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ergamon Org. Geochem. Vo l. 21, No. 2, pp . 115-120, 1994Copy right 1994 ElsevierScienceLtdPrinted in G reat Britain. All rights reserved0146-6380/94 6.00 + 0.00

The effect of maturity on the relative abundances of cadalene andisocadalen e in sediments from the Gippsland Basin Australia

ROBERT ALEXANDER, ROBERT I. KAGI, RA J K . SINGH a nd IMAMB. SOSROWIDJOJOCentre for Petroleum and Env ironmental O rganic Geochemistry, Curtin University of Technology,Box U1987, GPO, Per th 6001, Western Aust ra l ia

Rece ive d 2 7 O c to b er 1 9 9 2 ; r e tu r n ed fo r r ev is ion 9 D ece m b er 1 9 9 2 ; a ccep ted in r ev i s ed o r m 1 6 A u g u s t 1 9 9 3 )Abstract - -Two sedimentary sequences f rom the Gippsland Basin, Aust ra l ia have been analysed forcadalene and 3-isopropyl-l ,6-dimethylnaphthalene (isocadalene). The relative abundance of these com-pou nds changes withqncreasing depth (maturity) of the sediments and is associated with an increase inthe abu ndan ce of 1,6-dimethylnaphthalene relative to othe r dimethylnaphthalene isomers. These changeshave been attrib uted to d ealky lation and isomerization reactions of cadalene in the sediments becauseanalo gou s changes have been dem onstrate d in labo rato ry heating experiments in which cadalene wasconverted into a mixture containing main ly 1,6-dimethylnaphthalene and isocadalene when heated in thepresence of aluminum smectite. The ratio of isocadalene and cadalene m ay be a useful ma turity param eterfor some sediments and crude oils which contain these compounds.K e y w o r d s - - c a d a l e n e , 3-isopropyl-l ,6-dimethylnaphthalene, isocadalene, matu rity, maturity parameter,Gippsland Basin

INTRODUCTIONT h e d i s t r i b u t i o n o f i s o m e r i c a lk y l n a p h t h a l e n e s i nt h e b i t u m e n s e x t r a c t e d f r o m s e d i m e n t s u n d e r g om a j o r c h a n g e s a s b u r i a l d e p t h a n d s e d i m e n t t e m -p e r a t u r e i n c r e a s e . F o r b o t h d i m e t h y l n a p h t h a l e n e sa n d t r i m e t h y l n a p h t h a l e n e s t h e o r - su b s t i t u te d i s o m e r ss h o w a d e p l e t i o n r e l a t i v e t o t h e / / - s u b s t i t u t e dc o m p o u n d s w i t h p r o g r e s s i n s e d i m e n t m a t u r i t y( A l e x a n d e r e t a l . , 1985 ; R adke , 1987) . T hese e f f ec t sh a v e b e e n a t t r i b u t e d t o a c o m p l e x s e r i e s o f a c i d -c a t a l y s e d r e a c t i o n s i n v o l v i n g d e a l k y l a t i o n , a l k y l a t i o na n d i s o m e r i z a t io n t h a t r e s u l t in a m i x t u r e o f r e a c t i o np r o d u c t s . I s o m e r d i s t r i b u t i o n s i n t h e r e a c t i o n p r o d -u c t s a p p r o a c h t h e e q u i l i b r iu m d i s t r i b u t io n i n w h i c ht h e m o r e s t a b l e f l -s u b s t i t u te d i s o m e r s p r e d o m i n a t e .T h e s e c h a n g e s h a v e b e e n u s e d t o d e v e l o p m a t u r i t yi n d i c a t o r s , b a s e d o n c o m p o u n d r a t i o s , t h a t c a n b eu s e d t o i n f e r t h e m a t u r i t y o f s e d i m e n t s a n d , i n t h ec a s e o f c r u d e o i l s , t o i n f e r t h e m a t u r i t y o f th e s o u r c er o c k s a t t h e t i m e o f h y d r o c a r b o n e x p u l s i o n (c f.R adke , 1987) .

