UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

POTENTIAL FOR GENERATION OF NATURAL GAS I N SEDIMENTS O F THE CONVERGENT M A R G I N O F THE

ALEUTIAN TRENCH AREA

K e i t h A. Kvenvolden 1

Roland von Huene 1

Open-File Report 84-404

The report is p r e l i m i n a r y and has n o t been reviewed for conformity w i t h U.S . Geological Survey e d i t o r i a l s t a n d a r d s .

enl lo Park, CA

Table of Conten t s

Page

L i s t of Tab les , Appendices and Plates...................-............2 L i s t of F i g u r e s ...................................................... 3

I n t r o d u c t i o n , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . .4 Seismic Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 I n t e r p r e t a t i o n s of Se i smic Records ...................................6 Geochemical Analyses and R e s u l t s ............l........................9 ...... E s t i m a t e of Geothermal G r a d i e n t r..........................-...ll Assessment of P o t e n t i a l f o r Gas Accwrmlations.......................12 Smmry ..........................................,..................15 Acknowledgements .............................................-.*..*.15 ................................................... References . . . . . . . 16 T a b l e s ........................................................*.....19 F i p r e s .............................................................20 .......................................................... Appendices 3 5 P l a t e s ............................................................ (Fold out )

L i s t of Tables

Table 1. Summary of Geochemical R e s u l t s by Sample S i t e s . . . . . . . . . . ..19 T a b l e 2. R e s u l t s o f Analyses o f Keroqen. ........................... 19 Table 3, Geothermal G r a d i e n t s i n OC/km Determined from Base

o f Gas Hydrate Reflector (BSR on Marine Se i smic Records.. .l9

L i s t of Appendices

Appendix 1. L i t h o l o g i c Logs o f DSDP Cores Showing samples f o r t h i s r e p o r t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

......... Appendix 2. V i t r i n i t e R e f l e c t a n c e o f Nine S e l e c t e d Samples 50

List o f P l a t e s

P l a t e 1. Seismic L i n e 111 from G r i d A showing P r o g r e s s i v e Enhancement of In format ion f r o m Stacked Record t o a Migrated Time Record t o a Depth S e c t i o n .

P la te 2. L i s t i n g o f Geochemical Results.

L i s t of F igures

Figure 1. P l a t e t e c t o n i c map of t h e Circum-Pacific r eg ion showing major p l a t e boundaries i nc lud ing t h e Aleu t ian Thrus t , t h e s i t e of t h e Aleut ian Trench subduct ion zone.. . . . . . . . . . .20

Figure 2. Map showing l o c a t i o n s of g r i d s A and B o f seismic l i n e s and s i n g l e se i smic l i n e s C and D. Seismic l i n e s wi th in g r i d s A and B are i n d i c a t e d a t expanded scale....................21

Figure 3 . Representa t ive t i m e sect i -ons f o r s e i smic grid A (F ig . 2 ) . .. 22 Figure 4. Depth Sec t ion o f Seismic l i n e 111 from grid A (Fig. 2) ..... 2 3

Figure 5. Representa t ive t i m e a n d depth s e c t i o n s for s e i smic grid B ................................................... (Fig. 2 ) 24

................... Figure 6. Seismic l i n e s C ( l i n e 13) and D (F ig . 2 ) 2 5

Figure 7. Pyrograms from TEA/FID ana lyses of 40 samples analyzed i n .............................................. t h i s r e p o r t . . 26

F igure 8a. P r o f i l e s wi th depth of o rgan ic geochemical r e s u l t s a t ................................................... s i t e 178 27

b. P r o f i l e s w i th depth o f o rgan ic geochemical r e s u l t s a t s i t e 180............................................... 28

c . P r o f i l e s w i th depth of o rgan ic geochemical results a t s i t e 181...............,.................................. 29

d. P r o f i l e s w i th depth of o rgan ic geochemical r e s u l t s a t s i t e 183................................................. 30

e. P r o f i l e s w i th depth of o rgan ic geochemical r e s u l t s a t s i t e 1 9 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1

Figure 9. van Krevelen diagram showing H I (mg HC/g OC) and 01 (mg C02 /g OC) f o r n ine s e l e c t e d samples ........................ 32

Figure 10. Examples of BSR on se i smic r e c o r d 51 from g r i d B (F ig . 2 ) . . 33

Figure 11. Isotherms a t t empera tures of 50°, 150°, and 250°C based on a cons t an t h o r i z o n t a l and v e r t i c a l geothermal g r a d i e n t of 30DC/km and superimposed on se i smic l i n e 11 1 ( F i g . 4 ) . ..... 34

P o t e n t i a l f o r Generat ion of Natura l Gas i n Sediments of t h e Convergent Margin o f t h e

Aleu t ian Trench Area

ABSTRACT

Sediment be ing subducted i n t h e e a s t e r n p a r t of t h e convergent margin of t h e Aleut ian R e n c h has a p o t e n t i a l t o gene ra t e l a r g e volumes of n a t u r a l gas, perhaps a s much a s 2 . 8 x l o 6 m3 of methane per km3 of sediment, even though t h e con ten t of o rgan ic carbon i n t h e sediment i s very low, averag ing about 0.4%. This high p o t e n t i a l f o r gas genera t ion r e s u l t s p r imar i l y from the enormous volume of sediment undergoing subduction. Along t h e e a s t e r n Aleut ian Arc-Trench system a 3 - b t h i c k s h e e t of sediment i s be ing subducted a t a r a t e of about 60 km p e r m i l l i o n years . We eszimate, based on cons ide ra t i ons of t h e s t a b i l i t y requirements f o r gas hyd ra t e s observed a s anomolous r e f l e c t o r s i n some of our se i smic records , and on one measurement i n a deep we l l , t h a t the geothermal g r a d i e n t i n t h i s reg ion i s about 30°C/km. Such a g rad i en t sugges t s a temperature regime i n which the maximum gas genera t ion i n t h e subduct ing sediment occurs beneath t h e upper s lope. Thus t h e sediment of t h e upper s l o p e , as opposed t o t h a t of t h e s h e l f and lower s l o p e , could be t h e most p rospec t ive f o r gas accumulation i f s u i t a b l e r e s e r v o i r s a r e p re sen t .

