanes from kerogen nor for cracking of long-chain rz-alkanes. The present distribution patterns of the biochemicals probably reflect closely the nature of the origirml organic matter. The general strong dominance of long-chain (Cs,) la-alkanes; the lack of evidence for diagenetic change; and the absence of correlation between abundances of n-alkanes and n-fatty acids (among both the longer- and shorter-chain compounds), lead to the conclusion that at least the long-chain n-alkanes were largely deposited as such in the sediment, having originated in land-plant material, remains of which are abundant in the samples. In the upper 170 ft. (possibly marine), ra-alkanes with chain lengths below C&ebecome important, suggesting greater significance of aquatic life as a source of organic matter at the time of deposition, a conclusion which is in general accord with the geological history of the basin, although this history is not well known, While the abundances of isoprenoid alkanes show no depth-trend, the ratios of the C&e isoprenoid (phytane) to the isoprenoids C,s (pristane), C&sand C, all increase or decrethsetogether from one depth level to another. This is co~on&nt with the prevailing view of a common derivation from phytol. Pristane abundances indic&e that high relative concentrations of this compound do not necessarily indicate a marine depositional environment, as has been claimed. &Cl3 values from the non-marine part of the formation have 8 mean of -2&3%,,. This value is considered to be inherited directly from land-plant organic matter.

DAWSONRiver Drilling (DRD) No. 6 well is one of a series of continuously cored holes drilled by the Geological Survey of Queensland, Australia, to provide stratigraphic information about the Surat Basin (Fig. 1). This basin, which has commercial hydrocarbon accumulations, is situated in the south-central part of Queensland: its geology is described in MORANand Gussow (1963), JENSEN et al. (1964) and THE

HO~ETOO~~

(1967).

DRD 6 well, which was drilled in the north eastern part of the basin, was spudded in Hutton Sandstone, and penetrated the following sequence, nearly the whole of which is regarded as non-marine: Lower Jurassic

Triassic

Hutton Sandstone Evergreen Shale Precipice Sandstone unconformity Rewan Formation

* Deceased 885

575 302

ft ft ft

12

ft

141

R. T. MATHEWS,X. P. Iuu&, K. S. JACKSONand R. B. Joas

886

I

I’

149-E

148-E

I

15O.E

1

Fig. 1. Simplified geological map of part of Surat Basin.

The Evergreen Shale in DRD 6 consists (GRAY, 1968) of black, dark grey and grey shale, siltstone and sandstone (Table 1). A few thin coal layers and laminae occur at intervals in the lower 400 ft. Leaf and stem material is abundant in most samples. The interval between 126 and 153 ft below the formation top (Westgrove Ironstone Member) is distinguished regarded as marine.

by the presence

of oolitic

chamosite,

and is

In this study the relative concentrations of n-alkanes C,, to C,, have been determined in eleven samples of shale core distributed over a thickness of 550 ft of the Table 1. Description of Evergreen Shale Depth below formation top (ft)

Interval

Samples

thickness (ft)

from interval

6-126

126

126-153

27

153-385

232

Nos. 6-8

386-485

100

-

485-576

90

Nos. 9-11

Nos. 1-3 No. 4

Interval description (after GRAY, 1968)

Dark shale, siltstone, sandstone becoming more sandy towards top. Oolitic shale with siltstone, sandstone; thin beds of oolitic chamosite. Thin-bedded shale, siltstone, sandstone, with a few coal laminae. Sandstone, with a few coal and shale laminae. Dark shale, with siltstone and sandstone; thin coal layers.

Hydrocarbons and fatty acids in the Evergreen Shale, Surat Basin, Queensland

887

Evergreen Shale; in addition, a-fatty acid relative concentrations (C,, to C,) have been determined in seven of these samples.

