Migracao_petroleo_BR

8/3/2019 Migracao_petroleo_BR

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Advances in OrganicGeochemistry 1991Org . Geochem. Vo l. 19, No s 1 -3, pp. 13-27, 1992Printed in G rea t Britain. All rights reserved 0146-6380/92 $5.00+ 0.00Copyright 1992Pergamon Press Ltd

G e o c h e m i c a l a s s e s s m e n t o f p e t r o l e u m m i g r a t i o n p h e n o m e n a o n areg iona l s ca l e : case s tudie s f rom Braz i l ian marg ina l bas ins

Lu Iz A . F . TRINDADE* and SIMONC. BRASSELLtDepartment of Geology, Stanford University, Stanford, CA 94305-2115, U.S.A.

Abstract--Geochemical effects of secondary migration on marine oils derived from a single source ofenhanced salinity have bee n investigated in the Sergipe-Alagoas Basin, but no molecular migrationcharacteristics that are independent of source constraints wer e identified. M ixed oils from the P otiguarBasin, derived from both lacustrine freshwater and m arine source be ds of enhanced salinity revealcompositional trends which are related to source rock features and maturity conditions. Heterogeneitiesin their oil compositions can be related to the history o f migration and reservoir filling. Also, short-termstratigraphic variations seen fo r biomarker compositions observed in core samples of source rocks fromthe Potiguar Basin help explain anomalies in the trends defined by the migrated mixed oils in the Basin.K e y w o r d s - - d e p o s if i o n a l environment parameters, stratigraphic variability, secondary migration effects,biomarkers, mixed oils, compositional variability

INTRODUCTIONTh e e ffect s o f sou rce-dep enden t fac to r s and in - rese r -v o i r a l t e r a ti o n o n p e t r o l e u m c o m p o s i t i o n s a r e w i d el yrecogn ized and accep ted (Tis so t and Wel te , 1984) .Series o f g ross compo s i t iona l charac te r i s t i cs o f b io -m a r k e r p a r a m e t e r s w h i c h a r e i n d i ca t i ve o f s u c he ff ec ts h a v e b e e n p r o p o s e d a n d r o u t i n e l y e m p l o y e d i ng e o c h e m i c a l e v a l u a t i o n s o f s e d i m e n t a r y b a s in s . T h eg e n e t i c f a c t o r s w h i c h c o n t r o l t h e c o m p o s i t i o n o fexpe l led pe t ro leum inc lude the depos i t iona l env i ron -m e n t a n d m a t u r a t i o n o f th e s o u r c e r o c k . T h e p r e v ai l -ing depos i t iona l cond i t ions can be re f l ec ted by thep r e se n c e o r a b u n d a n c e o f c o m p o n e n t s t h a t c h a r a c te r -i ze o rgan i sms wh ich a re spec i f i c to wel l descr ibeddepos i t iona l env i ronmen ts (e .g . Bras se l l and Eg l in -ton , 1986 ; Vo lkm an , 1988) . Th e matu r i ty o f thes o u r c e r o c k c o n t r o l s t h e b u l k c o m p o s i t i o n o f t h eexpe l led pe t ro leum and the s t e reochemica l con f igu r -a t i o n s o f it s b i o m a r k e r s w h i c h d i f f er i n t h e ir t h e r m a ls tab i l i ty (e .g . Mackenz ie e t a l . , 1980, 1981).I n - r e s e r v o i r a lt e r a t i o n o f p e tr o l e u m s m a y r e s u l tf r o m b i o d e g r a d a t i o n , w a t e r w a s h i n g o r t h e r m a lc rack ing . B iodeg rada t ion invo lves the se lec t ive bac-t e ri a l u p t a k e o r m o d i f i c a t i o n o f c o m p o n e n t s w i t h i nreservo i red pe t ro leums (Ph i l ipp i , 1977 ; Connan ,1 98 4) w h e r e a s w a t e r w a s h i n g r e f e r s t o t h e r e m o v a l o ft h o s e c o m p o u n d s t h a t a r e m o r e s o l u b l e i n w a t e r(Lafargue and Barker , 1988) . In ce r ta in geo log ica lse t tings , the com pos i t io n o f the rese rvo i red pe t ro leumc a n b e a f f e ct e d b y ig n e o u s i n t r u s io n s o r f u r t h e r b u r i a lo f t h e r e s e r v o i r t o h i g h e r t e m p e r a t u r e z o n e s , l e a di n g*Petrobrfis/Cenpes/Divex, Cidade Universitfiria, Qd 7, Ilhado Fundfio, Rio de Janeiro, IL l, CE P 21910, Brazil.tPresent add ress: Biogeoehemical Laboratories, Depart-ment of Geological Sciences, Indiana University, Bloom-ington, I N 47405-5101, U.S.A.

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t o t h e r m a l c r a c k i n g o f h i g h e r m o l e c u l a r w e i g h t h y -d r o c a r b o n s a n d e n r i c h m e n t o f li g h t e n d s ( e .g . E v a n se t a l . , 1971; Tissot and Welte, 1984).