T h e m a t u r i t y i n d i c a t o r s b a s e d o n r e l a ti v e c h a n g e si n a l k y l n a p h t h a l e n e i s o m e r c o n c e n t r a t i o n s a n dt h o s e b a s e d o n s i m i l a r c h a n g e s i n t h e d i s t r i b u t i o no f a l k y l p h e n a n t h r e n e i s o m e r s , h a v e b e e n w i d e l yu s e d a l t h o u g h s o m e a n o m a l i e s h a v e b e e n r e p o r t e d( C a s s a n i e t a l . , 1 9 88 ). I n o u r e x p e r i e n c e s u c h m a t u r i t yi n d i c a t o r s h a v e s h o w n e r r a t i c b e h a v i o u r w h e na p p l i e d t o s e d i m e n t s c o n t a i n i n g a b u n d a n t p l a n tr e m a i n s o f J u r as s i c a n d y o u n g e r a g e ( A l e x a n d e re t a l . , 1 9 9 2 ) . T h i s m a y b e c a u s e d b y a h i g h b u tv a r i a b l e i n p u t t o t h e s e d i m e n t s o f a s p e c i f ic i s o m e r

( o r i t s p r e c u r s o r ) t h e c o n c e n t r a t i o n o f w h i c h i s u s e di n c a l c u la t i n g th e m a t u r i t y p a r a m e t e r . F o r e x a m p l e ,v e r y h ig h i n p u t s o f 1 - m e t h y l p h e n a n t h r e n e a r e s o m e -t i m e s o b s e r v e d i n s e d i m e n t s c o n t a i n i n g c o n i f e r r es i n s( A l e x a n d e r e t a L , 1 9 9 2 ; H e p p e n h e i m e r e t a l . , 1992);i n s u c h c a s e s m a t u r i t y i n d i c a t o r s c a l c u l a t e d u s i n gr e l a t i v e c o n c e n t r a t i o n s o f t h i s c o m p o u n d w i l l b ed i s t o r t e d . I n o r d e r t o o v e r c o m e s u c h s i t u a t i o n s ,m a t u r i t y i n d i c a t o r s n e e d t o b e f o r m u l a t e d t o i n c l u d ep r o d u c t p r e c u r s o r r e l a t i o n s h ip s .

C a d a l e n e o c c u r s w i d e l y i n s e d i m e n t s ( B e n d o r a i t i s ,1974; Baset e t a l . , 1 9 8 0 ; R a d k e e t a l . , 1984 ; N ob lee t a l . , 1 99 1) . C o m p o u n d s w i t h t h e c a d a l e n e c a r b o ns k e l e t o n o c c u r i n p l a n t s , b r y o p h y t e s a n d f u n g i( B o r d o l o i e t a l . , 1 98 9 ). T h e y a r e e s p e c i a l l y a b u n d a n ti n p l a n t r e s in s , f o s s il r e s in s a n d e s s e n t i a l o il s d e r i v e df r o m l a n d p l a n t s ( G r a n t h a m e t a l . , 1 9 83 ; v a n A a r s s e ne t a l . , 1 9 9 0 ) . F o r t h e s e r e a s o n s t h e p r e s e n c e o fc a d a l e n e i n s e d i m e n t s h a s b e e n i n t e r p r e t e d a s i n d i -c a t i n g a n i n p u t t o t h e s e d i m e n t s o f l a n d p l a n t s(B endora i t i s , 1974 ; R adke , 1984 ; S im one i t , 1986) .T h e s e r e p o r t s c o n c e r n s e d i m e n t a r y o r g a n i c m a t t e rd e p o s i t e d a f t e r J u r a s s i c t i m e s w h i c h c o r r e s p o n d s t o ap e r i o d i n e v o l u t i o n a r y h i s t o r y w h e n p l a n t s w e r ew i d e s p r e a d .

I n t h i s p a p e r w e r e p o r t o n t h e i d e n t i f i c a t i o n o fc a d a l e n e a n d 3 - i s o p r o p y l - l , 6 - d i m e t h y i n a p h t h a l e n e( s e e S t r u c t u r e 1 ) i n a s e d i m e n t a r y s e q u e n c e a n ds h o w t h a t a u n i f o r m c h a n g e o c c u r s i n t h e i r r e l a t i v ea b u n d a n c e s w i t h i n c r e a s e i n s e d i m e n t m a t u r i t y .B e c a u s e t h e r e l a t io n s h i p o f t h e se t w o c o m p o u n d s i st h a t o f p r e c u r s o r a n d p r o d u c t f o r a r e a c t i o n i n v o lv i n gi s o m e r i z a t io n o f t h e i s o p r o p y l g r o u p , t h e r a t i o o f t h e

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116 ROBERT ALEXANDERet al.

