Relation between n-alkane distributions and effective coalification temperatures in some Permian shales

Gcochimjca

et Cosmochimica

Acta.

1975. Vol.

39. pp. 1237 to 1243.

Pergamon Press. Printed inGreatBritain

Relation between n-alkane distributions and effective coalification temperatures in some Permian shales REGINALDT. MATHEWS

Department of Geology, University of Melbourne, Melbourne, Australia

A. C. &oK Department of Geology, University of Wollongong, Wollongong, Australia

and R. B. JOHNS Department of Organic Chemistry, University of Melbourne, Melbourne, Australia (Received

5 June

1974; accepted

in revised form

30 December

1974)

Abstract-In Permian shales of the Sydney Basin, Australia, n-alkane distributions have been compared with effective coalification temneratures (ECTs) estimated from vitrinite reflectivities. The upper, non-marine part of the section’shows evidence ‘of progressive cracking (shift of n-alkane maximum toward shorter chains and tendency to eliminate longer chains) as depth and ECT increase; but this trend is not maintained in the underlying marine section. All samples show lack of a maximum in the longer-chain n-alkane distribution. Possible reasons discussed are (i) a cracking rate of long chains greater than their formation rate; (ii) a need for higher temperatures than the rock has so far undergone to produce a new crop of long chains; or (iii) exhaustion of the straight-chain generating potential of the kerogen. Pyrolysis experiments may be effective in testing these possibilities. The linking of ECI’s to alteration stages of sediment hydrocarbons opens the possibility of comparing these stages among formations which differ in age and organic and inorganic composition, and among basins of diverse geological history. INTRODUCTION of organic matter as a result of temperature rise due to burial is generally considered the most important factor in the formation of hydrocarbons in the sedimentary crust, so much so that a high geothermal gradient is often regarded as one basin characteristic favouring the economic occurrence of oil and gas (e.g. WEEKS, 1972). An aspect of thermal alteration which has received attention is the cracking of normal alkanes. HENDERXIN et al. (1968) studied this process under laboratory conditions. A few authors have investigated thermal alteration of fatty acids in which product n-alkanes were also cracked (EISMA and JURG, 1967; SHIMOYAMAand JOHNS, 1971, 1972). Usually these processes were carried out in the presence of clays or other mineral catalysts. To compensate for the necessarily short term of the experiments, higher temperatures than are normally encountered in sedimentary basins have been used, in the range of about 200-375°C. A disadvantage in the use of the more elevated temperatures is that they may set off reactions which would not occur under natural conditions, no matter how prolonged the time available. THERMALalteration

Some attention has been paid to evidence of cracking of n-alkanes in rocks under geological conditions. In Tertiary sediments in California, PHILIPPI (1965, Figs. 4 and 5) found that as depth increased, a sharp maximum in the n-alkanes at Cz9 was replaced in the Los Angeles Basin by a broad maximum in the C,,-C& region, and in the Ventura Basin by a well-defined maximum at Ct,. At the same time the ratio of long-chain to short-chain n-alkanes in the range Ci5-Q4 decreased with depth, until, as far as can be judged from Philippi’s graphs, the proportion became fairly constant in the deepest rocks, beyond about 3OOOm in the Los Angeles Basin, or about 4000m in the Ventura Basin, which has the smaller geothermal gradient of the two. ALBRECHT and OURISZQN(1969) studied depth variation in nalkane distributions in a thick, homogeneous marine shale in the Douala Basin, Cameroon. The age ranges from upper Cretaceous (Turonian) to Palaeocene. At 1200m depth (present temperature 62°C) the nalkanes peaked at C,, and did not extend above C,, which is about the expected distribution in a marine sediment in which the organic matter has been subjected to little metamorphism. At 1SOOm (present

