8/12/2019 1997 Alexander Etal 1997

1/8

Pergamon Org. Geochem. Vol. 26, No. 5/6, pp. 391-398, 1997 1997 ElsevierScienceLtdAll rights reserved. Printed n Great BritainPII S0146-6380(97)00006-5 0146-6380/97 17.00 + 0.00

K i n e t i c s a n d m e c h a n i s m o f t h e t h e r m a l e l i m i n a t i o n o f a l k e n e s f r o msecond ary s tany l and tr i terpeny l e s ters imp l ica t ions for sed im entaryp r o c e s s e sROBERT ALEXANDER, PAUL G. KRALERT, IMAM B. SOSROWIDJOJO andROBERT I. K A G I

Australian Petroleum CRC/Centre for Petroleum and Environmental Organic Geochemistry, School ofApplied Chemistry, Curtin University of Technology, GPO Box U1987, Perth, 6001, Western AustraliaReceived 25 July 199 4; returned to author fo r revision 21 October 1994; accepted 3 January 1997 )

Abstract--The liquid phase thermal decompositions of a secondary stanyl ester and a secondary triter-penyl ester have been investigated in the temperature range from 275 to 320C. First-order rate con-stants have been measured and the activation energies and frequency factors for the processesdetermined. The reactions of naturally occurring sedimentary esters were also investigated by heatingan oil shale at 295C in the presence of tetralin for various periods of time. Analysis of the reactionproducts showed that A2-sterenes and da-sterenes were produced at a rate consistent with their for-mation from secondary stanyl esters by a concerted reaction mechanism. The extent of reaction of sec-ondary triterpenyl esters in a sedimentary sequence from the Gippsland Basin, Australia, was alsoinvestigated. A correspondence was observed in the depth-calculated-for-reaction obtained using thelaboratory-derived kinetic parameters and the measured depletion of triterpenyl esters in the soluble or-ganic matter in samples from the sequence. 1997 Elsevier Science LtdKey words- -s tany l esters, triterpenyl esters, biomarker kinetics, Gippsland Basin, Australia, sterenes

INTRODUCTIONCarboxylic esters occur widely in natural productsthat become incorporated into sediments. They aresufficiently refractory to survive diagenesis and arepresent in sedimentary rocks that enter the maturityzone where thermally induced reactions of organiccompounds take place. Alkyl esters undergo a con-certed elimina tion reaction to yield an alkene a nd acarboxylic acid, and are an important target forresearch for two reasons. Firstly, concerted reac-tions are insensitive to catalytic effects from themineral or maceral matrix of the rock, and the kin-etic parameters of these processes are therefore po-tentially useful for inferring the possible thermalhistories of sedimentary sequences in basin model-ling studies. Secondly, release of alkenes from car-boxylic esters during the maturation processprovides a reactive species that can readily undergocarbocation rearrangement reactions, the productsof which are commo n in hydrocarbons from maturesedimentary rocks. A knowledge of the timing ofester elimin ation might therefore shed light on themechanism of formation of these hydrocarbonswith carbon skeletons that are not of natural pro-duct origin.

The kinetics and mechanism of the thermal de-composition of esters with long-chain primary alkylgroups have been reported (Alexander e t a l . , 1992).

This paper compared the reactions of sedimentaryalkyl esters heated under both laboratory and natu-ral conditions and suggested that the reactions weresimilar in both systems. The data suggested thatsuch well-characterised reactions can be used forreconstructing the thermal history of sediments (cf.Alexander e t a l . , 1991).Carboxylic esters with secondary alkyl groups areusually more thermally labile than the correspond-ing primary alkyl esters (Richardson and O'Neal,1972). They react by a concerted mechanism similarto the primary alkyl esters (Taylor, 1979;Richardson a nd O'Nea l, 1972) as shown inReaction Scheme 1 and are, therefore, potentiallyuseful for providing information about the thermalhistory of sediments. Due to their increased thermallability, they are especially useful in sediments thathave been subjected only to mild heating.

Secondary alkyl esters with the alkyl group in theform of a six membered ring are of special interestbecause such compo unds occur widely in sediments.Steryl esters, for example, have been reported inRecent and contemporary sediments from marineenvironments (de Leeuw e t a l . , 1983; Wakeham e tal. , 1984). Fatt y acid esters of pentacyclic triter-penoids and of steroids have also been found inlacustrine sediments with a terrestrial input(Cranwell, 1986; Cranwell and Volkman, 1981).More recently steryl chlorin esters have been idento

391

8/12/2019 1997 Alexander Etal 1997

2/8

8/12/2019 1997 Alexander Etal 1997

3/8

Thermal elimination of alkenes from esters 393H e a t i n g e x p e r im e n t s

H e a t i n g e x p e r i m e n t s w e r e p e r f o r m e d i n t w o s e p -a r a t e s t u d i e s . O n e i n v o l v e d t h e s t a n y l e s t e r ( I ) a n dt h e t r i t e r p e n y l e s t er ( I I ) ; t h e o t h e r a n o i l s h al e f r o mJ u l i a C r e e k , A u s t r a l i a ( P a t t e r s o n et al., 1986).Kinet ic s tudies us ing s ingle compounds

