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Org. Geochem.Vol. 18, No . 5, pp. 641--645, 1992 0146-6380/92 5.00 + 0.00Printed in Grea t Britain. All rights reserved Copyright 1992Pergamon Press Ltd

High diahopane and neohopane abundances in a b iodegraded crudeo i l f rom the Bar row Sub bas in o f W es tern A us t r a li a

CARIM ARM ANIOS,ROBERTALEXANDER nd ROBERT . KAGICentre fo r Petroleum and Environmental Organic G eochemistry, Curtin U niversity of Technology,

G.P.O. Box U1987, Perth 6001, Western Australia

Abstract--Three crude oils from the Barrow Sub-basin of Western Australia have b een analysed forhopanoid biom arkers using GC -M S techniques. Enhanced abundances of nordiahopane and diahopaneand norneohopane relative to hopanes have been observed in the most severelybiodegraded sample. Thesedifferences have been attributed to the greater resistance of the diahopanes and norneohopane to m icrobialdegradation.

K e y w o r d s - - h o p a n e sdiahopane, norneohopanes, microbial degradation, Barrow Sub-basin, petroleumbiomarkers

INTRODUCTION

Biodegrad a t ion o f c rude o i l invo lves the p rogress iveu t i l i za t ion o f spec i f i c c l a s ses o f hydroca rbons bybac te r i a (of . Co nna ne t a l . 1980). Where th e tempe ra-ture of the reservoir i s less than 75C bacter ia cansurvive and funct ion (Phi l ippi , 1977) . The reservoirmus t a l so be in con tac t wi th m e teor i c wa te r s s ince th isi s be l i eved to be the ma jo r mechan i sm by whichbacter ia gain access to the reservoir (Bai leye t a l .1973).

B iodegrad a t ion o f a c rude o i l l eads to a sequen t i alr emova l o f n -a lkanes , i sopreno id and o the r b ranche da lkanes , and even tua l ly some cyc l i c a lkanes andaromat i c s a s cond i t ions fo r mic rob ia l ox ida t ion be -com e inc reas ing ly f avourab le (Reed , 1977; Alexandere t a l . 1983; Volkm ane t a l . 1983a, 1984; Peters a ndM oldow an , 1991) . C lea r ly, an unde rs t and ing o f thec o n d i t io n s u n d e r w h i c h t h e c o m m o n b i o m a r k e r s a r eal tered is essentia l for re l iable appl icat ion of b io-marker s to p rob lems invo lv ing the co r re l a t ion o f

b iodegraded c rude o i l s ( c f . Vo lkmane t a / . 1983a;Pe te r s and Moldowan , 1991) .

The hopano ids (F ig . 1 ) a re pen tacyc l i c a lkaneswhich a re found wide ly in sed imen ta ry rocks andpe t ro leum. Th i s c l a s s o f b iomarker, a l though qu i t eres is tant to bacter ia l a t tack, i s eventual ly degradedunder op t ima l r e se rvo i r cond i t ions (Volkmane t a l .1983a). I t has been sugge sted that the th erm al ly s table17~t(H),21f l (H)-hopanes (Fig . l , I -V III ) in cer ta inc i rcums tances can undergo a bac te r i a l ly med ia teddemethylat ion a t C-10 to form the 17~t(H),21f l (H)-25-norho pane ser ie s (F ig . l , IX-X V I) (Volkm ane t a l . 1983a ; Pe te r s and M oldow an , 1991) . A l tho ughth i s p rocess has no t been rep l i ca ted so f a r underl abora to ry cond i t ions , the iso la t ion o f a hopan o idf rom recen t sed imen t s con ta in ing a ca rboxy l g roupat C-10 (Albrecht , 1990) indicates that oxidat ion a t

th i s pos i t ion can occur under na tu ra l cond i t ions .Such p rocesses , a long wi th subsequen t deca rboxy la -t ion, could therefore provide a viable route to the17~, (H )-25- norh opa nes .