1237 (i.(‘.A. 39,%

(

R. T. MATHEWS, A. C. COOKand R. B. JOHNS

1238

temperature 72°C) Cz2 and above made up about 50 per cent of the distribution (Ci2-Q,,). This was interpreted by the authors as the result of thermal generation of heavy n-alkanes from kerogen; the possibility of major differences in the composition of the original organic matter at the two levels was not discussed. At 2170m (present temperature 95°C) the ratio of long to short chains in the n-alkane distribution was little less than at 1500 m but at 2740 m (present temperature about 110°C) the n-alkanes between about Cz2 and C30 had largely disappeared. The authors attributed this disappearance to cracking of the longer chains. In the Douala Basin the decline of the long-relative to short-chain n-alkanes occurs over a depth range of 600m at most, a much more abrupt effect than in the Californian rocks, where the interval of change is a minimum of around 25OOm. The geothermal gradient in the Douala section is intermediate between the average gradients of the two Californian basins, but the Douala rocks are of course considerably older. T~sso~ et al. (1971, Fig.9) show n-alkane distributions in lower Jurassic shales of the Paris Basin down to about 25OOm. Chains of length Cl2 and above fall rather irregularly in relative abundance as depth increases. Finally, WELTE(1964, Fig. 2) found a tendency for high molecular-weight n-alkanes to decrease in relative importance with depth in Carboniferous and Devonian claystones near Miinster, Germany, although in this case the compounds affected were in the region of n-C,,. Table 1. Upper part of stratigraphic column of northern Sydney Basin TRIAsSlC

PEFJ4V.N

Newcastle Coal neasures

In the present investigation, n-alkane distributions were examined in a number of shale cores from two wells, Terrigal No. 1, near Gosford, and Woodberry No. 1, near Newcastle, in the northern part of the Sydney Basin, Australia. The cores come from the non-marine Newcastle and Tomago Coal Measures, the Kulnura Marine Tongue, and the marine Mulbring Siltstone and Branxton Formation, all of upper Permian age (Table 1). The n-alkane distributions were correlated with equivalent coalification temperatures (ECTs) for the cores and other samples, estimated from vitrinite reflectivities measured in oil. EXPERIMENTAL The procedure for the hydrocarbon analyses of the shale cores followed closely that described in MATHEWS et al.

(1972). In addition to the chemical investigations, petrographic studies were made on a series of 15 coalhearing samples from Terrigal No. 1, and four from Woodberry No. 1.

RESULTS Hydrocarbon analyses

Table 2 shows the relative concentrations of n-alkanes from about C,0-C,2 to about C,, in the analysed cores. No appreciable odd/even preference is shown in any of the samples. Coal petrography

Most of the coals are inertinite-rich. The major part of the vitrinite-rich material occurs as coal inclusions (‘scares’) in normal sediment. Exinite is rare to absent. The general range of coal material present in the samples from the marine parts of the succession is similar to that in the samples from the non-marine sediments. The marine samples generally contain pyrite. Table 3 lists the mean maximum reflectivities measured at 546nm in immersion oil of refractive index 1516. In order to minimize type variation, measurements were made only on material referable to vitrinite A (BROWNet al., 1964).

DISCUSSION AND CONCLUSIONS Tomago Coal Measures (includes Kulnura

Marine Tongue)

Terrigal No. 1 spudded in Newcastle Coal Measures; Woodberry No. 1 in the lower part of the Tomago Coal Measures. The Kulnura Marine Tongue is absent from the Woodberry section.

Effective coalification temperatures (ECTs) were estimated from measurements of reflectivity in oil. The correlations of reflectivity with volatile matter yield published in the International Handbook of Coal Petrology (1963) were employed as a basis for estimating volatile matter yield in order to use the data of KARWEIL(1965) to calculate the ECTs. The Handbook correlations closely resemble those published by [email protected](1973, Fig. 5, scales C and D). The ranks found for the coal samples conform with the major basin pattern of coalification. It is reasonable to suppose that the temperature rise which produced the coalification is associated with the amount of cover

1239

n-Alkane distributions and effective coalification temperatures

Table 3. Oil-reflectivities of vitrinite

Table 2. Relative percentages of n-alkanes in shale samples. (Concentrations expressed as percentages of the sum of the areas under the peaks representing the n-alkanes) C.+Xl NO. s-la

10

Tl

11

13

14

15

16

17

0.22

1.70

4.07

6.04

7.14

7.70

8.26

T2

0.25

2.41

5.30

8.21

10.27

10.83

10.34

9.24

T3

1.10

2.04

2.78

3.57.

4.93

6.80

7.51

8.32

0.75

2.88

6.31

9.96

12.33

12.05

T5

0.53

3.54

7.00

9.74

10.96

10.47

9.21

8.06

T6

5.71

9.43

12.26

12.45

11.50

10.04

8.32

6.96

0.37

1.29

4.22

7.72

10.24

10.80

T4

T7 T8

1.89

4.80

6.99

7.87

8.66

8.34

8.47

7.77

T9

1.64

2.97

4.11

5.51

7.25

8.36

8.72

8.75

"1

2.69

6.14

7.69

8.91

9.67

8.78

9.04

7.79

YZ

3.74

7.66

9.46

10.36

10.41

9.29

8.36

6.97

w3

3.58

7.01

7.70

8.16

7.83

7.83

6.73

6.43

Clrbon No.