INTRODUCTION

The theo ry of p l a t e t e c t o n i c s provides conceptual models which can be powerful t o o l s i n t h e exp lo ra t i on f o r o i l and gas r e sou rces of c o n t i n e n t a l margins of t h e wor ld ' s oceans. Cont inenta l margins have been broadly c l a s s i f i e d based on r e l a t i v e movements among c r u s t a l p l a t e s i n t o pas s ive ( d i v e r g e n t ) and a c t i v e (convergent) types. Pass ive margins gene ra l l y c o n s i s t of broad she lves , s l opes , and r i s e s and a r e c h a r a c t e r i z e d by l a c k o f s e i s m i c i t y . I n c o n t r a s t , a c t i v e margins do n o t always have she lves and a r e a s s o c i a t e d with deep t r enches , volcanoes, and ear thquakes. Although p a s s i v e o r d ive rgen t margins have the r e q u i s i t e s f o r t h e genera t ion and accumulation of s i g n i f i c a n t q u a n t i t i e s of o i l and gas (Thompson, 19761, t h e r e q u i s i t e s f o r s i g n i f i c a n t hydrocarbon r e sou rces i n sediments of a c t i v e , convergent margins are poor ly understood. New information from s t u d i e s dur ing t h e l a s t t e n yea r s sugges t s t h a t a c t i v e margin s e t t i n g s could con ta in si tes of important gas and p o s s i b l y o i l accumulations (Thompson, 1976; Roberts, 1981; Gwilliam, 1982) . The p e c u l i a r n a t u r e of convergent margins, c o n s i s t i n g of t e c t o n i c a l l y complicated subduction complexes, does n o t l end i t s e l f t o "convent ional" petroleum exp lo ra t i on s t r a t e g i e s , and many of t h e b a s i c t e c t o n i c mechanisms ope ra t i ng i n subduct ion zones are only p a r t i a l l y understood.

S t u d i e s of se i smic r eco rds and samples from t h e Deep Sea D r i l l i n g P r o j e c t (DSDP) have e s t a b l i s h e d t h e r e a l i t y of ex t ens ive subduct ion of sediment a long a t l e a s t s i x modern convergent margins:

Japan Trench ( S c i e n t i f i c Pa r ty , 1980 ) Nankai Trough ( S c i e n t i f i c Party, 1980; Karig, Kagami, e t a l . , i n press) Mariana Trench (Hussong and Uyeda, 1981) Middle America Trench (Watkins, Moore, e t a l . , 1981; Aubouin, von Huene,

et al., 1982; 1983)

Barbados Ridge (Moore, Bi jou-Duval, e t a l . . 1981 Aleu t ian Trench (von Huene, 1979; von Huene, e t a l . , 1983 )

A t t h e s e margins l a r g e volumes of o c e a n i c sediment a r e b e i n g subducted, and t h e o r g a n i c m a t t e r i n t h e s u b d u c t i n g sediment w i l l undergo a l t e r a t i o n s a s t h i s sediment p lunges deeper and s u b s u r f a c e t e m p e r a t u r e s i n c r e a s e . Hydrocarbon gases, p a r t i c u l a r l y methane, a r e expec ted t o r e s u l t from t h e s e thermogenic a l t e r a t i o n p r o c e s s e s . Although t h e r e a r e f e w d i r e c t i n d i c a t i o n s o f the rmal g r a d i e n t s w i t h i n subduc t ion zones , model s t u d i e s show t h a t t e m p e r a t u r e s w i l l e v e n t u a l l y reach s u f f i c i e n t l e v e l s f o r t h e g e n e r a t i a n o f g a s (Delong and Fox, 1977) . The upward m i g r a t i o n o f g a s from t h e subduc ted sediment t o t r a p s w i t h i n t h e f r o n t o f t h e c o n t i n e n t a l margins seems l i k e l y c o n s i d e r i n g t h e Paleogene age of t h e sediment and t h e s u r p r i s i n g s t a b i l i t y o f t h e t e c t o n i c environment i n many modern convergen t margins , i e , t h e A l e u t i a n , Middle America, and Japan Trenches.

