Evidence for Gloeocapsomorpha prisca in Late Devoniansource rocks from Southern Alberta, Canada

Martin G. Fowlera,*, Lavern D. Stasiuka, Mark Hearnb, Mark Obermajera

aGeological Survey of Canada Calgary, Natural Resources Canada, 3303-33rd Street NW, Calgary, Alberta, Canada T2L 2A7bShell Canada Ltd., 400-4 Avenue S.W., Calgary, Alberta, Canada T2P 2H5

Abstract

Organic-rich intervals within the Late Devonian Camrose Member of southern Alberta, Canada show opticaland chemical characteristics very similar to those of Ordovician samples whose organic matter is dominated byGloeocapsomorpha prisca-derived alginite. This includes saturate fraction gas chromatograms dominated by n-alkanes

with a pronounced odd carbon number up to C19, and much lower abundances of C19+ n-alkanes, acyclic isoprenoidsand biomarkers. Microscopy reveals these samples contain G. prisca-like alginite with a morphology that previouslyhas been suggested for the growth of this organism under higher salinity conditions which is in agreement with the

depositional environment of the Camrose Member. One unusual feature of the G. prisca-rich samples is the very highconcentration of C21 and C23 n-alkylnaphthalenes and n-alkylbenzenes in the aromatic fractions of their extracts. Theorigin of these compounds is not known but their occurrence could be related to the high concentrations of C21 and C23

n-alkyl phenols that have been noted in pyrolysates of Estonian kukersites by previous workers. Other CamroseMember extracts show very different distributions that are more typical for carbonates deposited under anoxic, highersalinity conditions. Many oils that occur in Nisku Formation reservoirs in the Enchant-Hays field in southern Albertashow saturate fraction gas chromatograms and other evidence that indicate an important contribution to these oils

from the G. prisca-rich intervals. These are thought to have a contribution from Camrose Member source rocks.Hence, caution should be used in only attributing G. prisca geochemical characteristics to Cambro-Ordovician oils as,on rare occasions, they can also occur in younger source rocks and their derived oils.

Crown Copyright # 2004 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Gloeocapsomorpha prisca is the main contributingorganism to many Cambro-Ordovician hydrocarbon

source rocks where it can make up to greater than 90%of the organic matter (e.g. Reed et al., 1986; Hoffman etal., 1987; Fowler, 1992). Such source rocks are often

called kukersites (Hutton, 1987) after the OrdovicianEstonian kukersites in which Zalessky (1917) firstrecognized G. prisca. Kukersite-type deposits often

make excellent source rocks with high TOC (totalorganic carbon) contents and HI (Hydrogen Index)

values indicating Type I organic matter. In other Cam-

bro-Ordovician sediments, the contribution from G.prisca is diluted by input from other organisms andthese have been termed diluted kukersites (Fowler,

1992). There has been considerable dispute over whattype of organism G. prisca was (e.g. Reed et al., 1986;Hoffman et al., 1987; Foster et al., 1989; Fowler, 1992;

Stasiuk and Osadetz, 1990; Derenne et al., 1992;Wicander et al., 1996; Blokker et al., 2001). Mostauthors now favour that it was a probable photo-

synthetic coccoidal cyanophyte, possibly related to theextant Entophysalis major, that had both planktonic andbenthic life habits although some still believe it is relatedto the extant green algae, Botryococcus braunii (e.g.

Derenne et al., 1992).G. prisca-rich rocks and their derived oils show a

number of distinctive geochemical characteristics. This

0146-6380/$ - see front matter Crown Copyright # 2004 Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.orggeochem.2004.01.017

Organic Geochemistry 35 (2004) 425–441

www.elsevier.com/locate/orggeochem

* Corresponding author. Tel.: +1-403-292-7038; fax: +1-

403-292-7159.

E-mail address: mfowler@nrcan.gc.ca (M.G. Fowler).

is best displayed by their saturate fraction gas chroma-tograms which are dominated by n-alkanes up to C19

with a pronounced odd carbon number predominance.C20 and higher molecular weight n-alkanes are in much

lower concentrations and tend to show no or a veryslight odd over even carbon number preference. Whileimmature samples have low to moderate concentrations

of acyclic isoprenoids, higher maturity source rockextracts and oils show extremely low concentrations ofacyclic isoprenoids. Polycyclic biomarker compounds

are also present in very low concentrations. However,monocyclic alkanes are present in higher abundancethan in most other oils, with n-alkylcyclohexanes often

the second most abundant group of compounds afterthe n-alkanes. ‘Diluted kukersites’ differ from these byshowing a less pronounced odd carbon number pref-erence of the C13–C19 n-alkanes and greater amounts of

C19+ n-alkanes and acyclic isoprenoids (e.g. Fowler,1992). Pyrolysates of G. prisca rich sediments also show

distinctive characteristics. Some are heavily dominatedby n-alkyl species, often showing an odd carbon numberpreference with a low abundance of C19+ compounds(Douglas et al., 1990; Derenne et al., 1992; Blokker et

al., 2001), while Estonian kukersite pyrolysates appearto contain a higher abundance of phenols which byanalogy with extant Botryococcus braunii could be G.

prisca adapting to more saline conditions (Derenne etal., 1992).G. prisca was widespread in tropical epi-continental

seas during Cambro-Ordovican time with kukersites ordiluted kukersites known to occur on five present daycontinents (Fowler, 1992). There are very few reports of

G. prisca occurring in post-Ordovician sediments (e.g.Cramer and Diez de Cramer, 1972) with most olderreports that suggested a range in age up to Tertiaryappearing to be erroneous (Fowler, 1992; Wicander et

al., 1996). This led Fowler (1992) to suggest that G.prisca was part of the Ordovician-Silurian extinction

Fig. 1. Map showing location of Camrose Member/Nisku Formation source rocks in southern Alberta and the location of the

Enchant-Hays oil field. Note each township is 6�6 miles square so the same rocks occur roughly over a 120�70 mile (200�120 km)

area.