82

3

I : 1 6 - D i m e t h y l - 4 - i s o p r o p y l n a p h t h a l e n e I1 : 1 6 - D i m e t h y l - 3 - i s o p r o p y l n a p h t h a l e n eCADALENE) ISOCADALENE)

Structure 1.

r e l a t iv e c o n c e n t r a t i o n s o f t h e se t w o c o m p o u n d si s p o t e n t i a l l y a m o r e r e l i a b le i n d i c a t o r o f s e d i m e n tm a t u r i t y t h a n s o m e o f t he a r o m a t i c m a t u r i t yi n d i c a t o r s c u r r e n t l y i n u s e .

EXPERIMENTAL

S a m p l e sT h e s a m p l e s u s e d i n t hi s s t u d y w e r e o b t a i n e d

a s d r i l l c u t t i n g s f r o m e x p l o r a t i o n w e l l s i n t h eG i p p s l a n d B a s i n, A u s t r a l i a . T h e c r u d e o i l s w e r ef r o m t h e s a m e b a s i n . R o c k s a m p l e s w e r e a l l f r o mt h e L a t r o b e F o r m a t i o n o f L a t e C r e t a c e o u s ag e . T h i sf o r m a t i o n c o n s i s t s o f i n t e r b e d d e d c o a l s , c a r b o n -a c e o u s s h a l es a n d s a n d s o f n o n - m a r i n e o r i g i n . F o ra d e t a i l e d d e s c r i p t i o n o f t h e t e c t o n i c h i s t o r ya n d b a s i n d e v e l o p m e n t r e a d e r s a r e r e f e r r e dt o T h o m p s o n ( 1 9 8 5 ) , H e g a r t y et al. (1 9 8 5 ) a n dre fe re n c e s th e re in .Preparat ion of cadalene and i socadalene

C a d a l e n e a n d i s o c a d a l e n e w e r e p r e p a r e d a c c o r d i n gt o m e t h o d d e s c r i b e d b y E i s e n b r a u n et al. (1971).B r i e f ly , t h e s y n th e s i s o f c a d a le n e in v o lv e d aF r i e d e l - C r a f t s r e a c t io n o f p - c y m e n e a n d m e t h y l -s u c c i n i c a n h y d r i d e t o g i v e 4 - ( 2 - m e t h y l - 5 - i s o p r o p y l -p h e n y l ) 2 - m e t h y l b u t a n o i c a c i d , t h i s a c i d w a s c y c l i z e dw i t h p o l y p h o s p h o r i c a c i d t o g i v e 2 , 5 - d i m e t h y l - 8 -i s o p r o p y l - l - t e t r a l o n e w h i c h , in t u r n , w a s r e d u c e d t ot h e c o r r e s p o n d i n g a l c o h o l w i t h l i t h i u m a l u m i n u mh y d r i d e a n d t h e n d e h y d r a t e d a n d a r o m a t i z e d t o g i v ec a d a l e n e . I s o c a d a l e n e w a s s y n t h e si z e d u s i n g a s i m i l a rs e t o f r e a c t i o n s s t a r t i n g w i t h m - c y m e n e a n d m e t h y l -s u c c in i c a n h y d r i d e .