T h i s r e p o r t summarizes o u r s t u d i e s which t e s t e d t h e i d e a t h a t convergent margins can be t h e s i t e o f s i q n i f i c a n t hydrocarbon gas g e n e r a t i o n and accumulation. W e chose t h e convergen t margin o f t h e A l e u t i a n Trench, a t t h e northern-most edge o f t h e P a c i f i c P l a t e (F ig . 11, a s a n a p p r o p r i a t e a r e a f o r i n v e s t i n g c o n c e p t s o f gas g e n e r a t i o n and m i g r a t i o n i n a subduc t ion zone. P rev ious DSDP d r i l l i n g (Kulm, von Huene, e t a l . , 1973; Creager , S c h o l l , e t a l . , 1973) and e x t e n s i v e g e o p h y s i c a l and g e o l o g i c a l s t u d i e s (von Huene, 1979; P l a f k e r e t a l . , 1982; von Huene, e t a l . , 1983) p r o v i d e b a s i c i n f o r m a t i o n r e l e v a n t t o t h i s problem. Along t h e A l e u t i a n Trench, s t r u c t u r a l s t y l e s a s s o c i a t e d w i t h subduc t ion a r e d i v e r s e r a n g i n g from subduc t ion complexes w i t h n o a c c r e t i o n ( P l a f k e r , e t a l . , 1982) t o complexes w i t h e x t e n s i v e a c c r e t i o n as w e l l as sediment subduc t ion (von Huene, 1979; von Huene, e t a l . , 1983) . I n a d d i t i o n t o t h i s b a s i c g e o l o g i c a l and g e o p h y s i c a l i n f o r m a t i o n , some geochemical d a t a i n t h e form o f o r g a n i c ca rbon d e t e r m i n a t i o n s on sediments of t h e A l e u t i a n Trench system were a v a i l a b l e from DSDP Legs 18 and 19 (Bode, 1973a,b) . For this r e p o r t w e have done t h e fo l lowing :

a. Reprocessed m u l t i c h a n n e l s e i s m i c d a t a from p r e v i o u s U . S . G . S . s t u d i e s t o c l a r i f y t h e t e c t o n i c framework o f t h e A l e u t i a n subduc t ion complex, t o image deeper p a r t s of t h e A l e u t i a n s u b d u c t i o n zone, and t o e s t i m a t e volumes o f sediment i n v o l v e d i n subduc t ion .

b. Est imated t h e thermal g r a d i e n t from t h e b a s e o f g a s h y d r a t e r e f l e c t o r on s e i s m i c r e c o r d s , from a v a i l a b l e d r i l l data, and from g e n e r a l i z e d i n f o r m a t i o n a l o n g o t h e r convergen t margins .

c. Compiled i n f o r m a t i o n on o r g a n i c m a t t e r i n sed iments sampled d u r i n g DSDP Legs 18 and 19, and i n s e l e c t e d d redge samples from U.S.G.S. c r u i s e S-79-WG.

d. Assessed t h e p o t e n t i a l f o r g a s o c c u r r e n c e b a s e d on k i n d s and volumes o f sed iments , t y p e s and amounts of o r g a n i c m a t t e r , geothermal g r a d i e n t s , and t h e s t r u c t u r a l framework o f t h e A l e u t i a n subduc t ion complex.

SEISMIC SURVEYS

The s e i s m i c r e c o r d s u s e d f o r t h i s r e p o r t were t a k e n Erom two g r i d s ( A and B) of s e i s m i c l i n e s and two s i n g l e s e i s m i c l i n e s (C and D) a c r o s s t h e A l e u t i a n convergent margin n e a r Kodiak I s l a n d ( F i g . 2 ) . Many r e c o r d s from t h e s e g r i d s and l i n e s have been p r o c e s s e d t o c l a r i f y and deepen t h e s e i s m i c i n f o r m a t i o n w e l l beyond t h a t of t h e o r i g i n a l , more r o u t i n e l y p r o c e s s e d r e c o r d s . Cons iderab le s t r u c t u r a l v a r i a t i o n i n t h e a c c r e t e d sed iment can been s e e n on t h e s e s e i s m i c l i n e s ( F i g s . 3 and 4). An i m p o r t a n t c o n c l u s i o n Erom our s t u d y o f t h e s e r e p r o c e s s e d r e c o r d s i s t h a t a l a r g e amount of sediment i s p r e s e n t l y

being subducted beneath t h e a c c r e t i o n a r y complex. Thus al though t h i s p o r t i o n of t h e Aleut ian convergent margin i s t r u l y a c c r e t i o n a r y a t t h e p r e s e n t t ime , sediment subduct ion i s a n on-going and impor tan t process .

F i e ld work. A l l of t h e records were s h o t with t h e se i smic system aboard t h e RV S.P. LEE dur ing t h e f i e l d seasons of 1976, 1977, and 1981. The se i smic system was a tuned a r r a y of five air-guns t o t a l l i n g 21.7 l i ters, a 2,400 m - 24 group s t reamer , and a GUS 4300 d i g i t a l r eco rd ing instrument . In a l l surveys, s h o t p o i n t s were l oca t ed by s a t e l l i t e nav iga t ion supplemented by doppler-sonar and Loran-C f i x e s .

Data process ing . The d a t a w e r e p rocessed i n two phases. I n i t i a l , r o u t i n e p roces s ing was done t o gene ra l s t anda rds f o r CDP se i smic da t a i n records where t h e s t r u c t u r e i s r e l a t i v e l y simple. I n t h e second phase, t h e da t a w e r e r eprocessed and enhanced by migra t ion and depth conversions t o r e s t o r e t r u e s t r u c t u r a l r e l a t i o n s .