426 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

event that is often linked with a major glaciation.However, in recent years there have been crediblereports of G. prisca in post-Ordovician sediments.Abrams et al. (1999) found Silurian and Lower

Devonian intervals showing geochemical characteristicssimilar to those of Ordovician kukersites in the north-eastern Timan Pechora Basin of Russia, although the

presence of G. prisca was not visually confirmed. Inaddition, oils found in Lower Devonian-Carboniferousreservoirs in part of this basin have characteristics simi-

lar to kukersite derived oils and were thought to bepossibly sourced from these units. Abrams et al. (1999)also report J.M. Moldowan (personal communication,

1996) finding G. prisca characteristics in rock extracts asyoung as Middle Devonian. G. prisca has been recog-nized microscopically in Middle Devonian Elk PointGroup and Upper Devonian Woodbend Group rocks in

eastern Alberta, generally as a minor component oforganic-rich intervals (Nowlan et al. 1998a,b; Chow etal., 1995; Stasiuk, 1999a).

During the course of our study of Devonian Petro-leum Systems of the WCSB (Western Canada Sedimen-tary Basin), units enriched in G. prisca were also noted

in Late Devonian units, especially in the CamroseMember of the Winterburn Group, southern Alberta(Figs. 1 and 2), some of which gave extracts with char-

acteristics similar to those of Ordovician kukersites(Fowler and Stasiuk, 1999). It is these samples and theirderived oils that are the subject of this paper.

2. Methodology

Aliquots of possible source rocks were dried andpowdered for Rock-Eval/TOC analysis. The instrumentused was a Delsi Rock-Eval II pyrolysis unit equipped

with a total organic carbon (TOC) analysis module.Duplicate or muliple analyses of the core samples weremade. Rock samples selected for extraction were pow-dered and extracted using azeotropic chloroform:

methanol (87:13) mixture for 24 h. Rock extracts andoils were treated with approximately 40 volumes ofn-pentane to precipitate the asphaltenes. The deasphalted

extracts and oils were fractionated using open columnchromatography (3/4 activated alumina and 1/4 acti-vated silica gel with an adsorbent:sample mass ratio of

100:1). Saturates were recovered by eluting with 3.5 mlof pentane/g of adsorbent. Aromatics were recovered byeluting with 4 ml of 50:50 pentane-dichloromethane/g

adsorbent and the resins were recovered with 4 ml ofmethanol/g adsorbent.The gasoline range hydrocarbons (iC5–nC8) in the oils

were analyzed on a HP5890 gas chromatograph con-

nected to an OI Analytical 4460 Sample Concentrator.A small amount of the whole crude oil was mixed withdeactivated alumina and transferred to the Sample

Concentrator. The gasoline fractions were then passedonto the Gas Chromatograph equipped with a 50 mHP-1 column, with siloxane gum used as a fixed phase.The initial temperature was held at 30 �C for 10 min and

then programmed to 45 �C at a rate of 1 �C/min. Thefinal temperature was held for 25 min. The elutinghydrocarbons were detected using a flame ionization

detector. Saturate fractions were analyzed using gaschromatography (GC). A Varian 3700 FID gas chro-matograph was used with a 30 m DB-1 column. The

temperature was programmed from 60 to 300 �C at arate of 6 �C/min and then held for 30 min at 300 �C. Theeluting compounds were detected and quantitatively

determined using a hydrogen flame ionization detector.Gas chromatography–mass spectrometry (GC–MS) wasperformed on a VG 7070 mass spectrometer with a gaschromatograph attached directly to the ion source. The

Fig. 2. Stratigraphic nomenclature for Late Devonian units in

southern Alberta.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 427

instrument was controlled by an Alpha Workstationusing Opus software. A 30 m fused silica column (DB-5,J&W Scientific) was used for GC separation. The temp-erature was initially held at 100 �C for 2 min and then

programmed at 40 �C/min to 180 �C and at 4 �C/min to320 �C. After reaching 320 �C the temperature was heldfor 15 min. The mass spectrometer was operated with a

70 eV ionization voltage, 100 mA filament emissioncurrent and interface temperature of 280 �C. Massfragment ions monitored during single ion monitoring

experiments included m/z 177.1638, 191.1794, 217.1950,218.2028, 231.2106 and 259.2262. GC-MS analyses ofaromatic hydrocarbons were performed with a HP 6890

Series GC coupled to a HP 5973 Series Mass SelectiveDetector operated in selective ion monitoring (SIM)mode. The GC was fitted with a 30 m�0.25 mm i.d. DB-5column. The temperature was initially held at 100 �C for

2 min and then programmed at 3 �C/min to 300 �C andheld for 10 min. The mass spectrometer was operatedwith a 70 eV ionization voltage, 100 mA filament emis-

sion current and an interface temperature of 280 �C.Identification of peaks was aided by full scan GC–MSanalyses of selected samples using a Micromass Auto-

spec-TOF instrument in normal GC–MS mode. Py-rolysis-gas chromatography (py-GC) was achieved usinga Horizon Instrument Curie Point Pyrolyzer with a wire

with a Curie temperature of 610 �C on the injector ofa HP5890 gas chromatograph. The products wereseparated on a 30 m DB-1 column using a temperatureprogram of 0–300 �C at 6 �C/min.

Organic-rich core samples were prepared for incidentlight microscopy of dispersed organic matter by mount-ing rock fragments in epoxy resin. Prior to mounting,

small saw-cut core fragments (�1 cm cubes) were initi-ally hand polished while drill cuttings were crushed to1–5 mm particulates. Prepared as such, samples were

then put into 1 inch diameter circular plastic moldsensuring, where required, perpendicular to bedding,parallel to bedding or random orientation of the parti-culates. The moulds were filled with epoxy (cold setting

epoxy consisting of 2:1 resin and hardener) and left tocure for 24–48 h. The pellets were ground and polishedon a Buehler Ecomet IV polishing machine.

Dispersed organic matter (macerals) was analyzedand classified using white and fluorescence incident lightmicroscopy. Three reflected light microscope systems

were used for organic petrologic analysis: a Leitz MPVII system, a Zeiss UMP system, and a Zeiss Axioplan IIsystem. Fluoresence microscopy was done using water

immersion objectives with total magnification up to1200�. A Zeiss Invert 410 confocal laser scanningmicroscope equipped with 488, 543, and 633 nm lasersare also used, particularly for studying detailed maceral

morphology (2D and 3D perspectives) of liptinite andamorphinite macerals under magnification up to 8000�(see Stasiuk, 1999b).