F o r k i n e t i c m e a s u r e m e n t s , r e a c t i o n s w e r e c a r r i e do u t i n e v a c u a te d a n d se a le d 8 0 m m x 2 m m i .d .g l a s s a m p o u l e s p r e v i o u s l y d e a c t i v a t e d a c c o r d i n g t ot h e p r o c e d u r e o f A l e x a n d e r et al . (1988) . In a typ i-c a l e x p e r i m e n t , a m p o u l e s w e r e c h a r g e d w i t h t h ee s t e r (3 0 /~ g ) a n d a s u f f i ci e n t q u a n t i ty o f D e x s i l3 0 0 G C ( c a . 4 0 0 g , A l l t e c h A s s o c i a t es ) t o a f f o r da n i n e r t m e d i u m a t e l e v a t e d t e m p e r a t u r e s .D o t r i a c o n t a n e ( 3 0 g ) w a s a d d e d a s a n i n t e r n a ls t a n d a r d . B a t c h e s o f s a m p l e t u b e s w e re h e a t e d a tc o n s ta n t t e m p e ra tu re (__ .0 .5 C ) o v e r th e r a n g e 2 7 5 -3 2 0 C i n a m o d i f i e d V a r i a n 2 7 00 g a s c h r o m a t o -g r a p h ( K a g i et al., 1 9 9 0 ) a n d r e m o v e d a t a p p r o p r i -a t e t i m e i n t e r v a l s . A f t e r r a p i d c o o l i n g , t h e c o n t e n t sw e r e e x t r a c t e d w i t h d i c h l o r o m e t h a n e ( 3 2 0 0 1 ).C o n v e r s i o n o f t h e p r o d u c t a c i d t o t h e t r i m e t h y l s il y le s t e r d e r i v a t i v e b y t r e a t m e n t w i t h N , O - b i s ( t r i -m e t h y l s i l y l ) t r i f l u o r o a c e t a m i d e ( S u p e l c o , I n c . ) f a c i l i -t a t e d a n a l y s i s o f t h e m i x t u r e b y g a sc h r o m a t o g r a p h y ( G C ) . C o n c e n t r a t i o n s o f r e ac t a n ta n d p r o d u c t s w e r e d e t e r m i n e d b y c o m p a r i s o n o fc h r o m a t o g r a m p e a k a r e a s w i t h t h a t o f th e i n t e r n a ls t a n d a r d a n d a p p l y i n g th e a p p r o p r i a t e d e t e c t o r re -s p o n s e c o r r e c t i o n f a c t o r s . I n a l l e x p e r i m e n t s t h et o t a l m a s s o f u n r e a c t e d e s t e r a n d p r o d u c t s c o n s t i -t u t e d 9 0 - 1 0 0 % o f th e i n i ti a l a m o u n t o f re a c t a n t .P l o t s o f r e a c t a n t c o n c e n t r a t i o n ( e x p r e s s e d a s a l o g -a r i t h m ) v e r s u s t i m e a t v a r i o u s t e m p e r a t u r e s T w e r ec o n s t r u c t e d t o d e t e r m i n e f i r s t - o r d e r r a t e c o n s t a n t sf r o m t h e c o m p u t e d l i n e o f b e s t f i t a c c o r d i n g t o t h er e l a t i o n s h i p b e l o w ( e q u a t i o n 1 ), w h e r e C o i s t h e i n -i t i a l c o n c e n t r a t i o n o f r e a c t a n t , C t h e c o n c e n t r a t i o na t t i m e t , a n d k t h e r a t e c o n s t a n t .

I n C / C o ) = - k t (1 )T h e s e r e s u l t s w e r e t h e n u s e d t o p r e p a r e a p l o t o f

In k a g a in s t l I T f r o m w h i c h t h e a c t i v a t i o n e n e r g y( E ) a n d t h e f r e q u e n c y f a c t o r ( Z ) f o r t h e r e a c t i o nw e r e c a l c u l a t e d f r o m t h e g r a d i e n t a n d i n t e r c e p t r e -s p e c t iv e l y u s i n g t h e A r r h e n i u s e q u a t i o n ( e q u a t i o n 2 ) ,w h e re R i s t h e g a s c o n s ta n t .

k = Z e - E / R r (2 )

Ex p e r i m e n t s w i t h o i l s ha l eP y r o l y s i s e x p e r i m e n t s w e r e c o n d u c t e d o n a p r e -

e x t r a c t e d c a l c a r e o u s o i l s h a l e f r o m t h e J u l i a C r e e kd e p o s i t ( T o o l e b u e F o r m a t i o n ) p r o v i d e d b y D r C .B o r e h a m , A G S O , C a n b e r r a . A s a m p l e (2 0 g ) o f t hes h a l e w a s g r o u n d i n a d i s c m i l l t o p a s s t h r o u g h a