EXPXWMENTAL Alkanes The experimental procedure for alkanes was similar to that reported by JOHNSet al. (1966). All solvents were distilled fractionally before use and tested on a gas-liquid chromatograph (GLC) for possible contaminants. The unbroken core samples were first cleaned with distilled water atrd then sonicated (Branson Sonifier Model S-125, Branson Instruments Inc.) for 15min in a 4:l mixture of benzene:methanol to remove any surface contaminants. The COIW were then dried in an oven briefly and crushed in a ring-crusher to -150 mesh. Crushing time was not more than 2 min. The powder, in about 60 g lots, was sonicated for 30 min in a 250 ml centrifuge bottle with 200 ml of a 4: 1 benzene:methanol solution, The centrifuge bottle was cooled in an ice bath during the sonication. After extraction the suspension W&S centrifuged for 15 min at 2000 rev/mm, and the supernataut liquid separated. The whole process was repsatedd. The extracts were combined and the solvent evaporated under vacuum at 20% to leave a residue which was heated and stirred with an aliquot of n-heptane (5 ml). This procedure was repeated a further three times to ensure maximum extraction of the heptane-solubles. The heptane solution was centrifuged to remove any solids, and the solvent evaporated under vacuum at 20°C to about 2 ml. This sample was chromatographed on a column (2 x ?Jin.) of heptane-washed alumina, using rr-heptane as eluant. Fractions (5 ml) were collected, and those showing no ultra-violet absorption about 2700 A (usually the first two fractions) were combined to give the Total Alkanes fraction. The Total Alkanes were fractionated into la-Alkanes and Branched-Cyclic Alkanes in the conventional manner (JOHNS et al., 1966) by heating with 5 A molecular sieve in dry benzene. The three fractions of hydrocarbons were analysed by gas-liquid chromatography on Lzn Aerograph 1520 instrument using a flame ionisation detector. The 5 ft x 4 in. copper columns were packed with 5 ‘A SE-30 on 80-100 mesh ~hromosorb W. The gas flow (nitrogen) was 25 ml per mm, and each run was temperature-programmed from 50’ to 315% at 8% per mm. Au aerograph A90P-3 cbromatograph with a thermal conductivity detector and nitrogen gas as carrier was used to isolate and purify the isoprenoid alkanes. The cohmms used were 15 ft x 4 in. 1% SE-30 on Chromosorb W followed by re-injection of isolated ‘cuts’ on a 15 ft x g in. 5 % S-ring polyphenylether (6 PPE) on Chromosorb W. The purified material was finally identified by injection on to a 15 ft x & in. 6 PPE cohmm with hehum gas as carrier, in a PerkinElmer 2708 GUTmass spectrometer. The C,e, f&s, C,, and Cse isoprenoids were identified by comparison of their mass spectral fragmentation patterns with authentic samples. The ratios (Table 3) of the four isoprenoids were determined from the area under the curve (approximated to by peak height x half-height width) for each isoprenoid in the Branched-Cyclic chromatogram from a 15 ft x 4 in. 1% SE-30 on Chromosorb W column. These peaks were shown to be homogeneousby subsequentgas-liquid chromstography on & 15 ft x ) in. 6 % 6 PPE on Chromosorb W cohunn using helium as carrier gas, followed by mass spectrometry. Fatty acids Approximately 250 g lots of crushed shale were refluxed in 600 ml of methanol contammg 50 g of potassium hydroxide in solution. After 6 hr, about 200 ml of the suspension was decanted, a further 200 ml of methanol added, and refluxing continued for a further 6 hr. The total extract suspensions were combined, centrifuged at 2000 rev/mm for 15 mm, and the extract solution decanted. This solution, with 50 ml of distilled water and 200 ml of oz-heptane, was added to a separating funnel and shaken vigorously for 15 mm. The aqueous alkaline layer was run off, and re-extracted with la-heptane five times. The fatty acids were obtained from the aqueous layer by acidifying to pH 2 with hydrochloricacid and extraction three times with 20oml of sz-heptane. The combined extracts were evaporated to dryness and labelled the ‘total fatty acid’ fraction.

R. T. MATHEWS,

888

X. P. IUUAL, K. S. JACKSON and R. B. JOHNS

For GLC analysis, the fatty acids were esterified by heating with 3 to 5 ml of boron trifluoride-methanol solution on a water bath for about 5 min; the esters were then extracted into a heptane layer. This extract was reduced to about 2 ml by rotary evaporation and eluted through a short column of alumina with about 30 ml of n-heptane. Using the evaporation procedures employed for the hydrocarbons, the elute was evaporated down and labelled the ‘methyl ester’ fraction. The fatty acid esters were analysed by gas-liquid chromatography on an Aerograph 1520 instrument as in the case of the hydrocarbons, except that a 10 ft column was used. Components were identified by co-injection of standards, followed in the case of samples 1, 5, 8 and 11, by mass spectrometry, as for isoprenoids, except that the column used in the Perkin-Elmer 270B GLC-mass spectrometer was 12 ft by 8 in. stainless steel packed with 5 % SE-30 on 80-100 mesh Chromosorb W. RESTJLT~ Sample locations and some analytical data (total extract, total alkanes, total carbon, non-carbonate carbon, and sulphur) are given in Table 2. Table 2. Core locations, total extract, total alkanes, carbon and sulphur Sample No.