T h e s p e c t r a l p r o p e r t i e s o f t h e s y n t h e s i z e d c o m -p o u n d s w e r e s i m i l a r t o t h o s e o f c a d a le n e a n di s o c a d a l e n e r e p o r t e d b y A d a c h i et al. (1 9 8 1 ) a n dB f h l m a n n a n d Z d e r o ( 1 9 7 6 ) .I s o la t ion o f a r om at i c hy dr oc ar bon fr ac t i ons

C r u d e o i l a n d s o l u b l e o r g a n i c m a t t e r e x t r a c t e df r o m d r y c r u s h e d r o c k s u s i n g d i c h l o r o m e t h a n es o lv e n t w e re f i r s t d i s s o lv e d in h e x a n e a n d f i l t e r e d

t h r o u g h a s h o r t c o l u m n o f a c t i v a t e d a l u m i n a t or e m o v e v e r y p o l a r c o m p o u n d s . T h e f i l t r a te w a s t h e ns u b j e c t e d t o l i q u i d c h r o m a t o g r a p h y p r o c e d u r e s u s i n ga M e r c k - L o b a r G r o b e A ( 2 4 0 - 1 0 ) L i c h r o p r e p S i 6 0 ,4 0 - 6 3 m c o l u m n c o u p l e d t o a W a t e r s M i l l i p o r eM o d e l 5 1 0 d o u b l e p i s t o n p u m p . A l k a n e s w e r e e l u t e df r o m t h e c o l u m n u s i n g h e x a n e a n d t h e a r o m a t i cc o m p o u n d s e l u t e d b y r e v e r si n g th e f l o w t h r o u g h t h ec o l u m n u s i n g t h e s a m e s o l v e n t .Ana lys i s o f aromat ic hydrocarbon f rac t ion

G C - M S a n a ly s i s o f t he a r o m a t i c h y d r o c a r b o nf r a c t i o n w a s c a r r i e d o u t u s i n g a n H P 5 8 9 0 c a p i l l a r yG C c o u p l e d t o a n H P 5 9 7 0 s e r i e s M S D - c o m p u t e rd a t a s y s t e m , f i t te d w i th a 6 0 m x 0 .2 5 m m i . d . D B -5( J & W ) f u s e d s i li c a c a p i l l a r y c o l u m n . T h e G Cw a s t e m p e r a t u r e p r o g r a m m e d f r o m 7 0 t o 3 0 0 C a t3 C / m i n , f o l l o w e d b y a n i s o t h e r m a l p e r i o d o f 2 5 m i n .T y p i c a l M S c o n d i t i o n s w e r e E M v o l t a g e 1 9 0 0 Ve l e c t r o n e n e r g y 7 0 e V a n d s o u r c e t e m p e r a t u r e 2 5 0 C .S a m p l e s w e r e i n j e c t e d u s i n g a n H P 7 6 7 3 A i n j e c t o r .H e l iu m w a s u s e d a s c a r r i e r g a s a t a l i n e a r v e lo c i tyo f 3 0 c m / s . C a d a l e n e s w e r e a s s i g n e d b y m a s s f r a g -m e n t o g r a p h y o f th e m / z 1 9 8 m o l e c u l a r i o n a n db y c o m p a r i s o n o f r e t e n t i o n t i m e s w i t h t h o s e o fs t a n d a r d s .H e at i ng e x pe r i m e n t

T h e a l u m i n u m s m e c t i t e u s e d i n t h e p r e s e n t s t u d yw a s m a t e r i a l p r e p a r e d f o r a n e a r l i e r s t u d y ( A l e x a n d e ret al . 1 98 4) a n d d e s c r i b e d a s a l u m i n u m m o n t -m o r i l l o n i t e .

C a d a le n e (1 m g ) w a s s e a l e d in a P y re x g l a s sa m p o u l e o f a p p r o x . 2 .7 m l c a p a c i t y u n d e r a n i t r o g e na t m o s p h e r e w i t h f in e l y d i v i d e d a l u m i n u m s m e c t i t e(1 0 m g ) a n d h e a te d a t 1 6 0C fo r 1 2 h . T h e a m p o u lew a s t h e n c o o l e d , o p e n e d a n d t h e c o n t e n t s e x t r a c t e dw i t h d i c h l o r o m e t h a n e ( 2 m l ) . T h e s o l v e n t w a s r e -m o v e d b y c a r e f u l d i s t il l a t i o n a n d h e x a n e ( 5 0 0 1 )c o n t a i n i n g t h e e x t e r n a l s t a n d a r d u n d e r c a n e ( 0 .3 5 / ~ g)w a s a d d e d . A n a l i q u o t o f t h is s o l u t i o n w a s t h e na n a l y s e d b y G C t e c h n i q u e s . T h e c o m p o u n d s r e p -r e s e n t e d i n t h e r e s u l t a n t c h r o m a t o g r a m a c c o u n t e d

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Effect of ma turi ty of cadalene and isocadalene 117f o r 7 5 o f t h e i n i t i al m a s s o f c a d a l e n e a n d c o n s i s t e do f t h e fo l l o w i n g : c a d a l e n e (2 2 ) , is o c a d a l e n e ( 6 5 ) ,1 6-dimethylnaphthalene 4 ) a n d u n i d e n t i f i ed c o m -p o u n d s ( 9 ) .