1 0 0 / ~ m s i e v e a n d e x t r a c t e d b y u l t r a s o n i c a t i o n w i t hd i c h l o r o m e t h a n e ( 1 5 0 m l ) f o r 2 h . F l o t a t i o n o f t h el e ss d e n s e m a t e r i a l u s i n g a q u e o u s z i n c b r o m i d e s o l-u t i o n ( d e n s i t y 2 .1 g c m - 3 ) p r o v i d e d a n o r g a n i ce n r i c h e d f r a c t i o n . D r i e d s u b s a m p l e s ( 1 0 0 m g ) o ft h i s o r g a n i c r i c h m a t e r i a l w e r e p l a c e d i n t o d e a c t i -v a t e d 8 0 m m x 7 m m i . d. g l a s s t u b e s t o g e t h e r w i t hs q u a la n e (1 2 . 5 /~ g , i n t e rn a l s t a n d a rd ) a n d r e d i s t i l l e dte t r a l in (2 0 0 1 ) . T e t r a l in w a s a d d e d a s a r a d ic a ls c a v e n g e r t o i n h i b i t a lk e n e f o r m a t i o n f r o m f r e e r a d -i c a l s . A m p o u l e s w e r e t h e n e v a c u a t e d a n d s e a l e d ,a n d p l a c e d i n a c o n s t a n t t e m p e r a t u r e o v e n a t 2 9 5 Cfo r 0 . 5 -7 2 h .

W h e n q u e n c h e d , t h e l i q u i d p y r o l y s a t e w a s r e c o v -e r e d b y f i l t r a t i o n a n d c o m b i n e d w i t h t h e r e s i d u ew a s h i n g s ( 3 x 2 m l d i c h l o r o m e t h a n e ) . F o l l o w i n ge v a p o r a t i o n o f t h e v o l a t i l e s o l v e n t o n a s a n d b a t h ,t h e s o l u t i o n w a s f u r t h e r c o n c e n t r a t e d t o s m a l lv o l u m e ( c a . 2 5 / d ) b y c a r e f u ll y r e m o v i n g t h e b u l k o fth e t e t r a l in u n d e r a s t r e a m o f n i t ro g e n a t 7 5 C .P y r o l y s a t e s w e r e s u b s e q u e n t l y s e p a r a t e d b y t h i n -l a y e r c h r o m a t o g r a p h y ( T L C ) i n t o a l i p h at i c( R f > 0 . 7 5 ) , a r o m a t i c B y 0 . 0 5 - 0 . 6 5 ) a n d p o l a r

R f < 0 . 0 5 ) f r a c t io n s o n a c t iv a t e d a lu m in a (6 0G F 2 54 n e u t ra l , t y p e E , M e rc k ; 0 . 6 m m ) , d e v e lo p in gw i t h n - h e x a n e ( t e t r a l i n b a n d R f 0 . 6 5 -0 . 7 5 ) . T h e a l i -p h a t i c f r a c t i o n w a s f u r t h e r s 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 ( L i C h r o p r e p S i 6 0 , 4 0 - 6 3 / ~ m , M e r c k ;n - h e x a n e e l u a n t ) p r i o r t o G C a n a l y s i s i n o r d e r t ore m o v e t r a c e s o f t e t r a l in .GC analysis

P r o d u c t s w e r e d i s s ol v e d in n - h e x a n e a n d a n a l y s e du s i n g a H e w l e t t P a c k a r d 5 8 9 0 A g a s c h r o m a t o g r a p hf i t t e d w i th a 6 0 m x 0 . 2 5 m m i . d . W C O T fu s e d - s i l i c ab o n d e d m e t h y l s i li c o n e p h a s e c o l u m n ( D B - 1 , J & WS c ie n t i f i c ) a n d a f l a m e io n iz a t io n d e te c to r .H y d r o g e n 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 f l o wv e l o c i t y o f 3 0 c m s - l . S a m p l e s w e r e i n j e ct e d o n - c o l -u m n a t 4 5 C a n d t h e o v e n w a s t e m p e r a t u r e p r o -g r a m m e d t o 2 8 0 C a t 4 C m i n - l . D a t a a c q u i s i t i o na n d p r o c e s s i n g w e r e m a n a g e d u s i n g a P C - b a s e dD A P A s o f t w a r e p a c k a g e ( D A P A S c i e n t i f i cS o f tw a re ) .Py r o l y t i c m e t hy l a t i on - gas c hr om at ogr aphy

O n - l in e p y r o l y ti c m e t h y l a t i o n - g a s c h r o m a t o g -r a p h y w a s c a r r i e d o u t o n t h e a s p h a l t e n e s e x t r a c t e df r o m s e d i m e n t s f r o m t h e o f f s h o r e r e g i o n o f 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 ( f o r s a m p l e d e t a i l s s e eA l e x a n d e r et al., 1 9 91 ), w i th a P y ro je c to r m ic ro fu r -n a c e (S G E S c ie n t if i c ) i n t e r f a c e d v i a th e c a p i l l a ryi n l e t s y s t e m t o a H e w l e t t P a c k a r d 5 8 9 0 A g a s c h r o -m a t o g r a p h f i t te d w i t h a 2 5 m x 0 .2 m m i .d . W C O Tfu s e d - s i l i c a c ro s s - l in k e d m e th y l s i l i c o n e p h a s e c o l -u m n ( H P - 1 , f i l m t h i c k n e s s 0 . 3 3 / a m , H e w l e t tP a c k a r d ) . I n a n a d a p t a t i o n o f a m e t h o d d e s c r i b e db y C h a l l in o r (1 9 8 9 ) , s a m p le s ( c a . 3 m g ) w e re p re -p a r e d a s s o l i d p e l le t s m o i s t e n e d w i t h 2 5 % w / w a q u -