3. Results

3.1. Geology

The regional geology of the Winterburn Group insouthern Alberta is summarized in Kissling (1996). Inthe area that extends from Townships 1 to 15 and Ran-

ges 1 to 25W4 (Fig. 1), over which we sampled 22 cores,the Frasnian-aged Camrose Member/Nisku Formationstrata (Fig. 2) are represented by a �30 m thick

sequence that displays a shallowing-upward profile, withdolomitized marine shelf facies to peritidal, lagoon andtidal flat facies. Three mappable, potential source rock

units have been identified within the Camrose Member/Nisku Formation section in this area, two in the Cam-rose Member and one in the Nisku Formation (Fowleret al., 2001). Here we concentrate on the evidence for G.

prisca within the Camrose Member potential sourcerock intervals as there was little or no evidence for G.prisca within the Nisku Formation potential source

interval in this area.

3.2. Optical evidence for the occurrence of G. prisca

Macerals within potential source rocks of the Cam-rose Member/Nisku Formation in southern Alberta are

derived from both marine and terrestrial sources andsuggest variable and fluctuating paleodepositional con-ditions (Fowler et al., 2001). Coccoidal alginite maceralsand shallow water organic facies are an important

component of some Camrose Member potential sourcerocks (Fig. 3). Several varieties of coccoidal alginitemacerals appear to be present in these rocks (e.g. G.

prisca, Botryococcus, Pediastrum-like?), but the majorityhave morphological features consistent with G. prisca(Fig. 3) and are hereafter referred to as G. prisca. G.

prisca alginites are very abundant and dominant in theCamrose Member at 10-4-1-9W4 (1185.75 and 1185.95m), at 15-35-7-22W4 (1942.5 m) and at 13-32-7-20W4(especially at 1665.9 m) (Fig. 3). Similar G. prisca-

dominated organic facies also occur in the CamroseMember at 16-31-10-4W4 (1379.37 and 1379.58 m) and6-22-6-23W4 (2324.9 and 2325.8 m), although the algi-

nites at these locations, and particularly the latter, are‘softened’ and largely ‘altered’ due to thermal alterationand probable S-enrichment (Fig. 3). Petrographic evi-

dence for in situ oil generation from G. prisca alginite isparticularly clear at the 7-22W4 location where solidbitumen and crude oil inclusions are abundant (Fig. 3).

Stasiuk et al. (1993) described three G. prisca ‘micro-facies’ in Late Ordovician rocks of the Saskatchewanportion of the Williston Basin, two disseminated (A andB) and a stromatolitic facies. Disseminated A micro-

facies consists of small (generally <5 to 20 mm), thin-walled G. prisca alginite colonies typically consisting ofno more than 12 individual cells. Disseminated B

428 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

Fig. 3. Photomicrographs of dispersed organic matter within Ordovician kukersites, Williston Basin (a–c) and Nisku Formation/

Camrose Member source units (d–l), Alberta. Incident, conventional fluorescent light (a, d–k), and laser scanning fluorescence

microscopy (LSM; b,c); water immersion. Scale on photo a is for all photos except b and c (scale on photo b). (a). G. prisca showing

mainly open cell morphology (o) but also some closed (c) morphology. (b, c). LSM image of open and closed cellular morphology in

G. prisca. (d). Solid bitumen (b) associated with G. prisca enriched organic facies. (e–h). Abundant small to medium colonies of coc-

coidal G. prisca alginite (Gp) associated with intense yellow-fluorescing Prasinophyte alginite (p) in photo e. (i–l). Thermally mature G.

prisca alginite (Gp) associated with abundant bitumen. Low (k) and high (l) magnification images showing thermally mature G. prisca

associated with abundant bitumen (b) and crude oil inclusions (hcfi) in late calcite cements from 7-20W4 location. Note the ‘softening’

of G. prisca with closed cellular structure resulting from thermal maturity effects.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 429

microfacies is similar although it consists of larger G.prisca colonies (up to 100 mm) with much thicker cellwalls and development of the colony by a thickened‘sheath’. Disseminated A is thought to represent the

earlier growth stage of G. prisca, while disseminated B isa more advanced growth stage under more stressfulconditions, probably related to the development of algal

blooms (Stasiuk and Osadetz, 1990; Stasiuk, 1999a).The stromatolitic G. prisca microfacies are characterizedby algal mats with upper surface microtextures, such as

domes, pinnacles, pustules or phototactic structures(Stasiuk, 1999a). This could form via the disseminated Bstage or directly at the sediment–water interface. The dis-

seminated B and stromatolitic microfacies are the mostvolumetrically important in the Ordovician of theWilliston Basin (Stasiuk and Osadetz, 1990).The cell size, cellular organization and shape of the G.

prisca colonies observed in the Camrose Member aresimilar to those previously described for the Ordovician(e.g. Stasiuk and Osadetz, 1990) (Fig. 3). However, there

are some notable differences. Only the disseminated Aand B G. prisca microfacies were observed while thestromatolitic microfacies rarely occurs in the Camrose

Member/Nisku Formation samples or any other Devo-nian samples where we have noted G. prisca. A closedand open morphology characterizes disseminated B G.

prisca alginites in Ordovician sediments of the WillistonBasin but almost all the disseminated B alginite noted inthe Upper Devonian rocks is characterized by a closedmorphology (Fig. 3). Closed and open cellular mor-

phology can easily be distinguished using both conven-tional fluorescence microscopy and confocal laserscanning fluorescence microscopy (Fig. 3a–c), the latter

is within the same range of magnification as used byDerenne et al. (1992) to demonstrate the two morpho-types of G. prisca alginite. The alginite colonies in the

Upper Devonian samples also tend to be considerablysmaller than those found in Ordovician sediments,ranging from �5 to 50 mm in diameter (Fig. 3).

3.3. Extract characteristics

Camrose Member extracts show variable saturate

fraction gas chromatogram (SFGC) characteristics.These range from a sample (Fig. 4a) that gave a SFGCclosely resembling those from Ordovician G. prisca rich

samples to those that show no odd or even carbonnumber preference with high abundances of acyclic iso-prenoids and biomarkers (Fig. 4c) and those that show

intermediate characteristics (Fig. 4b). The samples withhigher concentrations of higher molecular weight n-alkanes and biomarkers in their SFGCs have biomarkerdistributions characterized by low pristane/phytane

ratios, a relatively high abundance of C20+ acyclic iso-prenoids and a m/z 191 mass chromatogram (Fig. 5c)that shows a pronounced C34 homohopane prominence,

a relatively high abundance of gammacerane and lowamounts of tricyclic and tetracyclic terpanes. These arecharacteristics that are generally associated with car-bonate and elevated salinity depositional environments

(e.g. Peters and Moldowan, 1993, pp. 141–142, 209)which is in agreement with those interpreted for theCamrose Member (Fowler et al., 2001). Sterane carbon

number distributions vary from a slight C27 pre-dominance to a pronounced C29 predominance. Micro-scopic examination of these samples indicates little to no

contribution from G. prisca to their organic matter.As noted earlier, some samples have organic matter

dominated by G. prisca and hence resemble Ordovician

kukersite deposits. The location that showed the bestcombined optical and chemical evidence of a G. priscacontribution in this study was the lower potential sourceinterval of the Camrose Member at 13-32-7-20W4.

