Lacustrine organic facies. A biomarker study using multivariate statistical analysis

AdvancesIn Orllnic Geochemistry1989 Org. Geochem.Vol. 16, Nos I-3, pp. 197-210, 1990 Printed in Great Britain

0146-6380/90$3.00+ 0.00 Pergamon Press pie

Lacustrine organic facies. A biomarker study using multivariate statistical analysis H. IRWINand T. MEYER Statoil, Geolab, Forus, P.O. Box 300, N-4001, Stavanger, Norway

(Received 19 September 1989; accepted 15 January 1990) Abstract--Four distinct depositional settings in the Devonian Orcadian Basin of Scotland have been characterised by their mineralogy and organic geochemistry. With the help of multivariate statistical analysis, the data visualise the organic matter source input, the depositional environment and the level of thermal maturity. Principal component analysis proved invaluable for extracting information from a data set composed of a large number of parameters. The thermal maturity of the majority of the samples lies within a restricted range in the oil window and biomarker composition reflects predominantly organic input and depositional environment. Basin margin deposits are deafly differentiated from deeper water central lake deposits. Salinity and anoxicity variations are seen within the latter type of deposits.

Key words--lacustrine O.M., data analysis, biomarkers, mineralogy, maturity, Devonian, Scotland.

INTRODUCTION The organic geochemistry of the lacustrine rocks of the Devonian Orcadian Basin of N.E. Scotland has received increasing interest in recent years because of their significant source potential (Parnell, 1985; Marshall et al., 1985; Peters et al., 1989) and characteristic biomarker composition (Hall and Douglas, 1983; Duncan and Hamilton, 1988). In the present study 55 samples have been evaluated by bulk organic geochemistry and mineralogical analyses and 27 were selected for molecular analyses. The aim has been to characterise sources of organic matter and relate them to depositional conditions which varied in both time and space. Multivariate statistical analysis after Kvalheim and Karstang (1987) has been applied in order to extract more information from the data set and as a tool to visualise the compositional variation within and between sedimentologically defined depositional settings. A detailed account of the geochemistry is in preparation (Irwin and Meyer).

EXPERIMENTAL METHODS

The analytical methods used in this study are in accordance with Statoils' standard methods as these are given in the manual Organic Geochemistry Standard Analytical Procedure Requirement and Reporting Guide, revised version of June 1988. The methods are agreed upon by Statoil, Norsk Hydro, Saga Petroleum and the Norwegian Petroleum Directorate as those methods to be employed in standard studies carried out on geological samples from Norwegian North Sea wells. Rock samples were crushed in a centrifugal mill for about one minute to ensure a sample size of less than

63/~m prior to weighing into LECO crucibles and analysis for the total organic carbon content (TOC) in a LECO IR-212 Carbon Determinator equipped with an HF-100 Induction Furnace. Any carbonate present was removed by treatment with 10% HCI and washing with distilled water. Approximately 100mg aliquots of the finely crushed rock samples were pyrolysed by a method described by Espitali~ et ai. (1977) using a modified Rock-Eval II instrument. The method employs rapid temperature programming up to 330°C to release and quantify free hydrocarbons (S1) in the rock samples, and then heating to 550°C using a temperature gradient of 25°C/min to liberate and quantify pyrolysable hydrocarbons ($2) from the kerogen of the rock samples. The recorded temperature (/'max) at maximum hydrocarbon generation during the pyrolysis can be used as a thermal maturity indicator. Selected and finely crushed aliquots of rock samples (approx. 20-50 g) were extracted in a Tecator Soxtec HT-system using a mixture of methylene chloride and methanol (93:7% by vol) in extraction thimbles that were pre-extracted and rinsed. The extraction procedure involved boiling for 1 h in the extraction solvent system before rinsing for a period of 2-3 h. An activated copper band was placed in the extraction cup to remove elemental sulphur during the sample extraction. The extracts were filtered into preweighed flasks, and the extraction solvent removed by means of a Bfichi Rotavapor System with the waterbath kept at less than 30°C and using a vacuum controller to ensure a pressure of slightly above 200 mbar. The amounts of extractable organic matter (EOM, C15 + ) were obtained by gravimetry when constant weight was attained. The EOM was thereafter redissolved in 197