8/12/2019 1997 Alexander Etal 1997

4/8

394 Rober t Alexander et al .e o u s t e t r a m e t h y l a m m o n i u m h y d r o x i d e s o l u t i o n( 2 .0 / A ) a n d i n t r o d u c e d d i r e c t l y i n t o t h e p y r o l y s e r ,w h i c h w a s m a i n t a i n e d a t 4 0 0 C . H e l i u m w a s u s e da s b o t h p y r o l y s i s a n d c a r r i e r g a s ( l i n e a r v e l o c i t y3 0 c m s - l ) , a n d i n je c ti o n s w e r e s p li tl es s ( l m i nh o l d ) . T h e o v e n w a s p r o g r a m m e d f r o m 1 0 C( 2 m i n ) t o 2 1 5 C a t 4 C m i n - 1 a n d t h e r e a f t e r a t2 C m i n - t t o 2 8 0 C ( 15 m i n ) . E l u t i n g c o m p o u n d sw e r e m o n i t o r e d b y a H e w l e t t P a c k a r d 5 9 7 0 B m a s ss e l e c t iv e d e te c to r i n th e fu l l d a t a a c q u i s i t i o n m o d e ,s c a n n in g f ro m 4 0 to 5 4 0 a m u in 1 .3 s cy c le s. T y p ic a lm a s s s p e c t r o m e t e r o p e r a t i n g c o n d i t i o n s w e r e : e l e c -t r o n m u l t i p l i e r v o l t a g e 1 7 5 0 V ; e m i s s i o n c u r r e n t2 2 0 /~ A ; e l e c t ro n e n e rg y 7 0 e V ; s o u rc e t e m p e ra t u re2 2 00 C . P y ro ly s i s p ro d u c t s w e re id e n t i f i e d f ro m th e i rm a s s s p e c t r a .C o m p u t e r m o d e l l i n g

A c o m m e r c i a l l y a v a i l a b l e m o d e l l i n g p a c k a g e( P D I / P C f r o m I E S , J u l i c h , G e r m a n y ) w a s u s e d t oo b t a i n b u r i a l h i s t o r i e s a n d t o p r o d u c e t h e v a r i o u sh e a t f l o w - t i m e r e l a t i o n s h i p s t h a t b e s t d e s c r i b e t h ep r e s e n t t e m p e r a t u r e s i n t h e s e d i m e n t c o l u m n .

RESULTS ND DISCUSSI ON

K i n e t i c sC h o le s t a n -3 /~ -y l n o n a n o a te ( I ) a n d 3 /~ -ac e to xy -

1 7 ~ - c a r b o x y m e t h y l - l u p a n e ( I I ) w e r e s e l e c t e d a se x a m p l e s o f e s t e r s w i t h s e c o n d a r y a l k y l g r o u p s a n dw i th s u f f i ci e n t ly lo w v o la t i l i t i e s t o e n a b le m e a s u re -m e n t s t o b e m a d e o n l i q u i d p h a s e p r o c e ss e s . T h es a m p l e s w e r e h e a t e d f o r v a r i o u s p e r i o d s o f t i m e a tp r e c i se l y r e g u l a t e d t e m p e r a t u r e s , a n d t h e r e a c t i o nm i x t u r e w a s a n a l y s e d u s i n g c a p i l la r y G C t e c h n i qu e sto d e te rm in e th e e x te n t o f r e a c t io n s . In a l l e x p e r -i m e n t s u s e d f o r k i n e t i c m e a s u r e m e n t s , c h r o m a t o -g r a m s o f t h e r e a c t i o n m i x t u r e s sh o w e d n o

s i g n if i c a nt p e a k s f r o m e x t r a n e o u s c o m p o u n d s a n dc o n v e r s i o n o f t h e e s t e r s t o c a r b o x y l i c a c i d s a n da l k e n e s w a s e s s e n ti a l l y q u a n t i t a t i v e . R a t e c o n s t a n t sf o r t h e t h e r m a l d e c o m p o s i t i o n o f t h e s t a n y l e s t e rw e r e d e t e r m i n e d o v e r t h e t e m p e r a t u r e r a n g e 2 7 5 -3 2 0 C . In a l l c a s e s th e r e a c t io n p ro d u c t s w e re th eA 2 -s t e re n e ( I I I ) a n d th e A 3 -s t e re n e ( IV ) w h ic hfo rm e d in a 2:1 r a t io . F ig u re l a s h o w s a p lo t o f t h er e s u l ts o b t a i n e d f r o m o n e s e t o f e x p e r i m e n t s : t h el i n e a r r e l a t i o n s h i p e x h i b i t e d b y t h e v a r i a b l e s i n t h i sp l o t w a s t y p i c a l o f t h e k i n e t i c d a t a o b t a i n e d a t a l lt e m p e r a t u r e s , a n d s h o w e d t h a t t h e p r o c e s s c a n b ea d e q u a t e l y d e s c r i b e d b y a f i r s t - o r d e r k i n e t i c t r e a t -m e n t . In th e c a s e o f t h e t r i t e rp e n y l e s t e r ( I I ) , as i n g l e r e a c t i o n p r o d u c t , c o m p o u n d V , w a s o b t a i n e di n a l l e x p e r i m e n t s . R a t e c o n s t a n t s f o r t h e t h e r m a ld e c o m p o s i t i o n o f t h i s e s t e r w e r e o b t a i n e d f r o m r a t ep l o t s o f s i m i l a r q u a l i t y t o t h a t s h o w n f o r t h e s t a n y le s t e r ( F i g . l a ) . F i g u r e l b s h o w s t h e A r r h e n i u s p l o t sfo r t h e s t a n y l a n d t r i t e rp e n y l e s t e r s . T h e a c t iv a t io ne n e rg ie s (E ) a n d f r e q u e n c y f a c to r s (Z ) fo r t h e r e a c -t io n s w e re c a l c u la t e d u s in g th e s lo p e s a n d in t e r -c e p t s. T h e d e t e r m i n a t i o n o f E a n d Z h a s b e e nc a r r i e d o u t w i t h s i m i l a r p r e c i s i o n to o u r p r e v i o u sk in e t i c s tu d ie s (K a g i e t a l . 1 9 9 0 ; A le x a n d e r e t a l .1 9 9 2 ) a n d a re s u i t a b le fo r m e a n in g fu l i n t e rp re t a t io nw h e n u s e d in g e o lo g ic a l s y s t e m s . T h e k in e t i c p a r -a m e t e r s f o r t h e t w o s u b s t r a t e s a r e s h o w n i n F i g . l b .

