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Or anic Geochemistrr, Vol. 2 pp. 33 43Pergamon Press Lid 1980. Printed in Great Britain

r imary migrat ion theory of petroleum and i tsapplication to petroleum exploration

KOICHI AOYAGI an d TADASHIASAKAWA*Cen tral Technical Labora tory, Japan Petroleum Exp lorat ion Co. Ltd.3-5-5 M idoriga oka , Hamura-machi , Nishitama-gun, T oky o, Japa n

Received 10 July 1978; accepted in revised orm 30 March 1979)Ab strac t--Cu rren t concepts of oi l and gas generat ion b y therm al decomposit ion of kerogen arereviewed. Primary oi l migrat ion mechanisms requiring large quanti t ies of water to serve as a carrierfor the movem ent of oi l from source ro ck to reservoir are discussed. Previous investigators regardedthe expulsion o f interlayer water from mo ntm ori l lonite and mo ntm ori l lon ite-i l l i te mixed-layer mineralsby transformation during diagenesis as the m ost imp ortan t source o f the carrier water .The process o f diagenesis is subdivided into three stages based on the results of experimental com pac-t ion studies on mo ntm ori l lonite clay, on studies of the expulsion mechanism of interlayer and interst i t ialwaters, and on the observed changes in po res and m ineral grains in argi l laceous sediments durin gthese stages. As a result , we conclude d that the migra t ion of oi l chiefly occurs durin g the late com pactionstage whe n the sediment 's poro si ty ranges from 30 to 10 .This conclusion implies that a large amou nt o f oi l can move du ring the period wh en the act ivegeneration of oi l corres pon ds with the prima ry migrat ion o f carrier water . Fo r instance, in the oi l-bear-ing Miocene sediments of the Ak ita area of northern Japan, the oi l generation tem peratu re rangedfrom 100 to 150C and the average migrat ion depth was between 1 300 and 2600m. Therefore, thepossib i l i ty of large oi l poo l formation wil l be high in the ba sin where the pale o-geo therm al gradien tis abo ut 5.0C/100m, In fact , i t is expected that the paleo -geoth erma l gradie nts in the vicini ties oflarge oil fields will be very close to this value.Predic t ion of the type and amo unt of hydrocarbons in the explora t ion area wi ll be poss ible byan examinat ion of the pa leo-geothermal gradient in the area and by a s tudy of the re la t ion be tweenabsolute porosi ty and burial depth of argi l laceous rocks.

I N T R O D U C T I O NT h e t a s k o f e x p l a i n i n g t h e p e t r o l e u m g e n e r a t i o n ,m i g r a t i o n a n d a c c u m u l a t i o n m e c h a n i s m s h a s b e e n a ni m p o r t a n t a n d s t im u l a t i n g p r o b l e m f o r p e t r o l e u mg e o l o g i s t s a n d g e o c h e m i s t s .

P o r f i r ' e v (1 9 74 ) r e p o r t e d t h a t s o m e g e o l o g i s t s a n dg e o c h e m i s t s f r o m U . S . S . R . a n d e a s t e r n E u r o p e h a v es u p p o r t e d t h e i n o r g a n i c t h e o r y o f p e t r o l e u m ; t h e yb e l i e v e t h a t p e t r o l e u m h y d r o c a r b o n s w e r e f o r m e du n d e r t h e t h e r m o d y n a m i c c o n d i t i o n s o f t h e u p p e rm a n t l e . H o w e v e r , m a n y r e s e a r c h e r s f a v o r t h e o r g a n i co r i g i n th e o r y o f p e t r o l e u m w h i c h i m p l i e s t h a t t h es o u r c e m a t e r i a l s o f h y d r o c a r b o n s o r i g i n a t e d f r o mb u r i e d o r g a n i c m a t t e r i n a r g i l l a c e o u s s ed i m e n t s.T h e r e a r e t w o a s p e c t s o f t h e o r g a n i c o r i g i n t h e o r y ,t h e p r o t o - p e t r o l e u m ( B r o o k s , 1 93 6) a n d t h e k e r o g e n( A be l s on , 1963) t he o r i e s . I n t h i s pa pe r , c onc e p t s w i l lb e b a s e d o n t h e k e r o g e n t h e o r y w h i c h i s p r e s e n t l ya c c e p t e d b y m a n y r e s e a r c h e r s .

U n d e r t h e i n f lu e n c e o f i n c r e a s i n g o v e r b u r d e n p r e s -s u r e a n d g e o t h e r m a l t e m p e r a t u r e , k e r o g e n i n a r g i l l a -c e o u s s e d i m e n t s w i ll g e n e r a t e p e t r o l e u m h y d r o -c a r b o n s b y t h e r m a l d e c o m p o s i t i o n . M a n y g e o l o g i s t sb e l ie v e t h a t c a r r i e r w a t e r i s n e c e s s a r y f o r t h e p r i m a r ym i g r a t i o n o f p e t r o l e u m f r o m s o u r c e t o r e s e r v o i r

r oc ks , e s pe c i a l l y in t he c a s e o f o i l ( H e db e r g , 1964) .* Present add ress: Tec hnolo gy Research Centre, JapanNat ional Oi l Corp., 2 -3-1 3 Torano mon , M inato-ku,Tokyo, Japan.

S o m e g e o c h e m i s t s , h o w e v e r , h a v e o b j e c t e d t o t h i sc onc e p t on t he ba s i s t ha t o i l i s on l y ve r y s l i gh t l y s o l -ub l e i n w a t e r ( Mc A ul i f f e , 1965) . O t h e r i nve s t i ga t o r sh a v e d i s p e l le d th i s o b j e c t i o n b y s h o w i n g t h a t p e t r o -l e u m s o l u b i l i ty i n w a t e r c a n b e a p p r e c i a b l e a t e l e -v a t e d t e m p e r a t u r e s ( P ri c e, 1 9 7 6 ) . M a n y A m e r i c a npe t r o l e um ge o l og i s t s ( Pow e r s , 1967 ; B ur s t , 1969 ; a ndP e r r y a n d H o w e r , 1 9 7 2 ) b e l ie v e t h a t t h e i n t e r l a y e rw a t e r o f m o n t m o r i l l o n i t e re l e a s e d b y t r a n s f o r m a t i o nd u r i n g t h e l a t e s t a g e o f d i a g e n e s i s b e c o m e s t h e c a r r i e rw a t e r e s s e n t i a l f o r p r i m a r y m i g r a t i o n . R e c e n t l y , t h ew r i t e rs o f t h e p r e s e n t p a p e r s u g g e s t e d t h a t i n a d d i t i o nt o i n t e r l a y e r w a t e r , in t e r s t i t i a l w a t e r e xp e l l e d f r oms e d i m e n t s d u r i n g t h e m i d d l e s t a g e o f d i a g e n e s i s a l s o

p l a y s a n i m p o r t a n t r o l e i n t h e p r i m a r y m i g r a t i o n o fo i l ( A oy a g i a nd A s a k a w a , 1977).