198

H. IRWINand T. MEYER

tetrahydrofuran (1:3 w/v) and 40 times (by vol.) of n-pentane was added before storage of the solutions for approx. 8 h in the dark and at ambient temperature to ensure asphaltene precipitation. The asphaltenes (ASPH) were gravimetrically quantified after filtration of the solution and removal of solvent residues. The deasphalted extracts were further separated into saturates (SATS), aromatic (ARO) and polar (NSO) fractions by medium pressure liquid chromatography (MPLC) after a method slightly modified from that described by Radke et al. (1980) employing a dual column system and n-hexane as the mobile phase. The precolumn in the present system was packed with cyanopropyl coated 5-10 #m silica particles, while the main column consisted of 40-63/zm silica gel Type 60 as described by the authors above. As the NSO compounds are retained on the precolumn during this preparative method, these components can be recovered by back-flushing the system using a methylene chloride methanol mixture (50:50% by vol.). The fractions were gravimetrically quantified after having removed the elution solvent. The SATS fractions were further analysed on a Hewlett Packard 5890A gas chromatographic system equipped with a flame ionizing detector (FID) and fitted with a 25 m (0.25-0.32 mm i.d.) fused silica capillary column coated with a 0.2/zm thick film of dimethyl polysiloxane as the bonded stationary phase. Helium (approx. 1 ml/min) was used as carrier gas, and the samples were injected in the splittless mode. The gas chromatograph oven was kept at 80°C for 1 min before following a temperature increment of 4°C/min up to 300°C where it was held isothermal for a period of approx. 20 min. The gas chromatographic analyses were recorded and processed by means of a VG Multichrome laboratory data system, based on a DEC PDP 11-73 computer/dual RD 53 and RL 02 disk drive system. The GC/MS analyses of steranes and hopanes were carried out on the SATS fractions employing an integrated VG 7250 double focusing, high resolution GC/MS/DS machine to function according to the SMIM (Selected Metastable Ion Monitoring) method first published by Warburton and Zumberge (1983) and further developed experimentally by Brooks et al. (1984) and Meyer et al. (1984). The chromatographic separations prior to the mass spectrometrical analyses were carried out on a Hewlett Packard 5890A gas chromatograph, operated in a similar fashion to that used in the SATS fraction GC analyses above. Bulk X-ray diffraction (XRD) analyses were carried out on powder mounts using a Philips 1771 instrument. Identification of peaks was done by reference to international standard analyses (Berry et al., 1974). The data are semiquantitative and calculated from peak heights. The multivariate data handling tool employed in this study was commercially developed under the

name SIRIUS by Kvalheim and Karstang (1987), and the samples were compared by means of the Principal Component Analysis (PCA) method, The first three principal components are presented in this study as the visualisation of the main variation in the sample set. KEROGEN TYPE AND DEPOSITIONAL SETTING

All samples were collected at outcrop from depths which eliminated or minimised effects of subaerial weathering. The sample loctions and stratigraphic intervals are indicated in Figs 1 and 2, respectively. The samples have been divided into 4 main categories according to age and sedimentary associations. These 4 groups are delineated on the PCA plots (Figs 4, 7 and 8) which are discussed in the following chapters. The range of kerogen types is indicated in Fig. 3. Group 1. The Lower Old Red Sandstone playa lake deposits

Deposition took place in mainly isolated playafilled basins of limited extent (Mykura, 1983); e.g. the foetid Strathpeffer lake deposits (Location 1). These samples contain less than 0.8 wt% Total Organic Carbon (TOC), composed of Type III kerogen with a significant amount of black, reworked or oxidised organic material. The high production index (PI) suggests that the free hydrocarbons may not be in situ

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Fig. 3. A plot of hydrogen index against T,= for samples from the Orcadian Basin. (Numbers refer to locations in Fig. 1). • Group l - - L o w e r Old Red Sandstone Playa lake deposits ( l , 32); x Group 2--Marginal and alluvial associated lake deposits (2, 8, 33); [O Group 3--"Central" lake deposits (10, 11, 15, 16, 17, 24, 26, 28, 29, 30, 31); • Group

4--John O'Groats Group lake infill deposits (19, 27).