Depth below formation top (ft)

1 2 3 4 5 6 7 8 9 10 11

3 50 110 129-135 167 248 337 365 525 526 551

Total extract

Total alkanes

(ppm)

(ppm)

1350 1850 2700 308 1150 1350 1850 500 11,800 3250 725

300 200 800 n.d. 55 115 250 40 n.d. 1100 109

Total C( %) 1.55 1.75 2.35 3.00 1.40 164 1.46 0.77 26.00 2.61 1.22

NonCarbonate C( %) 1.32 1.39 2.03 2.55 1.09 1.31 1.19 0.43 n.d. n.d. 0.94

0 0.1 0.1 0.2 0.1 0 O*l 0 0.44 0 0

n.d. = not determined. In all cases except No. 9 the weight of shale used for the hydrocarbon extractions was 200 g. In the case of No. 9 the weight used was 85 g. Sample 4 comes from the oolitic ironstone interval (Westgrove Ironstone Member). Sample 9 is from a thin coaly layer.

Table 3 shows relative percentages of n-alksnes in the range C,, to C,, for each sample, together with odd-over-even carbon preference indices (C.P.I.) (COOPER and BRAY, 1963). n-Alkanes in the C&,-C,, range are present in much greater concentration than those of lower molecular weight (e.g. sample 7, Fig. 3) in all samples except No. 1. Sample 1, which comes from 3 ft. below the top of the Evergreen formation, is exceptional in having a principal maximum at C,, (Fig. 2) ; and samples 2, 3, 4 and 5, while they maximize above C2,,, have a greater relative concentration of nslksnes in the lower molecular weight region than the deeper samples, excepting Nos. 9 and 11, in which the relative abundance of shorter-chain n-alkanes rises. The branched-cyclic alkanes show maximum concentrations in the pristane and the squalane regions, and the relative importance of these maxima, seems to vary in random fashion. The abundance of branched-cyclic alkanes relative to n-alkanes is

Hydrocarbons

and fatty acids in the Evergreen

Shale, Surat Basin, Queensland

889

Table 3. Relative percentages of rr-alkanes, and carbon preference indices Sample No.

n-Alkene carbon No.

1

2

3

4

5

13 14 16 16 17 18 19 20 21 22 23 24 26 26 27 28 29 30 31

6-S 9.0 14.5 15.0 11.6 5-7 3.3 1.6 l-3 1.3 l-8 2.2 2.8 2-9 4.0 3-2 6.4 2.7 6.0

l-6 2.7 3.3 2.8 2.3 1.7 1.9 l-4 2.0 3.1 7-5 10.0 13.6 11.1 12.9 5.9 7.8 3.2 6.5

3.0 3.0 3.7 4.0 4.7 4.2 4.1 3.9 5.2 4-c 8.1 4.7 12.2 4.5 11.7 3-4 7.0 2.2 5.3

0.4 0.7 1.0 1.9 3.6 4.6 6.2 4.7 5.8 5.1 8.4 4.4 10.8 6.0 11.2 4-2 9.6 3.4 9.9

0.5 1.8 3.2 3.6 3.4 2.6 2.3 1.5 l-6 1.0 3.2 3.4 7.8 6.0 13.6 7.4 16.7 6-8 14.4

C.P.I.

lmi

2.95

2.73

2.19

1.63

6 0.6 0.8 l-9 l-9 l-8 1.3 1.3 1.0 1.3 1.3 4.3 3.3 13.2 6.6 21.2 6-6 17.9 3.7 9.8 3.32

I

8

1.5 1.2 1.3 1.5 1.9 2.0 3.0 2.3 3.0 3.1 6.5 5.2 11.1 6.6 13-7 7.9 14.7 5.6 8.1

1.6 0.9 1.0 1.3 l-7 1.7 2.5 2.0 2.3 2-7 4.7 6-3 9-7 7.7 16.0 9.2 15.0 5.9 9.6

2.02

l-88

9 2.1 3.6 4.5 4.6 4.8 3.7 4.3 3.2 3.3 3.2 5.5 4.1 12.9 4.8 11.3 4.3 9.4 3.2 7.4 2.85

10

11

0.9 0.9 1.0 1.0 1.2 1.2 2.0 1.6 2.7 2.8 12.9 5.1 16.5 4.8 14.7 6.3 16.5 3.0 5.9

2.5 2.5 3.5 4.2 4.5 3.5 3.6 2.6 2.9 2.9 6.3 4.2 7.3 5.1 10.8 6.5 14.6 5.2 6.6

3.47

2.13

Concentrations expressed as permntages of the mm of the area8 under the peaks representing the n-slkmea.

small in most of the cores. They are comparatively important in sample 1 and in sample 4 (oolitic ironstone), as well as in the ‘coal’ (sample 9) mentioned below. Relative concentrations and concentration ratios of the isoprenoids C16, C,,, C,, (pristane) and C,, (phytane) are listed in Table 4. 1 ft Sample 9 is from a layer of impure coal (61.2 per cent ash), approximately

Fig. 2. Sample 1; n-alkanes.