Seismograph Se rv i ce Corporat ion d i d t h e i n i t i a l p roces s ing o f data c o l l e c t e d during 1976; Grant Geophysical Corporation processed d a t a c o l l e c t e d du r ing 1977; and t h e U.S. Geological Survey a t Menlo Park processed d a t a from both 1977 and 1981. The process ing sequence c o n s i s t e d of demult iplexing, e d i t i n g , v e l o c i t y a n a l y s i s , NMO c o r r e c t i o n and s t ack ing , deconvolut ion, f i l t e r i n g , AGC, and d i sp l ay . These d a t a were analyzed and r epo r t ed i n pre l iminary form i n va r ious p u b l i c a t i o n s (von Huene, 1979). In t h i s p re l iminary work many important s t r u c t u r a l f e a t u r e s were masked by d i f f r a c t i o n s , and more d e t a i l e d r ep roces s ing inc lud ing migra t ion was r e q u i r e d before t h e s e f e a t u r e s could be p rope r ly reso lved . An example of t h e p rog re s s ive improvement i n in format ion from a s t acked r eco rd t o a migrated t ime r eco rd and u l t i m a t e l y a depth s e c t i o n a r e shown i n P l a t e 1.

Reprocessing began wi th a more d e t a i l e d a n a l y s i s of t h e s t a c k i n g v e l o c i t i e s . The impxoved v e l o c i t i e s were t hen used t o r e s t a c k t h e d a t a p r i o r t o migra t ion , u t i l i z i n g t h e Digicon D I S C O system a t t h e USGS i n Denver, Colorado. Wave equa t ion migra t ion was then app l i ed . Each r eco rd was migrated a t f o u r o r f i v e cons t an t v e l o c i t i e s ranging from 1450 m / s t o 1850 m/s . Th i s sequence of migrated r eco rds was t hen in spec t ed t o determine t h e b e s t migra t ion v e l o c i t y of each p o r t i o n of t h e record. Migrat ion v e l o c i t i e s d i f f e r e d markedly from s t a c k i n g v e l o c i t i e s i n a r e a s of complex s t r u c t u r e where large changes i n d i p occur over sma l l d i s t ances . Often t h e b e s t migra t ion v e l o c i t y f o r each small a r e a could n o t be a p p l i e d t o produce a s i n g l e r eco rd because i nve r s ion of i n t e r v a l v e l o c i t y va lues causes t h e migra t ion a lgor i thm t o f a i l . Thus, each f i n a l migrated r eco rd was a compromise and was t h e best r eco rd t h a t could be assembled i n one s e c t i o n u s i n g t h e Digicon sof tware , b u t l o c a l complex s t r u c t u r e s were c l e a r e r on some of t h e cons t an t v e l o c i t y d i sp l ays . Where s t r u c t u r e was dep ic t ed most c l e a r l y , w e made t r a c i n g s t h a t comprise t h e f i n a l composite l i n e drawing. Reduced t ime r eco rds from grids A and B a r e shown on F igures 3 and 4.

INTERPRETATION OF SEISMIC RECORDS

The se i smic r eco rds were i n t e r p r e t e d s imultaneously wi th t h e p roces s ing o f each record. The s t acked r eco rds were s t u d i e d t o i d e n t i f y a r e a s where t h e c l a r i f i c a t i o n of s t r u c t u r e and e l imina t ion of d i f f r a c t i o n s would extend t h e imaging and thereby enhance t h e in format ion f o r i n t e r p r e t a t i o n of geologic s t r u c t u r e . In t h i s manner, each change i n v e l o c i t y and s c a l i n g parameters was monitored wi th r e s p e c t t o t h e improvement of s p e c i f i c s t r u c t u r a l f e a t u r e s , a n d a s e r i e s of i n t e r p r e t a t i o n s were made on succes s ive ly improved records .

General ly , migra t ion c l a r i f i e d s t r u c t u r e s i n t h e f i r s t 2 or 3 seconds of

t h e r e c o r d b u t w a s n o t a s e f f e c t i v e i n deeper pa r t s o f t h e record . Fo lds a t t h e beg inn ing o f t h e subduc t ion zone were c l e a r l y imaged when masking d i f f r a c t i o n s were c o l l a p s e d . The m i g r a t i o n v e l o c i t y t h a t b e s t c o l l a p s e d d i f f r a c t i o n i n a r e a s o f f o l d i n g w a s commonly d i f f e r e n t from t h e v e l o c i t y o f t h e rock measured by s e i s m i c r e f r a c t i o n o r from v e l o c i t y measurements on DSDP d r i l l c o r e s . F a u l t s were i n t e r p r e t e d based on t r u n c a t i o n s , changes i n t h e d i p s o f r e f l e c t i o n s , and o c c a s i o n a l , sha l low f a u l t p l a n e r e f l e c t i o n s .

The igneous o c e a n i c c r u s t , or basement o f t h e s u b d u c t i n g p l a t e , i s d e f i n e d a c o u s t i c a l l y by h i g h ampl i tude r e f l e c t i o n s , low f requency s i g n a l r e t u r n s , and an i r r e g u l a r , d i f f r a c t i o n - produc ing s u r f a c e . These f e a t u r e s give t h e basement r e f l e c t i o n a d i s t i n c t i v e c h a r a c t e r . However, t h e basement i s g e n e r a l l y b u r i e d by more t h a n 2 h a n d as much a s 4 lan o f sediment b e f o r e e n t e r i n g t h e subduc t ion zone. Thus, down t h e subduc t ion zone a s t h e o v e r l y i n g sediment r a p i d l y t h i c k e n s , t h e basement r e f l e c t i o n becomes d i f f i c u l t t o f o l l o w e s p e c i a l l y where a c c o u s t i c d i s p e r s a l i n a complexly deformed o v e r l y i n g sediment sequence h a s s c a t t e r e d t h e r e f l e c t i v e energy. The maximum d e p t h s t o which we were a b l e t o image t h e basement a r e a b o u t 7 km, and g e n e r a l l y a r e a s o n a b l e r e c o r d c l a r i t y was o b t a i n a b l e a t 4-5 km d e p t h s ; however, r e l a t i v e s t r u c t u r a l s i m p l i c i t y h i g h l y i n f l u e n c e d d e p t h and c l a r i t y o f imaging.