I n a d d i t i o n t o t h e s e c o m p a r a t i v e l y w e ll u n d e r s t o o dprocesses , f rac t iona t ion e f fec t s as soc ia ted wi th them i g r a t i o n o f p e tr o l e u m s m a y a l s o a f f ec t t h e i r c o m p o -s i t ion . Bo th bu lk and mo lecu la r var ia t ions ex i s tb e t w e e n t h e o i l e x p e l l e d b y p r i m a r y m i g r a t i o n a n dthe sou rce rock b i tumen (Tis so t and Pe le t , 1971 ;L e y t h a e u s e r e t a l . , 1983, 1986, 1988a, b) . Ho we ver,c o m p o s i t i o n a l m o d i f i ca t i o n s r e la t e d t o s e c o n d a r y m i -g r a t i o n a r e a s m u c h a m a t t e r o f d e b a te a s t h em e c h a n i s m s t h a t e f f e c t t h e m . C h a n g e s i n p e t r o l e u mc o m p o s i t i o n a l o n g m i g r a t i o n p a t h w a y s h a v e b e e na t t r ibu ted to geoch romatog raph ic e f fec t s (Se i fe r t andM o l d o w a n , 1 9 8 1 ; C a r l s o n a n d C h a m b e r l a i n , 1 9 8 6 ;K r o o s s e t a l . , 1991), losses in to so lu t ion (McAuliffe ,1980 ; Lafa rgue and Barker , 1988) and phase-con -t ro l l ed par t i t ion (S i lverman , 1965 ; Lar te r and Mi l l s ,1991) . How ever , m an y l imi t ing fac to rs com pl ica te ther e c o g n i t io n a n d i n t e r p r e t a t io n o f s u c h f r a c t i o n a t i o nef fec t s and gene t ic fac to rs o r in - rese rvo i r a l t e ra t iona n d m i x i n g m a y a l s o o b s c u r e t h e m .L a b o r a t o r y s i m u l a ti o n o f p e t r ol e u m m i g r a t io nt h r o u g h p o r o u s m e d i a h a s b e e n s h o w n t o p r o d u c ec o m p o s i t i o n a l m o d i f i c a t i o n s i n b o t h b u l k c o m p o -s i t ions and a t the mo lecu la r l eve l (Bon i l l a and Enge l ,1986, 1988; Phi lp and Engel , 1987; Zhusheng e t a l . ,1988 ; Bro th ers e t a l . , 1991). Ho we ver , the effects seeni n s u ch s i m u l a t io n e x p e r i m e n t s a r e m o r e m a r k e d t h a nthose suppor ted by f i e ld observa t ions , p robab ly be-c a u s e t h e y a r e p e r f o r m e d u n d e r c i r c u m s t a n c e swhe re in bo th the sca le and t im e o f such p rocesses a red r a m a t i c a l l y r e d u c e d ( D u r a n d , 1 98 8) .F i e ld i n v e s ti g a t io n s o f c o m p o s i t i o n a l m o d i f i c a t i o n sr e l a t e d t o m i g r a t i o n p h e n o m e n a n e e d t o b e c o n -d u c t e d o n a p p r o p r i a t e g e o l o g i c a l s y s t e m s w h e r e a


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1 4 L u I z A . F . T R IN D A DE a n d S IM O N C . B R AS SE L Ls ingle source ro ck w i th wel l-def ined geochem ica l andb iom a r ke r f e a tu r e s ha s ge ne r a t e d o i l s wh ic h ha vebe e n a c c um ula t e d in im m a tu r e s t r a t a a long we l lc ons t r a ine d m ig r a t ion t r e nds . S uc h c ond i t ions oc c u rin the Sergipe-Alagoas and Pot iguar Basins (Braz i l )where source rocks a re loca ted in of f shore s t ruc tura llows a nd the pe t r o l e um s the y ge ne r a t e d ha vem ig r a t e d to sha l low l a ndwa r d im m a tu r e s t r a t a( B a b insk i a nd S a n tos , 1987; S a n tos N e to e t a l . , 1990).O i l s a m ple s f r om the S e r g ipe - Ala goa s B a s in a r ethough t t o be de r ive d f r om sou r c e r oc ks de pos i t e dunde r c ond i t ions o f e nha nc e d sa l in ity ( B a b insk i a ndSantos , 1987; Mel lo e t a l . , 1988a). In contras t , m ixedoi ls de r ived f rom both lacus t r ine f reshwate r andm a r ine e nv i r onm e n t s o f e nha nc e d sa l in i ty we r e r e -c ove r e d f r om the P o t igua r B a s in whe r e c om po-s i t iona l d i f fe rences re la ted to source and matur i tya r e p r odu c e d by d i f f e re n t e x te n t s o f m ix ing ( Tr inda dee t a l . , 1 9 9 2 ). H y d r o c a r b o n b i o m a r k e r c h a ra c te r iz -a t ion o f a s ign if ic an t num be r o f sou rc e r oc k o r ga n ice x t r a c t s a nd o i l s f r om bo th ba s in s ha s be e n p r e -viously descr ibed (Santos and Rodr igues , 1986;B a b insk i a nd S a n tos , 1987 ; M e l lo e t a l . , 1988a , b ;S a n t o s N e t o e t a l . , 1990; Mel lo an d M axwell , 1991;

T r i n d a d e e t a l . , 1992) a nd the m os t d i a gnos t i c pa r -amete rs for these environments a re presented inTab le 1 . This s tud y seeks evidence fo r com posi t io na lm od i f i c a t ions po te n t i a l ly a t t r i bu ta b le to s e c onda r ym ig r a t ion p r oc e s se s in the S e r g ipe - Ala goa s a ndPot iguar bas ins . I t a lso a ims to contras t suchmigra t ion-dependent d i f fe rences in b iomarker d is t r i -bu t ions w i th the c om pos i t iona l he t e r oge ne i t i e s ob -se rved or ident i f ied in mixed oi ls of the Pot iguarBasin.

E X P E R I M E N T A LThe m a in sc ope o f t h i s s tudy c ons i s t s o f o r ga n icge oc he m ic a l a na lyse s o f sou r c e r oc k b i tum e ns a ndr e se r vo i r e d pe t r o l e um s . The va r ious s a m ple s we r esubm i t t e d to s e ve r a l p r e pa r a t ive s t e ps a nd p r o -cedures , w i th the objec t ive of b iom arke r charac te r iz -

a t ion and quant i f ica t ion . Source rock ident i f ica t ionwa s ba se d on to t a l o r ga n ic c a r bon c on te n t ( TOC ) ,f o l lowe d by R oc k- E va l py r o lys is u s ing s t a nda r d p r o -cedures (Espitali6 e t a l . , 197 7) and vitr inite ref lec-tance . These ana lyses were per formed a t Pe t robr f isR e se a r c h C e n te r.

