Determination of molecular structure of kerogens using 13C NMR spectroscopy: II. The effects of thermal maturation on kerogens from marine sediments

Geochimrca et Cosmochimica Ada Vol. 56, pp. 2725-2742 Copyright 0 1992 Pergamon Press Ltd.Printed in U.S.A.

00167037/92/$5.00

+ .oO

Determination of molecular structure of kerogens using 13CNMR spectroscopy: II. The effects of thermal maturation on kerogens from marine sediments R. L. PATIENCE,* A. L. MANN,+ and I. J. F. POPLETT BP Research Centre, Chertsey Rd., Sunbury-on-Thames,

Middlesex, TW16 7LN, UK

(Received March 20, 199 1; accepted in revisedform March 25, 1992) Abstract-Solid state 13C nuclear magnetic resonance (NMR) spectrometry has been used to analyse kerogens isolated from marine sediments, to obtain information about relative changes in average molecular structures with increases in thermal maturity. Three suites of samples, all of which vary from immature to mature with respect to petroleum generation, were investigated: (a) seven samples of the Cretaceous Brown Limestone Formation (BLF), Gulf of Suez; (b) six from the Miocene Monterey Formation (MF),

California; (c) seven from the Upper Jurassic to Lower Cretaceous Kimmeridge Clay Formation (KCF), UK continental shelf (UKCS). Each NMR spectrum has been quantified in terms of fourteen different carbon types. The immature KCF samples have a somewhat higher initial aromaticity (f.) than immature representatives of the other two suites, perhaps due to a slightly greater terrestrial organic input. With increasing maturity, only a modest increase infa occurs in all three suites, until petroleum generation commences. The latter results in a sharp increase inf., because alkyl carbon types are progressively lost from kerogen. No preferential loss of particular alkyl carbon types is seen within the resolution of the method. The percentage of heteroatom-bonded carbon (to 0 or S) declines consistently with increasing maturation, and prior to the onset of petroleum generation. The distribution of aromatic carbon types changes substantially with increasing maturity, in that the relative abundances of bridgehead (ring junction) and protonated aromatic carbons increase, whereas phenolic and alkylated aromatic carbon decline or remain roughly constant, respectively. The data acquired have been used to monitor the hydrogen budget during maturation. Firstly, aromatisation reactions seem to occur during petroleum generation (increasing aromaticity is not simply a concentration of existing aromatic carbon) and, secondly, sufficient or excess hydrogen is liberated during these reactions to “heal” alkyl bonds broken during generation. INTRODUCI’ION

EXPERIMENTAL Sample Origins and Preparation

NUCLEAR MAGNETIC RESONANCE (NMR) spectroscopy is amongst the most powerful techniques available in chemistry for structural elucidation. One of the many areas of its ap plication is the study of sedimentary organic matter, such as oil shales and coals, where it has been used to provide information about bulk features such as aromaticity (f.) and, more recently, average molecular structures (TREWHELLA et al., 1986; MANN et al., 199 1, and references therein ). The latter has been facilitated by the development of sophisticated solid state 13C-NMR methods, such as the approach reported previously (MANN et al., 199 1) which used an improved carbon type analysis, based on earlier work of TREWHELLA et al. ( 1986), to determine the molecular composition of different kerogen types. In this, the second part of a study on kerogen structure, the effect of thermal maturation on molecular structure of marine sediment-derived kerogens (i.e., from a dominantly algal/ bacterial source) has been investigated. Three suites of samples were analysed: the Kimmeridge Clay Formation (KCF) from the North Sea (UKCS), the Brown Limestone Formation (BLF) from the Gulf of Suez (Egypt), and the Monterey Formation (MF), offshore California, USA. Each sample set covers the maturity interval from immature to mature, with respect to petroleum generation.

Kimmeridge Clay Formation: All seven samples are marine elastic sediments from the central, or the southernmost part of the northem North Sea (UKCS) and are Late Kimmeridgian to Early Portlandian in age. Brown Limestone Formation: The seven samples from the Gulf of Suez, Egypt, are Cretaceous marine carbonates and have been described previously by BARWISEet al. ( 1984). Monterey Formation: All six samples are Miocene marine siliceous sediments from the Santa Barbara Channel Basin, offshore California, USA. Burial depths and basic geochemical (pyrolysis and elemental analyses) data for all twenty samples are given in Table 1. Kerogen concentrates and solvent extracts of the KCF and MF sediments were prepared as described by MANN et al. ( 199 I ). The BLF kerogen concentrates were prepared as described by BARW~SE etal.(1984).

Kerogen and Extract Analysis The kerogen concentrates themselves were analysed for their elemental (C, H, N, and 0) composition, pyrolysis parameters (Tmax, S2) and hence hydrogen indices (HI), gas:oil ratio (GOGI) from pyrolysis&C, as well as by dipolar dephased CP/MAS solid state 13C-NMR. The BLF samples, which had formed part of a previous NMR study (where only the degree of aromaticity,f., was determined, BARWISEet al., 1984), were not solvent extracted, because extract yields were thought to be low, although they are referred to here as “kerogens” for the sake of convenience. Solvent extracts from the KCF and MF samples were subjected to routine geochemical analysis: deasphaltening, high-performance liquid chromatography (HPLC) into saturates, aromatics and residues (also called polars, resins, or NSOs), gas chromatography (GC), and

* Present address: Geolab, Statoil a/s, 4001 Stavanger, Norway. +Present address: School of Applied Chemistry, Kingston Polytechnic, Penrhyn Rd., Kingston-on-Thames, Surrey, KTl 2EE, UK. 2725

2726

R. L. Patience, A. L. Mann, and 1. J. F. Poplett

of increasing maturity, particularly for the four deepest samples (Fig. 1a). Biomarker distributions obtained from GC-MS analysis of the saturates fraction of the soluble extract (Table 2) confirm that there is a significant increase in maturity with depth within a relatively constant organic matter type. For a review of the biomarker parameters used to evaluate source and maturity, see MACKENZIE( 1984). In terms of source, the shallowest and deepest samples are perhaps harder to interpret than the others, due to the high abundance of 17/3(H),21/3(H)-hopanes in the former (SEIFERT and MOLDOWAN, 1980) and the extensive cracking that has occurred in the latter. However, the remainder are characteristic of algal/bacterial organic matter accumulations associated with marine, siliciclastic sediments. As is common in sediment extracts from the North Sea, there are variable quantities of 28,30_bisnorhopane present in the majority of these samples (GRANTHAM et al., 1980; MACKENZIEet al., 1984; MOLDOWANet al., 1984). Its distribution appears to be more controlled by source than maturity as there is no obvious trend with depth, though even closely spaced samples (geographically) exhibit wide variations, e.g., the sample from 2340 m (low) and that from 2905 m (high), which are from nearby wells. The latter sample also has an unusually low hopanelsterane ratio ( MOLDOWANet al., 1985 ) , which may reflect a minor source variation such as a comparatively greater abundance of algal-derived organic matter. Both saturates and aromatics maturity parameters (Table 2) demonstrate maturity increases with depth. Two of the most commonly used are illustrated in Fig. 2a ( SEIFERTand MOLDOWAN, 1978; MACKENZIEet al., 1980, 1981). These would imply that petroleum generation commences some-

GC-mass spectrometry (GC-MS) ofthe saturates and aromatics fractions. Experimental details have been given previously (MANN et al., 1991). Solid State 13CCP/MAS NMR

The detaileddescription of this technique is given elsewhere(MANN et al., 199 1 ), including the NMR experimental details, the computerised peak fitting routine, an example of a real and fitted (simulated) spectrum, and an assessment of errors and quantitative reliability. Briefly, average molecular structures were obtained by a carbon type analysis technique involving the fitting of the simulated spectrum to the dip&r dephased spectrum, and then the deconvolution of the standard CP/MAS spectrum into individual carbon type resonances, using chemical shift data obtained from standard compounds. RESLJL’ISAND DISCUSSION Sample Selection The main aim of this study was to investigate the effects of thermal maturity on kerogen molecular composition. In order to do this, it was necessary to choose samples where lithological variations were minim&d. Geochemical data (elemental analysis, pyrolysis, and biomarkers) were used to demonstrate that the sample suites varied predominantly in degree of thermal maturity and that organofacies (i.e., the organic matter input and the depositional setting) changes within the three sample sets were minor. Kimmeridge Clay Formation A maturity trend was established in the first instance using a combination of depth, HI and GOGI (Table 1). The increase in GOGI and decrease in HI with increasing depth of burial of samples indicate that these samples represent a trend

FABLK

0ri

&in

5-k 80.