Recent ly, a new ser ies o fC 2 9 C 3 3h o p a n o i d s n a m e dd iahopanes (F ig. 1 , X IX -X X II I ) has been ident i fi edin c rude o i l (Moldowane t a l . 1991) . The diahopaneca rbon ske le ton d i ff e r s f rom the hopane ske le ton in

tha t i t has a m e thy l g roup o n C-15 ins t ead o f C-14 .This pap er is conce rned with the effect of b iode grada -t ion on the diahopanes and the recent ly ident i f iedn o r n e o h o p a n e ( M o l d o w a ne t a l . 1991) (Fig. 1,XVII I ) in th ree c rude o i l s f rom the Bar row Sub-basin , Western Austra l ia .

EXPERIMENTAL

C r u d e o i l s

Geolog ica l da ta fo r each o f the Bar row Sub-bas inoi ls are given in Table 1 . Detai led acco unts o f the

pe t ro leum g eo logy o f the Bar row S ub-bas in can befound in paper s by Crank (1973) , Thom as and Smi th(1974) and Thomas (1978) . Detai led s tudies on thegeochemis t ry o f the c rude o i l s have a l so been re -p o r t e d ( Vo l k m a ne t a l . 1983a). In brief, the Barrowsample is a parat f in ic l ight crude oi l which shows noev idence o f b iodegrada t ion . T he F l inde r s Shoa l andMard ie samples a re naph then ic c rude o i l s wi th com-posi t ions consis tent wi th thei r having been subjectedto seve re b iodegrada t ion and wa te r wash ing . TheDing o C lays tone o f Ju rass ic age is the pu ta t ive sourceof these c rude o i ls (Volkm ane t a l . 1983b).

I s o l a ti o n o f b r a n c h e d a n d c y c l i c a l k a n e s f r o m c r u d e o i lThe b ranched and cyc li c a lkanes were i so la t ed

f rom c rude o i l s amples us ing p rev ious ly r epor t edm e t h o d s ( Vo l k m a ne t a l . 1983a). Briefly, the alkanes

641

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642 CARIM ARMANIOSe t a l .

, , U l

X V I

1 7 a ( H ) , 2 1 p ( H ) - h o p a n e s

I R I = H R 2 = C H 3

I I R I - C 2 H 5 R 2 - C H 3

I l l R I - C 3 H 8 R 2 = C H 3

I V R I = C 4 H I 0 R 2 = C H 3

V R I - C 5 H 1 2 R 2 - C H 3

V I R I - C 6 H I 4 R 2 = C H 3

V I I R I - C T H 1 6 R 2 - C H 3

V I I I R I - C S H 1 8 R 2 - C H 3

1 7 a ( H ) , 2 1 ~ ( H ) - n o r h o p a n e s

I X R I = H R 2 - H

X R I = C 2 H 5 R 2 = H

X I R I - C 3 H 8 R 2 - H

X I I R I - C 4 H I 0 R 2 - H

X I I I R I = C 5 H I 2 R 2 - H

X I V R I = C 6 H I 4 R 2 - H

X V R I = C 7 H I 6 R 2 = H

X V I R I - C 8 H I 8 R 2 - H

1 8 a ( H ) , 2 1 ~ ( a ) - n e o h o p a n e s

X V I I R - H

X V I I I R - C 2 H 5

l l l l l l

I - l ~ I I I

1 7 ~ H) - d l a h o p a n e s

. I ~ r , , , , . - ~ 3 . 8X X I R C 4 H I 0

X X I I R C 5 H 1 2

X X I I I R = C 6 H I 4

X - X X I I I

Fig . 1 . Ho pan oids in pe tro leum.

w e r e s e p a r at e d u s in g c o l u m n c h r o m a t o g r a p h y w i t hs il ic ic a c i d . T r e a t m e n t w i t h m o l e c u l a r s i e v e s r e m o v e dt h e n - a l k a n e s l e a v i n g a f r a c t io n e n r i c h e d i n b r a n c h e da n d c y c l i c a l k a n e s w h i c h w a s f u r t h e r a n a l y s e d u s i n gG C - M S t e c hn i q u es .