T a b l e 1 . A s e l e c ti o n o f b i o m a r k e r p a r a m e t e r s t h a t a r e s o u r c e d e p e nd e n t . T h e n u m b e r s g i v e ns hou ld be r ega rded as gu ide l ines no t r ig id l im i t sB I O M A R K E R F E A T U R E

Odd/Even prefel'01~gP ds tane /P hy taneG a m m a e w a n e I n de xBis no rhopane IndexC35/C34 hopanesTr icyc l i c s I ndexC30 hopaneC27 steranesC30 s te r anes4-Me s texane Index[$--carotane

L A C U S T R I N E M A R I N E E N H A N C E DF R E S H W A T E R S A L I N I T Y

>1 1 < 1

< 5 0 > 6 0< 5 > 1 0< 1 > 1

> 5 0 < 5 02 0 0 - 3 0 0 p p m > 8 0 0 p p m

< 5 0 p p m > 8 0 0 p p mn .d . p r es .< 6 0 > 6 0n . d . > 2 0 0 p p m

O dd /E ven P re f e r ence = m eas u red fo r the r ange C16 to C22 .P r i s ~ y t a n e = p r i s t a n e /p h y t a n e m e a s u r ed f r o m R I C t ra c e .Gam ma cera ne In dex = gRrnmaem~me x 100 / 17a(H),21[ l(H)-hopane.B is no rhop ane Index = 28 ,30 -b i s no rhopane x 100 /17a(H ) ,2113(H ) -hopane .C35/C34 hopan es = 22S + 2 2R C35 17a(H),211~(I-l) -hopanes / C34 counterpar ts .Tr icyc l i c s I ndex = s um O f C l9 to C29 ( exc lud ing C22 and C27) t r i c y e ] i c terpanes / 17c~(H),2 [~H )-

hopane .(230 hopa ne = 17ct( I -l) ,2 [3(H)-hopane cal ibra ted to deutera ted s tandard.(227 s te~anes = 20S + 2 0R 5a(H ) ,14 z(H ) ,17a(H ) -cho les tanes ca l ib r a ted to deu te r a ted s t andards .C30 st e r anes = p r es ence o r abs ence o f 24 -pm py lcho les tanes a s de tec ted by G C-M S /M S .4 - M e s t e r a n e I n d e x = s u m o f a l l C 30 4 - m e t h y l s t e ra n e s i n m / z 2 3 1 f r a g m e n t o g r a m / 2 0 S + 2 0 R

5 a ( H ) , 1 4 a ( H ) , 1 7 a ( H ) - c h o l e s t a n e s.~-carotane = ~c aro tan e oal i lx ated o deuterated nohexatr icontane and s terane standards.


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Geochemical assessment o f petroleum migration 15A n a l y t i c a l p r o c e d u r e s f o r b i o m a r k e r d e t e r m i -

na t ion u t i l i zed es tab l i shed methods fo r ex t rac t ionand f rac t ion a t ion o f the samples . A de ta i led descr ip -t i o n o f t h e m e t h o d s e m p l o y e d i s gi v e n e ls e w h e re( T r i n d a d e et al . , 1992). In brief , source rocks weree x t r a c t e d b y S o x h l e t a n d t h e i r e x tr a c t s a n d p e t r o l e u ms a m p l e s w e r e f r a c t i o n a t e d b y t h i n - l a y e r c h r o m a t o g -raphy (s i l i ca , hexane) ; a l ipha t i c and a romat ic hyd ro -c a r b o n s w e r e c o l l e c t e d s e p a r a t e l y a n d p o l a rc o m p o u n d s w e r e o b t a i n e d a s a t hi r d f r a c t io n , b u t n o ti n v e s ti g a t ed f u r t h e r. C o m p o n e n t s o f th e h y d r o c a r b o nf r a c t i o n s w e re i d en t if i ed b y a c o m b i n a t i o n o f G C - M Sa n a l y s e s c o n d u c t e d i n f u l l d a t a , M I D a n d M S / M Sm o d e s u s i n g a F i n n i g a n T S Q 7 0 i n s t r u m e n t . A r a n g eo f d e u t e r a t e d h y d r o c a r b o n s t a n d a r d s w a s u s ed f o rb i o m a r k e r q u a n t i t a t i o n .

GEOLOGIC SETTINGHisto ry o f Pot iguar and Serg ipe -A lagoas Basins

T h e s t u d y e x a m i n e d o i l sa m p l e s f r o m t h e P o t i g u a rBas in , the eas te rnmos t bas in o f the Braz i l i an equa to -r i a l m a r g i n , a n d f r o m t h e S e r g i p e - A l a g o a s B a s i n i nthe Braz i l i an mer id iona l marg in (F ig . 1 ) . Desp i ted i f fe rences in the i r t ec ton ic se t t ing and r i f t ing p ro -