Fig. 3. Sample 7; n-alkanes.

above sample 10. Sample 9 is about 3+ times richer in total benzene-methanol extract than the richest normal shale core examined. It has a n-alkane distribution in which high molecular weight, members are dominant, but to which shorter-chain n-alkanes make a large contribution. In the ‘coal’ the branched-cyclic alkanes are much more abundant than in the shales. Seven of the samples (1, 2, 3, 4, 5, 8 and 11) were analysed for n-fatty acids (JOHNS et ad., 197 1). Relative concentrations of the acids from C,, to C,, are given in Table 5, together with even-over-odd carbon preference indices (C.P.I.) (KVENVOLDEN, 1966). 5

R. T. MATHEWS,X. P. Iauu,

890

K. S. JACKSONand R. B. JOHNS

Table 4. Relative concentmtions and concentration ratios of isoprenoid hydrocarbons

a8mple 1 2

3 4 6 6 7 8 9 10 11

Dqath below formation top (ft)

C10

C19

3 60 110 129-136 167 248 337 365 626 626 651

21.3 21.1 11.8 23.1 18.6 18.0 12.0 19.3 9.6 12.6 19.4

32.4 31.6 36.0 31.3 33.8 41.8 39.4 43.6 26.7 48.1 43.1

C18 24.4

23.2 21.7 30.9 25.1 21.7 16.6 16.6 40.0 15.6 20.8

C16 21.9

24.1 30.6 14.6 22.3 18.4 32.0 20.8 23.7 23.8 16.7

Ratio

Ratio

Ratio

Cm/C,,

Cm/C,*

Cm/C,,

oa

0.87

0.97

0.33 0.74 0.55 0.43 0.31 0.44 0.36 0.26 0.45

0.54 0.75 0*74 0.83 0.72 1.17 0.24 0.81 0.93

0.39 1.68 0.83 0.98 0.38 0.93 0.40 0.63 1.16

0.67

0.91

0.88

Concentrations are expressed &Bpercentages of the mnn of the areas under the peaks representing the four isoprenoids.

Table 5. Relative percentages of n-fatty acids, and carbon preference indices Sample No.

n-Fatty

acid carbon No.

1

2

3

4

6

8

11

6.7 5.5 21.4

4.5 3-2 18.3

4.2 6.1 31.4

6.1 13.1 6.6

3.3 12.6

6.8 24.6

14

b-6

16 16 17

4.3 11-4 4.3

1.7 l-7 9.3 2.9

1.6 1.6 8.6 2.6

0.9 1-6 9.7 1.8

18 19

12.6 4.7

9.2 4.0

7.8 3.9

7.6 3.3

7.2 6-O 12.2 7.9 14.7 5.3 12.8 1.8 4.4 0.6 0.9

6.5 3.8 7-5 6-3 17.0 4.9 19.8 2.2 9.2 0 0

6.7 4.4 6.3 3.9 7.9 2.8 7.1 1.2 2.4 0 0

2.37

3.46

2.66

20 21 22 23 24 26 26 27 28 29 30

6-7 4.6 6.5 6-b 6.7 4.5 6.3 3.5 3.6 3.3

6.6 4.9 9.0 6.9 14.8 6.6 14.4 2.6 6.3 0 0

C.P.I.

1.82

2.51

6.9

4.2 3-9

7.6 7.6 2.5

6.8

2.6

4.6 7.4 4.4 8.8 2.8 6.8 I.4 2.4

0.8 1.7 0.8 l-7 0.8 I.7 0 0

2.55

3.31

6.3

Conoentrations expressed as percentages of the mm of the areas under the peeks representing the n-fatty acids.

DISCUSSION AND CONCLUSIONS Origin of hydrocarbons The stratigraphy of the Surat Basin indicates that the depth of burial of the Evergreen Shale when sedimentation ceased in the north-eastern part of the basin was probably a minimum of about 4500 ft or a maximum of about 6500 ft. Despite the many wells drilled in the basin, equilibrium sub-surface temperature date are rare. In two wells around 150 miles from DRD-6, the geothermal gradients a,re