I n t h e p r e s e n t p a p e r , t h e p r o c e s s o f d i a g e n e s i s i sd i s c u s s e d f r o m t h e s t a n d p o i n t o f c h a n g e s i n m i n e r a lg r a i n s a n d p o r e s i n a r g i l l a c e o u s se d i m e n t s , a n d t h e s ec o n c e p t s w i ll b e a p p l i e d to t h e t e m p e r a t u r e a n d d e p t ho f p e t r o l e u m g e n e r a t i o n i n s e v e ra l J a p a n e s e a n d o t h e rb a s i n s . T h i s p r i m a r y m i g r a t i o n t h e o r y w i l l b ee xp l a i n e d in de t a i l a nd w e w i l l i l l u s t r a t e how t h i st h e o r y c a n b e a p p l i e d t o p e t r o l e u m e x p l o r a t i o n .

P R O C E S S O F D I A G E N E S I S I NA R G I L L A C E O U S S E D I M E N T S

D i a g e n e s i s is d e f in e d a s t h e p r o c e s s i n v o l v i n gp h y s i c a l a n d c h e m i c a l c h a n g e s i n s e d i m e n t a f t e r d e p o -s i t i on t ha t c onve r t s i t t o c ons o l i da t e 4 , r oc k ( A .G . I . ,

33

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34 K. AOYAGIand T. ASAKAWATable 1. Relationships between absolute porosity and burial depth of the Neogene argillaceous sedimentsin several Japanese basins and elsewhere e l l s M i t i M i t i A k i t a T a k a g i M i t i O k l a h o m aPo ro si ty (* /, -;T -,~ .~ Ha m a yu ch i N i i k a p p u O i l F i e l d s R - 3 K u b i k i U S A Ve n e zu e la

5 04 03 02 01

5

6 5 01 0 2 01 4 9 02 2 2 03 9 0 0

2 0 01 1 2 01 5 0 01 8 5 0

4 4 0 m

7 9 01 2 7 01 8 5 02 6 2 0

8 8 0 m

1 2 5 01 6 8 0

2 8 0 m

6 0 0

9 0 01 2 8 01 7 8 03 1 5 0

1 2 03 3 06 0 0

1 0 7 01 6 0 0

4 2

9 4 01 6 8 0

A g e M i o c e n e M i o c e n e M i o - P l i o P l i o c en e M i o - P l i o P e nn s yl . E o c - M i o- P e r m .

1960). Larsen and Chi l ing ar (1967), Rieke and Chi l -inga r i an (1974) and ma ny o the rs have w r i t ten a bou tthe variou s phases o f d iagenes is in argi l laceous sedi-ments . These publ ic a t ions indica te tha t pressure , tem-pe ra tu re and reac t ion t ime a re the phys ica l f a c to r s ,and pH , Eh and chemica l comp os i t ion o f s ed imen tf lu ids are the chemical fac tors control l ing diagenes is .A l l o f the s e fac to rs s hou ld be cons ide red in m on i to r -ing the proc ess o f diagenesis .

Prac tically, how ever , it is very difficult to id entifythe s tage o f d iagenes is by us ing a l l of the abo ve fac-tor s. Th us , ma ny re s ea rche rs u sed the ch ange o f abs o -lu te po ros i ty in s ed imen t to mo n i to r the p rog re s s o fdiagenesis , s ince the poro s i ty o f a rgi l laceous sedi-men t s i s inve rs ely p ropor t ion a l to the inc rea se o f bu r -ial depth.Poros i ty changes in sediments

The ch ange in abs o lu te po ros i ty o f a rg i l l aceouss ed imen t s i s con t ro l l ed by ove rburden p re s s u re ,wa te r -expu l sion m echan i s ms , r a t e s o f depos i tion , an dnature of sedim ent gra ins . Therefore , in the s t r ic tsense , re la t ionships be tween poros i ty and buria l depthof sediments in a specif ic bas in m ust be es tabl ishedfrom d ata f or tha t specific si te of depos i t ion. In gen-era l , however , i t i s a lso t rue tha t the re la t ions be tweenporo s i ty and depth in different bas ins with s imilargeologica l h is tor ies c lose ly resemble each other .

Tab le 1 indica tes the re la t ionship s be tween theabs o lu te po ros i ty and bu r i a l dep th o f Neogene a rg i l-l a ceous s ed imen t s in M i t i Hamayuch i and M i t i N i ik -appu o f H0kk a ido , s eve ra l o i l fi elds o f Ak i t a (M iya -zaki , 1966), and Tak agi R-3 (M atsuzawa, 1961 , 1962)and M i t i Kub ik i o f N i iga ta in J apan . In the Neoge neof J apan , the ave rage bu r i a l dep th o f the s ed imen twi th 50 po ros i ty co r re s ponds to abo u t 500 m; 40poros i ty co r re s po nds to 1000 m, 30 to 1400m, 20to 1800 m, 10 to 280 0m and 5 to 3200 m. Poro s i tychanges in C a rbon i fe rous to Pe rmian a rg i l l a ceous

rocks o f Ok lah om a in U .S .A . (A thy , 1930) and theEocene to M iocene a rg i l l a ceous s ed imen t s in Vene -zuela (Hedberg , 1936 ) are a lso show n in the table.Min eral t ransformat ion in sedimen ts