T h e r e a c t i v i t y o f t h e t r i t e r p e n y l e s t e r c o m p a r e dw i t h t h e s t e r y l es t e r i s l o w e r a t a l l t e m p e r a t u r e su s e d in th i s s tu d y (F ig . l b ) . B e c a u s e th e a c t iv a t io ne n e rg ie s fo r t h e tw o s u b s t r a t e s a re s im i l a r , t h ed i f f e re n c e i s d u e to th e f r e q u e n c y f a c to r s ( e n t ro p ie s )o f t h e p ro c e s s e s . T h e e n t ro p y d i f f e re n c e b e tw e e n th et w o r e a c t i o n s i s p r o b a b l y r e l a t e d t o t h e n u m b e r o fh y d r o g e n a t o m s o n t h e c a r b o n a d j a c e n t t o t h e e s t e rf u n c t i o n a l g r o u p t h a t c a n p a r t i c i p a t e i n t h e r e a c -t io n . In th e c a s e o f t h e s t a n y l e s t e r , fo u r f l -h y d ro -

- 0 . 5

r .- - - 1 . 0

1 . 5

0

o . o .

s tany l es te r a

ime (h)

_=

-9

- 1 0

-1 1

- 1 2

- 1 3

stanyl ester~'~, . \ ~ E= 195k j mo l l\ . . . , ~ ~ 2 48x 101ss'I

t r i t e r p a n y l e ~ \E = 195 kj tool 1 ,.,.,,. ~ .Z = 1 9 2 x 1 m2 s l ~ ~

i i I i ~ I i1 7 0 1 . 7 4 1 . 8 1 . 8 2

(1/1 ) x 10s (K 1)Fig. 1. Plots showing (a) typical kinetic data used for determining the first-order rate constants and (b)Arrhenius plots for the s tanyl and triterpenyl e s t e r s

8/12/2019 1997 Alexander Etal 1997

5/8

Thermal elimination of alkenes from esters 395gens are availab le, whereas in the case of the triter- 0.0penyl ester the presence of the g e m dimethyl groupat C-4 reduces the number of adjacent hydrogens totwo. o.sThe values obtained in this study for the acti-vation energies and frequency factors for the reac-tions of secondary esters are within the range of Avalues reported in the litera ture for gas phase reac- ~ -1.0tions of secondary alkyl esters. For example, acti- =vation energies and frequency factors for thedecomposition of secondary alkyl acetates in thegas phase are in the range 181-203 kJ mo l -] and -1.54 . 0 1 0 1 2 - 8 . 5 1 0 ] 3 S 1 respectively (Richardsonand O Neal, 1972; Scheer e t a l . , 1963), whileO Co nno r and Nace (1953) have reported kineticparamete rs of 184.6 kJ mol l and 5.1 x 1012 s-I forthe elimination reaction of cholesteryl acetate togive 3,5-cholestadiene over the temperature range281-329C.

R e a c t i o n o f s e c o n d a r y a l k y l e s t e r s i n k e r o g e nIn order to measure the sterenes released via a

concerted reaction process during pyrolysis of oilshale esters the experiment was carried out in thepresence of tetralin. Under these conditions freeradicals formed by other reaction pathways duringthe pyrolysis experiment are quenched by hydrogendonation from tetralin and consequently appear assaturated products; only the reaction products thatarise via concerted reactions should thereforeappear as alkenes. Figure 2 shows a rate plot forthe fo rmation of A2-cholestene from oil shale or-ganic matter heated at 295C in the presence of tet-rails. The value for the initial concentration Co) ofA2-cholestene was obtained from a plot of In C ver-sus time (equation 1) by extrapolation to zero timeand placing a greater weighting on the earlier datapoints in order to minimise the effects of secondaryreactions of cholestene. The expected rate of for-mation of the sterene calculated using the kineticparameters derived from measurements on chole-stan-3fl-yl nonanoate is shown as a broken line inFig. 2. Clearly, the measured rate of formation ofsterene from the oil shale is similar to the calculatedrate. It is apparent from these data that whenheated under laboratory conditions the stanyl esterswhich occur naturally in the oil shale react at asimilar rate to the single synthetic ester used forkinetic measurements. The result is consistent withprevious reports (cf. Alexander e t a l . , 1992) thatchanges in the nature of the alkyl moiety of the car-boxyl component within one class of ester (i.e.whether an ester of a primary or secondary alcohol)have only a minor effect on the kinetic parameters.We suggest therefore that the kinetic parametersmeasured in the laboratory for the stanyl nonanoateare similar to those of the naturally occurring stanylesters in the oil shale.