Hydrocarbons

and fatty acids in the Evergreen

Shale, Sumt Basin, Queenels;nd

891

respectively 0.64 and O+B°C per 100 ft of depth, and since the wells are on anticlines, these rather low values would probably be higher than for the su~~undi~ basin. In the DRD-6 region the basement is probrtbly shallower than in the area of the two wells mentioned, which should have an elevating effect on the gradient. The basin has not been subjected to severe tectonic action, nor to any igneous activity, except vulcanicity on the easternmost edge, far from the DRD-6 area. In the absence of me~~eme~ts it can at least be s&d that the Evergreen Shale in the DRD-6 region has not been subjected to high temperatures. ALBRECEF and Ou~rssaa (1969) interpreted their data, to show that in a thick marine shdaleof late Cretaceous age in the Douala Basin, Cameroon, generation of long-chain a-alkanes from kerogen began a;t depths where the temperatu.re is now about 75°C ; while in the lower Toarcien (lower Jurassic) of the Paris Basin, the threshold temperature for this process ws taken to be 60°C (Loves and Trsso~, 1967). However, the long-chain a-alkanes of the Evergreen Shale show marked odd-over-even predominance, a feature which is absent from ?Akanes generated by thermal degradation, and which productian of n-alkanea by heating of kerogen has evidently beon insufficient to mask. In the Douala Basin shale, cracking of longerchain m-alkanes apparently did not take place until the burial depth was beyond 7000 ft (present ~rn~e~t~re lOO”C a;nd above). The relatively low Go~~entration of shorter-chain n-alk~nes found in the non-marine (deeper) part of the Evergreen Shale suggests that the temperature history of this formation has been too mild for cracking to occur. No consistent variation with depth is apparent in the Evergreen Shale in compound abundances, in isoprenoid ratios, or in C.P.I. values for n-alkanes or for f&ty acids, Thus, despite its age, the orgxk. components of the Evergreen Shale seem to have undergone little d&genesis, and the hy~o~~rbon d~t~b~tions in the samples axe probably dire& reflections of the character of the organic matter supplied to the sediment at the time of deposition.

The n-alkane ~st~butions in the majority of samples are eh&r~~ter~~ by concentrations which are high in the C,,-C,, range, and very much smallex in the range below CaD,a typical distribution for sediments in whioh the bulk of the organic matter has been derived from land plants. Examining the possibility that the n-alkanes have been formed from f&y acids, a plot (Fig. 4) of relative concentrations of ?a-alkanes of carbon-number N superimposed on relative concentrations of n-fatty acids of carbon-number (M + 1) shows no correlation such as is observed in the Green River Shale and other rooks (KVENYOLDEN, 1807). (Significantly, in view of the evidence for lack of diagenotic change in the Evergreen Shale, the correlation ia about as good as shown by Kvenvolden for modern sediment from the San Nicolas Basin off the California coast.) The data, then, do not support the possibility of derivation of the a-aJkanes from B-fatty acids, Noting the abundance of fossil leaf material in the Evergreen samples, it seems unnecessary to look past the n-alkanes naturally present in land plant material (assuming data for modern plants to be applicable to ancient ones) as the source of the long-ohain s-alkanes in the Evergreen Shale.

R. T. MATHEWS,X. P. IGUAL, K. S. JACKSONand R. B. JOHNS

892

n-alkane carbon number I$

19

2L

29 1L

19

2$

Sample5 3c.sample1 20. 10-h ‘.a -._.-._.%__C..J+,*+gJ. -

\

_

2’3 25 30 15 20 25 n-fatty acid carbon number

30

3CSample 2

30 Sample 4 2010 -J&&&+/ 15

29

Sample 8

AA

fatty acid ---n-alkane -

Fig. 4. ra-Alkane and la-fatty acid relationships; the percent concentrations are relative within each group of compounds.

Possible effect of selective migration In the above arguments the assumption is made that the n-alkane distribution has not been affected by the primary migration of hydrocarbons out of the shale as burial depth increased. It is generally considered (e.g. SILVERMAN, 1965) that no differential adsorption of n-alkanes occurs during migration, but the possibility of this is raised as a result of comparing n-alkane distributions in the Evergreen Shale with those in crude oils from reservoirs within that formation (~THEWS et al., 1971). These oils have n-alkane maxima well below C&,, so that if the Evergreen Shale was the source of the oils, long-chain n-alkanes may have been left behind in the shale during the migration. In none of these cases however, is there sufficient well-control completely to define the reservoir, so there remains doubt that the Evergreen Shale actually was the source.

Isoprenoids It has been suggested that phytol could serve as a precursor of phytane (BENal., 1963) and isoprenoids of shorter chain-length, through a process of geochemical transformation (JOHNS et al., 1966). The best evidence to support this view is the data from the Green River Shale (ROBINSON et al., 1965), and the notably small abundance of the C,, isoprenoid in a Devonian sediment (MCCARTHY and CALVIN, 1967). In the Evergreen Shale there is no appreciable decrease with depth in the concentration of phytane, nor any consistent decrease in the ratio of phytane to the other isoprenoids. However, a plot of the ratios in Table 4 shows that in general all three ratios increase or decrease together, which is consistent with an origin for the isoprenoid alkanes by diagenesis of phytol as common precursor. An apparent contradiction does exist with other (unpublished) data on Recent sediments (JOHNS et al., 1970), results from which indicate at the least a dual origin for pristane, and raise the question as to whether the present concentrations of the isoprenoids are in fact a reflection of ecological conditions at the time of deposition. The development of these arguments will be taken up at a later date. DORAITIS et