eesses , bo th bas in s evo lved as par t o f a typ ica ld ivergen t , r i f t ed con t inen ta l marg in , and can bel inked to a s ing le evo lu t ionary geo log ica l h i s to ryre la ted to the open ing o f the Sou th At lan t i c .T h e S e r g i p e - A l a g o a s B a s i n h a s t h e m o s t c o m p l e t egeo log ica l reco rd o f a l l Braz i l i an marg ina l bas in s andi t s t ec ton ic sed imen ta ry reco rd can be d iv ided in th reemain s tages: ( i ) pre-ri f t , ( i i ) r i f t and (i i i ) post-r i f t( t rans i t iona l and ocean ic phases ; F ig . 2 ) , a s p roposedb y P o n t e a n d A s m u s ( 1 9 7 8 ) a n d O j e d a y O j e d a(1982) . The p re - r i f t s t age i s absen t in the Po t iguarBas in ; i t s s ed imen ta ry reco rd beg ins du r ing the r i f ts tage.( i ) Pre -r i f t s tage (Paleozo ic an d Jurass ic ) . I n t h eSerg ipe-Alagoas Bas in th i s s t age i s rep resen ted byPermo-Carbon i fe rous f luv ia l , g lac ia l and sha l lowm a r i n e s e d i m e n t s u n c o n f o r m a b l y o v e r l a i n b y aJu ras s ic sequence composed o f f ine to coarse e las t i csed imen ts depos i t ed under h igh ly oxygena ted con -d i t ions in b ra ided , f luv ia l fac ies as soc ia ted wi theo l ian and sha l low lacus t r ine env i ronmen ts (Lana ,1990).( i i ) R i f t s tage (Neocomian to Ear ly Apt ian) . T h er i f t s t age compr i ses l acus t r ine f reshwater sha les ,bo rdered by de l t a ic p rog rada t ions , a l luv ia l p la in sand fan de l t a depos i t s , wi th p rog res s ive f luv ia l

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Fig. 1. Loc ation and m ajo r structural elements of the Potiguar an d Sergipe-Alagoas basins, No rthe astBrazil.


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16 Lu~ z A. F. TRINDADE nd SIMONC. BRASSELLI.' EI o . R o . o IW

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i. O N E S N A , E[ ' ~ '] C O N G L O M E R A T E i '~ -' ~J ] H A LI T E ~ ] A N H Y D R IT E

f'2":1 ['7-'71S A N D S T O N E W I T H I ' ' 1 V O L C A N IC R O C K S I " ~'1 B A S E M E N TI N T ER B E D D E D S H A L EFig 2 Schematic stratigraphic chart of the Brazilian marginal basins (modified from Mello e t a l . , 1988b).

p r o g r a d a t i o n t o w a r d s t h e t o p ( M a t o s e t a l . , 1987;B e r t a n i e t a l . , 1990; Lana, 1990). At th is t ime, organicr ich sha les were depos i t ed in deep , anox ic f reshwaterl a k e s , w h i c h c o n s t i t u t e i m p o r t a n t s o u r c e r o c k s i nbo t h bas in s . T ec ton ic up l i f t o f th i s s equence c rea teda r e g i o n a l u n c o n f o r m i t y a t t h e e n d o f t h e r i f t s t a g e(Ber tan i e t a l . , 1990; Lana, 1990).

( i ii ) P o s t - r i f t s t a g e . This s t age can be subd iv idedi n t o a t r a n s i t i o n a l p h a s e a n d a n o c e a n i c p h a s e . T h et r a n s i t i o n a l p h a s e ( A p t i a n - A l b i a n ) w a s a t i m e o ftec ton ic qu iescence du r ing subs idence l inked toc r u s t a l c o o l i n g ( P o n t e a n d A s m u s , 1 97 8) . T o p o -g r a p h i c b a r r i e r s l e d t o t h e d e p o s i t i o n o f m i x e d c a r -bona tes , sha les , mar l s and evapor i t es in a res t r i c tede n v i r o n m e n t i n f l u e n c e d b y m a r i n e i n c u r s i o n s(Ber tan i e t a l . , 1 9 9 0 ; K o u t s o u k o s e t a l . , 1991) . Or-g a n i c r i c h c a l c a r e o u s b l a c k s h a l e s a n d m a r l s w e r edepos i t ed and co ns t i tu te the secon d se r ies o f imp or t -an t sou rce rocks wi th in the bas in s .T h e o c e a n i c p h a s e i s c h a r a c t e r i z e d b y t h e a d v e n t o fs e a f l o o r s p r e a d i n g , c o u p l e d w i t h c o n t i n u o u s s u b s i -dence , wh ic h f ina lly es tab l i shed oc ean ic cond i t ions inthe bas in . She l f ca rb ona te s and t ransg res s ive sha lesw e r e d e p o s i t e d f r o m t h e A l b i a n t o t h e U p p e r C r e t a -c e o u s f o l l o w e d b y a s e q u e n c e o f p r o g r a d a t i o n a lpro xim al coa rse s i liciclast ic facies , shal lo w shelf car -b o n a t e s a n d s h a l l o w t o d e e p m a r i n e p e l i t e s a n dt u r b i d i t e s f r o m t h e L a t e C r e t a c e o u s t o t h e H o l o c e n e(Ber tan i e t a l . , 1990; Lana, 1990).In the Serg ipe-Alagoas Bas in , th i s s tudy focuses ont h e s o u t h e r n p a r t o f th e b a s i n , w h e r e m i g r a t e d o i l sw e r e r e c o v e r e d f r o m t h e A r a c a j u H i g h ( F i g, 3 ) . I n t h eP o t i g u a r B a s i n , t h i s w o r k e x a m i n e d s a m p l e s f o r t h e

o f fsho re a rea and fo r the h inge l ines tha t def ine them a i n o n s h o r e g r a b e n ( F i g . 3 ) .