G analysis o f alkanes

T h e a n a l y si s o f th e t o t a l a l k a n e s b y g a s c h r o m a t o gr a p h y w a s c a r r i e d o u t u s i n g a H e w l e t t P a c k a r d 5 8 8 0 Ag a s c h r o m a t o g r a p h e q u i p p e d w i t h a 5 0 m x 0 . 22 m m

Ta b l e 1 . G e o c h e m i c a l d a t a p e r t a i n i n g to t h e c r u d e o i l s fr o m t h e B a r r o wS u b - b a s i n Vo l k m a ne t a l . 1983b)

R e s e r v o i r R e s e r v o i rP e t r o l e u m Ty p e R e s e r v o i r d e p t h a t ) a g e

B a r r o w P a r a f l in i c M u d e r o n g 2 01 0 L a t e

o i l Sha le Ju rass icF l i n d e r s N a p h t h e n i c B i r d r o n g 7 0 0 A p t i a nShoa l o i l SandsM a r d i e N a p h t h e n i c M a r d i e 7 7 A p t i a n

o i l G r e e n s a n d

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Hopanoid biomarkers in Barrow Sub-basin crude oil 643

Table 2. Ho pano id nomenclature and abbreviations (for structuresrefer to Fig . 1)

Abbreviation Structure NameC29DIA XX 14e H),I7a (H),21~(H)-nordiahopaneC29D XI 17~ H),21~(H)-25-norhopaneC ~ II 17" H),21~ H)-norhopaneC 2 9 Ts XV III 18,, H),21,g(H)-norneohopaneC~D IA XX 14~ H),17,,, H),21 g H)-diahopaneC3 III 17~ H),21fl(H)-hopane

i .d . fu sed s i l i ca co lumn coa ted wi th BP-5 s t a t i ona ryp h a s e ( S G E A u s t r a l i a ) a n d a n O C I - 3 o n - c o l u m nin jec to r (SGE Aus t r a l i a ) . Hydrogen was used a s t heca r r i e r gas a t a l i nea r f l ow ve loc i ty o f 30 cm/s . Th eo v e n w a s p r o g r a m m e d f r o m 7 0 t o 3 0 0 C a t 4 C /r a in .

G C - M S a n a ly s is o f b r a n ch e d a n d c yc li c a lk a n es

The ana lys i s o f b ranched and cyc l i c a lkanes by gasc h r o m a t o g r a p h y - m a s s s p e c t r o m e t r y w a s c a r r i e d o u t

u s i n g a H e w l c t t P a c k a r d 5 9 7 0 M S D e q u i p p e d w i t ht h e RT E / A d a t a s y s t e m . T h e G C M S w a s f i t t e d w i t ha 60 m x 0 .2 m m i .d . DB-5 ( J&W ) co lum n . The ov ent e m p e r a t u r e w a s p r o g r a m m e d f r o m 5 0 t o 2 7 4 C a t8C/ ra in , t hen to 300C a t 1C/ rain . Th e samples w erei n j e c t e d o n - c o l u m n u s i n g a H P 7 6 7 3 A a u t o s a m p l e r( H P ) a n d a n a l y s e d i n S I M m o d e .

RESULTS AND DISCUSSION

G a s c h r o m a t o g r a m s o f th e t o t a l a lk a n e s f r o mthe th ree r e l a ted c rude o i l s a r e show n in F ig . 2 , and Ta re s imi la r t o t hose p rev ious ly pub l i shed (V o lkm an

et a l . 1 98 3a ). T h e m a s s c h r o m a t o g r a m o f t h e u n d e -g r a d e d B a r r o w s a m p l e s h o w s a p r e d o m i n a n c e o f t h e

~

n-a lkanes , w h i le t hose o f t he b iodegrad ed c rude o i l s [l a c k n - a l k a n e s a n d c o m p r i s e u n r e s o l v e d c o m p l e x /mix tu re s o f b ranched and cyc l i c a lkanes .

ig . 3 shows par t ia lm / z 1 91 m a s s c h r o m a t o g r a m sof t he c rude o il s. T he d i ahopa nes and C29Ts wereas s igned on the bas i s o f the i r pub l i shed r e t en t iont imes r e l a t i ve t o t he 17~(H) -hopanes (Moldowanet al . 1991) . These a s s ignmen t s were suppor t ed bythe mass spec t r a i n so f a r a s a l l compounds a s s igneda s d i a h o p a n e s d i s p l a y e d p r o m i n e n tm / z 191 and

pa ren t i on r e sponses . The ex t ended d i ahopaneswere p re sen t i n concen t r a t ions t oo low to enab lem e a s u r e m e n t s t o b e m a d e .