In Fig . 1 , the phys ica l con di t ions (pressure, tem-pe ra tu re , and reac t ion t ime ) con t ro l l ing t r ans fo rma-t ion of v~rious minera ls in argi l laceous sedim entsdur ing d iagenes i s a re s ummar ized acco rd ing toA o y a g i et al (1975) and Aoyag i and Ka zam a (1977,1978). Fo r c l ay mine ra l s in the T e r t i a ry s ed imen t s o fJ apan , a p re s s u re o f 900 kg /cm z and a t empe ra tu reo f 104C a re requ i red fo r the t r ans fo rma t ion f rommo ntmo r i i lon i t e to mon tmor i l lon i t e - i l l i t e mixed- l aye rmine ra l and 920 kg /cm 2 and 137C fo r t r ans fo rma t ionfrom m ixed-layer m inera l to i l l i te . Fo r zeol ites,370 kg /cm 2 and 56C a re nece s s a ry fo r the t r ans fo r -mat ion from volcanic glass to c l inopt i lo l i te ,860 kg /cm 2 and 116C fo r the conve rs ion o f c l inop t i -lo l it e to ana lc i t e and /o r heu land i te , and 930 kg /cm 2and 138C fo r the t r ans fo rma t ion f rom ana lc i t eand /o r heu land i t e to l aumont i t e and /o r a lb i t e . Fo rs il ica m inera ls , 250 kg/c m 2 and 45C are req uired forthe t r ans fo rma t ion f rom am orp hou s s i li ca to low-c r is -toba l it e , an d 660 kg /cm 2 and 69C fo r the changef rom low-c r i s toba l i t e to low-qua r t z .The ave rage ca lcu la t ed geo the rma l g rad ien t inJapanese oi l f ie lds is 2 .89C/100m (Aoyagi andKazam a , 1978) . The re fo re , i n the Te r t i a ry s ed imen t so f J apan , a bu r i a l d ep th o f abou t 1000 m i s nece ss a ryfo r the t r ans fo rma t ion f ro m amo rpho us s i li ca to low-cris tobal i te , 1400 m for the v olcanic glass -c l in opt i lo-l i t e conve rs ion , 1900m fo r the low-c r i s toba l i t e - low-qua r t z conve rs ion , 3100 m fo r the mon tm or i l lon i t e - -mixed- l aye r mine ra l conve rs ion , 3500m fo r thec l inop t i lo l i t e -ana lc i t e and /o r he t l l and i t e conve rs ion ,4200 m fo r the mixed- l aye r m ine ra l - il l i te conve rs ion ,and 4300 m fo r the conve rs ion o f ana lc i te an d /o r heu -l and i t e to l aumont i t e and /o r a lb i t e .

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Primary migration theory of petroleum 35

v

500

1000-

o

C layM inera l s

_ M o n

Z e o l i t e s

Glass

C l i

Lau o r ~ b

S i l i c aM ine ra l s

Am or

L o w e r i

L o w - O t z

50

100

_

1E

Ez2o

Abbr . Mon = Montmor i l lon i teMix = Mon- l l l M ixed- layer M inera l] I I = l l l it e

G lass : Volcanic G lassCI i = C l inopt i lo l i t eAna = Analc i teHeu = Heuland i teLau = Laumont i teAIb = A lb i te

Amor = Amorphous S i l icaCr i = Cr is tobat i teOtz = Quar tz

Fig. 1, Overburden pressures and geothermal temperatures required for the diagenetic transformationsof clay minerals, zeolites and s ilica minerals in T ertiary arg illaceous sediments of Japa n (m odifiedafter Aoyagi and Kazama, 1977).

Proc ess o f diagenes is in sedimentIn Hedb e rg s (1936) po ros i ty s tudy , a s s how n in

Fig. 2 , the proc ess of com pac t ion in argi l laceous sedi-ments can be c lass i f ied in to the fol lowing four s tages ;mechan ica l r e a r rangem en t (90-75Y/o po ros i ty ) , de -water ing (75-35~o), mechanica l deformat ion (35--10~o)and recrys ta l l iza t ion (10-O~) s tages . Subsequent ly ,Weller (1959) and Burs t (1969) reported s imilar com-pac t ion mode l s .R ecen t ly , many repor t s on the expe r imen ta l com-pac t ion o f a rg i ll a ceous s ed imen t s have been p ub-l ished. Inami and Hoshino (1974) s tudied themechan ica l p rope r t i e s o f sed imen ta ry rock s in J apan ,and 3 yr later, they c lass i fied th e com pac t ion pro cessinto viscous , p las t ic and e las t ic s tages (Hoshino andInami , 1977) . B as ed on re s u lt s o f expe r imen ta l c om -pac t ion o f mon tmo r i l lon i t e c l ay by Aoyag i et al(1976), Aoyagi and Asakawa (1977) proposed the fol-lowing th ree s t ages o f d i agenes i s; e a r ly com pac t ion ,l a t e compac t ion and rec rys t a l l i z a t ion (o r t r ans fo rma-tion) stages.Ave rage re l a t ions h ips p ropos ed by Aoyag i andAs akawa (1977)among the changes in pa r t i c l e s , po re sand l iqu ids , and the ave rage bu r i a l dep th and geo -the rma l t empe ra tu re du r ing each s t age o f d i agenes i sin the Te r t i a ry a rg i l l aceous s ed imen t s o f J apan a resummarized in Fig . 3 .Th e first s tage of diagen esis is identified as the

ea r ly com pac t io n s tage. As the num ber o f g ra in con-tac ts in the sediment a re re la t ive ly low a t th is s tage ,the phys ica l prop erty o f the sedim ent wil l be viscous .The phys ica l p roces s con t ro l l ing th i s s t age i s compac -t ion by overburden pressure and i t wi l l cause themechan ica l r e a r rangemen t o f g rains . Sed imen t po ro -s i ty a t th is s tage ranges fro m 80 to 30~o. A largeam ou nt of oxidiz ing in ters t it ia l water resem bling seawater wil l be expel led durin g the e ar ly pa rt of th iss tage and reducing in ters t i t ia l and in ter layer waterfrom swell ing c lay minera ls wi l l be expel led rapidlyf rom the s ed imen t s du r ing the l a t e pa r t o f the s t age .M ine ra l neo fo rma t ions , wh ich cou ld reduce the s ed i -me nt s poro s i ty , a re genera l ly inac t ive a t th is t ime.Howeve r , t he t r ans fo rma t ion f rom amorphous s i l i c ato low-c r i stoba l it e a t t he m idd le pa r t o f the s t age andthe conve rs ion o f vo lcan ic g la s s to c l inop ti lo li t e a tthe la tes t par t of the s tage wil l occur .The se cond s tage of d iagenes is is te rme d the la tecom pac t io n s tage. A s tab le f ramework o f s ed imen tg ra ins w i l l be fo rmed by mechan ica l de fo rma t ion andan inc rea s e in num ber o f g ra in co n tac t s w i l l r e s u ltfrom the increase of ove rbur den pressure . Thus , thephys ica l charac ter is t ic of the sedim ent w ill changefrom viscous to plas t ic . Geologica l fac tors a ffec t ingthe s ed imen t du r ing th i s s t age a re compac t ion ,cemen ta t ion and t rans fo rma t ion . Th i s s t age i s cha rac -ter ized by poros i t ies rang ing fro m 30 to 10~o, the gra-