Hydrocarbons and fatty acids in the Evergreen Shale, Surat Basin, Queensland

893

ALBRECH~and OURISSON (1969) regard a relatively high concentration of pristane as ~dic~ting a marine origin for the organic matter of ;I sediment, snd in this eonnection cite BLXJ~SER (1963). Their contention is supported by the low relative concentrations of pristane found by ROBINSONet ab. (1965) in the Green River Shale. However, Table 4 shows that in the Evergreen Shale pristane is the principal isoprenoid in all samples except 9 (coaly layer) ; and in samples 6, 8, 10 and 1I, which come from what is regarded as the undoubted freshwater part of the formation, the concentration of pristane is more than double that of any of the other isoprenoids: therefore the general validity of its use as s, marine indicator must be regarded as questionable. Carbon isotope ratios

Carbon isotope ratios were determ~ed (by the institute of Nuclear Sciences, Department of Scientific and Industrial Research, New Zealand: Director, T. A. Rafter) on kerogen and benzene-methanol extract of sample 11. In addition, ratios are available (also determined by the above Institute) for samples from the lower part of the Evergreen Shale at two other localities in the Surat Basin (Juandah and Alton) ; and for shale cored at Moonie oil field in the non-marine Precipice Sandstone, which irnrne~~~ly underlies the Evergreen Shale. These ad~tion~l values support those found for the DRD-6 sample (Table 6). Table 6. Per mil WY3 ratios of shale samples (PDB limestone standard) Sample No. 11, DRD-6 well Shale from lower part of Evergreen formation (core sample from Juandah No. 1 well) Shale from lower part of Evergreen formation (core sample from Alton No. 3 well) Shale from Precipice Sandstone, close to base of Evergreen Shale (core sample from Moonie No. 12 well)

6-Crs (kerogen)

&Cl3 (extract)

-224.6 + 0-I

-26.5

f O-1

-27.1

f 0.1

-255

i 0.1

-27.6

f 0.1

-25.2

f O-1

-27.7

f O-1

Determinations made by Institute of Nuclear Sciences, New Zealand (Director: T. Rafter), The Precipice Sandstone (non-marine) underliesthe Evergreen Shale.

The &OS values of the non-marine sample 11 fall in the range of isotopic compositions published for organic matter from marine sediments {SILVERER and EPSTEIN,1958). However, DE~ENS (1969, p. 319) states that the carbon isotope composition of most ancient sediments, both fresh water and marine, is close to -26x,. He also gives the mean 6-CY3in Recent fresh water sediments as -25x,, of common land plants (modern and ancient) as -25%,, and of fresh water plankton as about -3O%,. SILVERMANand EPSTEIN(1958) found that 6-CY3ratios for the non-marine Green River Shale (Eocene) were -30*5%, for c~oroform-methanol extract, and -31*9%, for total organic matter, the low values being explained as due to derivation of the organic matter of the rock from algae living in s, stagnant environment. In any case, the Green River values fall in the fresh-water plankton range. The S-CY ratios for the Evergreen Shale, on the other hand, can be regarded