The pa r t i a l m / z 1 91 m a s s c h r o m a t o g r a m o f th eFl inders Shoal sample , shown in Fig . 3 , i s s imi lar totha t o f t he B a r row sample , excep t fo r a sma l l r e la t i veinc rease i n t he peak l abe ll ed C29D. The p re sence o f al o w c o n c e n t r a t i o n o f t h is c o m p o u n d i n F l in d e r s S h o a lc rude o i l ha s p rev ious ly been in t e rp re t ed a s r e su l ti ngf rom a m ix ing o f two c rudes , one a heav i ly deg radeds a m p l e w i t h s o m e 2 5 - n o r h o p a n e s a n d t h e o t h e r apa r t l y deg raded o i l (A lexande ret al . 1983 ; Vo lkmanet al . 1983a ) . The r ema in ing peaks in t he F l inde r sShoa l s ample ind ica t e t ha t t he 17~(H) -hopanes (C~ ,C30), t he n o rne oh opa ne (C29Ts) and the tw o d i a -hopanes (C29DIA and C30DIA) occu r i n s imi l a r p ro -po r t ions t o t hose found in t he Ba r row c rude o i l .

The pa r t i a lm / z 1 91 m a s s c h r o m a t o g r a m f r o m t h eM a r d i e s a m p l e s h o w s a d o m i n a n t p e a k f o r 2 5 -no rh opa ne (C29D) and the peaks fo r no rh op ane (C29)and hop ane (C3o) . In t e re s t i ng ly, t he d i aho panes(C29DIA, C~JDIA) and the no rneohopane (C29Ts )have a s imi lar d is t r ibut ion to those in the less de-

g raded samples . Th i s i nd i ca t e s t ha t t he d i ahopanesa n d n o r n e o h o p a n e s a r e m o r e r e si s ta n t t o b i o d e g r a d a -t i on than the 17~(H) -hopanes .

Ev idence tha t s ign i f i can t dep le t ion o f d i ahopanesa n d n o r n e o h o p a n e a l s o o c c u r s w a s p r o v i d e d b yc o m p a r i n g t h e r el at iv e a b u n d a n c e s o f h o p a n e a n dn o r n e o h o p a n e i n t h e u n d e g r a d e d B a r r o w s a m p l ew i t h th o s e o f n o r h o p a n e a n d n o r n e o h o p a n e i n th eM a r d i e s a m p le . I f h o p a n e h a d b e e n c o n v e r te d t o t h ed e m e t h y l a t e d 2 5 - n o r h o p a n e w i t h o u t a l t e r a t i o n o fn o r n e o h o p a n e , t h e n t h e p e a k l a be ll e d C 2 9 D i n t h em a s s c h r o m a t o g r a m f r o m M a r d i e s h o u l d b e o n l y

abo u t o ne h a l f t ha t l abe ll ed C30 in t he B a r row m ass

a r r o w U J

2O

it

I F l l n d e r s h o a l

L

R E T E N T I O N T I M E

Fig. 2. Capillary gas chromatograms of the alkanes isolatedfrom the crude oils. C2o n-alkane is indicated by (20), theisoprenoid alkanes pristane and phytane by (#), the Ci4 ~6

bicyclanes by (*), and the 25-norhop ane series by ( ^) .

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6 CARIMAglvL~baOSt a l

chrom atogra m, s ince the 191 fragm ent is der ived onlyfrom the D/ E r ing moiety of C29D while the pe aklabelled C30 has a 191 contribu tion fro m bo th the A/Band D/E r ing moiet ies . Clear ly the response f romC29D, shown in Fig. 3 , i s mu ch s t ronger (comp aredwith C29Ts) tha n pred icted, suggesting tha t nor-

neohopane and the d iahopanes a re degraded bu t a ta s lower ra te than the 17n(H)-hopanes .