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36 K. AOYAGI an d T. ASAKAWAP o r o s i t y

. i . )

9 0

8 0

7 0

6 0

5 0

4 0

3 0

2 0

1 0

Hedberg( 1 9 3 6 )

M e c h a n i c a lR e a r r a n g e m e n t

D e w a t e r i n g

MechancaD e f o r m a t i o n

RecrySlallization

W e l l e r( 1 9 5 9 )

( P l a s t i c )

( Gra in Contact )

G r a i nD i s t o r t i o n /

( C r u s h i n g )

B u r s t( 1 9 6 9 )

n

H o s h i n o &l n a m i ( 1 9 7 7 )

V i s c o u s

C o m p a c t i o n

P l a s t i cC o m p a c t i o n

A o y a g i 8 .Asakaw a ( 1977 )

Earlyompact o n

L a t eC o m p a c t i o n

E l a s t i c RecrystallizationC o m p a c t i o n

Fig. 2 Var ious con cepts of the process o f d iagenes is in arg i l laceous sedim ents as propo sed by d i f ferentinvestigators.

a o

M e c h a n i c a l R e a r r a -n g e m e n t o f P a r t i c l e sM e c h a n i c a l D e f o r m a -t i o n o f P a r t i c l e s

O T r a n s f o r m a t i o n o fu ' ) M i n e r a l s

N e o f o r m a t i o n o fM i n e r a l s

E x p u l s i o n o f ~ J J J J ~Q I n t e r s t i t i a l W a t e r

E E x p u l s i o n o fI n t e r l a y e r W a t e r

3o J P o r o s i t y ( O /o ) 8 0 3 0

A v e r a g e B u r i a l D e p t h ( m )

A v e r a g e T e m p e r a t u r e ( C )

S t a g e o f O i a g e n e s l s

0 - 1 4 0 0

1 5 - 5 5

E a r l y C o m p a c t i o n

3 0 1 0

1 4 0 0 2 8 0 0

5 5 9 6

L a t e C o m p a c t i o n

< 1 0

> 2 8 0 0> 9 6

R e c r y s t a t l i z a t i o n( T r a n s f o r m a t i o n )

Fig. 3 . Cha nges in pores l iquids and gra ins and rang es of average b ur ia l depths an d geotherm altempera tures dur ing each s tage of d iagenes is in the Ter t ia ry argi l lacous sediments of Japan.

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Primary migration theory of petroleum 37dua l expu l s ion o f sma l l amou n t s o f in t e r l aye r and in -ters ti tia l waters , and the neofo rm at ion o f variou smine ra l s in po re s paces . The t r ans fo rma t ion f romIow-c r i s toba l i te to low-qua r t z occu rs a t t he midd lepart of the s tage .The th ird s tage of d iagenes is is ca l led the ' recry s ta l -l i z a t ion ' o r ' t r ans fo rma t ion ' s t age . Va r ious au th igen icmine ra l s s uch a s mon tmor i l lon i t e - i l l i t e mixed- l aye rminera l , i l l i te , ana lc i te , heulandi te , laumonti te , anda lb it e, wh ich a re t r ans fo rmed f ro m o the r m ine ra lg ra ins , w i l l p roduce a s t rong rock f ramework . Theeffec t of comp act io n is extremely low a t th is t imebecaus e the rock f ramework can s uppor t t he ove r -bu rden p re s s u re . Thus , t he mos t impor tan t geo log ica lfac tors ac t ing during th is s tage wil l be the increasedgeo the rma l t empe ra tu re and reac t ion t ime and a l s othe mig ra t ion o f chemica l comp onen t s . Po ros i ty o fthe rock a t th is s tage is less than 10 . In ter lay er andinters t it ia l w aters can not be eas i ly expel led and wil lbe t rapped for a long period as foss i l water .

T E M P E R A T UR E A N D D E P T H O FH Y D R O C A R B O N G E N E R A T I O N

As a re s u lt o f advances in ana ly t i c a l t e chno log ys ince the mid-1960s, geoc hem ical s tudies of org ani cmatter in argi l laceous sediments a re avai lable forvarious bas ins in Japan and e lsewhere (Phi l ippi , 1967;Vas s oev ich e t a l . 1967; Albrecht and Ourisson, 1969;Tissot e t a l . 1971; Nixon, 1973; Asakawa, 1975).Thes e s tud ie s es t ab li s hed tha t t he gene ra t ion o f hy -d roca rbons in a rg i l l a ceous s ed imen t s occu rs a t a c e r -t a in dep th o f bu r i a l, and tha t t he com pos i t ions o fhyd roca r bon s in R ecen t s ed imen t s d if fe r f rom thos eof ancient sed iments and crude oi ls . Ancient sedi-men t s and c rude o i l s con ta in re l a t ive ly h igh concen-

t ra t io ns o f l i ght hydroca rbon s (C 2-C 9) , whe rea sR ecen t s ed imen t s con ta in ve ry sma l l am oun t s o f the s ehydroca rbons . In add i t ion , i t was found tha t odd ca r -b o n - n u m b e r e d n o r m a l p a r a f f i n s a r e p r e d o m i n a n t i nR ecen t s ed imen t s wh il e the con ten t s o f even and oddc a r b o n - n u m b e r e d n o r m a l p a r a f f i n s a r e a l m o s t e q u a lin crude oi ls (Bray and Evans , 1961). These g eochem i-ca l fac ts indica te tha t pe troleum is genera ted by thema tu ra t ion o f hyd roca r bon s in s ed imen t s a t a c e r ta indep th . B y s uch s tud ie s geo log i s t s and geochemis t shave conf i rmed the gene ra t ion theo ry o f pe t ro leumdur ing d iagenes i s and have iden t i f i ed the ke rogencom pon en t o f sed imen t s a s the p r inc ipa l s ou rce o fpe t ro leum.Hi tch on (1974) exp lains the p roces s o f ke rogen fo r -ma t ion in s ed imen t s a s fo l lows . Organ ic ma t t e r de -pos i t ed w i th ino rgan ic ma t t e r in an anae rob ic en -v i ronmen t w i l l change d i rec t ly to ke rogen a n d / o rhumic ma t t e r . Th i s change i s b rough t abou t a t s ha l -low dep ths (20 -30 m) by ex tensive b iochemica l a l t er -a t ion . The gene ra ted humic ma t t e r w i l l be po lymer -i zed to h igh -molecu la r we igh t ke rogen a s a r e s u l t o fhydro lys i s , deamina t ion , den i t rogena t ion and deca r -boxy la t ion reac t ions du r ing the ea r ly s t age o f d ia -genesis.