RESULTS AND DISCUSSIONS o u r c e r o c k c h a r a c t e r i z a t io n

I n a g r e e m e n t w i t h t h e i r c o m m o n g e o l o g i c e v o l -u t i o n , p e t r o le u m s o u r c e ro c k s f r o m b o t h b a s i n s o c c u rin equ iva len t t ec ton ic s t ra t ig raph ic sequences wi th inthe r i f t s t age and the t ra ns i t iona l ph ase o f the pos t -r i f t s t age (Cerque i ra , 1985 ; M el lo e t a l . , 1988a, b). Inthe r i f t s t age , p re -Ap t ian o rgan ic - r i ch , l acus t r inef reshwater , da rk g ray /b lack sha les were depos i t edw h i c h a r e c o m p o s e d m a i n l y o f T y p e I I k e r o g e n , w i t hT O C v a l u e s a v e r a g i n g 2 % a n d h y d r o c a r b o n s o u r c ep o t e n t i a l u p t o 4 0 k g H C / t r o c k . T h e y c a n a t t a i n700 m in the Scrg ipe-A lagoas Bas in and exceed 500 min the Po t iguar Bas in S h a le s a n d m a r l s o f t h e A p t i a n t r a n s it i o n a l p h a s eo f the pos t - r i f t s t age were depos i t ed in a res t r i c tedm a r i n e e n v i r o n m e n t o f e n h a n c e d s a li n it y . E x c e l le n ts o u r c e r o c k s a r e p r e s e n t c h a r a c t e r i z e d b y o r g a n i c -r i c h ( T O C v a l u e s u p t o 8 % ) c a l c a r e o u s b l a c k s h a l e sa n d m a r l s d o m i n a n t ly c o m p o s e d o f T y p e I I k e r o g e na n d w i t h $ 2 v a l ue s r e a c h i n g 3 0 k g H C / t r o c k .D e t a i l e d m a p p i n g o f t h e o r g a n i c r ic h n e ss a n dm a t u r i t y s t a g e o f t h e s o u r c e b e d s h a s p e r m i t t e d t h ede l inea t ion o f the i r loca t ions B o th bas in s posses st w o g e o c h e m i c a l l y c o n t r a s t i n g s o u r c e r o c k t y p e s,depos i t ed e i ther in l acus t r ine f reshwater env i ron -m e n t s a n d u n d e r m a r i n e c o n d i t i o n s o f e n h a n c e ds a li n it y , w h i c h h a s l e d t o t h e g e n e r a t i o n a n d a c e u m u -


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Geochemical assessment of petroleum migration( a t P O T 1 G U A R A S I N

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Vi "i 'K ~ ll m ~sEuENrFig. 3. Distributions of source rock(s) (modified from C erqueira, 1985 and Babinski and S antos, 1987)the locations of oil reservoirs from w hich samples were taken along the direction o f principal m igrationand the positions of the reservoirs within stratigraphic cross-sections (n.b. the vertical scales are b othgreatly exaggerated) for (a) the P otiguar Basin and (b) the Sergipe-Alagoas Basin.

l a t ion o f two d i f fe ren t o i l types and , in add i t ion , tot h e e x i st e n ce o f m i x e d o i ls s o u r c e d b y v a r y i n g p r o -p o r t i o n s o f b o t h r o c k t y p e s . A l s o , th e s o u r c e r o c k s i nbo t h bas in s l i e in o f fsho re s t ruc tu ra l lows (F ig . 3 ) andt h e p e t r o l e u m s t h e y g e n e r a t e h a v e m i g r a t e d o n s h o r et o s h a l l o w i m m a t u r e s t r a t a . P e t r o l e u m s a m p l e s w e r eco l lec ted f rom wel l s a long reg iona l c ro ss sec t ions toas ses s mig ra t ion e f fec t s (F ig . 3 ) .A s s e s s m e n t o f s tr a t i g r a p h i c v a r i a b i l it y i n s o u r c e r o c k s

S i g n i f i c a n t v a r i a b i l i t y i n t h e b i o m a r k e r d a t a f o r as i n g l e t y p e o f t h e d e p o s i t i o n a l e n v i r o n m e n t i s t o b ee x p e c t e d g iv e n th e i r d e p e n d e n c e o n t h e a b u n d a n c e o fp r e v i o u s o r g a n i s m s a n d t h e i n h e r e n t d i f fe r e n ce s in t h ef a c t o r s t h a t i n f l u e n c e b i o l o g i c a l p r o d u c t i v i t y a n ds p e c i a t i o n w h i c h a r e t e m p o r a l l y d e p e n d e n t . S h o r t -t e rm var iab i l i ty in sou rce- rock sys tems i s a wel lr e c o g n i z e d c h a r a c t e r i s t i c i n t h e m a r i n e r e a l m ( e . g .C u r i a l e a n d O d e r m a t t , 1 9 8 9 ; K a t z e t a l . , 1991);s ign i f i can t , sho r t - t e rm s t ra t ig raph ic var ia t ions canthere fo re be expec ted in a l acus t r ine o r res t r i c ted