These include samples from Location 2 which are associated with an alluvial fan. The organic content is negligible and the kerogen is Type III, with 5% black oxidised coaly fragments. The calcareous mudstones from Location 8 were taken from an alluvial plain area which received periodic lake transgressions. The mid-cycle samples have fair organic contents (> 1% TOC) and good hydrocarbon potential (PP 10kg HC/ton rock). The hydrogen index indicates kerogen Type II in these good samples and visual kerogen analysis indicates the presence of alginite-B (Leith, personal communication). One sample is included from the marginal location at Melby in Shetland (3363). It has around 1% TOC and petroleum potential of 3 kg HC/ton rock. Group 3. The Middle Old Red Sandstone "central" lake deposits These sediments were deposited in a large shallow lake covering Caithness and Orkney. Although there

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Fig. 5. Examples of chromatograms from lacustrine samples from the Orcadian Basin. Abbreviations: Pr--pristane; Ph--phytane; K---C25 regular isoprenoid; L--squalane; ? and ~--gamma and beta carotane; AI, AII, BI, BII terminology after Duncan and Hamilton (1988). are variations in lithology and organic content which reflect which part of the cycle has been sampled, this group of samples is typified by high organic contents (e.g. Locations 15, 16, 26 and 30) and kerogen Types I and II (Fig. 3) consisting of amorphous organic matter and alginite-B. Low organic contents in samples from, e.g. Locations 11, 13 and 24 may result from high maturity.

Group 4. The John O'Groats Group sediments In late Givetian times the lake was infilled by the John O'Groats and Eday Groups (Fig. 2). The organic content is below I wt% TOC and the hydrogen index indicates Type III kerogen. The kerogen is typically dominated by amorphous material. OG

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MINERALOGICAL

ANALYSIS

The results of principal component analysis of the normalised and autoscaled mineralogical data are shown in Fig. 4. The first principal component (PCI) accounts for 39.1% of the variance in the data; PC2 for 20.2% and PC3 for 19.1%. Altogether 78.9% of the variance is accounted for by these three components. The loadings plot shows which variables control discrimination along the PCs. Changes in the balance between inflow and outflow of the lake, controlled by both climate and tectonism, would have been reflected in lake transgression and regression and concomitant variations in sediment supply and salinity. These would have been recorded as lithological and mineralogical variations within the

202

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sedimentary deposits. Figure 4 can therefore be interpreted in terms of location within the lake basin. This interpretation is based on the assumption that the carbonate and pyrite are of primary or early diagenetic origin [see Duncan and Hamilton (1988), Janaway and Parnell (1989) and Irwin and Meyer (in preparation) for further discussion]. Group 2 samples 2/18 and 8/26 separate from the main group due to their high elastic and siderite contents. Sample 3363 separates out because of the relatively high carbonate content dominated by calcite. These characteristics are consistent with 3 different fresh water, marginal lake environments. Saline or hypersaline, reducing depositional environments are suggested for samples 1133, 1132, 3059 and 2857 from Group 3, along with those from Group 1 (1/15, 1/16) on the basis of their high dolomite and pyrite contents although the same effect could result from higher thermal maturity. This point is discussed later when considering the organic geochemistry. Variations in clastic input, the dolomite/calcite ratio and content of pyrite, chlorite and feldspar cause the spread in data for the remaining Group 3 samples. These variations are interpreted to reflect transgression and regression of the lake with accompanying salinity variations as indicated in Fig. 4. The John O'Groats Group samples do not separate from the main bulk of samples on the basis of their mineralogy. ORGANIC MATTER SOURCES Chromatographic data The chromatograms in this study were classified in a similar way to Duncan and Hamilton (1988). Examples of chromatograms are given in Fig. 5. Details of the parameters and the classification is presented in Irwin and Meyer (in preparation). A substantial algal/bacterial source input is indicated by a chromatographic distribution with Cmax between nC~7 and nCls (Malinski et aL, 1988); the