894

R. T.

MATHEWS,

X. P.

IUUAL,

K. S.

JACKSON

and R. B. JOHNS

as consonant with the evidence of the n-alkanes that at least in the lower part of the formation land plants were the main source of organic matter. GEOLOC+ICAL SIGNIFICANCE OF HYDROCARBON DISTRIBUTIONS The low total (benzene-methanol) extract of sample 4 (Ironstone Member) shown in Table 2 appears to conflict with the fact that this sample contains the highest proportion of organic carbon of all except the coaly layer. However, JUDSON and MURRAY(1956) have shown for Recent lake sediments that hydrocarbon content need not reflect total organic content, but depends also on oxidation conditions on the deposition bottom. The presence of oolitic chamosite in the Ironstone Member indicates an environment in which currents were active, and so probably oxygenated. Chamosite can form in an oxidising environment if the pH is not too high (CASTAGO and GARRELS,1950). If it is true as argued above that there is a relative lack of diagenetically generated hydrocarbons in the Evergreen Shale, the effects of an oxidising environment at time of deposition would still be apparent. In most of the samples from No. 6 downward, n-alkanes below C,, comprise around 10 per cent of the total (~&&ato n-Cal), except in samples 9 and 11. In sample 5 this proportion rises to about 17 per cent. It is still 17 per cent in sample 4, 27 per cent in sample 3, and reaches 65 per cent in sample 1. In sample 2 there is a reversion to a level of about 16 per cent. Thus sample 5 ushers in a rise in the importance of the n-alkanes below C,, which is in general maintained and increased until near the top of the formation they much outweigh in abundance the higher n-alkanes. In a case where diagenetic hydrocarbon generation can be ruled out, the balance between long and short-chain n-alkanes may be interpreted as a reflection of the relative amounts of land-plant and aquatic material supplied to the sediment, because aquatic organisms are on present knowledge a significant source only of short-chain n-alkanes, not greater than C,, (e.g. ORO et aZ., 1967; CLARKand BLUMER,1967). It is generally agreed that there was a marine episode at or near the end of Evergreen time, mainly on the evidence of the Westgrove Ironstone Member, with its oolitic chamosite. JENSENet al. (1964) consider that the base of the oolitic interval marks a sudden change to marine conditions, whereas the n-alkane distributions suggest that the land-plant contribution was declining, i.e. that marine influence was beginning to enter the basin, at the time of sample 5. Although the Ironstone interval is the only one for which there is good evidence that it is marine, its n-alkane pattern suggests that it does not represent the maximum of the transgression, and that land plant material was still the dominant type of organic matter being incorporated in the sediments in Ironstone Member time. Finally, from the dominance in it of shorter-chain n-alkanes, sample 1 (Fig. 2) should represent the highest rise of sea level, and thus the greatest recession of the shore from the neighbourhood of DRD 6 well, before the retreat of the sea, and the deposition of the supposedly continental Hutton Sandstone. In this interpretation, sample 2 would have to be regarded as representing a temporary fall of sea level, with a consequent increase in the amount of land plant material delivered to the site as distance from shore decreased. The nearby Auburn Complex (Fig. l), an igneous and metamorphic block, was probably dry land throughout, so that the maximum distance of the DRD 6 site from shore

Hydrocarbons and fatty acids in the Evergreen Shale, Surst Basin, Queensland

895

would hs,ve been less than about 20 miles, probably not sufficient to eliminate landplant debris from the sediment at any stage. Unfortunately the geological evidence for environmental conditions in the Surat Basin near the end of Evergreen time is meagre and indecisive. No definitely marine fossils have been found in the Evergreen Shale except a few arenaceous foraminifera associated with the Westgrove Ironstone Member in the Moonie oil field (EVANS, 1966). In DRD 6 well, Acritarchs appear rather abruptly at the time of deposition of the Ironstone Member, and taper off in numbers before disappearing at the base of the Hutton Sandstone (REISER and WILLIAMS, 1969). HO~ETOORN (1967) and JENSEN et al. (1964) regard the Ironstone Member as representing a marine episode, and consider that it was followed by reversion to continental conditions. The latter picture conflicts with the interpretation drawn above from the n-alkane distributions in the uppermost samples, where sample 1, from 3 ft below the top of the Evergreen Shale, corresponds to the greatest transgression of the sea; in which event the base of the overlying Hutton Sandstone would presumably be a regressive sand. To complicate matters, as far as n-slkanes are concerned the distinction to be drawn seems to be not between terrestrial and marine organic matter, but between terrestrial organic matter from dry-land plants and organic matter from aquatic organisms (mainly plants), marine or non-marine @IATHEWS et al., 1970). Therefore it is conceivable that in a lacustrine environment the same effects on n-alkane distribution might be produced that we are here ascribing to marine transgression. (Thus in samples g-the ‘coal’-and 11 from the non-marine part of the formation, the relative concentration of shorter-chain n-alkanes is comparable with that in sample 3, from the ?marine sequence. This can be interpreted as due to enlarged contribution from non-marine aquatic organisms at the levels of samples 9 and 11, especially in the former instance, since coals are the product of swampy environments.) Nevertheless, whatever is true in the case of the Evergreen Shale, there appear to be good grounds for suggesting that it may be possible to correlate sediment hydrocarbon distributions with basin history, where the latter is better known. Acknowledgernente-The investigation was financed partly by a joint grant to R. B. JOHNSand R. T. MATHEWSby the NufXeld Foundation, and this assistance is gratefully acknowledged. The writers thank the Geological Survey of Queensland for readily supplying samples and information; and the Australian Research Grants Committee for support (to R. B. J.). REFERENCES ALBRECHTP. and OKJRISSON G. (1969) Diagenese des hydrocarbures satures dans une serie sediment&e Bpaisse(Douala, Cameroun). Geochim. Cosmochim.Acta 33, 138-142. BENDORAITIS J. G., BROWN B. L. snd HEPNERL. S. (1963) Isolation and identification of isoprenoidsin petroleum. Proc. 6th World Petroleum Congress, sect. 5, pp. 13-29. BLUMERM. (1963) Pristane in zoophmkton, Science 140, 974. CASTAGOJ. R. and G~RRELSR. M. (1950) Experiments on the deposition of iron with special relevance to the Clinton iron ore deposits. Ewn. Qeol. 45, 755-770. CLARKR. C. and BLUER M. (1967) Distribution of n-paraffins in marine organisms and sediments. Limnol. Oceanogr.12, 79-87. COOPERJ. E. and BRAY E. E. (1963) A postulated role of fatty acids in petroleum formation. Beochim.Coemochim. Acta 27, 1113-1127. DEOENSE. T. (1969) Biogeochemistry of the stable carbon isotopes. In Organic Cfeochemistry, (editors G. Eglinton and M. T. J. Murphy), Chap. 12, pp. 304-329. Springer-Verlag.