As ove rbu rden p re s su re and geo the rm a l t empe ra -tu re inc rea s e , gen e ra te d ke rogen wi ll unde rg o me ta -morph ic changes s imi l a r to coa l i f i c a t ion . The mos timpor tan t geo log ica l f a c to r con t ro l l ing the p roces s i st empe ra tu re ; t he s econd mos t impor tan t f a c to r i sreac t ion t ime. Pressure is respons ible only for phys i-ca l changes s uch a s the vo lume t r i c dec rea s e o f wa te rand por es in the organic ma t ter (Teichmii lle r andTeichmii lle r, 1966). These ideas have been co nfi rm edby expe r imen ta l s tud ie s on ca rbo n iza t ion o f po ll engra ins and spores (Gut jar , 1966).

We i t e (1972) ha s p ropos ed the fo l lowing s equenceof even t s to exp la in the pe t ro leum gene ra t ion p roces sby the rma l decom pos i t ion o f ke rogen . Af te r expu l sionof ino rgan ic and o rg an ic ga se s such a s H2 0 , C O z ,HzS, N2 and C H4, ke rogen wi l l fo rm s o lub le o rgan icp roduc t s (b i tumen) and a newly gene ra ted in s o lub leke rogen . Thes e newly gene ra ted l iqu id p roduc t s andkerogen wil l aga in produce gases , kerogen, and l iquidp roduc t s inc lud ing hydroca rbons . The p roces s w i l lcon t inue repea ted ly fo r a long t ime , p roduc ing in -c rea s ing y ie lds o f hydro ca rbons , i n wh ich a r om a t i cc o m p o u n d s g r ad u a l ly b e c o m e m o r e p r o m i n e n t . A s are s u lt o f the s e con t inu ing p hys ico -chemica l r e ac t ions ,the ke rogen wi l l g radua l ly be changed to g raph i t e .

Tissot e t a l . (1975) presented the re la t ionshipsb e t w e e n h y d r o c a r b o n m a t u r a t i o n a n d b u r i a l d e p t hand geo the rma l t empe ra tu re o f s ed imen t s in va r iousbas ins of the world (see Fig . 4) . As sho wn in thefigure , i t i s genera l ly recognized tha t the geothermalt empe ra tu re nece s s a ry fo r the ma tu ra t ion o f hydro -carbons wil l decrease as reac t ion t ime increases .R ecen t ly , As akawa and Fu j i t a (1979) repor t ed tha tthe gene ra t ion o f hydro ca rbo ns in the M iocene s ed i-men ts of severa l Japan ese oi l f ie lds occ urred a t ap-p rox ima te ly 100C and reached i t s peak a t abou t150C

P RI MA R Y M I G R A T I O N O F P E T R O L E U MH Y D R O C A R B O N S

As s ta ted ear l ie r , the sedim ents m us t be bu ried toa ce r t a in min ima l dep th to p rov ide the t empe ra tu reneces s a ry fo r the gene ra t ion o f pe t ro leum h ydro -ca rbons f rom ke rogen . The re fo re , many inves t iga to r sh a v e b e en c o n c e r n e d a b o u t t h e m e c h a n i s m o f p r i m a r ymigra t ion o f gene ra ted pe t ro leum f rom s ource rocksto reservoirs . On e of the pr inc ipa l ques t io ns inv olvedthe origin of carr ie r w ater for the oil , since m anygeologis ts be l ieved tha t mo s t of e n ters t i t ia l waterin a rg i l l a ceous s ed imen t s wou ld have been expe l l edbe fo re the gene ra t ion o f pe t ro leum h ydro ca rbo nsf rom ke rogen and occu r red (A thy , 1930 ; Hedbe rg ,1936, and Levorson, 1954).

Powers (1967) s tud ied the mechan i s m o f compac -t ion in argi l laceous sediments and sugges ted tha t thein te r laye r wa te r r e l ea sed by the m on tmo r i l lon i t e - i l li t et rans fo rma t ion wou ld be impor tan t a s a c a r r i e rbecaus e the gene ra t ion dep th o f hydroca rb ons gene r -a l ly co r re s pond s to the t ime o f the m ine ra l t r ans fo r-

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115 Ce n t

Or i en ta l

I t I I I 0 0 2 O 0 I 0 0 2 O 0 t I I II 0 0 2 0 0 I 0 0 2 0 0m g /g C o r g a n i q u e

Fig. 4. Relationships betwee n the maturation of hydrocarbons an d the burial depth and geotherm altemperature of sediments in various b asins of the world (after Tissot e t a / 1975).marion s tage . Later , Burs t (1969) and Perry andHower (1972) inves t iga ted in de ta i l the re la t ionbe tween the dep th o f the mo n tmor i l lon i t e - i l l i t e t r ans -fo rma t ion and the mechan i s m o f expu l s ion o f wa te rf rom c layey s ed imen t s in the Gu l f C o as t and s up -por t ed Pow er ' s t heo ry ( s ee F ig. 5 ). The i r conc lus ionss hou ld be rega rded a s the theo ry o f p r ima ry m ig ra -t ion of pe troleu m durin g the la te s tage of d iagenes is.Burs t (1969) sugges ted tha t the in ter layer waterre leased from sediments with poros i t ies be tween 30and 16 was the pr inc ipa l vehicle for o i l migra t ion.

Recent ly , Aoyagi and Asakawa (1977) s tudied theaverag e geoth erm al gradien t (2 .89C/100 m) and verti -ca l poros i ty changes in argi l laceous sediments ofva r ious J apanes e ba s ins and examined the mode o fhydroca rbon gene ra t ion and the t r ans fo rma t ions o f

various minera ls during each s tage of d iagenes is(Fig . 6) . In addi t ion, based on the assumption tha tthe o r ig ina l po ro s i ty o f a rg i ll a ceous s ed imen t was80~o and tha t ha l f o f the rem a in ing 20~o g ra in vo lumewas com pos ed o f s wel ling mon tm or i l lon i te , t hey ca l -cu la t ed the vo lume change o f g rains and po re s pacein sediments durin g each s tage of d iagenes is (Fig . 7).They conc luded tha t t he act ive p r im ary mig ra t ion o fo i l i n a rg i l la ceous s ed imen t s o f J apan o ccu r red a t t hela t e compac t ion s t age when the s ed imen t po ros i tyrange d between 30 and 10~/, . This conc ept sho uld beca l led a s the theo ry o f p r ima ry mig ra t ion o f pe t ro -leum d uring the m iddle s tage o f diagenesis.