Hontaray

Pornutf (USA)

on

1325.9

2362.0 3651.0 4063.0 5002.0 5262.0 957.0

rdmaaton. Fornution (KumtJ

1521.0 2499.0 3019.0 3393.0 3575.0 4195.0

Kiamaridga

C-Y

Formution (UKCSJ

1.

PI

-

I

J

I anomlous

n.d.

Depth

(=J

1000.0 2340.0 2905.0 3995.0 3975.0 4299.0 4659.25

1.

mc

PrRoLrsrs

a1

3.9 6.4 i:f 4.1 1.7 4.7 f:(' 4.4 2.3 9.5 2.4

2.3 1.5 1.9 2.3 3.7 2.1

17.6 35.7 19.3 14.3 13.9 4.1

n.d. a.d. n.d.

32.0 39.5 40.4 43.7 14.3 23.4 6.9

n.d. n.d. n.d. n.d.

7.1 9.9

1.0

9”:: 7.4

f:: 6.9 4.7 2.2 1.6

4.2 6.3

91 81 + 92 data

= gas: oil ratio

-AL

Iii

from pyrolymia-QC

27.5 33.4 19.7 ii.6 22.3 7.1 4.3

459 553 503 411 335 239 690 940 (1188J 993 621 246 297 387 375 323 345 301 169 69

OP

ANALYSKS

PI'

swws

POR

8lvDY

Sulphur

22.3 26.6 26.3 16.7 29.2 29.5

1.6

0.11 0.24 0.27 0.29

n.d.

n.d.

n.d.

n.d

0.20 0.29 0.23 0.39 0.29

n.d. n.d. n.d. n.d.

n.d. n.d.

Go&

0.12 0.04 0.09 0.12 0.21 0.34

393.5 389.3

0.14

419.0 421.0 426.2 424.5

n.d. n.d. n.d. n.d.

n.d.

n.d. n.d. n.d.

n.d. n.d.

n.d. n.d.

0.03 0.04 0.13 0.17 0.17 0.24 0.37

a

Carbonat* (WttJ

max C’CJ

fti*J

- no data

2. 0ooI

52

Aw

410.5 406.0 425.0 427.9 433.0 430.5 433.0

0.12

n.d.

0.17 0.14 0.18 0.20 0.23 0.31 0.72

n-d. n.d. 11.9 0.5 8.5 0.5 1.2 7.3 3.1

htlrJ

2: 1.4 3.0 1.3

n-d. n-d. n-d. n.d. n.d.

n.d. a.d. n.d.

n.d. n.d. n.d. 6.4

Molecular structure of kerogen from marine sediment

2121

rise steadily from around 0.10 to 0.79, while diasterane relative abundance increases from 7.3 to 65.0, rising more sharply prior to petroleum generation. For the deepest sample, only phenanthrene parameters ( RADKE, 1988) could be determined due to its high maturity and associated biomarker cracking. This helps little in the estimation of the maturity of this sample; both these aromatic parameters show considerable scatter across the suite, which may be due to the presence of varying amounts of C,r -Cl7 contamination, probably from drilling fluids, seen in the saturates GC trace (not shown). To summa&, the pyrolysis, elemental, and biomarker analyses of these seven samples confirm firstly that they are representatives of essentially the same organic matter type and secondly that they vary predominantly in their thermal history, as exemplified by biomarker and pyrolysis data, with the onset of petroleum generation between 3000 and 4000 m. It is not possible to be more specific for such a restricted number of samples from the central North Sea, because the depth at which significant petroleum generation commences is not uniform across the region.

a)

Or

---l---~

Brown Limestone Formation

HI

b) aOm (ml

a,

0

looot

r

aKKt-

---

0.16

44

aa6

1I

em

ooo

~-_-‘._-__~_--~_

0.0

--.-L_---._r

a00

400

HI

--m

-606I

FIG. 1. Plot of HI and GOGI vs. depth of burial for samples from (a) Kimmeridge Clay Formation and (b) Monterey Formation sed-

iments.

where between the two samples at 2905 and 3895 m, which is in agreement with the depth indicated by the HI and GOGI trends. Other maturity indicators (e.g., MACKENZIE, 1984) also show systematic variations with depth, e.g., Ts/Tm ratios

These samples were chosen previously because of their lithological uniformity ( BARWISEet al., 1984). Bulk parameters, such as HI and GOGI, were used to check maturity trends. Only limited data were available for the precise samples analysed previously and again here by NMR. As a result, the “kerogens” were reanalysed. These data are presented in Table 3. GGGI values show an entirely consistent upward trend with increasing depth of burial, whilst the HI data are more scattered. F’yrolysis-GC traces show only small changes with depth in the distributions of the “oil” (X5) components. The two most noticeable features are a general simplification of the product distribution and a preferential loss of higher molecular mass compounds (Fig. 3), whilst retaining an overall appearance which is commonly associated with amorphous algal/bacterial kerogens deposited in sediments in a restricted marine environment. In these samples, therefore, depth is a reasonable measure of maturity. As for the KCF, the depth at which petroleum generation commences is not uniform in the Gulf of Suez because of the variable heat flow, but it commences at ca. 3000-3500 m, a conclusion which is supported by these data. Monterey Formation The basic pyrolysis and elemental analysis data for the whole sediments (Table 1) show that there was no significant change in HI or GGGI for the three shallowest samples ( 1325.9, 2362, and 3651 m), followed by a steady decline (HI) and increase (GOGI) for the three deeper samples (4063, 5002, 5242 m; Fig. lb). This suggests that these samples represent a good maturity trend with depth. The pyrolysisGC traces indicate that there are minimal organofacies variations. The onset of petroleum generation would be placed at about 3500 to 4000 m from these data although, as with the KCF and BLF, the depth at which it commences is not necessarily uniform across the basin.

2728

R. L. Patience, A. L. Mann, and I. J. F. Poplett

TABLE

2.

BIO-R

DATA

FOR MONTEREY

AND

KImRImE

CLAY

FORMATION

Sarqplt3 NO.

A

B

Bfomarker C

Monterey Formation (USA)

1. 2 3 4 5 6

n.d. 0.48 0.60 0.56 0.59 0.65

n-d. 0.00 0.34 0.25 0.33 0.38

n.d. 33:35:32 37:33:30 33:42:25 34:38:28 36:34:30

n.d. 0.40 0.17 0.32 0.16 0.19

Kinrmeridge Clay Pormation (UXCS)

1 2 3 4 5 6 7b

0.32 0.19 0.42 0.59 0.59 0.59 n.d.

0.00 0.05 0.16 0.57 0.54 0.54 n.d.

39:18:43 38:20:42 34:27:39 33:30:37 32:30:38 31:23:46 n.d.