Smplr

12

Tl

iii

19

7.74

6.09

20

6.45

21

5.68

22

5.49

23

6.00

24

5.16

7.59

6.62

5.14

4.45

4.72

3.29

2.92

2.51

7.76

7.86

7.30

6.53

6.72

5.72

4.89

4.57

T4

10.35

8.66

6.96

6.09

5.15

4.94

3.60

3.01

6.10

5.13

4.47

3.93

3.24

2.74

T-So Coal weasures

4.74

TZ

6.93

NNCmtle Coal Meel)sures.

25

T3

T5

TerrigalNo.1

T6

5.60

4.52

3.35

2.78

2.14

1.57

1.14

1.00

10.32

9.29

8.98

7.26

7.15

5.20

4.37

3.71

T8

6.80

6.42

6.13

4.74

4.16

4.82

2.94

3.38

T9

7.91

7.27

6.31

5.45

5.33

4.46

3.50

3.41

Ul

7.18

5.85

5.53

4.31

4.20

2.99

2.72

1.97

wz

5.99

5.01

4.47

3.81

3.61

2.54

2.08

1.66

Y3

5.91

6.W

5.48

5.42

5.29

4.41

3.43

2.93

C.rbcm

"0.

26

27

28

29

30

31

32

33

Suple

T1

4.19

3.47

2.97

2.19

1.78

1.06

0.72

0.35

0.26

T2

1.90

1.55

1.07

0.75

0.38

3.58

3.28

2.07

1.72

0.98

T4

2.47

1.84

1.35

0.88

0.41

T5

1.82

1.45

1.04

0.72

0.41

T6

0.70

T7

2.95

2.00

1.64

1.35

0.83

T8

1.64

2.02

1.05

0.59

0.54

T9

2.56

2.24

1.56

I.27

0.86

Wl

1.77

1.09

0.94

0.55

0.37

Y2

1.41

1.13

0.88

0.70

0.48

Y3

2.06

1.62

1.06

0.74

0.39

0.22

0.54

T = Terrigal well; W = Woodberry well. Stratigraphic locations of samples shown in Table 4. over the coals. In estimating the EC& the assumption is made that they were maintained for about 200 million years, which follows the generally held view of the history of the Sydney Basin. This is that sedimentation ended with the close of the Triassic, and that no important erosion occurred before major uplift at the end of the Tertiary. In this way, the ECT at 7315 m in Terrigal No. 1 well was estimated to have been 75X, and at 1219m, 95°C giving a gra-

0.82

0.18

1.02

0.82

0.19

769

1.06

0.89

0.19

775

1.07

0.88

0.19

807

1.07

0.90

0.16

808

1.07

0.89

0.18

827

1.05

0.88

0.18

827

1.04

0.90

0.14

839

1.06

0.88

0.18

910

1.13

0.95

0.18

936

1.13

0.97

0.16

970

1.18

1.00

0.17

1226

1.43

1.19

0.24

1227

1.42

1.15

0.28

1231

1.44

1.19

0.23

SSqle depth

Tonago Coal Bkas"reB

*rBUfOll ROMtiDll

I

T3

1.0

769

Woodberrq

PO-tiOn

2.31

T7

728

=

W

No.

Lx

152

1.21

221

1.59

628

1.11

650

1.25

1 %I,

Bircflecrmce

(coke mosaic present)

546nm; noi, = 1.516.

dient of 1°C in 24.4m. Using this gradient, EC% in the marine part of the Terrigal section (Kulnura Marine Tongue), were calculated. The EC? is the temperature which over a period of 200 million years would have produced the degree of coalilication found. The ECT will be less than the maximum temperature reached by the organic matter as a result of burial. In the Sydney Basin the temperature at any particular stratigraphic level, where not affected by igneous action, and assuming a stable geothermal gradient, would have risen until the end of the Triassic, when deposition is considered to have ceased. As sediment was thereafter eroded the temperature would have declined, very slowly until the Pliocene, then more rapidly as a consequence of uplift and accelerated erosion from that time. The temperature history would have been complicated by variations in rates of subsidence and deposition, and by a time-lag in the adjustment of sub-surface temperatures to vertical crustal displacements. The ECT has the advantage of absorbing a number of factors the effect of which would be impossible to determine separately.