. . . . .\\

. , %

-..~. ' . . . . . .

. 2 . C 0 t f ~ a , , J4 8 1 2 1 6T i m e h )

Fig. 2. Plots showing the similarity in the rate of release of20R cholest-2-ene from Julia Creek oil shale (data points)and the rate of reaction of the reference stanyl ester(dashed line).

R e a c t i o n s o f e s t e r s h e a t e d u n d e r g e o l o g i c a l c o n d i t io n si n s e d i m e n t s

Using the laboratory determined kinetic data, thedepth interval in a sedimentary sequence overwhich reaction occurs can be calculated using basinmodelling software. The result of such a calculationfor sediments heated under conditions typical ofnatural heating due to increased subsidence isshown in Fig. 3. The calculations used a uniformheating rate (1.7C Ma -l) and geothermal gradi ent(35 Ckm l) and a surface temperature o f 14C.The results show that reaction of the secondary sta-nyl ester occurs more readily in sediments thandoes reaction of the secondary triterpenyl esterwhich, in turn, occurs much more readily than thecorresponding reaction of primary alkyl esters(Alexander e t a l . , 1992) or the processes o f oil for-mation. Also shown in Fig. 3 is the calculatedextent of reaction for steroid aromatisation usingthe kinetic data of Mackenzie and McKenzie(1983). Clearly the reaction of the stanyl ester andthe aromatisation process both occur at similardepths (temperatures) in the sediment column. Theapparent coincidence of the two processes in thedepth profiles may be linked: the possibility that therate of formation of triaromatic steroids in sedi-ments is determined by the rate of formation ofunsaturated steroids from esters needs further inves-tigation; however since neither alkenes nor A-ringmonoaromatics are stable in mature sediments, thetriaromatic steroids might form from the reactionsof C-ring monoaromatic steroids that still containan ester fun ctionali ty at C-3.Depletion o f the esters of olean-3-ol with increasein depth in a sedimentary sequence from theGippsland Basin, Australia, is consistent withthe laboratory determined kinetic parameters for

8/12/2019 1997 Alexander Etal 1997

6/8

396 Robert Alexander e t a l .

PC I

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

A g e of o ldes t sediments = 8 3 M aS u r f a c e t e m p e r a t u r e = 1 4 CH e a t i n g r a t e = 1 . 7 C M a 1G e o t h e r m a l g r a d i e n t = 3 5 C k m 1

. o . -

- - - , - . , . . . , . . . , . - - , . - . . . . . . . . . .

'~ T-...,.....,.,,.

5 0 ~ I i i j0 0 . 2 0 . 4 0 . 6 0 8

Fraction of reaction

1 . 0

- - - - - p r i m a r y a l ky l e s t e r s. . . . .. . . . s e c o n d a r y s t a n y l e s t e r s. . . . s e c o n d a r y t r it e r p e n y l e s t e r s

steroid a rom at iza tionlabi le kerogen oi l format ion)

Fig. 3. A plot showing the calculated progress of various reactions with depth for a sedimentarysequence heated under conditions of a natural system. Kinetic parameters for each reaction type andtheir sources were: stanyl and triterpenyl esters, Fig. l b); primary alkyl esters Alexander e t a l . 1991);steroid aromatisation Mackenzie and McKenzie, 1983); labile kerogen Quigleye t a l . 1987).

ester elimination from a secondary triterpenylester. Figure 4 a) shows the calculated depth profilefor the e liminati on reaction of the triterpenyl esterto form the A2-triterpene for the Volador-1 well.This calculation was done using the same heat flowhistory that was previously reported to account forthe reaction of primary alkyl esters in this wellAlexander e t a l . 1991) and shows that alkenes

from reaction of secondary alkyl esters in the A-ring of a triterpenoid begin to form just before3000m and this process is near completion byabout 3500 m.