0.12 0.10 0.27 0.39 0.56 0.79 n.d.

origin

= no data I no data

duo due

to to

parameter D E

SEDIMENT

EXTRACTS

I

G

H

I

J

n.d. 0.76 1.05 1.29 0.79 0.74

n.d. 0.05 0.06 0.56 0.91 0.85

n.d. 0.37 0.23 0.60 0.62 0.74

n.d. 13 4 13 3 5

n.d. 1081 424 49 64 18

n.d. 49 55 47 70 79

0.53 0.57 0.46 1.05 0.77 0.52 1.09

0.04 0.10 0.47 0.09 0.82 0.67 n.d.

0.29 0.28 0.25 0.40 0.46 0.81 n.d.

7 21 33 48 50 65 n.d.

56 n.d. 323 28 27 7 n.d.

48 62 33 54 61 46 n.d.

contaminated extract ovennature sample

a b

n.d. n.d.

A B c

C,, hopano 22S/(22Rt22S) c,, mterane 209/120R*2OS) c,,:c,,:C,,aaa sterenes (%)

D

Ts/

E P G H I J

MPI c,, 2OR triaromatic steraae/(samo + C,, 2OR monoaromatic Sterane) c,. triaromatic ateranes/f*ame + C,, triaromatfc eteranes) p& steranss/all storanee (%) 28,30-blrmorhopane/hopans (%) Hopanos/fhopanes + atoraaee) (4)

/Te+Tm)

Determining the variation in maturity with depth of these samples from biomarker parameters (Table 2) is not straightforward, as is often the case in the exceptionally rapidly heated MF. The extract from the shallowest sample yielded anomalous results due to contamination, perhaps from drilling fluids. However, all the other bulk data for the kerogen itself-pyrolysis, py-GC, elemental analysis,&tend to suggest that the kerogen is not affected and is immature, consistent with its shallow depth. Thus, bulk data are included in this study, though extract data for this sample have had to be discounted. The remaining five samples behave broadly as might be anticipated with increasing maturity. Figure 2b shows how two commonly used molecular maturity parameters ( SEIFERT and MOLDOWAN, 1978; MACKENZIE et al., 1980; 198 1) increase with depth, confirming the trends shown by the pyrolysis parameters. In summary it is concluded that variation in organic input between the samples is minimal. The changes in properties that are Seen can therefore be attributed predominantly to increasing maturity with depth. All the parameters (GOGI, HI, and molecular parameters) point to the onset of petroleum generation at between 3500 and 4000 m. STRUCTURE

DETERMINATION

USING

13C-NMR

Quantitation

The accuracy of any quantitation in solid state 13C-NMR depends on the efficiency of the cross polar&ion in detecting carbon spins, and how this a&cts calculation offa and carbon structure distribution. This is a subject which has been discussed frequently in the literature (see, e.g., SNAPE et al., 1989 ) , and which was reviewed also by MANN et al. ( 199 1) . To summa&, calculations off ( = ArCO + ArC”C + ArCC + ArCH; Fig. 4) are believed to be valid except where the

H/C ratio is low, in which case fa is underestimated. The carbon type distributions obtained will be representative unless paramagnetic centres happen to be associated with a particular carbon type, although this is not believed to occur in reality. The carbon type analysis approach used here depends on calibration using chemical shifts of carbon types in standard compounds. In addition to the carbon types quantified here (Fig. 4)) there are a number of other compound types containing sulphur or nitrogen which have to be considered when interpreting the data, as they have not been included in the carbon type approach. These are ( 1) carbon attached to sulphur in alkyl or aryl-alkyl sulphides or in thiols (chemical shifts overlap with comparable oxygen compounds); (2) aromatic carbon attached to sulphur-i.e., thiophenes (overlap with aromatic carbon attached to alkyl sidechain); ( 3) aromatic carbon attached to nitrogen-i.e., pyrroles or pyridines (overlap as 2). Hence, the carbon type analyses will be alEcted for kerogens with high sulphur contents ( 1 and 2). It is not believed that ignoring nitrogen will alter results since this element is always present in insignificant concentrations. Variations in Kerogen Structure with Thermal Maturation

In all three cases (KCF, BLF, and MF), burial depth has been used for convenience as the indicator of increasing thermal maturation. Although it is recognised that the thermal history of each sample is not necessarily proportional to its current burial depth alone, the latter is an adequate parameter for illustration of the changes in molecular composition with thermal maturation. Changes

in

aromaticity (f,)

All three sample suites show the same general trend: a modest increase in f. prior to the onset of petroleum gener-

Molecular structure of kerogen from marine sediment

a) Depth (ml

1

0

loo0

moo -

4ow

-_

2129

geochemical data (Fig. la). The increase inf. with depth for the BLF kerogens is small down to ca. 3000 m, below which it increases rapidly. These data, therefore, give the same “dogleg” reported earlier by BARWISEet al. ( 1984). The MF samples again show the same characteristic kick inf., between 3600 and 4000 m in this case. Similar changes infa with thermal maturation have been reported in naturally buried (BARWISEet al., 1984; WITTE et al., 1988) and laboratory-heated sediments ( TAKEDAand ASAKAWA, 1988; WITTE et al., 1988), sediments which have been affected by igneous intrusions (DENNIS et al., 1982; SAXBY and STEPHENSON,1987), and for coals as “rank” increases (e.g., DAVENPORTand NEWMAN, 1985; DEREPPE et al., 1983; NEWMANet al., 1988, inter alia). These results therefore endorse the observation that the generation process cleaves off alkyl carbon preferentially, concentrating the bulk of the aromatics in the residual kerogen, together with any aromatics newly formed by aromatisation / condensation reactions (e.g., BARWISEet al., 1984). The nature of the alkyl carbon cleaved from kerogen, rearrangements of the aromatic structure, and the role of oxygen/sulphur are considered further in the following text. Changes in alkyl functionality

b)

---j :r

DopUb(ml

I

! !

I 1

\+

I I I

oooo-

I

4ooo-

oooo-

-cawcoo.ceoTmMc FIG. 2. Plot of two biomarker maturity parameters vs. depth of burial for extracts from (a) Kimmeridge Clay Formation and (b) Monterey Formation sediment samples.

ation followed by a rapid rise (Table 4 and Fig. 5). The increase in_&of kerogens in the KCF is particularly pronounced in the two deepest samples, indicating that petroleum generation is taking place below ca. 4000 m, consistent with the

Alkyl carbons are subdivided into a number of structural types -C, CH, CH2 (in midchain or ring), CH2 (adjacent to a methyl group), and CHs (attached to alkyl or aromatic C; Fig. 4). The changes in the relative abundances of these groups in the total organic carbon are given in Table 4 and shown in Fig. 6. The major features of these plots are that CH2 groups dominate the alkyl functionality, followed by lesser amounts of CH (e.g., branching points in acyclic isoprenoids or ring junctions in polycyclic structures). Both of these groups tend to decline with depth, as the total abundance of alkyl groups declines. The three data sets appear similar, in that the relative abundances of the various groups follow the same order ( CH2 > CH > CH3al > CH3arom > C), and there are no major preferential losses of particular structural types. The dominance of the alkyl groups by CHI in the kerogens is in agreement with data presented for a suite of coals ( DAVENPORTand NEWMAN, 1985), the laboratory heating experiments of TAKEDAand ASAKAWA( 1988), and the qualitative work on four marine kerogens of WITTE et al. ( 1988). However, ALEMANYet al. ( 1984) claimed that CH groups were dominant in coals, a situation complicated further by claims that the CH/CHl ratio is dependant on both rank (PAINTER et al., 1984) and the origin of the coal ( ERBATURet al., 1986). There does seem to be agreement that alkyl sidechains are cleaved during thermal maturation (TAKEDAand ASAKAWA, 1988; Wrr-r~ et al., 1988) and that, as a result, average chain lengths decrease (DEREPPE et al., 1983; DAVENPORTand NEWMAN, 1985; although there must be some doubt about the validity of chain-length calculations based on NMR data) and the proportion of CH3 groups increases. LAMBERTand WILSON( 1985) used the evidence ofdecreasing chain length to infer that the generation mechanism was therefore by betacleavage of alkyl groups attached to aromatic rings. However, the present data do not support an increase in the relative

2130

R. L. Patience, A. L. Mann, and I. J. F. Poplett TABLE3.

PylloLTSIS

mm

ELummL

AKALISIB

OP Km-8

FOR

m

STUDY

9-h

%e

1 2 3 4 5 6

51.8 49.1 35.7 45.0 a9.7 32.7

1 2 3 4 5 6 7

11.4 11.2 66.4 58.2 51.3 61.0 36.7

1 2 3 4 5 6 7

56.4 52.2 45.5 50.5 31.0 39.7 31.0

Origin

SUDplO No.