generally low n-alkane concentration above n-C25 and the frequent occurrence of significant ~- and fl-carotane (Tissot and Welte, 1984). An input from non-photosynthetic bacteria may be indicated by a particular abundance of nC25 (Han, 1970) and the stepped nature of some of the chromatograms is attributed to a specific organic matter from an unknown source or possibly bacterial reworking (Duncan and Hamilton, 1988). A maturity effect can be ruled out as both smooth and stepped chromatograms were obtained from samples within a few metres of each other. Some of the samples within the "central" lake deposits contain the C25 regular isoprenoid and squalane (K and L respectively in Fig. 5) along with - and fl-carotanes all of which indicate either high salinity or methanogenesis (Waples et al., 1974; ten Haven et al., 1988; Jiang Zucheng and Fowler, 1985). These samples form subgroup 3'. Group 1 samples also contain these characteristic compounds. The remaining samples do not contain these compounds and are interpreted as having been deposited in a fresh water, perhaps oxidising, environment. Biomarker data A representative example of SMIM fragmentograms for one selected depositional environment is given in Figs 6(a) and (b). Results of principal component analysis of the raw biomarker data which has been block normalised and autoscaled are shown in Fig. 7. PC1 accounts for 46.4% of the variance of the data; PC2, 16.0% and PC3, 9.0%. In Fig. 7 the effect of the variables on the principal components is shown by the loadings plot. The first and second principal components (PCI and PC2, respectively) are related to the relative content of regular steranes, rearranged steranes and hopanes. Furthermore the abundances of gammacerane, Ts and the unknown triterpane X carry the same type of information and further discriminate the samples.

Fig. 6(a). (Facingpage) Representative SMIM fragmentograms of steranes in a source rock extract from Group 3' "central" lake deposits with high salinity/anoxicity) from the Orcadian Basin. (Metastable transitions: M + - > m/z 217 for C27--C~0steranes.) Abbreviations (notation of steranes): C27 C28 C29 C30 s 24 s 24 s 24 s t - t - t - t e me e e pe r er tr rr a

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13fl(H), 17~t(H)-diasterane (20S) 13fl(H), 17a(H)-diasterane (20R) 13a (H), 178(H)-diasterane (20S) 13a (H), 178(H)-diasterane (20R) 5a (H),14~t(H),I 7~t(H)-sterane (20S) 5ct(H), 148(H), 178(H)-sterane (20R) 5ct(H), 148(H), 178(H)-sterane (20S) 5=(H), 14~t(H), 17a(H)-sterane (20R)

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Lacustrine organic facies The plot of PC1 vs PC3 shows further discrimination based on the content of the C2s and C29 steranes and diasteranes relative to the C27 counterparts. The Group 1 Strathpeffer stagnant lake samples are discriminated on the basis of their relatively abundant regular steranes indicating a particularly high algal organic content (Mackenzie et al., 1984). Reference to Fig. 4 indicates that these rocks are rich in pyrite and dolomite which along with the organic markers for high salinity confirms their deposition in a stagnant playa lake. Group 2 marginal deposits and Group 4 lake infill deposits are characterised by a higher proportion of hopanes signifying a higher content of lipids derived from aerobic bacteria (Comet and Eglinton, 1987) and/or a higher content of higher plant material (Mackenzie et al., 1984). Duncan and Hamilton (1988) also reported higher proportions of hopanes in fish-beds located at the margin of the Orcadian Basin. One "central lake" sample from location 15 (1546) groups together with the present marginal deposits. This sample is described as being stromatolitic and brecciated. Hopanes are also produced by cyanobacteria (Comet and Eglinton 1987) and may therefore also indicate the presence of cyanobacterial mats at the margins of the basin. The John O'Groats Group lake infill deposits (Group 4) form a group on the basis of their contents of gammacerane (G), the unknown C30 hopanoid (X) and Ts. As stated above these compounds appear to have something in common. Both X and Ts are maturity indicators (Cornford et al., 1986; Seifert and Moldowan, 1978, 1980) although X appears to

205

be related to terrigenous organic matter (Philp and Gilbert, 1986) and Ts may be related to saline depositional conditions (Mello et al., 1988). The precursor of gammacerane has not yet been confirmed but it may originate from protozoa (Hills et al., 1966; Henderson et al., 1969; Brassell and Eglinton, 1986) or bacteria (Mello et al., 1988) and is often associated with hypersaline sediments (ten Haven et al., 1985). The association of these three compounds suggests that both X and Ts may have a strong facies control in these samples. Group 3 "central" lake samples show a spread which is largely a result of small variations in the relative proportions of steranes, hopanes and diasteranes and particularly significant variations in the predominance of C2s and C29 relative to C27 steranes and diasteranes. This is illustrated by samples 1545-1553 from Pennyland Shore all of which have the same level of maturity. Only 5 of the 9 "saline/ anoxic" samples forming Group 3' are differentiated from the other Group 3 samples. The overlap in biomarker composition of the saline/anoxic and freshwater samples indicates that the organic source input was largely the same for both groups but that differences in the preservation potential of the environment (salinity and/or anoxicity) has caused usually the addition of only minor amounts of the special marker compounds discussed above. The samples which separate out from the main body of samples do so largely on the basis of their C28 sterane and diasterane content. They are samples which tend to be relatively enriched in dolomite, pyrite and silicate minerals (Fig. 4).