R. T. MATHEWS,A. C.

1240

It would be of interest to compare the gradient of the ECT in Terrigal No. 1 (which, if the estimates are correct, would be a true average palaeo-thermal gradient) with the present thermal gradient, but no temperature measurements are available from the well. If we make the reasonable assumption that in general the major supply of organic matter to the coal-measure shales was land-plant material, it is probable that the original syndepositional distribution of nalkanes in these rocks had a maximum above n-C,,. However, Table 4 shows that all the non-marine samples have single maxima well below n&,, and the position of the maximum shows a consistent trend toward shorter chain lengths as depth increases. [A similar shift was noted by LEY~EU~R and WELTE(1969, Fig. 5) in n-alkanes of coals of the Saar, but it does not appear to have been observed previously in shales, except that WEL~E (1964, Fig. 2) shows a slight difference in the position of the maximum in Carboniferous and Devonian claystones above and below 5OOOmdepth.] The absence of oddf even preference among the long-chain n-alkanes is an indication of a relatively advanced stage of metamorphism in the organic material. Table 4 also shows the relative concentration in the shale cores of n-alkanes of chain length C,, and over. In the non-marine rocks these long-chain hydrocarbons are comparatively well represented up to an ECT of SO-90°C, and since they show no odd/even

COOK

and R. B. JOHNS

preference, may well be of thermal origin, derived from degradation of organic residues in the sediment. If sample T2 is omitted there is a decline in the relative abun~n~ of long chains with depth in the Terrigal non-marine interval, and as in the case of the Douala Basin (ALBRECHTand OURISSON, 1969), it occurs over a comparatively short interval (500m). The anomalously low value for T2 is difficult to explain, especially as T2 seems to be the sample richest in visible plant remains, and might therefore be expected to have been well endowed with longchain n-alkanes when cracking of heavier compounds began. The simultaneous downward trends in the position of the n-alkane maximum and in the proportion of long chains in the non-marine rocks lead to the conclusion that the existing distributions are the result of progressive cracking with depth, In the Douala Basin, at a present temperature of 1lO”C, the bar diagram published as their Fig.2(d) by ALBRECHTand OLJR~SON(1969) shows the relative concentration of long-chain n-alkanes to be about 5 per cent. This distribution is reached in the non-urine part of the Terrigal section at an ECT of about 95°C (Table 4). Although ECTs are not available for the Douala Basin, the difference of 15°C between these temperatures suggests as a first approximation that the 5 per cent level for chains of length Czz and over would have been reached at least twice as quickly in the Douala Basin sample as in the Terrigal core. This

Table 4. Positions of n-alkane maxima and relative percentages of long chains

of

ET*

5a2866

8662252

12521416

O-221

Iolllwin~ BilUtOrU I)ranxton FONtiOn

221594 59c 915

Tl

701

T2

709

Cl5

19

77

T3

829

Cl7

31

79

c17

38

T4

94s

Cl6

24

84

1090

Cl4

16

90

T6

1204

%z

T7

1265

TB

1279

T9

1289

iI1

170

w2

348

if3

612

%I

16

‘ECT = effective coalification temperature. in Sample No. column, T = Terrigal well, W = Woodberry well.