WC WaiR)

o/c wnw

Extract 0

Izxt Yialb

Nontaray

1

Pormation

2

1.44 1.44 1.42 1.32 1.14 1.03

0.140 0.083 0.081 0.092 0.041 0.033

4.1 3.7 17.3 15.4 18.4 13.2

0.08 0.08 0.49 0.34 0.62 0.40

1.37 1.40 1.40 1.42 1.27 1.30 1.12

0.078 0.114 0.060 0.053 0.052 0.062 0.043

a.d. n.d. n.d. n.d. n.d. n-d. n-d.

n.d. n.d. n.d. n.d. n.d. n.d. n.d.

1.40 1.42 1.23 1.23 1.23 1.03 0.81

0.130 0.089 0.076 0.054 0.046 0.032 0.035

2.1 4.5 4.1 9.6 6.2 5.8 1.2

0.04 0.09 0.09 0.19 0.20 0.15 0.04

origin

NO. Ilont.*ay

Ponmtfon

(USAJ

Brown Limoatone Ponmtion

(EgyPtJ

Kinrmeridge clay

Pormation K'KCSJ

lVSA)

3 4 5 6

Brown

tone Pormation Limes

IEayptJ

Kf

rawridge Clay Pormation RnrCS)

I

)

n.d.

-

1 2 3 4 5 6 7

anomalous

T..,

H/C’

O/c'

392.5 391.8 419.5 409.0 426.8 427.5

1.32 1.17

68.9 49.5

555 469 367 374 232 151

1.01 0.64

0.095 0.103 0.055 0.130 0.045 0.085

n.d. n.d. n.d.

54.3 45.5 299.1 314.1 212.3 314.5 82.7

438 406 450 540 414 516 235

412.8 402.3 409.3 429.8 425.5 432.5 429.5

1.55 1.71 1.34 1.26 1.33 1.16 (1.05)

n.d. n.d. n.d. a.d. n.d. n.d. n.d.

7.5 3.2 llI3 6.3 5.8 3.1 2.4

200.3 188.9 130.3 144.0 83.8 57.6 20.2

355 362 286 I85 270 145 65

400.5 399.0 417.5 428.5 431.5 434.5 (510)

1.19 1.22 1.03 1.09 1.01 0.76 0.54

0.150 0.090 0.170 0.090 0.110 0.090 0.140

SP

HI

6.5 4.4 a.7 4.9 4.1 4.3

287.7 230.5 130.9 168.5

n.d. n.d.

91

ka t-’

.

n.d. n.d.

ka t.,

OC

1.11 1.09

data

. no data

1 from elemental 2 extract/l

analysfa

C

proportion of methyl groups with maturation, either because 1) it genuinely does not occur; 2) it may be small and less than experimental error; 3) it may not become significant until much deeper/higher maturities. Changes in aromatic functionality The carbon in different aromatic structures (Fig. 4) can be subdivided into four types: ArCO, aromatic C attached to H (ArCH), aromatic C attached to sidechain alkyl C ( ArCC ) , and bridgehead C ( ArC”C ) -which collectively constitutef,. The data are given in Table 4 and the changes with maturity displayed in Fig. 7. In general, the three data

sets show remarkable consistency. Prior to petroleum generation, the only systematic change in relative distributions is a decline in ArCO. The other noteworthy point is that the initial distribution of the carbon types in the immature KCF kerogens is different to that in both the MF and BLF kerogens. In the KCF, the dominant group is protonated aromatic carbon (i.e., benzene-type), whereas the latter ranks behind alkylated and ring junction aromatic carbon in relative abundance in immature MF and BLF samples. This may also support the observation, made in the preceding text and previously (MANN et al., 199 I), that the KCF has a somewhat greater terrestrial organic input, since lignin is dominated by single aromatic rings.

Molecular structure of kerogen from marine sediment

Ii

I

II

2731

L

3383.0m

1521.0m

3575.0m

2499.0m

4185.0m

FIG. 3. Pyrolysis-GC traces for kerogens from the Brown Limestone Formation. See text for experimental conditions.

3018.0m

In general, at the onset of generation, ArC”C and ArCH increase dramatically with increasing thermal maturation, whereas ArCC shows a small or no increase and ArCO continues to decline in all three suites. Bearing in mind that f. is also increasing, the increase in ArC”C and ArCH can be interpreted in two ways: either new aromatic structures are being formed, or the increase is due to a concentrationeffect as other aromatics are lost (e.g., to liberated petroleum). The relative importance of these two possibilities will be considered later. It has been noted previously that ArCH tends to increase with coal rank (WILSON et al., 1984) and during artificial maturation of kerogens ( TAKEDAand ASAKAWA, 1988) and that alkylated aromatic carbon decreases during the latter

process ( TAKEDA and ASAKAWA, 1988). TAKEDA and ASAKAWA( 1988), however, were unable to distinguish between aromatic carbons that were alkylated (i.e., ArCC in the nomenclature used here) and those that were in ring junctions (i.e., ArC’C), but it has been shown here that ArC”C increases. Other authors have reported the disappearance of phenolic compounds with increasing coal rank (e.g., NEWMAN et al., 1988) and that aromatics in general are formed during maturation by dehydrogenation ( DEREPPE et al., 1983), whilst BURGER et al. ( 1985) reported that the average number of condensed rings in coals increased from 2-14 to 48 as %C increased from 83 to 91%. WITTE et al. ( 1988) reported the same qualitative trend in the increase in protonated aromatic carbons and internal quatemary ‘(i.e.,

R. L.Patience, A. L. Mann, and I. J. F. Poplett

2732 FUNCTIONALITY

STRUCTURE Cdl-l\

CARBONYL

c,n=o

SYMBOL c-0

CARBOXYL

COOR

PHENOL

AtGO

BRANCHED

AROMATIC

ArCC

BRIDGEHEAD AROMATIC

ArC’C

PROTONATED AROMATIC

ArCH

OXY-METH

CHO

I NE

OXY-METHYLENE

Cf%&O-C/H

OXY-METHYL

pqJo-c

QUATERNARY ALIPHATIC

c-@-c

CH,O

C

c+

METHINE

METHYLENE

CH,O

(CZ)