Fig. 6(b). (Opposite) Representative SMIM fragmentograms of hopanes in a source rock extract from Group 3' "central" lake deposits with high salinity/anoxicity) from the Orcadian Basin. (Metastable transitions: M+--* m/z 191 for C27-C35 hopanes.) Abbreviations (notation of hopanes): 27A 18r,(H)-22,29,30-trisnorneohopane (Ts) 27B 17~t(H)-22,29,30-trisnorhopane (Tm) 27C 171/(H)-22,29,30-trisnorhopane 28A 17a (H),21fl (H)-28,30-bisnorhopane 28B 17fl(H),21,,(H)-28,30-bisnormoretane 28C 171/(H),211/(H)-28,30-bisnorhopane 29A 17~t(H),211/(H)-30-norhopane 29B 171/(H),21ct(H)-30-normoretane X Unknown hopane 30A 17ct(H),211/(H)-30-hopane 30B 171/(H),21~t(H)-30-moretane 30C 171/(H),211/(H)-30-hopane 31A 17~t(H),211/(H)-homohopane (22S) 31B 17ct(H),211/(H)-homohopane (22R) 31C 171/(H),21~t(H)-homomoretane 31D 171/(H),211/(H)-homohopane 32A 17~t(H),21fl(H)-bishomohopane (22S) 32B 17ct(H),211/(H)-bishomohopane (22R) 33A 17ct(H),211/(H)-trishomohopane (22S) 33B 17ct(H),211/(H)-trishomohopane (22R) 34A 17~t(H),211/(H)-tetrakishomohopane (22S) 34B 17ct(H),211/(H)-tetrakishomohopane (22R) 35A 17~t(H),211/(H)-pentakishomohopane (22S) 35B 17~t(H),211/(H)-pentakishomohopane (22R) G Gammacerane

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The diagenetic formation of the rearranged (dia-) steranes was confirmed by Sieskind et al. (1979) and Rubinstein et al. (1975), who describe a clay-catalysed protonation mechanism. The reaction occurs in clastic and argillaceous sediments. The diasterane to sterane ratio is also dependent on thermal maturity (Seifert and Moldowan, 1978). The absence of diasteranes has been interpreted as indicative of carbonate rich lithologies (McKirdy et al., 1984; Palacas et al., 1984) and the diasterane/sterane ratio for the present samples is generally low. Significantly higher contents of diasteranes are present in Group 4 samples which may reflect their different organic facies or elevated maturity. Mineralogically Group 4 cannot be differentiated from Group 3 samples (Fig. 4). This implies that the higher diasterane content of these samples is a result of thermal maturity rather than facies. Sample 2/18 (alluvial fan association) is discriminated from the central lake deposits by its very low sterane and diasterane content. Mineralogically it differs from the other samples by its high siderite content indicating a freshwater anoxic depositional environment low in calcium. It also contains a significant amount of terrigenous organic matter as indicated by its n-alkane distribution (Fig. 5).

ORGANIC MATrER MATURITY

Marshall et al. (1985) indicated that the maturity of the Orcadian Basin in general lies within the oil window. In addition, the low maturity of older stratigraphic horizons at the margins of the basin indicates that they have never been buried significantly (Parnell, 1985). There are problems with visual estimates of maturity because of the lack of true vitrinite, the presence of alginite-B and dominant amorphous organic matter tends to produce anomalously low reflectance values, and thick walled spores can complicate spore colour estimates and measurements. The relative maturity of the samples in this study has been studied using principal component analysis (PCA) which gives a maturity ranking based on several maturity parameters but which, in this case, also illustrates some interesting organic facies effects. Conventional and widely employed parameters for thermal maturity, based on biomarkers, were used along with Tma~ and HCTC. For an overview of biomarker maturity parameters see Mackenzie (1984), Cornford et al. (1986) and Tannenbaum and Aizenshtat (1984). PCI accounts for 32.3% of the variance in the data; PC2 for 26.3% and PC3 for 14.6%. Together these three components account for 73.2% of the variance. The results with interpretation are presented in Fig. 8. As shown by the loadings plots the scores on the first principal component (PCI) reflect the relative