R E S U L T S N D D I S C U S S I O N

C a d a l e n e a n d 3 - i s o p r o p y l - l , 6 - d i m e t h y l n a p h t h a l -e n e ( i s o c a d a l e n e ) w e r e p r e p a r e d u s i n g e s t a b l i s h e dp r o c e d u r e s ( E i s e n b r a u n et al. 1971) and used asr e f e re n c e s fo r i d e n t i f i c a t i o n o f t h e t w o c o m p o u n d si n s e d i m e n t e x t r a c t s a n d c r u d e o i l u si n g G C - M Ste c h n iq u e s . F ig u re 1 s h o w s m / z 1 9 8 m a s s c h ro -m a t o g r a m s f r o m a m i x t u r e o f th e s y n t h e si z e d c o m -p o u n d s a n d f r o m t h e a r o m a t i c f r a c t i o n o f a c r u d e o il( T u n a - 4 ) . T h e s t a n d a r d s h a d t h e s a m e r e t e n t i o n ti m e sa s t h e c o m p o u n d s r e p r e s e n t e d b y t h e s h a d e d p e a k s .W h e n t h e s t a n d a r d s w e r e c o - i n j e ct e d w i t h t h e

m / z 1 9 8

aT U N A 4 C R U D E O I L

C a d a l e n e I s o c a d a l e n eb

~ . r - ~

S T A N D A R D S

c)H E A T I N G

E X P E R I M E N T

PR E T E N T I O N T I M E m i n i

Fig. 1. Partia l m/z 198 mass chromatog rams showing peaksfor (a) eadalene and isocadalene in a Tuna-4 crude oil (b)a mixture o f the synthesized s tandards an d (c) the reactionproducts from heating cadalene with aluminum smeetite for12 h at 160C.

a r o m a t i c f r a c t i o n b o t h s t a n d a r d s a n d u n k n o w n s c o -e l u t e d u s i n g c o l u m n s c o a t e d w i t h b o t h B P - 5 ( S G E ,A u s t r a l i a ) a n d D B 1 7 0 1 ( J & W , S c i e nt i fi c ) s t a t i o n a r yp h a s e s . T h e u n k n o w n s a l s o h a d s i m i l a r m a s s s p e c t r at o t h e s t a n d a r d s .

F i g u r e 1 a l s o s h o w s a n m / z 1 9 8 m a s s c h ro -m a t o g r a m f r o m t h e r e a c t i o n p r o d u c t s o b t a i n e d b yh e a t i n g c a d a l e n e w i t h a l u m i n u m s m e c t i t e a t 1 6 0 Cf o r 1 2 h . C a d a l e n e a n d i s o c a d a l e n e w e r e t h e m a j o rr e a c t i o n p r o d u c t s f r o m t h e h e a t i n g e x p e r i m e n t .T h e o n l y o t h e r s i g n i f i ca n t r e a c t i o n p r o d u c t w a s1 , 6 - d i m e t h y i n a p h t h a l e n e . T h e s e r e s u l t s s h o w t h a td e a l k y l a t i o n a n d i s o m e r i z a t i o n a r e th e m o s t f a v o u r e dr e a c t i o n s a n d t h a t t h e i s o p r o p y l g r o u p i s m o r e r e a d i l yd i s p l a c e d th a n t h e m e t h y l g r o u p s u n d e r t h e s e a c i d -c l a y - c a t a l y s e d r e a c t i o n c o n d i t i o n s . A n a l y s i s o f t h e s ec o m p o u n d s i n s e d i m e n t a r y s e q u e n c e s s u g g e s t s t h a tt h e s e r e ac t i o n s m a y a l s o o c c u r u n d e r n a t u r a l h e a t i n gc o n d i t io n s in s e d im e n t s .