zCT*

EC22 c%

and above

I

I

I

/

74

T5

6.5

of

Ewlkme m(uimum

(%

I

n-Alkane distributions and effective coalification temperatures is in agreement with the difference between their ages, which are of the order of 100 million and 200 million years, r&pectively. In the deepest sample (about 25OOm, in the lower Jurassic of the Paris Basin) for which they give an n-alkane distribution (see the Bouchy pattern in their Fig.9), TIS.$~Tet al. (1971) show that in the range n-Cl4 to n-C,,, chain lengths of 22 carbons and above constitute about 25 per cent of the total. Again, no ECTs are available from the Paris Basin study. Since its history is so simple and thoroughly known, this basin offers an unusually good opportunity to correlate stages of organic metamorphism with EC% Determination of ECTs as a part of studies of alteration of organic matter in rocks should prove a valuable means of comparing organic metamorphic stages reached under similar time-temperature conditions in different formations, or in different basins, thus allowing a better opportunity of evaluating the influence of the original organic and mineralogical composition of rocks in guiding the course of changes in their organic component after deposition. The marine interval in Terrigal well did not afford core samples with sufficient vertical separation to show any systematic variation in n-alkane content resulting from temperature increase, the distance between the top and bottom samples being only 24 m. What is very surprising about these cores is their demonstration that the cracking trend indicated in the n-alkanes of the overlying coal-measures is not continued in the Kulnura Marine Tongue, where instead the rocks appear to be at a stage which was reached in the non-marine sediments at a depth abound 300m less, and an ECT about 15°C lower. An obvious inference is that this behaviour is related at least in part to the original nature and early diagenetic history of the organic residues in the marine rocks. Yet the studies mentioned above as disclosing progressive changes in n-alkane distributions with depth (PHILIPPI, 1965; ALBRECHTand OURISSON, 1969; TIS~~T et al., 1971) were made on marine sequences. An X-ray diffraction investigation of some of the Terrigal marine and non-marine samples revealed no marked differences in mineralogy at the present stage in the history of the rocks. Woodberry well passes through a dolerite intrusion of uncertain age between 270 and 305m. The presence of this ephemeral heat source in the section causes complications, but on the other hand has its own interest. At 221 m a large vitrinite phytoclast in shale is carbonized to a coke with fine and even mosaic structure and very low reflectivity (that is, for a coke). At 628m the microscopic appearance of coal in the section suggests an absence of any major local

1241

heating effects, and the rank accords well with the regional pattern (COOK, 1974). The ECT at this level is about 8O”C, and accordingly the analysed shale core W3 at 612m has a relative proportion of longchain n-alkanes similar to that found in sample T4, where the ECT is 84”C, although the position of the n-alkane maximum in W3 is somewhat lower (Table 4). It is noteworthy that although W3 is in a marine part of the Woodberry section (Branxton Formation), its n-alkane distribution correlates well with the progression of distributions in the non-marine part of the Terrigal sequence, unlike the samples from the Kulnura Marine Tongue. Whereas at the level of sample W3 the temperature history of the Woodberry section is back to normal, samples W 1 and W2, respectively, about 1OOmabove and 43m below the intrusion, show anomalous nalkane distributions which can be attributed to the geologically brief rise of temperature caused by the dolerite. Taking Terrigal samples as a guide, at both levels the relative proportion of long-chain n-alkanes and the position of the n-alkane maximum are lower than would be expected for ECTs less than 8O”C, and in fact comparison with Terrigal indicates for samples W 1 and W2 an ECI in the vicinity of 9O”C, meaning, of course, that the sill heated them well above this temperature. It seems probable that the n-alkane distributions found in the rocks examined in the present study have been ‘frozen’ much as they were at the culmination of the organic metamorphism. ALBRECHI and OURIS~ON(1969) found in a sample at 1500 m depth and a present-day temperature of 72”C, a bimoclal n-alkane distribution, with a strong maximum above and below n-C,,, and believed the long chains to have resulted from thermal action on the kerogen. If this result has general validity, it suggests that at low temperatures long-chain n-alkanes can be generated and remain untracked, or at least crack at a slower rate than their rate of formation. Thus in the Sydney Basin samples, reduced temperatures following the uplift of the basin perhaps give the possibility of modification of the culminating n-alkane distributions, but since no significant uplift occurred until Pliocene time, the period in which lowered temperatures could have operated (say, three million years) would doubtless have been too brief for any notable effect. From Table 4 it can be seen that of the Sydney Basin samples, none, even among those which have undergone the lowest ECT’s, shows a bi-modal nalkane distribution. PHILIPPI (1965), in shales of the LOS Angeles and Ventura Basins, found that a longchain maximum in n-alkanes existed in the youngest