A summary o f t h e main s t r u c t u r a l f e a t u r e s i n t h e r e c o r d s used f o r t h i s s t u d y i s s h m i n F i g u r e s 3 th rough 6. There a re d i f f e r e n c e s i n t h e r e f l e c t i o n s shown between t h e dep th and t i m e p r e s e n t a t i o n s because each h a s a d i f f e r e n t c h a r a c t e r even though t h e o r i g i n a l d a t a are t h e same. T h i s d i f f e r e n c e i s p r i m a r i l y caused by t h e compression o f s c a l e r e q u i r e d i n t h e upper , low-ve loc i ty p a r t o f t h e sediment sequence during t h e convers ion from t i m e t o d e p t h , and by t h e s t r e t c h i n g s e e n i n t h e lower , h i g h - v o l e c i t y sediment. I t was sometimes d i f f i c u l t t o t r a c e e x a c t l y t h e same r e f l e c t i o n s i n bo th t h e t ime and d e p t h p r e s e n t a t i o n s .

The s e i s m i c r e c o r d s of t h e e a s t e r n A l e u t i a n Trench show c o n s i d e r a b l e subduc t ion of sediment . The t h i c k n e s s o f sediment i n t h e t r e n c h i s from a l i t t l e more t h a n 2 ~III (von Huene, 1972) t o a b o u t 5 km ( l i n e 120, Fig . 3 ) p r io r t o e n t e r i n g t h e subduc t ion zone. The subduc t ion zone shows a v a r i e t y of s t r u c t u r a l s t y l e s , and even r e c o r d s o n l y 10-20 km a p a r t are s i g n i f i c a n t l y d i f f e r e n t . There fore , each g r i d of l i n e s i s d i s c u s s e d s e p a r a t e l y .

The sou thwes te rn most g r i d ( g r i d A , and F igs . 2 , 3 , and 4 ) d e p i c t s a p a r t o f t h e t r e n c h where sediment i s p r e s e n t l y from 2.5 t o 5 km t h i c k . About 1.5 k m o f t h e sediment i n t h e t r e n c h i s t u r b i d i t e and hemipe lag ic f i l l which i s 0.6 m i l l i o n y e a r s (m.y.1 o l d a t D S D P S i t e 180, j u s t a b o u t 200 km n o r t h e a s t i n t h e t r e n c h (von Huene and Kulm, 1973) . As t h e sediment s e c t i o n passes i n t o t h e t r e n c h , t h e f i r s t deformat ion i s marked by small p r o t o - r e v e r s e f a u l t s t h a t do n o t r e a c h t h e s u r f a c e ( F i g . 3 ) . Here, t h e s e c t i o n i s d i v i d e d i n t o 2 p a r t s ; t h e upper p a r t i s s c r a p e d o f f a t t h e de format ion f r o n t and a t t a c h e d t o t h e f r o n t o f t h e margin whereas t h e lower p a r t i s subducted. The d e p t h o f t h i s d i v i s i o n can v a r y , and t h e d i v i s i o n i s found b o t h above and below t h e b a s e o f t h e hemipe lag ic and t u r b i d i t e sediment f i l l i n g t h e t r e n c h a x i s . In r e c o r d 111 ( F i g s . 3 and 4 ) t h e subducted sediment i s 1.2 s e c (2 .0 km) t h i c k ; i n r e c o r d s 117 and 120 ( F i g . 3 ) t h e subduc ted sediment i s 2 .5 and 2.6 s e c t h i c k ; u s i n g t h e same v e l o c i t y a s i n r e c o r d 11 1, t h e t h i c k n e s s a l o n g r e c o r d s 117 a n d 120 i s 4.2 and 4.3 km, r e s p e c t i v e l y . An average t h i c k n e s s o f s u b d u c t i n g sediment a t t h e f r o n t of t h e margin i n t h i s grid i s 2 sec o r a b o u t 3 . 7 km. I n record 11 1 ( F i g s . 3 and 4 ) t h e subduc ted sediment t h i n s s l i g h t l y w i t h d i s t a n c e landward from t h e deformat ion f r o n t . Landward of t h e mid-slope area t h e s l o p e o f t h e t r e n c h s t e e p e n s , and t h e s t r u c t u r e i s mainly monoc l ina l (F ig . 4 ) . The mid- s l o p e a r e a marks a change i n tectonic regime where a t l e a s t the upper 6 km of

deformed sediment i s no l o n g e r b e i n g e x t e n s i v e l y t e c t o n i z e d by h o r i z o n t a l compression b u t i s r a t h e r b e i n g u p l i f t e d a s a r i g i d body. The change i n t e c t o n i c regime may mark a t r a n s i t i o n from l a r g e l y d u c t i l e deformat ion o f weak, o f f s c r a p p e d sediment w i t h e l e v a t e d p o r e - f l u i d p r e s s u r e a t t h e f r o n t of t h e margin t o a more rigid body b e i n g u n d e r p l a t e d by s u b d u c t i n g sediment (von Huene, 1979 and von Huene, e t a l . , 1983). Most p e r t i n e n t t o t h e t h e s u b j e c t o f t h i s r e p o r t i s t h a t w i t h i n t h e s o u t h e r n g r i d , which c o v e r s a 65 km s t r e t c h a l o n g t h e s l o p e of t h e t r e n c h , t h e t h i c k n e s s of subduc ted sediment v a r i e s between 2.0 and 4.3 km and a v e r a g e s 3 . 7 km. The subduc ted sediment h e r e i s mainly from a sequence of ocean b a s i n t u r b i d i t e s . A t DSDP s i t e 178, t h i s sediment sequence i s of Neogene a g e and c o n t a i n s massive hemipe laq ic mudstones and some s i l t and sand t u r b i d i t e s (vow Huene and Kulm, 1973).