F i g u r e 2 s h o w s p l o t s o f t h e r e l a t i v e a b u n d a n c e s o fi s o c a d a l e n e a n d c a d a l e n e , e x p r e s s e d a s f r a c t i o n - o f -r e a c t i o n f o r t h e c o n v e r s i o n o f c a d a l e n e t o i s o -c a d a l e n e , a g a i n s t sa m p l e d e p t h f o r t w o w e l ls f r o m t h eG i p p s l a n d B a s i n , A u s t r a l i a . T h e r e s u l t s s h o w t h a tw i th in c re a s e in s a m p le d e p th a n in c re a s e o c c u r s int h e v a l u e o f t h e i s o c a d a l e n e / c a d a l e n e r a t i o i n b o t hw e l l s . B e c a u s e i s o c a d a le n e h a s n o t b e e n r e p o r t e da s a n a t u r a l p r o d u c t , a n d i s a b s e n t o r p r e s e n t i nl o w r e l a t iv e a b u n d a n c e i n i m m a t u r e s e d i m e n t s , w es u g g e st t h a t i t i s f o r m e d i n s e d i m e n t s f r o m c a d a l e n eb y i s o m e r i z a t i o n r e a c t i o n s a s a r e s u l t o f s e d i m e n th e a t i n g a n d t h e c a t a l y t i c e f fe c ts o f c l a y s o r o t h e ra c i d ic c o m p o n e n t s o f t h e s e d i m e n t s .F u r t h e r s u p p o r t f o r a c i d - c a t a l y s e d r e a c t i o n s o fc a d a l e n e u n d e r n a t u r a l h e a t i n g w a s o b t a i n e d f r o mt h e d i s t r i b u t i o n s o f d i m e t h y l n a p h t h a l e n e s i n t h es e d i m e n t s a m p l e s . F i g u r e 2 a l s o s h o w s t h e c h a n g e i na b u n d a n c e o f 1 , 6 - d i m e t h y l n a p h t h a l e n e r e l a t iv e t o t h es u m o f 1 , 3 - d i m e t h y l n a p h t h a l e n e a n d 1 , 7 - di m e t h y l -n a p h t h a l e n e . T h e s e l a t t e r t w o i s o m e r s w e r e s e l ec t e df o r r e f e r e n c e p u r p o s e s f o r t h r e e r e a s o n s r e l a t e dt o a n a l y t i c a l c o n v e n i e n c e . T h e y c o - e l u t e w h e nc h r o m a t o g r a p h e d o n g a s c h r o m a t o g r a p h y c o l u m n sc o a t e d w i t h B P - 5 s t a t i o n a r y p h a s e ; t h e y e l u t e i m -m e d i a t e l y p r i o r t o 1 , 6 - d i m e t h y l n a p h t h a le n e , a n d t h e yo c c u r i n a p p r o x i m a t e l y s i m i l a r a m o u n t s i n m a n ys a m p l e s . F r o m t h e f i g u r e i t i s a p p a r e n t t h a t t h ep r o g r e s s i v e i n c r e a s e i n t h e r e l a t i v e a b u n d a n c e s o f1 , 6 - d i m e t h y l n a p h t h a l e n e o c c ur s o v e r a s i m i l a r d e p t hi n t e r v a l t o t h e c h a n g e i n r e l a ti v e a b u n d a n c e o fc a d a le n e a n d i s o c a d a le n e . In th i s r e s p e c t t h e o b s e r -v a t i o n s m a d e o n t h e r e a c t i o n t y p e s o c c u r r i n g i n t h es e d i m e n t s p a r a l le l t h a t f r o m t h e l a b o r a t o r y h e a t i n ge x p e r i m e n t d e s c r i b e d a b o v e .