1242

R. T. MATHEWS, A. C. CWK and R. B. JOHNS

sediments, and disappeared at a present temperature of 133°C in upper Miocene sediments of the Los Angeles Basin, and at 112°C in lower Pliocene sediments of the Ventura Basin. (Incidentally, the shortchain maximum, once established, does not change position with depth, unlike in the Sydney Basin shales.) In the lower Jurassic of the Paris Basin, TI~~QT et al. (1971) found only a broad low maximum in the longer chains of the shallowest sample, and this maximum disappears at 700 m depth. (Again, in these rocks there appears to be no change in the position of the short-chain maximum with depth.) As mentioned earlier, a pattern lacking a long-chain maximum was found by ALBRECHTand OURIS~~N (1969) in a deep Douala Basin sample. A similar distribution was reported by DOUGLAS et al. (1969) in a shale containing plants and some algae from the Scottish Carboniferous (maximum at C17). (A torbanite from the Scottish Carboniferous had a single maximum in the longer chains, at n-C,, to n-C13, but since this rock is described as consisting almost entirely of algal remains it is a special case, in which the scarcity of inorganic mineral matter may have restricted catalytic effects on the metamorphic process to a minimum.) No ECTs have been published for any of these instances. Again, the distribution under discussion has been found in non-marine Permian shales of the Cooper Basin (central Australia) which are under investigation in this laboratory. A single short-chain maximum seems thus to be common in normal shales where the organic matter is in an advanced state of thermal alteration. [An example of rocks where it does not occur is found in the Miinsterland 1 well (WELTE,1964, Fig. 2) where Carboniferous and Devonian claystones occurring down to great depths and high temperatures have a single long-chain maximum.] If this pattern, strongly skewed toward the long-chain end is the product of cracking, the low relative concentration of long-chain n-alkanes suggests (i) that the cracking process outpaces the formation of new long chains under more intense conditions of organic metamorphism; or (ii) that a limit has been reached to at least the longchain generating potential of the source material. Such a limit might be imposed either by exhaustion of the straight-chain components of the kerogen, which are regarded by DOUGLASet al. (1970) as the source of thermally-generated n-alkanes in sediments; or because detachment of straight-chain material from the kerogen could go no further in the range of temperatures to which the rocks were subjected. Regarding the relative rates of formation of long chains and their cracking, this explanation is certainly not supported by the strongly bi-modal distribution

found by ALBRECHTand OURISSON(1969) and mentioned at the beginning of this section of the discussion; although at higher temperatures the cracking rate perhaps draws ahead of the formation rate. LOSS of generating capacity by the kerogen suggests itself as an explanation for the curve published as their Fig. 1 by ALBRECHTand OURISSON(1969), which shows the quantity of saturated hydrocarbons extracted from their samples as rising to a maximum at about 2200~ and then falling steeply to near-zero as depth and temperature increase. Shale pyrolysis experiments are in progress in an attempt to distinguish among the possibilities mentioned. As described earlier, samples W 1 and W2 from the Woodberry well have undergone a natural pyrolysis, but since the time at which this occurred is not known, neither is the degree to which the n-alkane distribution produced by the intrusive heating has been modified by later burial-temperature

cracking.

Acknowledgements-Thanks are due to the Australian Research Grants Committee for supporting this investigation, and to the Geological Survey of New South Wales for ready co-operation in making available samples and information.

REFERENCES

ALBRECHT P. and OURISWNG. (1969) Diagtdse des hydrocarbures saturts dans une s&ie ldimentaire tpaisse (Douala, Cameroun). Geochim. Cosmochim. Acra 33, 138-142. E#OSTICKN. H. (1973) Time as a factor in thermal metamorphism of phytoclasts (coaly particles). 7th Cong. Int. Strat. Grol. Carb.. Krefeld, 1971, Compte Rendu 2, 183 193.

BROWNH. R., COOKA. C. and TAYLOR G. H. (1964) Variations in the properties of vitrinite in isometamorphic coal. Fuel 43, 111-124. COOK

A. C. (1974) Aspects of coal rank in the Sydney Basin. Abstracts, 9th Symposium on Advances in the Study of the Sydney Basin, University of Newcastle, Australia, pp. 8-9.

DOUGLAS A. G., EGLINT~NG. and MAXWELL J. R. (1969) The organic chemistry of certain samples from the Scottish Carboniferous Formation. Geochim. Cosmochin. Acta 33, 57%590.

DOUGLAS A. G., EGLINT~NG. and HENDERSON W. (1970) Thermal alteration of the organic matter in sediments. In Advances in Organic Geochemistry, 1966, (editors G. D. Hobson and G. C. Speers), pp. 369-388. Pergamon Press. EISMAE. and JURGJ. W. (1967) Fundamental aspects of

the diagenesis of organic matter atid the formation of hydrocarbons. Proc. 7th World Petrol. Congr., Vol. 2, pp. 61-72.

HENDERWN W., EGLINT~NG., SIMMONS P. and L~VELXXK J. E. (1968)Thermal alteration as a contributory process to the genesis of petroleum. Nature 219, 1012-1016.

n-Alkane distributions and effective coalification temperatures

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