e n v i r o n m e n t t h a t h a s b e e n s u b j e c te d t o m a r i n e i n c u r -s i on s . T h e y m a y r e s u l t f r o m l o c a l c h a n g e s i n p r e c u r -so r o rgan i sms , d i f fe ren t s ed imen ta t ion ra tes , changesi n t h e e n e r g y o f th e d e p o s i t i o n a l e n v i r o n m e n t , w a t e rleve ls , s a l in ity and oxyg ena t io n cons t ra in t s .S t ra t ig rap h ic var iab i l i ty was spec i fi ca l ly as sessed ina se ries o f samples f r om dep ths be tw een 2629 and2639 .5 m o f well E3 (F ig . 3 ) in the Po t ig uar Bas in(F ig . 4 ). Th e in te rva l i s com pose d o f f ine ly l am ina tedd a r k g r a y t o b l a c k s h a le s , w h i c h s h o w n o e v i d e n c e o fb i o t u r b a t i o n a n d a r e i n t e r b e d d e d w i t h f i n e s a n d -s tones (F ig . 4 ) . The sha les a re r i ch in o rgan ic mat te r( 1 . 6 -4 . 7 % T O C ) w i t h a d o m i n a n c e o f T y p e I / I Ik e r o g e n a n d w i t h h y d r o c a r b o n s o u r c e p o t e n t i a l s( 1 . 9 -2 5 . 5 k g H C / t r o c k ) t h a t b r o a d l y p a r a l le l t h e i rT O C c o n t e n t s [ F i g . 4 ( a ) ] . T h e c o n c e n t r a t i o n s a n dr e l at iv e a b u n d a n c e s o f s e le c t ed b i o m a r k e r s w e r emeasu red to charac te r ize th i s sou rce in te rva l . Thes t r a t ig r a p h i c v a r i a b i li t y s h o w n b y t h e r a n g e o f T O Cc o n t e n t s a n d R o c k - E v a l p y r o l y s i s d a t a i s m a t c h e d b ya l a c k o f u n i f o r m i t y o r s y s t e m a t i c v a r i a b i l i t y i n


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Fig. 4. (a) B asic geochemical parameters o f source rocks from w ell E3 in the Potiguar Basin showing thestratigraphic variability in geo che mic al parameters. T O C = to ta l organic carbon content (%);$2 -- hydrocarbon source potential from Rock-Eval pyrolysis (kg HC /t rock); HI = hydrogen index (mgHc /g OC). (b) Source and m aturity parameters derived from biomarker ratios (see Table 1 for definitionof indices and ratios), showing the stratigraphic variability of the source interval.b i o m a r k e r p a r a m e t e r s [ F i g . 4 ( b ) ] t h a t h a v e p r o v e du s e f u l i n t h e c h a r a c t e r i z a t i o n o f s o u r c e r o c k s a n dpe t ro leums in the Braz i l i an marg ina l bas in s (Mel lo e ta l . , 1988a , b ; Tr indade e t a l . , 1992) . The same b io -marker charac te r i s t i cs he lp def ine depos i t iona l pa le -o e n v i r o n m e n t s a n d t h e i r v a l u e s m a y b e e l e v a t e d i ns a m p l e s a s s o c i a te d w i t h c o n d i t i o n s o f e n h a n c e d s a l-i n i ty ( e. g. g a m m a c e r a n e i n d e x ; te n H a v e n e t a l . , 1985;M o l d o w a n e t a l . , 1985 ; Fu J iamo e t a l . , 1986; B rasselle t a l . , 1988 ; de Leeuw and S inn inghe Dams t6 , 1990)a n d / o r t h o s e f o r m a r i n e c a r b o n a t e s ( C3 5/C ~ h o p a n era t io ; e.g. t en H ave n e t a l . , 1985, 1988; Alba ig6s e t a l . ,

1986 ; Fu J iamo e t a l . , 1986; Brassell e t a l . , 1988) . Theda ta f rom wel l E3 shows majo r d i f fe rences in theva lues fo r the gammacerane index [F ig . 4 (b ) ] appar -en t ly re f l ec t ing a t rans i t ion f rom lacus t r ine to mar ineenv i ronm en t a nd d i f fe ren t level s o f s a l in i ty . S imi la r ly ,the b i sno rhopane index shows s ign i f i can t var iab i l i ty[F ig . 4 (b ) ] and max imizes in the on ly sou rce rocks a m p l e w i t h a g a m m a c e r a n e i n d e x > 4 5. T h e u n d e r -ly ing sands tone a t 2639 .5 m i s s t a ined by a ma tu re o i l[f. i t s st e rane i s om er iz a t ion - -F ig . 4 (b ) ] wh ich i s he ldt o d e r iv e f r o m g r e a t e r d e p t h s , h e n c e i ts g a m m a c e r a n eindex ( > 100 ) i s no t ind igenous . Th e var iab i l i ty in


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Geochemical assessment of petroleum migration 19C35/C34hop ane ra t ios for sam ples f rom wel l E3 re f lec tthe r a nge se e n f o r t he t r a ns i t i ona l s e q ue nc e o f th eP o t igua r B a s in ( S a n tos Ne to e t a l . , 1990; T r ind a de e tal . , 1992) and severa l o the r Braz i l ian margin a l bas ins(Mel lo e t a l . , 1988a, b) .Overa l l , these se lec ted b iomarker ra t ios s t ronglysugge s t t ha t t he b iom a r ke r c ha r a c t e r i s t i c s o f t hepe t r o l e um s de r ive d f r om suc h sou r c e r oc ks a r e un -l ikely to be c on ta ine d wi th in na r r ow ly de f ined r a nge s .T h e e f f e c ts o f s e c o n d a r y m i g r a t i o n