They ind ica ted tha t a l a rge am oun t o f the in t e r l aye rand in ters t i t ia l water expel led from the sediments dur-ing the ea r ly compac t ion s t age cou ld no t be invo lved

POWERS 1967)

E3

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Fig. 5. Expulsion o f water available for oil m igration during eac h stage o f diagenesis (after Perryand Hower, 1972).

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e m p e r a t u r e ( *C )75 100 125 150I I I

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Prim ary migrat ion theory of petroleum 39

T r a n s f o r m a t i o n S t a g eo f M i n e r a l s

G la s s t [

M o n L o w - C r i

C i l

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k a uI l l o rI A I b

A b b r : M o n = M o n t m o r i l l o r l i t eM i x = M o n - l l l M i x e d - l a y e r M i n e r a lI I I = I l l i t e

G l a s s = V o l c a n i c G l a s sC I i = C l i n o p t i l o l i t eA n a = A n a l c i t eH e u = H e u ( a n d i t eL a u = L a u m o n t i t eA I b = A i b i t e

A m o r = A m o r p h o u s S i l i c aC r i C r i s t o b a l i teO t z = Q u a r t z

Fig. 6. Relat ionships am ong the average geotherm al gradient, average porosi ty, generat ion of hydro-carbons, and transformation of various minerals during each stage of diagenesis in the Tert iary argi l la-ceous sediments of Japan (modified after A oyag i and Asakawa, 1977).

i n o i l m i g r a t i o n s i n c e t h e g e o t h e r m a l t e m p e r a t u r e o ft h e s t a g e w a s 1 5 - 5 5 C a n d w a s b e l o w t h a t r e q u i r e df o r o i l ge ne r a t i on . D ur i ng t he r e c r ys t a i l i z a t i o n ( o rt r a n s f o r m a t i o n ) s ta g e , th e g e o t h e r m a l t e m p e r a t u r ew ou l d r is e a bo ve 96C , w h i c h w ou l d r e s u l t i n a c t i veg e n e r a t i o n o f o i l a n d t h e r m o c h e m i c a l m e t h a n e . H o w -e ve r , t he i n t e r l a ye r a nd i n t e r s t i t i a l w a t e r c o u l d no tc o n t r i b u t e t o o i l m i g r a t i o n b e c a u s e t h e w a t e r c o n t e n ta t t h i s s t a g e w o u l d b e v e r y l o w a n d c o u l d n o t b ee x p e l l e d b e c a u s e o f t h e s e d i m e n t ' s l o w p e r m e a b i l i t y(Magara , 1975) .D u r i n g t h e l a t e c o m p a c t i o n s t ag e , h o w e v e r , o i l g e n -e r a t i o n i n J a p a n e s e M i o c e n e s e d i m e n t s c o u l d n o tha ve be e n s i gn i f i c a n t be c a us e t he ge o t he r m a l t e m -pe r a t u r e w a s on l y be t w e e n 55 a nd 96C . N e ve r t he l e s s ,a n y g e n e r a t e d o i l c o u l d e a s il y h a v e m i g r a t e d o u t w a r dt h r o u g h t h e p o r e s b e c a u s e a l a r g e a m o u n t o f c a r r i e rw a t e r , c o m p o s e d o f i n t e r l a y e r w a t e r f r o m m o n t m o r i l -l on i t e , c r y s t a l l i ne w a t e r f r om z e o l i t e s , a nd i n t e r s t i t i a lw a t e r f r om po r e s , w ou l d be e xpe l l e d f r om t he s e d i -m e n t s . A s i nd i c a t e d i n F i g . 7 , a l m os t e q ua l qu a n t i t i e so f i n t e rl a y e r a n d i n t e r s ti t ia l w a t e r w o u l d b e e x p e l l e df r om t he s e d i m e n t s . Th i s c on t r a d i c t s Pow e r ' s ( 1967)a s s u m p t i o n t h a t i n t e r la y e r w a t e r i s s o l e l y r e s p o n s i b l ef o r o i l m i g r a t i on .

O n t h e o t h e r h a n d , p o r o u s r e s e r v o i r s s u c h a s s a n d -

s t one s a nd t u f t s w i l l e xpe r i e nc e a de c r e a s e i n po r os i t yw i t h i n c r e a s in g b u r i a l d e p t h d u e t o c o m p a c t i o n a n dc e m e n t a t i on ( A oya g i , 1974) . The s e r oc ks , how e ve r ,s h o u l d r e m a i n s u f f ic i e n tl y p o r o u s t o r e t a in t h e p e t r o -l e u m h y d r o c a r b o n s e x p e l l e d w i t h c a r r i e r w a te r . I n a m ia n d H o s h i n o ( 19 74 ) r e p o r t e d t h a t p o r o s i t ie s g r e a t e rt h a n 1 5 ~ i n a r e n a c e o u s r o c k s g e n e r a l ly c o r r e s p o n dw i t h t h e e a r l y c o m p a c t i o n s t a g e i n a r g i l l a c e o u s r o c k s .T h e y c o n s i d e r e d t h a t t h e a r e n a c e o u s r o c k s w i t h p o r -o s i ti e s b e l o w 1 5 h a d a l r e a d y r e a c h e d th e r e c r y s ta l -l i z a t io n s t a ge , d u r i n g w h i c h t h e a c t iv e n e o f o r m a t i o no f a u t h i g e n i c m i n e r a ls o c c u r s a n d t h a t c o n d i t i o n s r e -s p o n s i b l e f o r t h e l a t e c o m p a c t i o n s t a g e i n a r g i l l a c e o u sr o c k s d o n o t a p p l y t o a r e n a c e o u s r o c k s . A s t h e M i o -c e ne r e s e r vo i r s o f t he J a pa ne s e o i l f i e l d s ge ne r a l l yc o n t a i n e d m u c h v o l c a n i c g l a ss , l a r g e q u a n t i t i e s o fm i n e r a l s s u ch a s ze o l i te s a n d c a r b o n a t e s w o u l d b ef o r m e d d u r i n g d i a g e n e si s . T h i s w o u l d r e s u l t i n a na b r u p t d e c r e a s e in p o r o s i t y a n d p e r m e a b i l i t y o f r e se r -v o i r s , s o t h a t i t m a y b e c o n c l u d e d t h a t p r i m a r y o i lm i g r a t i o n c o u l d n o t o c c u r a f t e r t h e r e c r y s t a l l i z a t i o ns t a g e i n r e s e r v o i r s h a s s t a r t e d . G a s m i g r a t i o n , h o w -e ve r , is pos s i b l e du r i ng t he r e c r ys t a l l i z a t i on s ta ge .