METHYLENE

ALIPHATIC

METHYL

AROMATIC

METHYL

C

CH

C-F&W

CH,W)

c-p@

C’+,

;CHm

CH,al

rate of loss of this heteroatom( s) associated with alkyl cleavage, or petroleum generation, a feature which was also observed by TAKEDA and ASAKAWA (1988). This does not necessarily imply a lack of involvement of heteroatoms in this process. Rather it shows that NMR cannot provide an answer, because the data are effectively normal&d to the material present (i.e., left after generation). Heteroatoms may or may not be lost and the consequences of cleavage on either side of the heteroatom in the linkage kerogen-0 (or S)-alkyl would produce different NMR responses by the residue. The carbon attached to oxygen can be subdivided into a number of carbon types-carbonyl (C=O), carboxyl (OR), ether/alcohol (CHfi), and phenolic ( ArCO) (Fig. 4)-and changes in their relative abundances with increasing maturity are given in Table 4. Carboxyl and carbonyl groups are particularly thermally unstable in comparison to ether/ alcohol and phenolic-type carbon-oxygen bonds. The principal difference between the KCF and the BLF/MF kerogens lies in the abundances of the phenolic ( ArCO) group relative to the alcohol/ether group (CH,O). Whilst CH,O (x = 2 and 3) groups are dominant in the BLF and MF kerogens (assumed to be a function of the higher lipid relative to lignin contribution), there is no significant difference in the relative abundances of ArCO and CH# in the KCF, implying a slightly greater contribution of lignin, and hence land plant material, to these sediments. A contribution from the latter source would also explain why the KCF kerogens appear to have somewhat higher initial aromaticities (ca. 33%) (lignin is also highly aromatic) than either the MF or BLF (ca. 27% ) .

CH,arom

Generation, Aromatisation, and the Hydrogen Budget

FIG. 4. Description of the carbon types quantified by NMR, in terms of functionality, structure, and symbol used.

ring junction) carbons with increasing maturation of a series of four Toarcian shales, a phenomenon which is here demonstrated on a much larger sample set. Changes in oxygen functionality The general increase in aromaticity (and hence decrease in alkyl carbon) with maturity in all three sample suites is associated with a decline in the amount of carbon in oxygencontaining carbon types (Table 4) and hence the total amount of carbon attached to oxygen (fo in Fig. 8). It should be remembered that “oxygen” may also be sulphur, due to the limits of NMR analytical resolution (see “Quantitation” section). In the case of these samples, the level of “organic” sulphur (total minus pyrite sulphur) is estimated to be between 33 and 150 atom% of the organic oxygen. Consequently, the oxygen functionality obtained from NMR spectroscopy should be a reasonable representation in most cases, but the overlap with relatively abundant sulphur means that the data should be interpreted with caution, particularly as two of the sample suites (BLF and MF) are supposedly high in organic sulphur. The decline in carbon attached to oxygen is regular and does not rapidly alter at the point of petroleum generation (Fig. 8), which suggests that there is no acceleration in the

The following sections have two aims. The first is to evaluate whether the combined geochemical and NMR data can contribute to understanding the extent to which aromatisation reactions take place during petroleum generation. The second is to assess whether these reactions are sufficiently extensive that enough hydrogen is liberated to “cap”, or heal, broken carbon-carbon bonds. The object of this latter exercise is to establish whether an organic source alone can provide the hydrogen required, or whether an inorganic source (e.g., Hz0 or minerals) must of necessity be invoked. To achieve these aims, the following calculations and assumptions have been made:

1) The loss of kerogen mass during diagenesislgeneration can be calculated from the change in HI (immature + mature) and the mass lost is converted into petroleum. 2) The percentage loss of mass can be used to calculate a theoretical increase inf. due to a concentration effect only. 3) It is assumed that the difference, if any, between this theoretical value off. and the actual measured value is due to aromatisation reactions (whilst taking into account the amount of aromatic carbon lost into/with the generated petroleum), which hence provides a measure of the aromatic carbon formed. 4) An assumption is made that one carbon atom “aromat&d” liberates one hydrogen “atom” (i.e., two hydrogens freed per double bond formed), which thus gives the total hydrogen liberated from a knowledge of the total aromatic carbon formed in 3 ) .

Molecular structure

TABLE 4. CARBON TYPE DATA FROX NWR FOR MONTEREY Origin

Noatuey Formation

(USA)

BrOWI

Liamstoao Ponnetioa (EwRt)

Kimmri dge clay Ponmtioa WXCS)

Origin

Monterey Formation (USA)

Browa Lfmwtoa~ Fonmtioa (Emt)

Kfnmoridge Clay Pornmtloa

VJKCS)

1 2

Data aonmlimd x = 2+3

&%Bipl* NO.

f.

1 2 3 4 5 6

27.1 30.1 33.9 42.0 52.2 63.1

1 2 3 4 5 6 7

AND

KIIWBRIDGB

Oxygoa-containing carbon typo8 cm0 COOR Arc-0 CHO 0.9 0.0

2733

of kerogen from marine sediment CL,AY WRMTION

err,&

Aromatic crrboa typo= Arcc Arc-C Arm

0.0 0.0 0.0

2.6 0.3 0.4 0.2 0.4 0.6

2.9 3.4 2.3 3.2 2.3 3.6

2.2 1.9 0.9 4.9 0.8 0.4

6.8 4.8 2.9 2.1

8.2 6.4 8.2 9.8 6.1 8.7

14.9 12.6 25.6 24.2

7.0 10.5 8.5 16.4 18.2 26.6

27.5 29.6 31.0 31.1 41.0 42.1 48.8

0.6 1.5 0.1 0.0 0.0 0.0 0.0

1.1 0.0 0.6 0.5 0.4 1.0 0.5

1.7 2.6 1.4 1.4 0.9 1.0 0.5

1.5 2.4 0.9 0.9 1.4 1.3 1.0

4.6 7.5 4.4 3.9 3.4 3.9 3.3

9.0 9.7 11.3 10.0 11.6 8.5 11.2

12.8 12.0 10.4 9.7 14.9 18.6 25.6

4.1 4.0 7.5 10.1 13.5 13.7 11.5

1 2 3 4 5 6 7

33.4 33.0 47.8 45.1 46.4 62.3 76.9

1.1 0.4 0.3 0.3 0.0 0.0 0.0

2.6 1.5 1.0 0.5 0.4 0.2 0.9

4.6 4.9 5.2 3.3 2.5 2.8 1.7

2.1 0.6 0.1 0.6 0.0 0.8 0.0

f:i 2.2 4.0 3.8 2.0 0.9

8.9 8.9

8.0 4.6 20.4 17.3 17.1 21.0 35.6

11.9 14.6 12.8 12.1 14.5 23.8 28.9

Sample

Alfphatic

carbon

No.

C

CR

0.8

0.0

32.0 37.6 34.0 29.0 25.2 16.6

::: 1.6 1.5 1.4 1.0 1.2

14.1 11.0 11.6 11.5 9.3 7.9 6.8

38.7 36.3 38.4 41.3 32.9 29.7 29.9

0.7 0.5 0.0 0.0 0.2 0.0 0.0

11.7 14.0 7.4 8.5 7.8 5.3 2.1

27.6 30.4 25.3 26.2 27.0 16.0 8.2

0.7 0.5 0.1 0.2 0.0

12.3 11.8 10.2 9.3

typoa Cn,(C2) =I&

56::

Z:i

KERoOEN#

CH,arom

C&d

3.9 3.5 3.4 4.2 2.9

3.4 2.9 4.1 3.6 3.7 3.3

5.7 4.8 5.4 4.3 4.1 3.7

f:i 4.4 2.9 3.9 3.0 2.5

2.9 3.9 3.7 2.9 2.5 5.2 2.9

3.5 3.6 3.7 3.7 3.9 5.6 2.9

i:'o

f:,"

6.4 7.0

3.5 ::1"