T h e c h a n g e i n r e l a ti v e a b u n d a n c e o f c a d a l e n e a n di s o c a d a l e n e is p o t e n t i a l l y u se f u l a s a p a r a m e t e r t oi n d i c a t e m a t u r i t y f o r p e t r o l e u m s o u r c e r o c k s b e c a u s et h e m a t u r i t y z o n e i n w h i c h t h e c h a n g e s t a k e p l a c ec o r r e s p o n d s t o t h e m a t u r e z o n e o f s o u r c e r o c k sw h e r e o i l a n d g a s f o r m a t i o n o c c u r s . T h e m a t u r i t y

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118 ROBERT ALEXANDER t a l

D e p t h( m )

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Fig. 2. Plots showing the change in relative abundance of isocadalene and cadalene, 1,6-dimethyl-naphthalene and ethylcholestane diastereomers 20S, 20R) with depth for the Grunter-I O)and Volador-1 O) samples.

data shown in Table 1 suggests that peak oil gener-ation occurs in the Volador-1 well at a depth ofapprox. 4000 m. At this depth the inte rpola ted Rovalue is approx. 0.9, there is significant change in theCPI values, and the sterane paramete r is approac hingthe steady state value. In the Gru nter-1 well, using thesame criteria, peak oil generation occurs at a depthof approx. 3500m. At both locations we inferthat significant oil generation probably commencesat depths approx, 500m shallower than peak oilgeneration. These results suggest to us that the iso-cadalene/cadalene parameter has values in the basinof approx. 0.15 in the zone where initial oil formationoccurs, but its value increases markedly nea r peak oilgeneration and continues increasing to values above0.4 in the post-mature zone. Data for the crude oilsamples Table 2) also support these conclusions. TheSnapper-4 crude oil is the lowest maturity crude oiljudging from the maturity values shown. The Tuna-4

and Wirrah-1 crude oils however have been releasedfrom more mature source rocks possibly near peakoil generation and Tuna-I represents a crude oil/condensate from even higher maturity source rocks.These examples serve to illustrate the wide rangeof values of the cadalene/isocadalene parameter inthe zone of oil formation and suggests that it isa sensitive measure of maturity in this importantmaturity zone.

The data for MPI-I, TNR-1 and DNR -I shown inTable 1 illustrates the variability and erratic natureof some of the conventional aromatic maturityindicators when applied to these sediments. Theparameter based on conversion of cadalene to iso-cadalene may be a useful alternative for sediments ofthis type. Clearly this parameter is limited in itsapplication to sediments that co ntain cadalene. Theseare sediments with a significant input o f land plant sa n d usually those of Jurassic age and younger.

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Effec t o f ma t u r i t y o f cad a l en e an d i so cad a l en eTable i. Geochemic al data f or the sedimentary sequence

119

Sterancs~Sample Sediment 20Sdepth (m) temp. (C) Ro(%)* CPI 20S + 20R MPI-I TNR- I DNR-1V o l a d o r - I3033 -- -- 1.65 0.15 0.56 0.10 2.623159 -- -- 1.84 0.19 -- -- --3280 - - 0.60 . . . . .3315 -- -- 1.57 0.23 0.47 0.85 3.003370 - - 0.71 . . . . .3405 2.27 0.53 0.55 4.133498 -- -- 2.29 0.21 0.54 0.66 2.533525 -- 0.73 2.31 0.33 0.39 0.56 1.083655 - - 0.76 . . . . .3750 0.82 . . . . .3768 1.98 0.403820 0.473875 -- 0.85 . . . . .3920 0.89 . . . . .3985 -- 0.83 . . . . .4040 152* . . . . . .4070 -- -- 1.47 0.50 0.51 0.66 2.084080 158 . . . . . .4090 0.88 . . . . .4191 - - - - 1.35 0.50 0.50 0.74 2.804245 162 -- 1.25 0.50 0.49 0.72 2.724359 -- -- 1.20 0.54 -- -- --G r u n t e r - l1910 - - - - 0.78 -- 0.29 1.03 7.322170 - - -- 1.70 0.14 0.37 1.02 0.872290 1.26 0.15 0.41 0.93 3.652395 -- -- 1.16 0.26 0.60 0.66 3.852550 . . . . 0.60 1.08 5.592675 - - - - 2.01 0.23 0.61 -- 3.562775 - - -- -- 0.20 0.71 1.07 4.432861.3 1151 . . . . . .2925 0.23 0.843019 1205 . . . . . .3 0 3 0 2 . 1 2 0 . 3 5 0 . 8 6 0 . 7 9 3 . 5 2