I n t h e p r e c e d i n g d i s c u s s io n , w e e s t a b l i s h e d t h e p o r -o s i t y a n d m i n e r a l c h a n g e s , t h e i n t e r l a y e r a n d i n t e r -s t i ti a l w a t e r e x p u l s i o n m e c h a n i s m s , a n d t h e g e n e r -

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40 K. AOYAGI nd T. ASAKAWAedimentat ion

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l ; i : i '. i . ~ s ..~....:...... :; -om 9 5 %

Fig. 7. Volum etric changes of grains and pores in argillaceous sedim ents during eac h stage of diagenesis(after Aoyagi and Asakawa, 1977).a t i o n o f p e tr o l e u m h y d r o c a r b o n s i n J a p a n e se T e r t ia r ys ed imen t s whos e ave rage ca lcu la t ed pa led -geo the rma lgrad ient was 2 .89C /100m . Fo r different pa led-geo the rm a l g rad ien t s, howeve r , t he pa t t e rn o f changeswi ll be cons ide rab ly m od i f ied . Fo r in s tance , a s s uminga g rad ien t o f 6.0C /100 m, the t r ans fo rma t ion s f rommontmor iUon i t e to mixed- l aye r mine ra l and f rom vo l -can ic g l a s s to c l inop t i lo l i t e wou ld occu r a t bu r i a ldepth s of ab ou t 1500 and 700 m, respectively . Thisimpl i es tha t t he neo fo rma t ion o f s uch m ine ra l s a smixed- l aye r mine ra l and c l inop t i lo l i t e wou ld s t a r t a tan ear l ie r s tage of d iagenes is (mainly a t the ear ly co m-pact ion s tage). On the other han d, i f the gradient w as2 .0C /100m, the neo fo rm a t ion o f the s e mine ra l sw o u l d b e g in a t d e p t h s o f a p p r o x 4 5 0 0 a n d 2 1 0 0 m ,respect ive ly . Under these condi t ions the t ransforma-t ion o f the s e mine ra l s wou ld occu r a t a l a te r s t ageof diagenes is (mainly a t the recrys ta l l iza t ion s tage) .The re fo re , t he pa led -geo the rma l t empe ra tu re mus t beproperly es tabl ished for each bas in s tudied.

A P P L I C A T I O N O F M I G R A T I O N T H E O R YT O P E T R O L E U M E X P L O R A T I O N

As s t a t ed be fo re, Aoyag i and As aka wa (1977) p ro -pos ed tha t t he p r imary mig ra t ion o f o i l occu rs du r ingthe l a te com pac t ion s t age when the po ros i ty o f arg i l-l a ceous s ed imen t s ranges f rom 30 to 10%. S ince com -pac t ion i s t he mos t impor tan t f a c to r con t ro l l ing th i ss tage, t he bu r i a l dep th requ i red fo r p r im a ry o i l m ig ra -t ion will be gove rned b y the vert ica l chan ge of por-

os i ty o f a rg i l l aceous s ed imen t s in e ach ba s in . O n theo the r hand , the mos t impor tan t f a c to r in f luenc ing thegene ra t ion o f hydr oca rb ons f rom ke rogen i s geo the r -ma l t empe ra tu re . R ecen t ly , As akaw a and Fu j i t a(1979) repor t ed tha t t he gene ra t ion o f o il i n M iocenes ed imen t s o f J apan began a t app rox 100C andreached i ts peak a t about 150C. Therefore , the poss i -b i l it y o f fo rm a t ion o f hydroc a rbo n poo l s in a g ivenexp lo ra t ion a rea can be eva lua ted f rom pa leo -g_eo-the rma l t empe ra tu re da ta a t bu r i a l dep ths whe re theporos i ty o f a rg i l la ceous s ed imen t s range f rom 30 to1o .

For example , f rom a po ros i ty -bu r i a l dep th s tudyof M ioc ene a rg i l l a ceous s ed imen t s in the Ak i t a a reao f no r the rn J apan (M iyazak i , 1966), i t was conc lud edtha t the p r ima ry mig ra t ion o f o i l i n th is a rea o ccu r redat buria l d epths be tw een 1300 and 2600 m (see Fig . 8) .F igu re 9 ind ica te s how the range o f o i l gene ra t iontempe ra tu re s a t t he mig ra t ion dep th w i l l va ry acco rd -ing to changes in the pa led -geo the rma l g rad ien t inthe area .

As s hown in F ig . 9 , i f t he pa led -geo the rma l g rad ien twas 7 .0C /100m, the geo the rm a l t empe ra tu re d u r ingpr imary m ig ra t ion o f o i l wou ld range f rom 119 to223C. Since th is tempera ture range is h igher thanthat expected for o i l genera t ion , the potent ia l forform ing la rge oi l poo ls in the area w ill be low. Theform at ion of gas pools , however , i s st il l poss iblebecaus e the gene ra t ion o f the rmochemica l me thanecan t ake p lace a t t he se t empe ra tu re s . A s s uming a g ra -dient of 3 .0C/100 m, the tem pera tu re a t the requ ired

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Primary migration theory of petroleum 41

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0 4

o

Fig. 8 . Relationships between absolute porosity and burial depth in the Miocene argillaceous sedimentsof Akita oil f ie lds, Japan after M iyazaki, 1966).p o r o s i t y w o u l d r a n g e f r o m 5 4 t o 9 3 C , w h i c h i s t o ol o w f o r t h e g e n e r a t io n o f o il a n d t h e r m o c h e m i c a lm e t h a n e . B i o c h e m i c a l m e t h a n e c a n b e g e n e r a t e d a c -t i v e ly a t t h e s e t e m p e r a t u r e s , th u s t h e f o r m a t i o n o fg a s p o o l s w i l l b e p o s s i b le . I f th e g r a d i e n t w a s5 . 0 C / 1 0 0 m , t h e t e m p e r a t u r e d u ri n g p r im a r y m i g r a -t i o n o f o i l w o u l d b e i n t h e 8 0 - 1 4 5 C r an g e. I n t h i sc a s e , th e p o s s i b i l i t y o f f o r m i n g l a r g e o il p o o l s i n t h ea r e a i s h i g h s i n c e o i l g e n e r a t i o n f r o m k e r o g e n i s v e r ya c t i v e a t t h e s e t e m p e r a t u r e s .I n t h e A k i t a a r e a , t h e r e a r e m a n y o i l f i e l d s s u c ha s t h e Y a b a s e , T o y o k a w a , S a r u k a w a , F u k u b e z a w a ,T s u c h i z a k i - o k i , H a c h i m o r i a n d o t h er s . C u m u l a t iv ep r o d u c t i o n s o f c r u d e o i l a n d n a t u r a l g a s t h r o u g h t h ee n d o f 1 9 6 9 i n t h e s e f i e l d s a r e l i s t e d i n T a b l e 2 . S i n c e