5.1 4.6

7.1 5.9 6.4

4.9 2.7

i:'B 3.9

5.7 4.3

4.9

1::: 12.4 14.7 10.7

8.9 9.9

to 100%

5) A calculation is made to establish the amount of hydrogen required to heal the bonds broken during generation of the mass of petroleum calculated in ( 1). Following these steps, the difference between the amount of hydrogen liberated in (4) and the amount required in (5) is compared. 6) Finally, the H/C ratio of reservoired petroleums is compared with the calculated H/C ratio of generated products. Potential Oil Yield, f. and Aromatisation There is a strong correlation between 1 - f. and potential oil yield in shales ( MACIELet al., 1978, 1979; BARRON,1982; MIKNIS et al., 1982a,b; HAGAMANet al., 1984, MIKNIS and MACIEL, 1984). This can also be observed when HI is plotted against /. for the MF and KCF kerogens, for which the mlevant data exist (Fig. 9). A critical issue is whether the increase in kerogenf. as HI declines is due to selective concentration of aromatic carbon as alkyl groups are lost or to the formation

of new species. To establish this, the expected f., assuming concentration only, is calculated for a given change in HI, and then compared with the observed f.. To do this, the kerogen mass lost is calculated for given ranges of HI values. Changes in HI during thermal maturation have been used here, rather than TOC (cf. WITTE et al., 1988) or Sr values (cf. COOLESet al., 1986), because TOC and Sz are far more variable in a given environment of deposition than HI (see, e.g., data for the Peru upwelhng region in PATIENCEet al., 1990). Thus, for a given source interval, the change in TOC due to maturity effects alone cannot be distinguished from variations in initial TOC values. The Relationship Between Changes in HI and Loss of Kerogen Mass In order to calculate the expected aromaticity of a given kerogen after petroleum generation has begun, it is important to be able to calculate the fraction of the initial kerogen mass

2734

R. L. Patience, A. L. Mann, and 1. J. F. Poplett 80

70

50

f, (%)

Monterey Fm

--e

Clay Fm _-z__Kimmeridge Brown Limestone Fm

50

40

30

20 1000

2000

3000

4000

5ooo

Depth (m) FIG. 5. Changes in f. with depth of burial for the three sample suites analysed.

which has been lost. To achieve this, it has been assumed that the initial total organic mass (TOM) can be split, conceptually at least ( COOLES et al., 1986), into two components. For a given HI value, therefore, X = fraction of TOM which can be “generated”; (as measured by pyrolysis), and 1 - X = “inert” fraction.

After a period of generation, the HI declines to a new value. Our two-component model now consists of Y = fraction of the original TOM which can still be generated, and 1 - X = inert fraction. The fraction of the initial TOM which has been generated is X - Y. But, HI = (Sz - lCKl)/TOC.

a) KimmeridgeClay Fm. 40

c

l ___-----

.-4

-* 4 .

30

(I

‘*__-----

I,)

+C -c

20

- +

%C

++

CH

- all CH2 CHBarom CH3al

10

0 r so0

1000

1500

2ca

2m

3000

3500

4000

4500

5Ooa

Depth (m) FIG. 6. Changes in alkyl carbon types as a function of burial depth for the three sample suites analysed.

Molecular structure of kerogen from marine sediment The maximum theoretical value for HI (i.e., when X = 1) is taken to be ca. 1200. This is based on the assumption that the TOM contains 85% by mass organic carbon on average, i.e., TOM X 0.85 = TOC (COOLES et al., 1986). In the hypothetical case of a sample that generates all its organic carbon, then Sa = lO*TOM or Sz = (lO*TOC)/ 0.85. Hence, HI = (lo* lOO)/O.SS = 1200, approximately. For any other initial value of HI (HI,), therefore,

2135

HI1 = x. 1200

(1)

And for any final value of HI ( HIr), Y l-x+y’

HIF -= 1200

(2)

b) Brown Limestone Fm.

z= EH - + -A+

~:j

+> loo0

SC0

- all CH2 CHBarom CH3al

, 1500

3om

2!ioo

2wo

3!500

4ooo

4500

Depth(m) c) Monterey Fm.

l.

\

‘e- _ _ _ - _

‘9

+C 9.1

- + -A+

l

Ot’. loo0

lsw

2ooa

2500

3mo

3500

4mo

Depth (m) FIG. 6 (Continued)

4500

5mo

5500

-

CH all CH2 CHBarom CH3al

2136

R. L. Patience, A. L. Mann, and I. J. F. Poplett a) Kimmeridge Clay Fm. 40

10

0 loo0

2m

XQO

4ow

5ooo

Depth (m) FIG. 7. Changes in aromatic carbon types as a function of burial depth for the three sample suites analysed.

MF as the HI changes from 550 to 150 during thermal maturation and 27% of the original TOM has been lost for the KCF as the HI changes from 375 to 65.

Substituting Eqn. ( 1) into Eqn. (2) for X, l---

HII 1200

Observed and Predicted f. Values

Rearranging, (1200-HIr)*Y=(l

-s)*HIr

Therefore, y = (1200 - HII) (HI,) (1200 - HIr)*(

(3)

Combining Fqns. ( 1) and ( 3),

X-

--(HI,) ‘=(1200)

( 1200 - HIi) (1200-HI,)*0

(HIr)

= HIt*( 1200 - HIr) - HIr*( 1200 - HIr) 1200*( 1200 - HIr) Finally, X-Y=

HIi - HIr 1200 - HIr ’

This relationship between changes in HI and the fractional loss in organic matter which results is demonstrated graphically in Fig. 10, for a range of initial HI values. From the pyrolysis data for these sample suites (Table 3 ) , it has been estimated that the MF and the KCF have “initial” (i.e., immature) HI values for the kerogens of 550 and 375, respectively, and “final” (i.e., when mature) values of 150 and 65, respectively. Thus, using Eqn. (4) to calculate X - Y, an estimated 38% of the original TOM has been lost for the

The effect that this mass loss has onf, for a given kerogen depends on two factors: ( 1) the composition of the petroleum generated and (2) whether aromatics are formed or merely preserved in the residual kerogen. Here, first, only the former of these two factors is considered. In theory, the petroleum generated can have an aromaticity value in the range from zero (i.e., wholly alkyl) up to the value of the initial source organic matter. (It is assumed that the generated products will not have afa value greater than that of the kerogen.) The aromaticity value for the immature source organic matter can be calculated from Fig. 9 and is approximately 24% for HI = 550 (MF) and 32% for HI = 375 (KCF). If the petroleum generated over the HI interval has anS, equal to the initial kerogenf, (i.e., 24% for the MF and 32% for the KCF) then the final kerogenf, will be the same as the initial fa (assuming no formation of aromatics in the kerogen); if the petroleum has no aromatic content (the other theoretical extreme), then final kerogen fs = 24.0 [ 1 /( 1 0.38)]% = 38.8% for MF, = 32.0 [l/( 1 - 0.27)]% = 43.6% for KCF. Evidence on the aromaticities of reservoired petroleums (e.g., BARWISEet al., 1984), and other compositional data on petroleum, suggests thatf. values of 15-20% are typical. Hence, it is likely that the aromatic contents of products generated would be far closer to the initial kerogen fa figure than to 0%. The observed values off. for the mature kerogen from Fig. 9 are 60.2% for the MF and 76.0% for the KCF. Thus, for

2731

Molecular structure of kerogen from marine sediment b) Brown Limestone Fm.

25

m-

-A--_

0 1oM)

500

1500

---W---c.+a__ 2ooo

Dep;hym)

-. -w 3ooo

4wo

3500

4

4500

c) Monterey Fm.

--8---_

.---

----___.p

----

.lwo

2mo

4ow

3ow

5ow

6oca

Depth(m) FIG.