An a s se s sm e n t o f t he in f lue nc e o f s e c onda r y m i -g r a t ion on o i l c om pos i t ion on a r e g iona l s c a l e wa sm a de us ing sa m ple s r e c ove re d f r om se ve r a l a c c um u-la t ions a long c r os s s e c t ions f r om the de e p sou r c erocks to sha l low rese rvoir s as i l lus t ra ted by ther e g iona l c ros s s e c tions o f bo th the P o t igua r a ndSergipe-A lagoas Basins (F ig . 3) . Th e oi l samples f romthe Sergipe-Alagoas Basin inves t iga ted here in a resole ly der ived f rom enhanced sa l in i ty sources ,whe r e a s those f r om the P o t igua r B a s in a re m ixe d tovar iable extents .Va r ious f a c to r s c ou ld p r oduc e c ha nge s in pe t -r o l e u m c o m p o s i t i o n s a l o n g m i g r a t i o n p a t h w a y s ,specif ically ( i) geochromatographic effects (Seifer ta n d M o l d o w a n , 1 9 8 1 ; C a r l s o n a n d C h a m b e r l ai n ,1986 ; Kr ooss e t a l . , 199 1), ( i i) losses into solu tion(McAulif fe , 1980; Lafa rgue and Barker , 1988 ) and( ii i) phas e-contro l led par t i t ion (S i lverman, 1965; N o-b le e t a l . , 1991 ; La r t e r a nd M i ll s, 1991) . The sese pa r a t e p r oc e s se s m a y dom ina te in d i f fe r e n t geo log i-ca l se tt ings , but the i r e f fec ts may be obscu red bygene t ic or genera t ive fac tors or by in- rese rvoir a l te r -a t ion a nd m ix ing . En t r a inm e n t o f e x t r a ne ous c om -pounds f r om sha l lowe r s e d im e n t s du r ing pe t r o l e umm igr a t ion c a n p r ov ide a f u r the r oppor tun i ty f o rc om pos i t iona l c ha nge s to oc c u r ( P h i lp a nd Gi lbe r t ,1982; Jaff6 e t a l . , 1988; Bac e t a l . , 1990), a l tho ugh thep r es e n ce o f a n a c h r o n o u s c o m p o n e n t s m a y a i d r e co g -ni t ion of th is process .No f rac t iona t ion e f fec ts tha t can be conc lus ive lyr e l a t e d to s e c onda r y m ig r a t ion ha ve be e n r e po r t e d ,m o s t p r ob a b ly due to the a bse nc e o f s tudie s f oc use don appropr ia te geologic se t t ings . Here , composi t iona lmodif ica t ions have been examined in s ingle sourcedoi ls of the Sergipe-Alagoas Basin , in a sea rch forc h a n g e s t h a t m i g h t b e p r o m o t e d b y a n y o f t h ea f o r e m e n t ione d m e c ha n i sm s ; i n a dd i t ion , m ixe d o i l sf r om the P o t igua r B a s in ha ve be e n e xa m ine d to a s se ssthe e x te n t o f o i l m ix ing a nd the d i r e c tion o f m i -g r a t ion .

G e o c h r o m a t o g r a p h i c e f f e c t s . G e o c h r o m a t o g r a p h ycan be descr ibed to occur in geologica l sys tems tha tc ons i s t o f two o r m or e im m isc ib l e pha se s , i nc lud inga s t a t iona r y pha se ( o r pha se s ) a nd a m ob i l e pha se( K r o o s s e t a l . , 1991) . Ef fec ts a t t r ib uted to geoch ro-m a tog r a phy a r e w e ll re c ogn iz e d in l a bo r a to r y e xpe r -iments and s imula t ion exerc ises , but on a bas ina lsca le the process can be he ld to be controvers ia l andappears subject to signif icant restr ictions.

La bor a to r y s im u la t ions o f pe t r o l e um m ig r a t ionth r ough wa te r - f re e sys te m s ha ve p r oduc e d a b r oa dr a nge o f m od i f i c a t ions , i n bo th the i r g r os s c om po-s i t ions an d a t the m olecu la r leve l (Boni lla and Enge l,1986, 1988; P h i lp a nd Engel, 1987; Z h u s h e n g e t a l . ,1988) . However , molecula r f rac t iona t ion is s ignif i -cant ly reduced in wate r -wet exper iments , r esul t ingon ly in c ha nge s in the c om pos i t ion o f c om pou ndclasses (Brothers e t a l . , 1991) . Expe r im e n t s w i thsyn the t i c c r ude o i l ha ve de m ons t r a t e d tha t ba s i cn i t r oge n c om pounds a r e the m os t p r one to ge oc h r o -m a tog r a ph ic in t e r a c t ions a nd tha t t he p r opor t ion o fn - a lka ne s r e l at ive to bo th a r om a t i c s a nd su l f u r c om -pounds is enhanced in the leas t re ta rded f rac t ions(Brothers e t a l . , 1991).F ie ld s tudies show l i t t le composi t iona l f rac t ion-a t ion a t t r ibutable to migra t iona l e f fec ts , apparent lyfor two reasons: ( i ) the rocks a re wate r -wet and,the r e f o r e , p r ov ide l im i t e d oppor tun i ty f o r m ine r a lsur faces to e f fec t geochromatographic separa t ions ,a l though the a q ue ous e nv i r onm e n t f a c i l i t a t e s t hep r e f e re n t i a l l os s o f t he m o r e w a te r -so lub le hydr oc a r -bons ; a nd ( i i ) a dso r be d c om pounds w i l l be c on t inu -ously desorbed by addi t iona l f lu id inf lux; thus , i t i ssm a l l d i s c on t inuous poo l s o r s t r inge r s o f m ig r a t ingpe tro leu m tha t a re mos t likely to exper ience geochro-matographic e f fec ts (Durand, 1988) .Spec if ic molecula r indica tors of pe t ro leum mi-g r a t ion p r op ose d p r e v ious ly ne e d to be v i ewe d wi thr e ga r d f o r sou r c e o r m a tu r i ty c ons ide r a tions . A b io -m a r ke r m ig r a t ion inde x ba se d on va r i a t ions in s t e r -ane s te reochemis t ry was def ined by Se ife r t andM oldowa n ( 1981) who a r gue d tha t t he r a t io o f14fl(H),17fl(H) to 14~ (H),17 ~(H ) C29 sterane s( /~f l/ fl fl + ~ ) could be enhanced a t a g iven m atur i tyleve l by increas ing m igra t ion d is tance , b ecause o f them or e p l a na r sha pe o f t he 14~( H) , 17~( H) i som e r s .Th i s a s se r t ion wa s suppor t e d by the e va lua t ion o fadso rpt ion f ree energies f or s te ranes in the labo ra to ry(Car lson a nd Ch amb er la in , 1986) and by f ie ld s tudiesin bo th the No r th S e a (M i le s, 1990) a nd the K a r a m a yoi l f ie ld , China (Huang e t a l . , 1990) . However , thepro po rtio n of 14fl(H),17fl(H)-steranes relative to14~ ( H) , 17~( H) - s t e ra ne s i s m a tu r i ty de pe nde n t( M a c ke nz ie e t a l . , 1982). Also , h igher re la t ive abun -dan ces o f 14fl(H),17fl(H)-steranes occ ur in depo si-t iona l se t tings o f enhanc ed sa l in i ty ( ten Hav en e t a l . ,1985 , 1986) , de m o ns t r a t ing the e nv i r onm e n ta l c on -t ro ls on the i r abundance , which can be readi ly ex-p la ine d by the r e c ogn i t ion tha t t he y o r ig ina t e f r omA7-s tenol precursors (Peakman e t a l . , 1989).Ev alu atio n of v ariatio ns in the /~/~//~fl + ~ C29sterane ra tio fo r the single-sourced oils of the Sergipe-Ala goa s B a s in shows no a ppa r e n t t r e nd wi th inc re a s -ing dis tance f rom source [F ig . 5(a ) ] . Most of them e a su r e m e n t s f o r the o i ls wh ic h ha ve m ig r a t e d up to49 km l ie be tween 0 .3 an d 0 .4, indica t ing t ha t ther a t io c a nno t be u se d a s a m ig r a t ion pa r a m e te r i n theSergipe-Alagoas Basin . However , the va lues for20S/(20S + 20R) 14~ (H) ,17~ (H)-s te rane ra t ios a re