t h e p a l e o - g e o t h e r m a l g r a d i e n t s i n t h e a r e a h a v e n o ty e t b e e n s t u d i e d , t h e p r e s e n t g e o t h e r m a l g r a d i e n t s f o re a c h o i l f i el d a r e g i v e n i n t h e t a b l e . T h e s e g e o t h e r m a lg r a d i e n t s w e r e o b t a i n e d f r o m t h e s t u d i e s o f O h g u c h ie t a l 1 9 7 0) , A i b a 1 9 7 7) , a n d T a n a k a a n d S a t o 1 9 7 7) .T h e m a x i m u m t e m p e r a t u r e m e a s u r e d d u r i n g e l e c t r i cl o g g i n g g e n e r a l l y d o e s n o t i n d i c a t e t r u e s u b s u r f a c et e m p e r a t u r e s b e c a u s e t h e r m a l e q u i l i b r i u m b e t w e e nb o r e h o l e m u d a n d f o r m a t i o n w a s n o t a t t a i n e d p r i o rt o t h e s e m e a s u r e m e n t s . T h e r e f o r e , t h e g e o t h e r m a lt e m p e r a t u r e s r e p o r t e d b y O h g u c h i e t a l 1 9 7 0 ) w e r er e c a l c u l a t e d f o r e a c h w e l l b y t h e f o r m u l a e s t a b l i s h e db y T a n a k o a n d S a t o 1 9 77 ),

A s s h o w n i n t h e t a b l e , t h e p r e s e n t g e o t h e r m a l g r a -d i e n t s i n t h e s e o i l f i e l d s r a n g e f r o m 3 . 2 t o

G e o t h e r m a l T e m p e r a t u r e ( C )

1 5 5 0 1 0 0 1 5 0 2 0 0 2 5 0- t= I

G e n e r a t i o n '~ . T e m p e r a t u r e E a r l y C o m p ~ . c t i o n Z o n e

0 o o :: o o .

v

o

_ . . :.: :.:,' ~ 2 6 2 0

4 0 0 0I 0 0 1 5 0

Fig. 9 . Changes in oil generation temperature ranges at migration depth for various paleo-geothermalgradients in the Miocene sediments of Akita oil f ie lds, Japan.

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42 K AOYAGIand T. ASAKAWATable 2. Cumulative productions of crude oil and natural gas through the end of 1969 and presentgeothermal gradients in various oil fields of Akita, Japan

0 i I P i I d Y a b a s e K u r o k a w a K a m i h a m a T o y o k a w a S a r u k a w a F u k u b e z a w a T s u c h i z a k i . o k i i H a c h i r n o r i

O i l P r o d u c t i o n / * 7 8 0 ~ Z Z O 1 ~ 9 0 9 6 0 7 2 0 1 7 0 1 5 0 I 1 0( x i 0 3 K L )a s P r o d u c l i o n I 0 / 0 I 0 0 ~ 0 t , 0 I 0 0 2 0 2 0 1 0( x 1 0 ~ M 3 )

G G S O 0 t ) - - - - / * . l 8 2 ) 3 1 . 2 3 ) / * 3 5 3 ) 3 1 9 3 ) 5 7 / * 3 )( C t I 0 0 m ) t

I ) A i b a ( 1 9 7 7 )2 ) T a n a k a a n d S a l o ( I 9 7 7 )3 ) R e c a l c u l a t e d f r o m O h g u c h i e t a l . ( 1 9 7 0 )

5.7C/100m. The geothermal gradient in Yabase oilfield, which is the most productive in the area, is5.0C/100m. Therefore, assuming that the paleo,geothermal gradients during the primary migrationof oil in the area were similar to the present geother-mal gradients, it appears that oil generation at therequired migration depths in these oil fields was notat a maximum level with the exception of the Yabaseoil field.

The possibility of forming large oil pools will behigh in the basin where the maximum generation ofoil corresponds with the stage of primary migrationindicated by burial depth. In other words, a suitablepaleo-temperature is essential for the formation of o ilpools in the area. Therefore, for exploration purposes,it is indispensable to determine the paleo-geothermalgradient as well as the relationship between absoluteporosity and burial depth of argillaceous sedimentsin each basin. From these studies, the pred iction ofoil pool occurrences in the exploration area will bepossible,

CONCLUSIONIn the present paper, the writers studied in detail

the process of diagenesis as indicated by the changesof pores and grains in argillaceous sediments andclassified the process into early compaction, late com-paction and recrysta llization transformation) stages.We also explained the process of hydrocarbon gener-ation from kerogen and discussed the relationsbetween generation temperature and depth of petro-leum occurrences in argillaceous sediments of Japanand elsewhere.

As a result, we concluded that the primary migra-tion of oil would occur during the late compactionstage when the porosity of argillaceous sedimentsranged from 30 to 10~.

From this theory of primary migration, we con-cluded that the correspondence of the stages of pri-mary migration and generation of oil were indispens-able for the formation of large oil pools in the explor-ation area. To establish these stages of generation an dmigration, it is important to study the relationshipbetween porosity and burial depth of argillaceoussediments and to determine the paleo-geothermal gra-dients in each basin.

A c k n o w l e d e m e n t s - - T h e writers would like to thank JapanPetroleum Exploration Company JAPEX) for permissionto publish the paper. Dr. T. ONITSUKAof our laboratory,Dr. K. HOSmNO of the Geological Survey of Japan, Dr.Y. FUJITAof Teikoku Oil Company, and Dr. S. R. SILVER-MAN of Chevron Oil Field Research Company read themanuscript and offered important comments to improvethe manuscript. Misses T. TAKAMASAand S, KANEKO ofour laboratory drafted the figures and tables. We thankthem very much.R E F E R E N E S

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