7.

the MF (initial HI = 550, final HI = 150)) the final kerogen f. predicted would be 24.0% iff. of the petroleum generated is 24%, whilst the predicted value for the kerogen is 38.8% if the petroleum f. is 0%. In practice, the observed kerogen fa is 60.2%. Similarly for the KCF (initial HI = 375; final HI = 65 ), the predicted final kerogenf. is 3 1.8%if the generated petroleum hasf. of 3 1.81, whilst the predicted value is 43.6% if petroleumf, is 0%. These values contrast with the observed kerogen f. of 76.0%. Hence, the observed final value offs is around 20-358 higher for the MF, and around 30-45% higher for the KCF,

(Continued) than the value predicted by assuming that aromatic species are concentrated, and not formed, in the kerogen during generation. On the basis of these calculations, therefore, extensive aromatisation reactions are taking place during generation. The Hydrogen Budget-Hydrogen

Generated

It is reasonable to assume that these aromatisation reactions are at the expense of alkyl structures in the kerogen, in which case hydrogen must be liberated in some form (not necessarily as hydrogen gas). Some of this hydrogen will be needed for

2138

R. L. Patience, A. L. Mann, and I. J. F. Poplett

.-o=-a--

*

2000

IWO

3000

4000

-

Monterey Fm Kimmeridge Clay Fm Brown Limestone Fm

5000

Depth (m) FIG. 8. Changes in the total amount of carbon attached to oxygen (h) with depth of burial for the three sample suites analysed.

the increase in H/C ratio when, e.g., alkyl structures are cleaved off to form alkanes in petroleum; thus, e.g.,

Alkyl-CC-X-Ker

+ 2H + Alkyl-C-H + H-X

-Ker

(i)

(alkyl + hydrogen + alkane) where X is an unspecified atom (C, 0, N, S ).

100

fa (%I

r

1

Whether the hydrogen released by aromatisation is enough to complete this type of reaction, or is even in excess, depends on the extent of aromatisation, the calculation of which is in turn dependant on an assumption of the aromatic carbon lost to petroleum generated. The percentage of the original organic (alkyl) carbon which is converted to aromatic carbon (f,) can be approximated by this equation: f+ = aromatic C in final kerogen + aromatic C in petroleum - initial aromatic C. For the MF,f, = (60.2 X 0.62) + (f. oil X 0.38) - 24.0, where f.oil can range between 0 and 24%. Hence, fm= 13 to 22% for& oil = 0 to 24%, respectively. For the KCF,f, = (76.0 X 0.73) + (f. oil X 0.27) - 31.8, wheref, oil can range between 0 and 31.8%. Hence, fQ= 24 to 32% forf. oil = 0 to 31.8%, respectively. Assuming that, on aromatisation -CH2---CH2-

I

0’

0

100

200

200

400

600

600

700

HI -

MontwoyFm

+-KCF

FIG. 9. Plot of HI vs. f, for the Kimmeridge Clay and Monterey Formation kerogens.

+ -CH=CH-

+ 2H,

(ii)

then one carbon atom “aromatised” yields one hydrogen atom. For the MF, if 13-22 carbon atoms per 100 undergo aromatisation, then 13-22 hydrogen atoms are liberated. But, from the mass balance, 38 carbon atoms per 100 generate petroleum. (This assumes that there is no substantial difference in the carbon content, as a weight percentage of the total organic matter, between the kerogen and the petroleum. Given the other assumptions made in this argument, it is probably reasonable to disregard any variation in carbon content.) The available new hydrogen per carbon atom in products (H:C) for reaction (i) above is therefore in the range 13138 to 22138 or approximately 0.33 to 0.6. This suggests

2739

Molecular structure of kerogen from marine sediment

- -A- -o-

-V-t+

0

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

HI= 1100 HI=800 HI= 480 HI=600 HI= 200

1 .oo

C REMOVED (FRACTION) FIG. 10. The relationship between changes in HI and the fractional loss in organic matter which results for a range of initial HI values. that there are ca. 0.33 to 0.6 atoms of hydrogen atom of carbon generated.

liberated per

For the KCF, if 24-32 carbon atoms per 100 undergo aromatisation, then 24-32 hydrogen atoms are liberated. But, from the mass balance, 27 carbon atoms per 100 generate petroleum. The available new hydrogen per carbon atom in products (H:C) for reaction (i) is therefore in the range 24/ 27 to 32127 or approximately 0.9 to 1.2. This suggests that there are ca. 0.9 to 1.2 atoms of hydrogen liberated per atom of carbon generated. The Hydrogen Budget-Hydrogen

Required

The quantity of hydrogen made available by aromatisation may be compared to the amount estimated to be required to cap C-X (X = C, N, 0, S) bonds cleaved during generation. To do this, it is necessary to have some idea of the composition, e.g., in terms of carbon number distribution, of petroleums typical of those generated from the source rock type under consideration. This type of data may be obtained from reservoir fluid studies, although it should be stressed that these are reservoired and not generated fluids, and may therefore differ in composition, particularly in that the former tend to have higher saturated hydrocarbon contents than the latter. Table 5 gives the composition of three contrasting petroleum samples expressed as mol%. Two of these are marine sourced oils, from the North Sea (i.e., Kimmeridge Claysourced) and the Gulf of Suez (carbonate lithology ), whereas the third comes from a lacustrine source (Suphan Buri Basin, onshore Thailand). To simplify the argument, it is assumed that each organic molecule that ends up in petroleum starts out as an alkyl

group attached to kerogen and, on generation, it requires the addition of one hydrogen atom to cap the cleaved end. In the present discussion, what really matters is the ratio of (H taken up):(C in the petroleum); the precise nature of the product, be it acyclic alkane, cycloalkane, or arene, is not significant. More important, however, is the carbon number distribution of the oil; waxy oils, with a dominance of long carbon chains will have a lower hydrogen demand per carbon atom than more gassy oils. From the data in Table 5, this demand can be estimated from the following assumptions: 1) Each methane molecule generated requires one H per C, 2) Each ethane molecule generated requires one H per two C atoms; 3) Each propane molecule generated requires one H per three C atoms, etc. TARLR 5. COXPOSITION

&VLW

component North Saa 36.1 7.0

8.8

3.7 3.0 2.6 4.0 4.6 4.3 3.9

::“o 15.0

321

OF TYPICAL RESRRVOIRRD FLUIDS Oil mInp1a Oulf of 9U.z 47.4 10.3 7.2 5.1 ::: 3.1 i:: 1.9 1.4 i::