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20


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22 LuIz A. F. TRINDADEand SIMONC. BRASSELLBarker, 1988). Water washing is dominant in thereservoir and does not appear to be prominent duringpetroleum migration from source to trap (Lafargueand Barker, 1988).Independent of the mechanism involved, the Ser-gipe-Alagoas Basin oils are altered to variable ex-tents. The relative importance o f the > n-C~2 fractionof the oils varies significantly among the samples, asshown by their whole oil GC traces (e.g. those in Fig.7), but no systematic trend in the alteration of the oilswith distance of migration is evident. Examination ofthe values for various biomarker ratios (Fig. 7) in thesamples provides direct evidence of the lack of anyalteration trend. For example, the abundances of themore readily degraded n-alkanes of lower molecularweight (e.g. Snowdon and Powell, 1979; i.e. n - C 1 2 / n -C25) appear to be greater in samples SE #6 andSE 4~ 5 which have intermediate migration distancesthan in the samples at the extremes of the migrationpathways. Similarly, the n-C17/pristane values showno decrease which can be held to reflect the preferen-tial degradation of n-alkanes relative to acyclic iso-prenoid alkanes (e.g. Bailey e t a l . , 1973). All of thesamples were stored under comparable conditions,but their storage was not strictly controlled andmonitored raising the possibility that some indetermi-nate factors or effects may have been influential.Overall, water washing and biodegradation are un-doubtedly more prevalent and effective in reservoiredaccumulations of petroleum than is possible duringmigration given that biodegradation, in particular,depends on the infiltration and replenishment ofmeteoric water and can be limited by temperature.Thus, it seems unlikely that the extent ofoil alterationcan be a function of the relative distance of mi-gration.

Despite such limitations, there are some indicationsthat progressive water washing can occur with in-creasing distance of migration. In this study com-pounds within the gasoline range could not be usedto assess the influence of water washing because theyhave been removed from most of the samples. Hence,heavier compounds were examined. The ratiophenanthrene/Z methylphenanthrenes shows a gen-eral decrease with increasing distance of migration[Fig. 8(a)], in apparent agreement with the preferen-tial removal of the more soluble phenanthrene (cf.naphthalene; McAuliffe e t a L , 1980). Also, there is atrend for the phenanthrene/n-C~s ratio to decreasewith increasing distance of migration [Fig. 8(b)]which is consistent with loss of aromatics prior tosaturates during the water washing process (Lafargueand Barker, 1988). The process of biodegradation, onthe other hand, removes n-alkanes prior to aromatichydrocarbons (Connan, 1984), and seems not to havebeen responsible for such a modification.

P h a s e - c o n t r o l l e d f r a c t i o n a t i o n . Phase behavior rep-resents another factor that can influence petroleumcomposition during secondary migration (Durand,1988; England and Mackenzie, 1989). Petroleums

encountering lower temperature and pressure regimesduring their upward migration can experience gasexsolution from their liquid phase, thereby affectingthe distribution of oil and gas accumulations withinthe ba s in a nd promoting fractionation of low molecu-lar weight compounds (Thompson, 1988). This effectwas first described for Mahakam Delta oils (Silver-man, 1965) and more recently, experimental simu-lations suggest that such partition can also affectaromatic hydrocarbon distribution and sterane car-bon number distributions (Larter and Mills, 1991).

Phase-controlled fractionation, however, can beexpected to occur in gas condensate-rich systems,where it depends on the gas/oil ratios (GOR) of thepetroleum expelled from the source rocks (Englandand Mackenzie, 1989; Diippenbecker and Horsfield,1990). Furthermore, phase-controlled fractionationwill occur when petroleum migration is primarilyvertical from deep sources to shallow accumulationsin reservoirs. The depth at which the two phases areestimated to begin to separate is called the migrationbubble point and can be used to predict single-phaseor two-phase accumulations (Noble e t a L , 1991).In the Sergipe-Alagoas Basin, no molecular frac-tionation related to a phase change is observed, whichmay be related to the low GOR of the petroleums(predominantly


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