The segment o f t h e t r e n c h surveyed i n g r i d B ( F i g s . 2 and 5 ) i s a l s o c h a r a c t e r i z e d by c o n s i d e r a b l e subduc t ion o f sediment ; however, because t h e s t r u c t u r a l s t y l e d i f f e r s from g r i d A , t h e n a t u r e o f t h e sediment b e i n g subducted may also d i f f e r . I n f a c t , it h a s been s u g g e s t e d t h a t c o n s i d e r a b l e t r e n c h f i l l and even s l o p e sediment may b e subduc ted a l o n g t h i s segment o f t h e t r e n c h (von Huene, 1979). Th i s p r e v i o u s s t r u c t u r a l i n t e r p r e t a t i o n h a s been modif ied, however, based on t h e more r e c e n t p r o c e s s i n g r e s u l t s . The f o u r r e c o r d s s t u d i e d from g r i d B (F ig . 5 ) show a v a r i a b l e s t r u c t u r a l s t y l e b u t each demons t ra tes subduc t ion o f a sediment s e c t i o n t h a t v a r i e s i n t h i c k n e s s from 1.2 t o 3.3 km a t t h e f r o n t o f t h e margin. I n r e c o r d s 71 and 72/62, t h e subduc t ion i n v o l v e s b o t h t h e t r e n c h f i l l and t h e u n d e r l y i n g ocean b a s i n sediment sequence; t h e zone o f sediment subduc t ion a p p e a r s t o b reak th rough t h e lower s l o p e and c o u l d i n v o l v e some s l o p e sediment . Landward o f t h e deformat ion f r o n t t h e maximum t h i c k n e s s on a s u b d u c t i n g sed iment s e c t i o n i s about 4 km i n r e c o r d 71. An a v e r a g e t h i c k n e s s o f a l l l i n e s i n t h e g r i d i s 2.75 km, and can v a r y from 1.2 km t o 4.0 km.

One r e c o r d a t l i n e C (Fig. 2 ) , e q u i v a l e n t t o r e c o r d 13 (F ig . 6), shows a v e r y t h i c k a c c r e t e d s e c t i o n , and no s u b d u c t i n g sediment i s obvious: however, t h e basement s u r f a c e w a s n o t imaged. Thus, we cannot de te rmine how much sediment i s subduc ted a l o n g r e c o r d 13, and whether r e c o r d 13 i s r e p r e s e n t a t i v e o f t h e a r e a .

I n c o n t r a s t t o l i n e C , nearby l i n e D, (F ig . 2 and 6 ) shows subduc t ion o f t h e complete sed iment s e c t i o n ( P l a f k e r e t a l . , 1982). Although t h e s e i s m i c r e c o r d does n o t show a wel l -developed decol lement , t h e age r e l a t i o n r e q u i r e d by t h e s t r a t i g r a p h y of a w e l l on t h e s h e l f compels such a n i n t e r p r e t a t i o n . On t h e s h e l f , t h e d r i l l h o l e o f f Middleton I s l a n d p e n e t r a t e d a sediment sequence of a t l e a s t l o w e s t Eocene age (Rau, e t . a l . , 1977, K e l l e r , e t a l . , i n press). The Oligocene/Miocene t i m e h o r i z o n can b e c l e a r l y fo l lowed from t h e d r i l l h o l e t o t h e b a s e o f t h e t r e n c h s l o p e ; t h e p o s i t i o n o f t h e base of t h e Eocene i s i n f e r r e d from i t s d e p t h i n t h e d r i l l ho le . From DSDP h o l e 180 t h e Quaternary s e c t i o n i s t r a c e d t o t h e b a s e o f t h e s l o p e and must p a s s benea th t h e f r o n t o f t h e margin ( P l a f k e r e t a l . , 1982). If t h i s r e c o r d i s r e p r e s e n t a t i v e o f a segment o f t h e t r e n c h , t h e r e i s a 2.7 km t h i c k sediment s e c t i o n b e i n g subducted which i n c l u d e s t h e uppermost t r e n c h sediment. T h i s s e i s m i c r e c o r d shows t h a t t h e subduc t ion of sediment may i n v o l v e t h e t o t a l s e c t i o n i n t h e t r e n c h a s h a s a l s o been demonstra ted i n t h e Middle America Trench o f f Guatemala (Aubouin e t a l . , 1984). The p r o x i m i t y o f l i n e D t o l i n e C ( F i g . 2 1 , which f a i l e d t o r e c o r d sed iment subduc t ion , emphasizes t h e v a r i a b i l i t y p o s s i b l e from t o t a l sediment subduc t ion t o t o t a l sediment a c c r e t i o n .