The abundance of oleanyl esters in the asphalteneisolates of samples from the Volador-1 well wasdetermined by derivatisation- pyrolysis gas chroma-tography Challinor, 1989; Kralert e t a l . 1995). Asis clear from Fig. 4 b), the abundance of the oleanylester in these samples decreases with increas ingdepth. Unfortunately, suitable samples were notavailable from shallower sections which could beused to establish the onset of this reaction; however,the available data does suggest that depletion of theoleanyl esters in the soluble organic matter SOM)

8/12/2019 1997 Alexander Etal 1997

7/8

T h e r m a l e l i m i n a t i o n o f a l k e n e s f r o m e s t er s 3 9 7

3 O O O

e

E 3 4 0 0 0

19, 217-227.Ka gi , R . I . , A le xa nd e r , R . a nd To h, E . 1990) K ine t i c sa n d m e c h a n i s m o f t h e c y c l i s a t i o n r e a c t i o n o f ortho-m e t h y l b i p h e n y l s . I n Advances in Organic Geochemistry1989 e d s B . D u r a n d a n d F . B 6 h a r . OrganicGeochemistry 16, 161-166.King , L . L . a nd R e pe ta , D . J. 1991) Nove l pyrop he o-phorbide s t e ry l e s te r s in B la c k Se a se dim e nts .Geochimica et Cosmochimica Acta 55, 2067-2074.Kra le r t , P . G . , A le xa nde r , R . a nd Ka gi , R . I. 1995) Ani n v e s t i g a t i o n o f p o l a r c o n s t i t u e n t s o f k e r o g e n a n d c o a lu s i n g p y r o l y s i s - g a s c 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 yw i t h in situ m e t h y l a t i o n . Organic Geochemistry 2 3 , 6 2 7 -639.de Le e uw, J . W. , R i jps t r a , W. I . C . , Sc he nc k, P . A . a ndVolk m a n , J. K . 1983) Fre e , es te r if i e d a nd r e s idua lbound s te ro l s in B la c k Se a Uni t 1 s e d im e nts .Geochimica et Cosmochimica Acta 47, 455-465.

A c t i v a t i o n e n e r g i e s a n d f r e q u e n c y f a c t o r s f o r t h ee l i m i n a t i o n o f a l k e n e s f r o m c h o l e s ta n - 3 f l - y l n o n a n o -a t e a n d 3 f l - a c e t o x y - 1 7 f l -c a r b o x y m e t h y l - l u p a n e h a v eb e e n d e t e r m i n e d a c c u r a t e l y u n d e r l a b o r a t o r y h e a t -i n g c o n d i t i o n s . S e d i m e n t a r y s e c o n d a r y s t a n y l e s t e r sr e a c t w h e n h e a t e d u n d e r l a b o r a t o r y c o n d i t i o n s i n am a n n e r c o n s i s t e n t w i t h t h e s e k i n e t i c p a r a m e t e r s .S e d i m e n t a r y t r i t e r p e n y l e st e rs , w h e n h e a t e d u n d e rn a t u r a l c o n d i t i o n s i n s e d i m e n t s , a r e d e p l e t e d i n s o l -u b l e o r g a n i c m a t t e r S O M ) in a m a n n e r c o n s i s t e n tw i t h t h e l a b o r a t o r y d e r i v e d k i n e ti c p a r a m e t e r s .Associate Editor - - B . S i m o n e i t

A le xa n de r , R . , F i she r , S . J . a nd Ka gi , R . I . 1988) 2 ,3-Dim e thylb iphe nyl : k ine t i c s of i t s c yc l i s a t ion r e a c t iona n d e f fe c ts o f m a t u r a t i o n u p o n i t s re l a ti v e c o n c e n t r a t i o nin s e dim e nts . In Advances in Organic Geochemistry 1987e ds . L . Ma t ta ve l l i a nd L . Nove l l i . Organic Geochemistry13, 833-837.Ale xa n de r , R . , Ka gi , R . I . a nd K ra le r t , P . G . 1993)K i n e t i c s o f t h e t h e r m a l e l i m i n a t i o n o f s t e r e n e s f r o m5= H) - s ta nyl e s ter s : im pl ic a t ions for ge oc he m ic a l pro -cesses in sediments . In Organic Geochemistry: PosterSessions from the 16th International Meeting on OrganicGeochemistry Stavanger 1993 e d K . O y g a r d , p p . 1 8 6 -1 8 9. F a i t h H u r t i g t r y k k , O s l o .A le xa nde r , R . , Kra le r t , P . G . , Ma rz i , R . a nd Ka gi , R .I . 1 9 9 1 ) A g e o c h e m i c a l m e t h o d f o r a s s e s s m e n t o f t h et h e r m a l his tor ie s of s e d im e nts : a two-we l l c a se s tudyf r o m t h e G i p p s l a n d B a s i n , A u s t r a l i a . APEA J . 3 1 , 3 2 5 -332.

h a s o c c u r r e d i n t h e z o n e p r e d i c t e d u s i n g t h e l a b o r a -t o r y k i n e t i c d a t a .

5 0 0 0 I I I I ~ I f0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 0 1 0 2 0 3 0

T r i te r p a n y l e s t e r O l e a n a n - 3 - o l e s t e r i f ie d )alcu la ted f ra c t io n o f r e a c tio n n g g - 1 S O M )