Fig. 4. Saturate fraction gas chromatograms of Camrose

Member extracts from southern Alberta. C15, C20 and C25 are

the C15, C20 and C25 n-alkanes, respectively; Pr and Ph

are pristane and phytane; H is 17a(H)-hopane, and peak

indicated with an asterisk is C17 n-alkylcyclohexane.

430 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

Three samples of interlaminated organic-rich mudstoneswere taken between 1665.0 and 1665.90 m depth. Thisinterval contains abundant shell fragments (brachiopodand bivalve) and fish fragments (e.g. fish bones). The

maceral assemblage for this interval (Fig. 3) is domi-nated by G. prisca disseminated A and B alginite withvery minor amounts of Prasinophyte alginite and acan-

thomorphic acritarchs. The depositional environment isthought to have had oxygenated surface waters withperiodically more restricted anoxic bottom water condi-

tions, possibly related to times of increased G. priscaproductivity. These samples have TOC contents rangingfrom 10.60 to 16.76% and HI values from 797 to 864

mg HC/g TOC. Their Tmax values are 439–441�C which

are higher than might be expected for the level ofmaturity at this location (�0.70% vitrinite reflectance

equivalent: Stasiuk and Fowler, 2002) but not for Type Iorganic matter.The SFGC of the sample from 1665.9 m (8842) is very

similar to those of Ordovician kukersites (Fig. 4a). It

shows a pronounced odd carbon number predominanceof the C17 and C19 n-alkanes and low amounts of C20+

n-alkanes, acyclic isoprenoids and biomarkers. It also

has relatively high concentrations of n-alkylcyclohex-anes with the peaks of these compounds clearly appar-ent in the SFGC (Fig. 4a). A m/z 82 mass

chromatogram (Fig. 6a) shows an odd carbon numberpredominance of these compounds over the C15-C23

range with an abrupt decrease in concentration after

C23. This is a somewhat different distribution for thesecompounds compared with other North Americankukersite extracts and derived oils which usually show a

Fig. 5. m/z 191 and m/z 217 mass chromatograms showing the distributions of terpanes and steranes, respectively, for Camrose

Member organic-rich samples from southern Alberta. Peaks identified in Table 2.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 431

distribution similar to that of the n-alkanes (Fowler etal., 1986; Fowler, 1992). An extract of the Late Ordovi-cian Collingwood Member from southern Ontario hasbeen reported with a predominance of C23 and C25 n-

alkylcyclohexanes but with C14–C31 homologues alsopresent in moderate to high abundance (Fowler, 1992).The biomarker distribution of sample # 8842 (Fig. 5,

Table 1) is different to most of the other CamroseMember/Nisku Formation organic-rich intervals thatshow no optical or chemical evidence of a major G.

prisca contribution. This includes higher abundances ofC27 and C29 17a(H)-hopanes relative to C30 17a(H)-hopane, lower gammacerane and C34 homohopane prom-

inence, lower Ts/Tm, much higher diasterane/regularratio and a higher abundance of short-chain steranes(Fig. 5a, Table 1). Steranes are in much lower abun-dance than hopanes. While these characteristics cannot

be attributed to a G. prisca contribution with any cer-tainty, some of these characteristics have been pre-viously noted for Ordovician kukersites and their

derived oils, including the predominance of diasteranesover regular steranes (Longman and Palmer, 1987;Fowler, 1992), a relatively higher abundance of C27 and

C29 hopanes (Zumberge, 1987) and the large pre-dominance of hopanes over steranes (Fowler, 1992). It isnoteworthy that the Family C oils of Abrams et al.

(1999), which have SFGCs suggesting that they have a

source rock containing a significant G. prisca contribu-tion, show many similar biomarker characteristicsincluding a C34 homohopane predominance, a muchgreater abundance of the C24 tetracyclic terpane than

the C23 tricyclic terpane and relatively high abundancesof diasteranes compared with the regular steranes.Evidence for a contribution from G. prisca to other

Camrose Member samples is less obvious. A samplefrom 10-4-1-9W4 (sample #8664, Table 1) that opticallyshowed a minor persistent contribution from G. prisca

has intermediate geochemical characteristics betweenthe 13-32-7-20W4 kukersite-like sample and samplesthat show no evidence of a G. prisca contribution. It has

a slight odd carbon number preference up to C19 andlower abundance of acyclic isoprenoids and biomarkers(Fig. 4b). This sample also has a higher concentration ofmonocyclic alkanes than most other samples showing a

pronounced odd carbon number preference up to C23

similar to the 13-32-7-20W4 sample (Fig. 6b). n-Alkyl-cyclohexanes are present in other samples (e.g. Fig. 6c)

but in much lower abundance and with a different car-bon number distribution. The 10-4-1-9W4 sample alsoshows intermediate biomarker characteristics including

only a minor C34 homohopane prominence, a high C29/hopane ratio, high diasteranes/regular steranes and C21

sterane/C29 sterane and low gammacerane/hopane

ratios (Fig. 5b, Table 1). A number of other samples

Table 1

Geochemical data for Camrose Member/Nisku Formation extracted samples and a typical crude oil from the Nisku Formation

reservoir (sample #2091)

GSC #

8664 8842 8686 8666 8685 2091

Location

10-4-1-9W4 13-32-7-20-W4 6-29-12-12W4 11-27-8-17W4 6-29-12-12W4 3-18-13-14W4

Depth (m)