GEOCHEMICAL ANALYSES AND RESULTS

The o r g a n i c m a t t e r i n sediment a s s o c i a t e d w i t h t h e a r e a of t h e Aleu t ian Trench can p r o v i d e a g u i d e t o t h e o r g a n i c m a t t e r t h a t i s b e i n g subduc ted benea th t h e p r e s e n t convergent margin. F o r t y samples have been s t u d i e d i n o r d e r t o a s c e r t a i n t h e c o n t e n t and p r o p e r t i e s o f t h e o r g a n i c m a t t e r i n sediment p o s t u l a t e d t o b e i n v o l v e d i n one way o r t h e o t h e r i n t h e subduc t ion p rocess . T h i r t y f o u r o f t h e s e samples came from f i v e DSDP s i t e s (178, 180, 181, 183, and 1921, and s i x samples came from two USGS d redge s i t e s (2 and 4 ) . F i g u r e 2 shows t h e l o c a t i o n o f t h e s e sampl ing s i t e s r e l a t i v e t o t h e s e i s m i c g r i d s which w e r e d i s c u s s e d p r e v i o u s l y . The b a s i n a l sed iments a t s i t e s 178, 180, and 183 a r e b e l i e v e d t o b e e q u i v a l e n t t o sed iments c u r r e n t l y undegoing subduc t ion a t t h i s a c t i v e margin. Sediments sampled a t S i t e 181 a r e lower slope d e p o s i t s which may have been a c c r e t e d d u r i n g t h e subduct ion. S i t e 192 i s on a seamount where t h e sediment c o v e r may b e r e p r e s e n t a t i v e o f b a s i n a l sed iments i n v o l v e d i n s u b d u c t i o n a t t h e w e s t e r n end of t h e A l e u t i a n Trench. Dredge samples 2 and 4 came from o u t c r o p s on t h e upper s l o p e . These samples may b e l i t h i f i e d e q u i v a l e n t s of o l d e r sed iments undergo ing subduct ion.

DSDP samples. S e l e c t i o n of 34 samples from t h e DSDP r e p o s i t o r y was guided by t h e f o l l o w i n g c o n s i d e r a t i o n s : samples from DSDP Leg 1 8 were chosen from S i t e s 178, 180, and 181; samples from DSDP Leg 19 were chosen from S i t e s 183 and 192. These sites w e r e s e l e c t e d because t h e sed iments a t t h e s e s i t e s appear t o be e q u i v a l e n t t o sed iments c u r r e n t l y invo lved i n some stage of subduct ion. I n d i v i d u a l samples w e r e s e l e c t e d on t h e b a s i s of p o s i t i o n i n t h e core, o r g a n i c carbon c o n t e n t , and on a v a i l a b i l i t y , w i t h t h e samples c o n t a i n i n g t h e h i g h e s t amount o f o r g a n i c ca rbon b e i n g p r e f e r e n t i a l l y c o l l e c t e d . Approximately 40 c c o f sediment were removed for each sample. Weights o f samples ranged from 6 2 t o 114 g. Appendix 1 shows t h e l o c a t i o n w i t h i n e a c h DSDP c o r e where samples were recovered . The l i t h o l o y i e s and a g e s of t h e samples a re g iven i n Appendix 1 and on P l a t e 2.

USGS samples. Six dredge samples from U.S.G.S. c r u i s e S-79-WG were ana lyzed ( P l a t e 2) . Five samples , i n c l u d i n g t h x e e mudstone and two d o l o m i t e s were o b t a i n e d from s i t e 2, and one sample, a massive l i m e s t o n e was u s e d from s i t e 4. These upper s l o p e , o u t c r o p samples ranged i n a g e from middle Eocene t o e a r l y P l e i s t o n c e . The wate r d e p t h s a t sites 2 and 4 a r e 1 8 0 0 and 2200 m, r e s p e c t i v e l y .

Orsan ic carbon. For t h e 3 4 D S D P samples used i n t h i s s t u d y , a n e s t i m a t e of t h e o r g a n i c carbon c o n t e n t ( P l a t e 2 ) was a v a i l a b l e t h r o u g h c o m p i l a t i o n s by Bode (1973a,b) which c o n t a i n o r g a n i c ca rbon v a l u e s f o r e q u i v a l e n t c o r e samples. Our d e t e r m i n a t i o n s by high t e m p e r a t u r e o x i d a t i o n t e c h n i q u e s , u t i l i z i n g a LECO a n a l y z e r , a r e l i s t e d i n P l a t e 2 and are i n remarkable agreement w i t h t h e v a l u e s o b t a i n e d by Bode (1973a ,b ) . Th i s c l o s e agreement e s t a b l i s h e s a h i g h l e v e l o f c o n f i d e n c e i n t h e r e p o r t e d o r g a n i c ca rbon values.

T o t a l carbon. T o t a l ca rbon d e t e r m i n a t i o n s were made on 40 samples ( P l a t e 2 ) . The resul t s show t h a t a l l b u t t h r e e samples c o n t a i n o n l y s m a l l ( less t h a n 0.37%) c a r b o n a t e carbon. Three d redge samples c o n t a i n 6.3 t o 9.1% t o t a l carbon, and t h e s e samples are c l a s s i f i e d as dolomi te o r l imes tone .