maturity being most strongly affected by %tiff, % Ts, %TtX and T ~ . (For definition of the parameters see Figs 6(b) and 8). As stated earlier Ts and X may be also significantly dependent on organic facies in these samples. An added complication is that 5a(H), 14fl(H)-steranes may form from 7 3-sterols as early diagenetic products in hypersaline environments (ten Haven et al., 1986) so producing anomalously high sterane maturity values (%tiff). The variation in the first three parameters is significant whereas Tmax may not be a sensitive maturity parameter because most of the range of values for these samples can be seen at individual localities (Fig. 3). The scores on PC2 are most affected by Tmax, %20S, %22S, %~fl C29 hopanes (ab29), %~fl C30 hopanes (ab30) and HCTC. %22S, ab29, and ab30, are not considered to be sensitive for ranking the maturity of this data set and probably carry most information about facies. This is because the equilibrium ratio has been reached by most samples (for %22S, >60%; whilst ab 29 and ab30 in our experience have generally reached around 90% by the oil window). Tmaxand HCTC are both affected by kerogen type and bitumen staining in addition to maturity. They were included here to see if they showed a relationship to maturity in this case. Sample 1/15 separates out because of its unusually high HCTC and anomalously low Tm~x (migrated hydrocarbons?) which causes the anti-correlation of these two parameters in Fig. 8. A similar set of plots omitting HCTC and Tmaxproduced the same distribution of samples along PCI and PC2 with the exception that sample 1/15 did not separate out. Figure 8 is preferred as it shows that inter-sample variation for the rest of the data set is, for the most part, small. The scores on PC3 are interpreted to be significantly dependent on facies being controlled largely by %Ts, DIST (diasterane/sterane ratio), ab29 and ab30. %Ts and DIST are controlled by both maturity and facies as mentioned earlier. The distribution of objects on the PC plots (Fig. 8) reflects relative maturity more than facies as shown by the overlap of the different facies groups. There is a limited spread of objects along PC2 which is probably largely facies controlled. However the spread of objects along PC3 indicates a more significant facies effect. The facies component inherent in the plots is illustrated by the variation in scores on PC1 for samples from location 15 which should have the same maturity. A line of equal maturity has been drawn through these samples. A similar line has been drawn through the John O'Groats samples from location 19 (1940-1942). Sample 8/26 appears to be the least mature, estimated at around 0.5% Ro from %20S, %TtX and %131~-steranes. Samples 2756 and 3059 appear to be the most mature (around 1.0% Ro). From Fig. 8 one would conclude that the majority of these samples from the Orcadian Basin have a very similar level of maturity and therefore variation due to facies will be dominant.

Lacustrine organic facies CONCLUSIONS - - M u l t i v a r i a t e statistical analysis has proved useful in analysing the variation in organic and mineralogical composition of lacustrine source rocks from predefined depositional settings. - - I n c o r p o r a t i o n of the interpretation of these variations in the " P C " plots gives a clear visualisation of differences in organic input and depositional environment. --Principal component analysis highlights which parameters are the most significant in discriminating between samples. It has aided environmental interpretation by elucidating relationships between organic components and between mineralogical and organic variation. --Principal component analysis gives a good indication of the relative maturity of source rocks which contain little vitrinite, even though it illustrates the facies effect on the biomarker parameters used for maturity. The majority of the present Orcadian basin samples were confirmed to have a maturity within a limited range within the oil window. Acknowledgements--The authors are indebted to Statoil for their permission to release the present information for publication. The organic geochemical analyses were skilfully performed by Ms Ann Elin Gilje, Statoil Geological Laboratories, and the biomarker analyses were carried out by Ms Unn Endresen and Mr Petter Jensen, Rogaland Research Institute. We are grateful for the efforts of Dr Chris Cornford, IGI, who collected the samples for us, and thanks also to the two referees whose comments helped to improve this manuscript. REFERENCES

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