3125 2.01 0.38 0.54 0.87 4.483260 -- -- i .67 0.45 0.46 -- --3450 -- -- 1.44 0.49 0.50 0.64 3.263540 -- -- 1.30 0.52 -- -- --3676 -- -- 1.36 0.53 0.61 0.62 3.453681 -- -- 1.22 0.54 0.57 -- --3800 1435 - - 1.19 0.57 0.60 0.72 3.75*Data from Stainforth (1984).#Reservoir temperature data from Burns e t a l . (1987).5Estimated from temperature gradient data of Ozimic e t a l . (1987).Data from Alexander e t a L (1991).Definitions and methods of measurement: Ro, vitrinite reflectance; CPI, 0.5(EC25 .. . C31/~ C24 .. .

C30 +gC 25 ... C31/~C24 ... C32) (GC); steranvs, 208 and 20R diastereomers of5a(H),14a(H),17 ,(H)-vthylcholestane m / z 2 1 7 ) ; MPI -I, (1.5 x 2-MP + 3-MP)/(0.667 x P + l-M P + 9 - M P ) m / z 1 7 8 and 192); TNR- I, 2,3,6-TMN/ (1,3,5-T MN+ 1,4,6-TMN) m / z l 7 0 ) ;DNR-1, (2,6-DMN + 2,7-D MN)/I, 5-DMN m / z 1 5 6 ) ; - - , no data.

Table 2. Geochemical maturity data for the crude oils

Gippsland oil

Acyclic alkanes Stvranes Triterpanes Aromat icPr 208 Ts Mor I

CPI nCl 7 208 + 20R Tm Hop MPI-I I + CSnapp er-4 1410 m 1.22 0.85 [ 0.34 0.30 0.20 0.80 0.11Tun a-4 1400.5 m * * 0.40 0.36 0.14 0.70 0.22Wirrah -I 204 6m 1.20 0.52 0.44 0.46 0.10 0.83 0.43Tun a-I 219 1.5 m 1.13 0.59 0.52 1.40 0.09 0.99 0.56*Biodegraded.tPartly biodegraded.Definitions and methods of measurement: P r / n C t T , pristane/heptadecane (C-C); CPI, 0.5(I;C25 ...

C31fl~C24 ... C30 + ~C25 . .. C31/ZC24 ... C32) (GC); steran~, 208 and 20R diastereomersof 5~(H),14 ,(H),lT~(H)-cthylcholestan r e ~ z 2 1 7 ) ; Ts/Tm, 18,,(H)-trisnorneohopane/17a(H)-trisnorhopane m / z 191 ); Mo t/H op, 17~ (H),21 ~ (H)-morvtane/17,, (H),21 #(H)-hop ane i n / z 191);MPI- I , ( I .5 x 2 -MP+3 -MP) / (0 .6 6 7 x P+I-MP+9 -MP) m / z 1 7 8 and 192); I/(I+C),isoocadalene/(isocadalene + cadalene) m / z 198).

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120 ROBERT ALEXANDER e t a lCONCLUSIONS

3 - I s o p r o p y l - 1 , 6 - d i m e t h y l n a p h t h a l e n e i s o c a d a l e n e )h a s b e e n i d e n t i f i e d i n s e d i m e n t s f r o m t h e G i p p s l a n dB a s i n, A u s t r a l i a a n d i s s u g g e s te d t o a r i se f r o mt h e a c i d -c a t a l y s e d r e a r r a n g e m e n t o f c a d a l e n e i n t h es e d i m e n t s .

I n t h e t w o s e d i m e n t a r y s e q u e n c e s s t u d i e d t h e r e l a -t i v e a b u n d a n c e o f c a d a l e n e a n d i s o c a d a l e n e c h a n g e dw i t h i n cr e a s e i n s a m p l e d e p t h m a t u r i t y ). T h e r e l a t i v ea b u n d a n c e s o f c a d a l e n e a n d i s o c a d a l e n e i n c ru d e o i l sf r o m t h is b a s i n a r e c o n s i s t e n t w i t h t h e i r e x p u l s io nf r o m s e d i m e n t s w i t h i n t h e o i l w i n d o w .Assoc ia te Ed i tor - -B . R. T. SIMONEIT

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