1185.75 1665.90 1276.55 1418.06 1269.72 1331.5-1386.0

TOC%

7.15 16.76 2.26 4.19 3.72 n/a

HI

895 856 765 800 784 n/a

%HC

21.71 23.35 40.85 17.87 25.46 76.48

S/A

0.29 0.70 0.53 0.26 0.28 0.62

Pr/Ph

1.20 n.d. 0.69 0.71 0.83 0.58

Pr/17

0.27 0.02 0.21 1.14 0.27 0.12

S/(S+R)

0.46 0.45 0.47 0.27 0.46 0.45

dia/reg

0.61 1.94 0.75 0.10 0.68 0.86

21/29

0.41 0.79 0.44 0.08 0.32 0.68

27:28:29

47:10:43 43:13:44 33:14:53 38:11:51 15:15:70 37:16:47

Ts/Ts+Tm

0.21 0.13 0.17 0.21 0.27 0.28

29/hop

0.75 0.74 0.72 0.49 0.73 0.76

23/hop

0.10 0.07 0.06 0.29 n/d 0.32

g/hop

0.17 0.11 0.14 0.52 0.35 0.11

alkylarom

high high high low high high

21/21+23

0.56 0.74 0.69 0.98 0.98 0.77

N/a—not applicable; TOC—% total organic carbon; HI—Hydrogen Index;%HC—weight percent hydrocarbons; S/A—saturated/

aromatic hydrocarbons ratio; Pr/Ph—pristane/phytane ratio; Pr/17—pristane/n-C17 ratio; S/(S+R)—5a(H),14a(H),17a(H) 20S/

(20S+20R)—C29 sterane; dia/reg- 13b(H),17a(H) 20S—C27 diasterane/5a(H),14a(H),17a(H) 20R—C27 sterane; 21/29—C21 sterrane/

5a(H),14a(H),17a(H) 20R—C29 sterane; 27:28:29—normalized relative abundance of C27, C28 and C29 regular steranes based on aaaisomers; Ts/Ts+Tm—18a(H)-trisnorhopane/(18a(H)-trisnorhopane+17a(H)-trisnorhopane); 29/hop—17a(H)-norhopane/

17a(H),21b(H)-hopane; 23/hop—C23 tricyclic terpane/17a(H),21b(H)-hopane; g/hop- gammacerane/17a(H),21b(H)-hopane;

alkylarom-relative abundance of n-alkylaromatics in aromatic fraction; 21/21+23—C21/(C21+C23) n-alkylnaphthalenes.

432 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

have characteristics intermediate between those of thekukersite-like sample (sample #8842) and samplesinterpreted to have no or very little G. prisca contribu-tion (e.g. sample #8666). These were interpreted to have

a significant G. prisca contribution which is also sup-ported by their aromatic hydrocarbon distributions andpyrolysis-gas chromatography data discussed below.

The major compounds in the aromatic fractions ofmany of the rock extracts are C21 and C23 alkylatedaromatic hydrocarbons. This is shown for a lower

potential source rock sample from 6-29-12-12W4 (sam-ple #8686) in Fig. 7. The mass spectra of the majorcompounds are very simple with the two major peaks

being the base fragment ion (m/z 92, 141, 155) and aM+ ion. The C21 and C23 n-alkylbenzenes and n-alkyl-naphthalenes are the major compounds with loweramounts of C21 and C23 alkylmethylnaphthalenes. The

carbon number distributions of the n-alkylnaphthalenesand n-alkylbenzenes show a strong tendency to mirroreach other for this sample set (Fig. 8), suggesting these

compounds have a similar origin. The relative amountsof C21 and C23 compounds varies. All samples show apredominance of the C21 compounds except one which

shows a slight predominance of C23. Extracts of thepotential source rock at the base of the Camrose Mem-ber have lower values for the C21/(C21+C23)parameterthan other source rock intervals (Table 1). Thus there

could be a palaeoevironmental control on this pa-rameter. This could be related to the different, deeperwater organic facies previously noted for this interval

compared with other Camrose/Nisku organic-rich unitswhich were deposited under more restricted shallowerconditions (Fowler et al., 2001). The m/z 92 and 141

mass chromatograms show that lower homologues arein very low abundance with a slight odd carbon numberpredominance over the C15–C19 range. Sample #8842,

which is one of the higher maturity source rock samplesexamined here, shows greater amounts of C20 and loweralkyl aromatic compounds than the other samples withan odd carbon number preference (Fig. 8). These may

be derived from cracking of the C21 and C23 compoundsor their precursors. There is possible evidence in thesemass chromatograms of extremely low abundances of

C23+ homologues, up to C30 for the alkylbenzenes andup to C25 for the alkylnaphthalenes. The m/z 155 masschromatogram shows the presence of multiple isomers

for the alkylmethylnaphthalenes. Aryl isoprenoids (i.e.1-alkyl-2,3,6-trimethylbenzenes) which are thought tobe derived from photosynthetic green sulphur bacteria

(Summons and Powell, 1987; Hartgers et al., 1994) canonly be detected in low abundance in samples withlower concentrations of the C21 and C23 alkylbenzenesbecause these latter compounds usually dominate the

m/z 133 and 134 mass chromatograms due to their con-siderably higher abundance. This is in contrast to mostof the other Devonian source rocks the Western Canada

Sedimentary Basin, such the Duvernay Formation,which have much higher concentrations of aryl iso-prenoids (e.g. Requejo et al. 1992).

The aromatic fraction of the Collingwood Membersample from southern Ontario mentioned earlier thathad a predominance of C23 and C25 n-alkylcyclohexaneswas also analyzed for comparison with the Camrose

Member extracts. C21 and C23 n-alkylnaphthalenes andn-alkylmethylnaphthalenes were not detected. Therewas a relatively high abundance of n-alkylbenzenes up

to C30 with the C23–C25 members in highest concentra-tions. Thus the distributions of the n-alkylcyclohexanesand n-alkylaromatics in the Ordovician Collingwood

sample are very different to the Upper DevonianCamrose Member samples.The concentration of the C21 and C23 benzenes and

alkylnapthalenes varies in the Camrose Member/NiskuFormation extracts. There does not seem to be anyobvious correlation with the source rock interval orproximity to anhydrite intervals. However, there does

seem to be some correlation with the presence of G.prisca alginite. A subjective assessment of the relativeabundance of these compounds is provided in Table 2.