273

Thailand 7.7 0.5 ::;: 2.1 1.4 2.4 4.9 4.2 4.0 ::: 61.0

345

2740

R. L. Patience, A. L. Mann, and I. J. F. Poplett

In other words, an average of one bond is broken in order to generate one petroleum molecule. The overall H required per carbon atom (H:C) will thus depend on the carbon number distribution of the generated product. One further assumption is also needed: the average composition of the Cl*+ fraction. An estimate can be made by simply dividing the average molecular weight of this fraction by 14 (equivalent to CH2). Thus, for the North Sea oil, this approximates to CZJ (though in fact the final outcome is fairly insensitive to whether it is C2, or CZ5; what controls it more strongly is the relative proportion of light and heavy components, i.e., its gas:oil ratio). For the other two petroleums, the averages are C2,, and C&, respectively. From the composition (expressed in mol%), the total number of carbon atoms involved in uptake of 100 hydrogen atoms (corresponding to generation of 100 petroleum molecules) can be estimated. For the North Sea oil, this is equivalent to number of carbon atoms = (36.1 X 1) + (8.8 X 3) + (7.0 X 2) + (3.7 X 4) + . . . + (15.0 x 23) = 684.0, number of H atoms = 100, hence, required H:C ratio = lOO/ 684.0 = 0.146. Had the C12+ fraction averaged CZI, the ratio would have been 0.153; had it averaged C&, the ratio would have been 0.140. Thus, the required H:C ratio is likely to lie somewhere in the range 0.14-o. 15. Analogous estimations for the other two oils show that, for the Suez and Thai oils, the required H:C ratios are in the ranges 0.23-0.25 and 0.05-0.06, respectively. Despite the uncertainties and assumptions made in these arguments, the conclusion must be that there is much more hydrogen generated during kerogen breakdown (H:C ratios are estimated to lie in the range 0.34-0.58 for the MF and 0.9- 1.2 for the KCF) than is needed to satisfy the demands of the products. However, the argument has as yet only considered the hydrogen involved in capping the ends of the generated petroleum. If each of the uncapped ends of the kerogen were also capped with hydrogen, then the required H:C ratios would be twice those calculated. However, it seems unlikely that all the loose ends are tied up in this way; one strong line of evidence which suggests this process does not always occur is that electron spin resonance (ESR) signals from kerogen are known to increase with maturity, especially during generation ( MARCHAND and CONARD, 1980). Hence, increasing numbers of free radicals are produced, and not all of the cleaved kerogen-alkyl bonds are capped on the kerogen side. No attempt has been made here to estimate what fraction of bonds this affects; all that can be said is that the required H:C ratios estimated above represent a lower limit and that they could be up to twice as great. Nevertheless, the conclusion still holds: at least as much hydrogen as is needed to cap broken bonds is available from aromatisation reactions; indeed it is likely that there will be an excess.

ratios for the petroleums as follows: North Sea 2.29, Gulf of Suez 2.46, Thailand 2.11. These values are calculated by assuming that all the petroleum is made up of alkyl carbon, which is clearly not the case. Furthermore, the acts of expulsion and migration tend

a)

_-.-.._._ -----_

r

lucmb ----

1.0

~~~---.___-_..._.--_---_._.. loo

a00

a00

400

HI

b) ,.&_

H/C mtb ____.__.

--...

_-__.

____

_-.

_ ..__

---

__._

-_

Hydrogen in Petroleum

More evidence for this last statement can be provided by comparing the “calculated” H/C ratios ofthe total generated products from the MF and KCF kerogens, with the “observed” H/C ratios for typical reservoired products obtained using compositional data in Table 5. The latter give H/C

FIG. 11. Plot of HI vs. H/C for the kerogens from (a) the Kimmeridge Clay Formation and (b) the Monterey Formation, using data from elemental analysis and NMR.

Molecular structure of kerogen from marine sediment to increase the alkyl content compared to that of the generated

products (i.e., saturated hydrocarbons are expelled in preference to aromatic and polar hydrocarbons; LEYTHAEUSER et al., 1988), whose H/C ratio is really of interest. In other words, these H/C ratios above are the maximum values, and they will be somewhat lower in reality for the generated products (i.e., prior to expulsion and migration, and with a significant aromatic carbon content). The “calculated” H/C ratios of all the products (gaseous and oil) from breakdown of the MF and KCF can be estimated from a plot of HI vs. H/C for the kerogens using data in Table 3. Two plots are shown in Fig. 1 la,b, using H/C data obtained for the KCF and MF, respectively. Each plot shows the results from both elemental analysis and NMR, which are somewhat different and therefore are treated seg arately below. Using Fig 11b for the MF, with an initial HI value of 5 50 and final value of 150, the estimated initial H/C is 1.3 1 (from elemental analysis) or 1.52 (from NMR); the final H/C is thus 0.89 (elemental analysis) or 1.06 (NMR). The H/C ratio of generated products can be calculated using either elemental analysis or NMR data. From elemental analysis, if the initial kerogen has 131 H per 100 C (i.e., H/C 1.31, Fig. llb),thefinalkerogenhas(89X.62)=55Hper62C (i.e., based on the calculations in the preceding text that the final kerogen has lost 38 carbon atoms per 100 initial carbons and has an H/C ratio of 0.89 from Fig. 11b). Therefore, the ~~o~~c~~have (by difference) ( 131 - 55) = 76 H per 38 C. The H/C ratio is thus 2.0. From NMR data, if the initial kerogen has 152 H per 100 C (i.e., H/C = 1.52 from Fig, 1 lb), the final kerogen has (106 X .62) = 66 H per 62 C (i.e., the final kerogen has lost 38 carbon atoms per 100 initial carbons and has an H/C ratio of 1.06 from Fig. 11b ) . Therefore, the products have ( 152 - 66) = 86 H per 38 C. The H/C ratio is thus 2.26. These estimated H/C values for generated products are similar enough (2.0 vs. 2.26) and are very close to the calculated maximum H/C ratio of “typical” (i.e., reservoired) petroleum products (see preceding text). For the KCF, with an initial HI value of 375 and final value of 65, the estimated H/C initial is 1.25 (from elemental analysis) or 1.43 (from NMR) (Fig. 1la). The estimated final H/C ratio is 0.56 (elemental analysis) or 0.83 (NMR; Fig. 1 la). The H/C ratio of generated products can be calculated, using the same process as for the MF, from elemental analysis data: if the initial kerogen has 125 H per 100 C, the final kerogen has (56 X .73) = 41 H per 73 C. Therefore, the products have (125 - 41) = 84 H per 27 C. The H/C ratio is thus 3. I 1. Using NMR data, if the initial kerogen has 143 H per 100 C, the final kerogen has (83 X .73) = 61 H per 73 C. Therefore, the products have ( 143 - 6 1) = 82 H per 27 C. The H/C ratio is thus 3.04. These calculated H/ C values of the generated products are close to each other (3.11 vs. 3.04) and are si~ifi~n~y higher than the calculated maximum H/C ratio of “typical” petroleum products. Hydrogen

Budget-Summary

In summary, the above calculations, although based on a considerable number of unavoidable a~ump~ons and approximations, lead one to the conclusion that aromatisation

2741

reactions do occur during petroleum generation, liberating hydrogen. The fact that calculated H/C ratios of products from kerogen breakdown are similar to or higher than those of typical reservoired fluids supports the argument that more hydrogen is liberated by these aromatisation reactions than is used up in “healing” C-X bond breaking during generation. This suggests that aromatisation is not driven by the need to abstract hydrogen to saturate carbon during C-X cleavage, but is a more the~~ynami~ly and kinetically favourable process, and secondly that free hydrogen (not necessarily as gas) is liberated during the generation process. It is perhaps significant that free molecular hydrogen is seen as a product of hydrous pyrolysis of kerogens ( ESPITALIEet al., 1984; ROHRBACKet al., 1984), although it is only rarely reported as a constituent of natural gas. These arguments do not preclude, of course, the possibility of hydrogen coming from inorganic source(s) (e.g., water). However, they do demonstrate that such inorganic sources are not essential. CONCLUSIONS NMR has been shown to be a powerful technique for dete~ining average molecular structures of kerogens. It has been used to analyse three suites of Type II kerogens which vary in thermal maturity. Maturity studies show that petroleum generation can be considered to equate to a simple loss of alkyl groups (dominantly) from the kerogen accompanied by new aromatisation reactions in the residual kerogen. The data support the suggestion that there is enough hydrogen generated by these aromatisation reactions to mend bonds broken during petroleum generation. In fact, there may well be a net generation of hydrogen (not necessarily liberated as free gas) from the organic matter during thermal maturation.

Acknowledgments-We would like to thank the following present or former BP staff for their invaluable assistance: Robin Jackson, Sandra Rodrigues, Chris Downes, Mary Keogh, and Andy Evans for geochemical analyses; Mike Taylor for helpful comments and discussions on the NMR data; and John Price for elemental analyses. Editorial handling: S. C. Brassell REFERENT

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