An at tempt was made to ident i fy degradat ionprodu cts f rom C29Ts and the diahopanes assumingthat they had undergone s imilar degradat ion pro-cesses to the 17~(H)-hopanes. However, a detailedanalysis of the M ardie crude oi l provided no evidenceof 25-nordiahopanes nor of a 25,30-bisnorneoho-pane. I t i s possible that these rearranged hopanes ,l ike the s teranes , are degraded to non-hydrocarbonproducts .

Al though the evidence s t rongly supports bio-

degradation effects as the most l ikely cause of theelevated re la tive abundances of 25-norhopanes , nor-neohopane and d iahopanes in the Mard ie sample ,an alternative explanation, based on differences inthermal s tabi l i ty of hopanoids can be proposed.M o l d o w a n e t a l (1991) reported that the rearranged

hopanes are mo re s table than the l t (H)-hopanesdue to re l ief of s ter ic s t ra in w hen the m ethyl group atCi4 is relocated to Cl5 (diahopanes), or when theme thyl gr oup Ct8 is relocated to C~7 (neoho panes).Using s imilar reasoning i t i s a lso probable thatremoval of the methyl group at C,0 would enhance

the s tabi li ty of the 25-norhop anes in com parison withthe 17~(H)-hopanes. Hence, a crude oil subjected toextreme thermal condi t ions might eventual ly becomerelat ively enr iched in 25-norho panes and rearrangedhopanoids .

Matur i ty assessment of severely degraded crudeoils such as the Mardie sample is difficult becauseman y o f the compou nds used as m atur i ty ind ica tor sare either absent or severely affected by biodegrada-t ion. Alkylnaphthalenes and alkylphenanthrenes inMardie are e i ther of very low abundan ce or h ave beensignificantly altered by biodegradation rendering

them unrel iable for matur i ty ra t ios . Volkmane t a l

(1984) however, reported that the re la t ive proport -ions of sh ort chain (C2~, C22) and long chain (C29)monoaromatic s teranes were s imilar in a l l threecrude oi ls and typical of matu re source rocks. Theseresults suggest to us that the Mardie crude oil has

m Z 1 9 1C 3 0

C 9

~ c 2 9 r , f lc29 hI^ [~A o hI^ ~ILC 2 9 D

L

I

I

I

F l n de r s S ~ a l

M a r d i e

w

R E T E N T I O N T I ME

Fig. 3. Partialm / z 191 m ass chromatogram s showing com parison of relative distributions of diahopanesto hopanes in selected crude oils from the Carnarv on B asin, (refer to Table 2 for hop anoid no menclature).

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Hop anoid b iomarkers in Bar row Sub-bas in c rude o i l 645

n o t b e e n s u b j e c t e d to u n u s u a l l y s e v e re th e r m a l c o n -d i t i o n s s u f fi c ie n t t o c a u s e t h e o b s e r v e d h o p a n o i dd i s t r i b u t i o n .

CONCLUSION

T h e r e la t iv e a b u n d a n c e s o f d i a h o p a n e s a n d 3 0 -n o r n e o h o p a n e r e la t iv e to t h e 1 7 g H ) -h o p a n e s h a v eb e e n s h o w n t o b e e n h a n c e d i n a s ev e r e ly b i o d e g r a d e dc r u d e o i l f r o m t h e B a r r o w S u b - b a s i n . T h e e ff ec t h a sb e e n p r e d o m i n a n t l y a t t r i b u t e d t o t h e s lo w e r r a te o fm i c r o b i a l d e g r a d a t i o n o f t he n o r n e o h o p a n e a n dd i a h o p a n e s c o m p a r e d w i t h t h e 1 7 ~ t H ) - h o p an e s .

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Volk man J. K. , Alexander R. , Kagi R. I . , Rowland S. J . andSheppard P. N. 1984) Biodegradation of aromatic hydro-carbo ns in crude oi ls from the B arrow S ub-basin ofWestern Austral ia .Org. Geochem.6, 619-632.