Fig. 6. Representative m/z 82 mass chromatograms of Cam-

rose Member organic-rich samples.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 433

Fig. 7. Aromatic hydrocarbon data for an organic-rich Camrose Member sample from 6-29-12-12W4 (1276.55 m); (a) Total Ion

Chromatogram (TIC), (b) m/z 92 mass chromatogram, (c) m/z 141 mass chromatogram, (d) m/z 155 mass chromatogram.

434 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

High abundance indicates that these are among themost abundant aromatic compounds in the extracts.Generally, these compounds are in either very high orvery low abundance. It is noticeable that the C21 and

C23 alkylaromatic species are more abundant in sampleswhere other geochemical characteristics suggested a sig-nificant G. prisca contribution based on SFGC char-

acteristics (n-alkane distributions, concentration ofacyclic isoprenoids and biomarkers) and biomarker fea-tures (e.g. minor C34 homohopane prominence, lower

gammercerane/hopane ratio). For example, these com-pounds are in high abundance in the kukersite-likesample #8842, but in much lower concentrations in a

sample such as #8666 which is believed to have little G.prisca contribution (Fig. 10).

High abundances of alkylated benzenes and naph-thalenes have been noted previously in sedimentsthought to be deposited under elevated salinity condi-tions or their derived oils (Connan et al., 1986; Williams

et al., 1988; Sinninghe Damste et al., 1993). They havebeen attributed a probable bacterial precursor. C21

compounds were found by Connan et al. (1986) and

Williams et al. (1988) to be in highest abundance in theirsamples. Sinninghe Damste et al. (1993) noted a varietyof alkylbenzene distributions in Mulhouse Basin sam-

ples including some that showed a large predominanceof C23 with the C25 also in much higher concentrationsthan other homologues. They did not observe the C21

and C23 predominance displayed by the CamroseMember samples.

Fig. 8. Aromatic hydrocarbon data for an organic-rich Camrose Member samples from 13-32-7-20W4 (1665.9 m) and 11-27-8-17W4

(1418.0 m).

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 435

The C21 and C23 alkylaromatic compounds in the G.prisca-rich Camrose Member samples may have a simi-lar origin to the high concentrations of n-alkyl phenols,especially C21 and C23, that have been noted in some

Ordovician kukersite pyrolysates from Estonia(Derenne et al., 1990; Blokker et al., 2001). These alkylphenols occur in much lower concentrations in py-

rolysates from another Ordovician kukersite deposit,the GuttenbergMember (Derenne et al., 1990; Blokker etal., 2001). This was attributed to G. prisca growing

under conditions of higher salinity during deposition ofthe Estonian kukersite by Derenne et al. (1990, 1992)and/or due to the higher maturity of the Guttenberg

Member (Blokker et al., 2001). The Camrose Membersamples that have significant G. prisca contributionwere likely deposited under conditions of elevatedsalinity, considering their close association with evapo-

rites, and thus may be the source of the C21 and C23

alkylaromatics.The results of pyrolysis-gas chromatography also

appear to show the influence of a G. prisca contributionto some samples. As indicated in Fig. 9, both the #8842and #8664 samples show similar pyrograms to an

Ordovician Yeoman Formation kukersite pyrolysatefrom the Williston Basin, Canada. The Ordoviciansample shows a decrease in abundance after C17 for

alkene–alkane doublets which is also shown by the twoCamrose Member samples, suggesting that a large pro-portion of the organic matter of these samples is likely

Fig. 9. Whole-rock pyrograms of (a) Upper Ordovician Yeo-

man Formation kukersite interval from southeast Saskatch-

ewan portion of Williston Basin and (b–d) Late Devonian

Camrose Member organic-rich interval samples from southern

Alberta, (b) G. prisca-rich sample from 13-32-7-20W4 (1665.9

m), (c) Moderately-rich G. prisca sample from 10-4-1-9W4

(1185.75 m) and (d) G. prisca-poor sample from 11-27-8-17W4

(1418.06 m). 12 and 17 are the C12 and C17 alkene–alkane

doublets, TMB is tetramethylbenzene.

Table 2

Biomarker compounds identified in saturate fraction mass

fragmentograms shown in Figs. 5 and 10

Peak

Compound

1

C23 tricyclic terpane

2

C24 tetracyclic terpane

3

18a(H),22,29,30-trisnorhopane(Ts)

4

17a(H),22,29,30-trisnorhopane (Tm)

5

17a(H),21b(H)-30-norhopane

6

17a(H),21b(H)-hopane

7

moretane

8

17a(H),21b(H)-homohopanes (S & R)

9

gammacerane

10

17a(H),21b(H)-bishomohopanes

11

17a(H),21b(H)-trishomohopanes

12

17a(H),21b(H)-tetrakishomohopanes

13

17a(H),21b(H)-pentakishomohopanes

a

pregnane

b

diacholestane 20S

c

5a(H),14a(H),17a(H)-cholestane (20S)

d

24-methyl-5a(H),14a(H),17a(H)-cholestane

e

24-ethyl-5a(H),14a(H),17a(H)-cholestane (20S)

f

24-ethyl-5a(H),14b(H),17b(H)-cholestane (20S & 20R)

g

24-ethyl-5a(H),14a(H),17a(H)-cholestane (20R)

h

24-propyl-5a(H),14a(H),17a(H)-cholestane (20S & 20R)

436 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

derived from G. prisca. Other Camrose Member/NiskuFormation samples that were thought not to have asignificant G. prisca contribution show very differentpyrograms (Fig. 9d) with much higher amounts of

higher molecular weight compounds. They all show arelatively large tetramethylbenzene (TMB) peak. Thiscompound is thought to be mostly derived from aryl

isoprenoids and their precursors found in Chlorobiacae(Hartgers et al., 1994). The #8664 sample shows a smallTMB peak that is not evident in the #8842 sample, once

again suggesting it was deposited in an intermediateenvironment between those of the G. prisca-rich and-poor intervals.