Fig . 4 . P lo t s showing a ) c a lc u la te d e x te nt of the t r i te rpe nyl e s te r r e a c t ion a nd b) m e a sure d c h a nge inole a nyl e s te r c on c e nt ra t ion in the so luble orga nic m a t te r SOM ) for the Vola dor -1 we ll .A le xa n de r , R . , Kra le r t , P . G . a nd K a gi , R . I. 1992)K i n e t i c s a n d m e c h a n i s m o f t h e t h e r m a l d e c o m p o s i t i o nof e s te r s in s e d im e nts . In Advances in OrganicGeochemistry 1991 eds C. B. Eckardt , J . R. Maxwell , S.R . L a r t e r a n d D . A . C . M a n n i n g . Organic GeochemistryCONCLUSIONS 19, 133-14 0.Aug us t ine , R . L . 1965) Catalytic Hydrogenation p. 63.A r n o l d , L o n d o n .Cha l l ino r , J . M. 1989) A pyrolys i s -de r iva t i s a t ion-ga sc h r o m a t o g r a p h y t e c h n i q u e f o r t h e s t r u c t u r a l e lu c i d a t i o nof som e synthe t i c polym e rs . Journal of Analytical andApplied Pyrolysis 16, 323-333.Cranw ell , P. A. 1986) Esters of acyclic an d polycyclic iso-

pre noid a lc ohols : b ioc he m ic a l m a rke r s in l a c us t r ine s e di -m e nts . In Advances in Organic Geochemistry 1985 edsD. Le ytha e use r a nd J . Rul lkot t e r , pp . 891-896.P e r g a m o n P r e ss , O x f o r d .Cra nwe l l , P . A . a nd Volk m a n, J . K . 1981) A lkyl a nds te ry l e s te r s in a r e c e nt l a c us t r ine s e dim e nt . ChemicalGeology 32, 29--43.Eckardt , C. B. , Pearce , G. E. S. , Keely, B. J . ,Kow alewsk a, G. , Jaff6, R. an d Maxw ell , J . R. 1992) Aw i d e s p r e a d c h l o r o p h y l l t r a n s f o r m a t i o n p a t h w a y i n t h ea q u a t i c e n v i r o n m e n t . I n Advances in OrganicREFERENCES Geochemistry 1991 eds C. B. Eckardt , J . R. Maxwell , S.R . L a r t e r a n d D . A . C . M a n n i n g . Organic Geochemistry

8/12/2019 1997 Alexander Etal 1997

8/8

398 R o b e r t A l e x a n d e r et al.Ma c ke nz ie , A . S . a nd Mc Ke nz ie , D . P . (1983)A r o m a t i z a t i o n a n d i s o m e r i z a t i o n o f h y d r o c a r b o n s i ns e d i m e n t a r y b a s i n s f o r m e d b y e x t e n s i o n . GeologicalMagazine 120, 417-470.N a c e , H . R . ( 1 95 1 ) A n i m p r o v e d h y d r o g e n a t i o n o f c h o le s -t e ro l to c hole s ta nol . Journal of the American ChemicalSociety 73, 2379.O Co nno r , G . L . a nd N a c e , H . R . (1953) Fur th e r s tudie so n t h e C h u g a e v r e a c t i o n a n d r e l a t e d r e a c t i o n s . Journalof the American Chemical Society 75, 2118-2123.Pa t t e r so n , J. H , Ra m sde n, A . R . , Da le , L . S. a nd Fa rd y ,J . J . (1986) Ge oc he m is t ry a nd m ine ra logic a l r e s ide nc e sof t r a c e e le m e nts in o i l sha le s f rom Jul i a Cre e k ,Que e ns la nd , Aus t r a l i a . Chemical Geology 55, 1-16.Prowse , W. G . a nd Ma xwe l l , J . R . (1991) H igh m ole c ula rc hlor ins in a l a c us t r ine sha le . Organic Geochemistry 17,877-886.Quig le y , T . M. , Ma c ke nz ie , A . S . a nd Gra y , J . R . (1987)K i n e t i c t h e o r y o f p e t r o l e u m g e n e r a t io n . I n Migration ofHydrocarbons in Sedimentary Basin s Collection

Colloques Seminaire e d B . Dol ige z , Vol . 45 , pp . 649-665. Edi t ions Te c hnip , Pa r i s .R ic ha rdson , W, H . a nd O Ne a l , H . E . (1972) The un im ole -c u l a r d e c o m p o s i t i o n a n d i s o m e r i z a t i o n o f o x y g e n a t e do r g a n i c c o m p o u n d s ( o t h e r t h a n a l d e h y d e s a n d k e t o n e s ) .I n Comprehensive Chemical Kinetics e d s C . H . B a m f o r dand C. F. H. Tipper , Vol. 5, pp. 381--446. Elsevier , NewY o r k .Scheer , J . C. , Kooyman, E. C. and Sixma, F. L. J . (1963)Ga s pha se pyro lys i s of a lkyl a c e ta te s . Recl. Trav. Chim.Pays-Bas 82, 1123-1154.Ta ylor , R . (1979) Pyrolys i s of a c ids a nd the i r de r iva t ive s .I n The Chemistry of Functional Groups Supplement Be d S . Pa ta i , P t . 2 , pp . 860-914. Wi le y , Ne w York .W a k e h a m , S . G . , F a r r i n g t o n , J . W . a n d G a g o s i a n , R . B .(1984) Va r ia b i l i ty in l ip id f lux a nd c om pos i t ion of pa r -t i c u la te m a t te r in the Pe ru upwe l l ing r e g ion . InAdvances in Organic Geochemistry 1983 eds P. A.Sc he nc k, J . W. de Le e uw a nd G . W. M. L i jm ba c h, pp .2 0 3 - 2 1 6 . P e r g a m o n , O x f o r d .