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EVANSP. R. (1966) Mesozoic stratigraphicpalynology in Australia. Awt. Oil Gas J. 12,58-63. GRAY A. R. G. (1968) Stratigraphic drilling in the Surat and Bowen basins, 19651966. Geol. Surv. Queensland, Rep. No. 22. HOUETOORN D. J. (1967) Jurassic reservoirs of the Surat basin. Proc. 7th World Petroleum Congress, Vol. 2, pp. 161-170. JENSENA. R., GREGORYC. M. and FORBESV. R. (1964) The geology of the Taroom 1: 250,000 sheet area and of the western part of the Mundubbera 1:250,000 sheet area, Queensland. Cwlth. Au&. Bur. Mm. Resources, unpub. Rec. 1964/61. JOHNSR. B., BELSKY T., MCCARTHYE. D., BURLIN~A~~E: A. L., HAUO P., SCHNOESH. K., RICHTERW. and CALVINM. (1966) The organic geochemistry of ancient sediments, pt. II. Beochim.Cosmochim. Acta 30, 1191-1222. JOHNSR. B., CAMERONW. E. and ONDER 0. M. (1970) Data from Organic geochemistry of Recent sediments, M.Sc. thesis (W. E. Cameron, 1969); and B.Sc. Hons. Rep. (0. M. Onder, 1970), University of Melbourne. JOHNSR. B., DEWAR N. and JACKSONK. S. (1971) Data from B.Sc. Hons. Rep. (N. Dewar, 1970); and Studies in diagenesis and environment, M.Sc. thesis (K. S. Jackson, 1971), University of Melbourne. JUDSONS. and MURRAYR. C. (1956) Modern hydrocarbons in two Wisconsin lakes. BUZZ. Amer. Assoc. Petrol. Geol. 40, 747-761. K~ENVOLDENK. A. (1966) Molecular distributions of normal fatty acids and paraffinsin some lower Cretaceoussediments. Nature 209, 573-577. KVENVOLDEN K. A. (1967) Normal fatty acids in sediments.J. Amer. Oil Churn.Sot. 44,628-636. LOUISM. C. and TISSOTB. P. (1967) Influencede la temperatureet de la pressionsur la formation des hydrocarburesdans les argiles a kerogene. Proc. 7th World Petroleum Congress, Vol. 2, pp. 47-60. MCCARTHYE. D. and CALVINM. (1967) The isolation and identification of the C,, saturated isoprenoid hydrocarbon 2,6,10-trimethyl-tetradecane from a Devonian shale. Tetrahedron 23, 2609-2619. MATHEWSR. T., BURNSB. J. and JOHNSR. B. (1970) Comparisonof hydrocarbon distributions in crude oils and shales from Moonie field, Queensland,Australia. Bull. Amer. Assoc. Petrol. Beol. 54, 428-438. MATHEWSR. T., BURNSB. J. and JOHNSR. B. (1971) An approach to identificationof source rocks. Awt. Petrol. Explor. Assoc. J. 11, 115-120. MORANW. R. and Gussow W. C. (1963) The history of the discovery and the geology of the Moonie oil-field, Queensland, Australia. Proc. 6th World Petroleum Congress, sect. 1; Awt. Oil Gas J. 10, 4448. OROJ., TORNABENE T. G., NOONERD. W. and GELPIE. (1967) Aliphatic hydrocarbonsand fatty acids of some marine and freshwater organisms. J. Bacterial.93, 1811-1818. REISER R. F. and WILLIAMSA. J. (1969) Palynology of the lower Jurassic sediments of the northern Surat basin, Queensland. Geol. Surv. Queensland, Publion. No. 339, Palaeontol. Papers No. 15. ROBINSONW. E., CONS J. J. and DINEEN G. U. (1965) Changes in Green River paraffins with depth. Beochim. Cosmochim. Acta 29, 249-258. SILVERMAN S. R. and EPSTEINS. (1958) Carbon isotopic compositions of petroleums and other sedimentary organic materials. Bull. Amer. Assoc. Petrol. Geol. 42, 998-1012. SIL~ER~N S. R. (1965) Migration and segregation of oil and gas. In $%kds in Subsurface Environments, Amer. Assoc. Petrol. Geol. Mem. 4, pp. 53-65.