3.4. Southern Alberta oils

The Nisku reservoired oils at Enchant-Hays in

southern Alberta (Fig. 1) are thus far the only majorfield discovered in the Nisku Formation in this region.In total, 21 oils from Nisku Formation reservoirs from

the Enchant-Hays area were analyzed for this study.Most of these oils show features that allow them to bedifferentiated from all other Devonian oils of the

WCSB. This is most apparent in their SFGCs whichcommonly show a high abundance of n-alkanes up toC17, often with a distinct odd carbon number pre-

dominance over the C15–C19 range, followed by a rapid

decrease in concentration to C18 and then a moregradual decrease in abundance with no odd or evencarbon number preference (Fig. 10). The samples tendto show a low abundance of acyclic isoprenoids and

biomarkers relative to n-alkanes. Most oils share othercharacteristics in common including, low saturate/aro-matic (<0.76) and pristane/phytane ratios (<0.82), low

amounts of rearranged steranes and hopanes comparedto their unrearranged counterparts, a predominance ofthe C24 tetracyclic terpane over the C26 tricyclic terpanes

and a significant gammacerane peak (Fig. 10c). Somealso show a C34 homohopane prominence. Aryl iso-prenoids are present in these oils as relatively minor

components. The C21 and C23 n-alkybenzenes and n-alkylnaphthalenes are among the most abundant aro-matic hydrocarbons in these oils. The C21 compoundsare in much higher concentrations than their C23

homologues, often by a factor of two or more (Fig. 10b).Enchant-Hays oils also have gasoline range hydro-

carbon distributions that differ from almost all the other

Devonian oils analyzed from Western Canada. Theyshow a very high abundance of n-alkanes relative toother compounds, with aromatics such as toluene pre-

sent in very low concentrations. This leads to someunusual values for gasoline range parameters such as thevery high heptane values which are more typical of

supermature oils (i.e. heptane values >30) (Thompson,

Fig. 10. Geochemical data for a Nisku Formation oil from southern Alberta (Hays 3-18-13-14W4, 1331.5-1386.0 m). (a) Saturate

fraction gas chromatogram (annotation of peaks as in Fig. 4), (b) partial aromatic hydrocarbon TIC; MN—methylnaphthalenes,

MP—methylphenanthrene, 21B and 23B—n-alkylbenzenes, 21N and 23N—n-alkylnaphthalenes (c) m/z 191 mass chromatogram,

(d) m/z 217 mass chromatogram; peaks identified in Table 2.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 437

1983). While low concentrations of aromatics can beattributed to water-washing, in the case of these Niskuoils it is thought to relate to their source organic matteras these gasoline range characteristics are also similar to

those shown by Ordovician kukersite derived oils fromthe Williston Basin (Fowler et al., 1998).

4. Discussion

Both optical and chemical evidence indicate a sig-nificant contribution from G. prisca to the organic mat-ter of Camrose Member potential source rock intervals.

Organic petrological observations show that the mor-phology of G. prisca in these Late Devonian samples isvery similar to that observed in many Ordoviciankukersite deposits. However, there is a strong pre-

dominance of G. prisca with the ‘closed morphology’within the Camrose Member samples.The extract characteristics of the samples believed to

have a major G. prisca contribution are also similar tothose of Ordovician kukersites. For example, SFGCswith a predominance of odd numbered n-alkanes up to

C19 and lower abundances of C20 and higher n-alkanes,acyclic isoprenoids and biomarkers. Saturated hydro-carbon biomarker distributions, especially those of the

most G. prisca-rich sample (sample #8842), also havesimilar features to those reported for Ordoviciankukersite samples. However, there is a significant differ-ence in the aromatic hydrocarbons with the Camrose

Member samples having high concentrations of C21 andC23 n-alkylnaphthalenes and n-alkylbenzenes whichhave not been previously reported for Ordovician sam-

ples. The origin of these compounds is still uncertain buttheir empirical association with G. prisca-rich samplessuggest that their precursors are biochemical compo-

nents of this organism. They may have a similar originto the C21 and C23 n-alkylphenols that are abundant inthe pyrolysates of Estonian kukersite (Derenne et al.,1990, 1992; Blokker et al., 2001).

Derenne et al. (1992) showed that increasing salinityfrom 0 to 10 g/l of NaCl could influence the morphologyof extant Botryococcus braunii and its production of

phenols. By analogy, they suggested that variations insalinity could be the cause of similar changes observedfor G. prisca in different Ordovician rocks. The range of

salinity chosen by Derenne et al. (1992) for theirexperiments with Botryococcus braunii reflect those inwhich this organism mostly thrives (i.e. freshwater to

brackish) and did not range up to normal seawatersalinity. There is considerable geological evidence thatG. prisca lived mostly in normal marine environmentsduring the Ordovician including for example, the Middle

Ordovician Guttenberg Member of the Decorah For-mation in Iowa (Ludvigson et al., 1996) and UpperOrdovician kukersite intervals in the Williston Basin,

Saskatchewan (Kendall, 1976; Stasiuk and Osadetz,1990). The Camrose Member intervals enriched in G.prisca are always associated with interbedded evaporitesand dolomites suggesting that the organism was living in

conditions of higher than normal salinity. Thus,although G. prisca is likely a different type of organismto Botryococcus braunii, the adoption of the closed cel-

lular morphology and the high abundance of C21 andC23 alkylaromatic compounds may represent a similarresponse to growth under higher salinity conditions for

this organism.The saturate fraction gas chromatograms, aromatic

hydrocarbon distributions and gasoline fraction hydro-

carbons all suggest that the organic-rich intervals thatare the source of most of the Enchant-Hays oils had asignificant G. prisca contribution. The saturated hydro-carbon biomarkers give more ambiguous evidence for

the origin of the oils. This can be explained by theCamrose Member/Nisku Formation potential sourcerocks containing several different organic facies (Fowler

et al., 2001). Hence, any oils that are derived from themmight reasonably be expected to have contributionsfrom units with different geochemical characteristics,

some indicative of a significant G. prisca contributionand others not. The SFGCs of the Enchant-Hays oilsfrom Nisku Formation reservoirs with the pre-

dominance of odd number n-alkanes up to C19

(Fig. 10a), and the relatively high abundance of C21 andC23 n-alkylaromatics (Fig. 10b) strongly suggest that G.prisca-rich intervals were a major source of hydro-

carbons for these Late Devonian oils. These source rockintervals tend to contain much lower concentrations ofbiomarkers than intervals where G. prisca made a lesser

contribution to their organic matter. Hence, the dis-tribution of biomarkers in the Enchant oils would likelyshow a greater influence from the non-G. prisca source

intervals than other compound classes which mightexplain why they have intermediate characteristics.This paper conclusively demonstrates the presence of

G. prisca in Late Devonian sediments in southern

Alberta using both optical and chemical evidence. Inaddition, G. prisca-like coccoidal algae have also beenoptically distinguished as a minor contributor to the

organic matter in other Middle and Late Devonian unitswithin the Western Canada Sedimentary Basin (WCSB)that do not show the chemical fingerprint for G. prisca

(Stasiuk, unpublished results). These include theMiddle Devonian Keg River and Winnipegosis forma-tions and Late Devonian Duvernay Formation in

central and northern Alberta, and the WinnipegosisFormation and Late Devonian Birdbear Formation,that is equivalent in age to the Camrose Member/NiskuFormation intervals discussed in this paper, in southern

Saskatchewan. However, in all of the other WCSBDevonian units G. prisca is only rarely found as a minorcomponent in organic-lean intervals deposited under

438 M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441

shallow water oxidizing conditions within carbonateplatform settings. This suggests that the precursor oftheir G. prisca alginite lived in the photic zone of mainlyshallow water depositional environments. Despite the

oxidative conditions and apparent low algae-biologicalproductivity within the water column during depositionof these sediments, some marine alginite macerals were

nonetheless preserved, probably as a result of selectivepreservation of highly resistant algaenan biomacro-molecules found in the outer cell walls derived from the

G. prisca precursor (e.g. Tegelaar et al., 1989). Prasino-phyte algae such as Tasmanites and Leiosphaeridia arethe main primary producers contributing to the organic

matter of the most of the Devonian source rocks in theWCSB such as the Duvernay and Keg River formations(Chow et al., 1995).The presence of G. prisca as a significant contributor

to Camrose Member potential source rocks and not toany of the several other Devonian source rocks in theWestern Canada Sedimentary Basin (Fowler et al.,

2001), allows us to speculate on the post-Ordovicianhistory of this organism. The geological record clearlyshows that G. prisca lost its position as a dominant

algae in the open marine tropical seas after the Ordovi-cian-Silurian extinction event. This probable cyano-phyte (Foster et al., 1989; Stasiuk and Osadetz, 1990;

Wicander et al., 1996) was replaced in this environ-mental niche by Prasinophytes and other green algae. Itsurvived in shallow water, near-shore environments thatare not typically considered conducive to source rock

development. It may also have had an ability to toleratehigher salinity conditions which are not as favourable toChlorophytes, and thus could have thrived in some of

these environments at least until the Late Devonianwhere it can be a major contributor to petroleum sourcerocks, such as those in the Camrose Member. This

speculation is obviously based on data from just onebasin but it will be interesting to see if other post-Ordovician occurrences of G. prisca are also associatedwith strata deposited under higher than normal salinity.

Reports to date such as that of Abrams et al. (1999) donot provide enough data to verify this.

5. Summary and conclusions

Some Late Devonian Camrose Member potentialsource rock samples show characteristics that suggest acontribution to their organic matter by the organism G.

prisca whose presence is very rarely noted in post-Ordovician strata. G. prisca characteristics are bestdemonstrated by a lower Camrose Member sample from13-32-7-20W4 that shows a saturate fraction gas chro-

matogram very similar to Ordovician kukersite samples,including an odd/even predominance of n-alkanes up toC19, much lower abundances of C19+ n-alkanes, acyclic

isoprenoids and biomarkers, and relatively highconcentrations of n-alkylcyclohexanes. Other samplesshow a range of characteristics intermediate between thekukersite and those more typical for carbonate envir-

onments. Microscopy confirms that G. prisca alginite isthe dominant component of the 13-32-7-20W4 sampleand major constituent of many other samples that show

the G. prisca chemistry. The morphology of the LateDevonian G. prisca is generally similar to that observedin Ordovician samples although the ‘closed’ cellular

morphology is more common which could be related togrowth of this organism under elevated salinity condi-tions. An unusual feature of many of the Camrose

Member/Nisku Formation extracts is the very highconcentrations of C21 and C23 n-alkylaromatic com-pounds, especially n-alkylnaphthalenes and n-alkylben-zenes in their aromatic fractions. This distribution does

not seem to have been previously reported and the originof these compounds is still uncertain. However, the C21

and C23 n-alkylaromatics are in higher concentrations in

those samples with a high G. prisca contribution to theirorganic matter and hence their occurrence could be rela-ted to the biochemistry of this organism. The n-alkylaro-

matics may have a similar origin to the C21 and C23

n-alkylphenols that have been noted in high concentrationsin some kukersite pyrolysates from Estonia that could

be due to G. prisca growing under higher salinity con-ditions (Derenne et al., 1990, 1992; Blokker et al., 2001).Thermally mature equivalents of the Camrose Mem-

ber/Nisku Formation potential source rocks are con-

sidered the source of the oils that occur in NiskuFormation reservoirs in the Enchant-Hays field ofsouthern Alberta. Saturate fraction and gasoline range

gas chromatograms, and the presence of the n-alkylaro-matics in the Enchant-Hays oils in relatively high con-centrations indicates an important contribution from G.

prisca-rich intervals.We have demonstrated conclusively using both opti-

cal and chemical evidence that G. prisca was a con-tributor to some very organic-rich intervals within the

Late Devonian Camrose Member. We have alsoobserved G. prisca optically as an occasional, minorcomponent in organic-lean intervals in other Middle

and Late Devonian units of the Western Canada Sedi-mentary Basin where its presence is not readily chemi-cally evident. Hence, G. prisca did survive the

Ordovician-Silurian extinction event but appears to losemuch of its ecological niche to prasinophytes and othergreen alga that dominate most of the organic-rich

Devonian intervals within the Western Canada Sedi-mentary Basin. These data and other reports (e.g.Abrams et al., 1999) indicate that caution should beused in unequivocally attributing a Cambro-Ordovician

age to sediments and oils having G. prisca geochemicalcharacteristics, as these can also occur in rocks as youngas the Late Devonian.

M.G. Fowler et al. / Organic Geochemistry 35 (2004) 425–441 439

Acknowledgements

We acknowledge the excellent technical support ofSneh Achal, Kim Dunn, Laura Mulder, Rachel Robin-

son and Ross Stewart at the NRCan GSCC OrganicGeochemistry and Organic Petrology Laboratory. Wethank Roger Summons and Sylvie Derenne for their

very helpful comments on an earlier version of thispaper. We also thank the GSC and the oil and gascompanies that financially supported the Devonian Pet-

roleum Systems of Western Canada Project for enablingus to undertake this research. This is Geological Surveyof Canada Contribution 2002251.

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