P y r o l y s i s . A l l 40 samples were ana lyzed by a t e m p e r a t u r e programmed p y r o l y s i s t e c h n i q u e (Thermal E v o l u t i o n A n a l y s i s o r TEA) i n which t h e p r o d u c t s o f p y r o l y s i s a r e measured by a f lame i o n i z a t i o n d e t e c t o r (FID), and t h e t e m p e r a t u r e of maximum p y r o l y s i s y i e l d i s de te rmined (Claypool and Reed, 1976). Pyrograms of t h e 40 samples a r e shown i n F i g u r e 7. I n g e n e r a l t h e p a t t e r n s a re r a t h e r non-desc r ip t and l a c k d i s t i n c t i v e peaks. The p a t t e r n s s i g n a l t h a t t h e o r g a n i c m a t t e r i n many samples i s somewhat u n s t r u c t u r e d and

l i k e l y immature . This p y r o l y s i s method p rov ides in format ion on t o t a l hydrocarbon y i e l d (measured i n pe rcen t of o rgan ic ca rbon) , v o l a t i l e hydrocarbons (measured i n p a r t s p e r m i l l i o n of t h e t o t a l hydrocarbon y i e l d ) , and Tmax i n degrees C r ep re sen t ing t h e temperature a t which t h e maximum amount of o rgan ic matter i s thermal ly decomposed. "Live carbon" i s a measure of t h e conten t of "hydrocarbon-prone" o rgan ic mat te r and i s obta ined by d iv id ing t h e t o t a l hydrocarbon y i e l d by t h e amount of o rgan ic carbon. "Live carbon" i s considered t o be t h e carbon t h a t w i l l , upon thermal evo lu t ion , s t i l l y i e l d a d d i t i o n a l hydrocarbon products such a s methane gas.

Discussion. The r e s u l t s of our o rgan ic geochemical ana lyses a r e l i s t e d i n P l a t e 2. These r e s u l t s show t h a t a l l of t h e samples analyzed con ta in low amounts (less than 1%) of o rgan ic carbon ( O C ) . The t o t a l hydrocarbon y i e l d i s a l s o low and ranges between 0.03 and 0.10%. With t h e except ion of f o u r samples, t h e amounts of " l i v e carbon" are l e s s t han 25%. m o s e four samples wi th #'live carbon" exceeding 25% are samples wi th t h e l e a s t amount of o rgan ic carbon. The v o l a t i l e hydrocarbon con ten t i s always less t h a n 2 0 0 ppm. Tmax ranges from 398 t o 523OC, b u t t h e pyroqrams (F ig . 7 ) from which t h e s e d a t a a r e taken are so i n d i s t i n c t i v e t h a t t h e r e s u l t s a r e gene ra l l y cons idered u n r e l i a b l e f o r i n t e r p r e t i v e purposes. P r o f i l e s wi th depth o f our geochemical r e s u l t s f o r t h e f i v e DSDP s i t e s a r e shown i n Figure 8a-e. These p r o f i l e s emphasize t h e low amounts of t h e va r ious geochemical parameters . Organic carbon decreases i r r e g u l a r l y wi th depth a t s i t e 178, 181, a n d 192. S i g n i f i c a n t t r e n d s o f o t h e r parameters a r e n o t obvious. The h i g h e s t amount of o rgan ic carbon (0.9%) was found a t a 220 m subbottom depth a t s i t e 183. 'Phis amount of o rgan ic carbon i s i n t e r e s t i n g from t h e p o i n t of view of source-rock eva lua t ion , bu t t h e low t o t a l hydrocarbon y i e l d makes t h e amount of " l i v e carbon" very l o w (8%) t h u s dec reas ing t h e p o t e n t i a l of t h i s sediment f o r hydrocarbon genera t ion .

Table 1 summarizes our r e s u l t s . The low amounts of o rgan ic carbon, t o t a l hydrocarbon y i e l d , " l i v e carbon", and v o l a t i l e hydrocarbons suggest t h a t t h e sediments w e have sampled and analyzed a r e poor p o t e n t i a l source sediments f o r petroleum, both o i l and gas. The average amount o f o rgan ic carbon i s equal t o o r less than 0.6%, a va lue cons idered near t h e lower l i m i t f o r p o t e n t i a l sou rces of hydrocarbons (Hunt, 1979). The amount of " l i v e carbon", i . e . , t h e carbon a v a i l a b l e fo r f u t u r e hydrocarbon genera t ion , i s Less t han 3 0 % which i n d i c a t e s , accord ing to work by Magoon and Claypool (19811, t h a t t h e o rgan ic ma t t e r i s prone t o gas genera t ion r a t h e r t han o i l genera t ion .

Nine samples w e r e s e l e c t e d f o r d e t a i l e d examination of o rgan ic ma t t e r t ype (Table 2). These samples were chosen because t hey appeared t o be r i c h e s t i n o rgan ic carbon, and t h e r e was s u f f i c i e n t sample on which t o c a r r y o u t t h e ana lyses . These samples were sub jec t ed t o a s p e c i a l i z e d p y r o l y t i c t echnique c a l l e d Rock-Eva1 ( T i s s o t and Welte, 1 9 7 8 ) . In a d d i t i o n , ou r ana lyses c o n s i s t e d of v i t r i n i t e r e f l e c t a n c e measurements, v i s u a l kerogen ana lyses by both t r ansmi t t ed and i n c i d e n t l i g h t , and eva lua t ion of t h e thermal a l t e r a t i o n index (TAI).

Rock-Eva1 ana lyses provided an independent measurement o f o rgan ic carbon and Tmax. I n a l l c a se s t h e Rock-Eva1 o rgan ic carbon value i s l a r g e r t han t h e va lue ob ta ined by TEA ( P l a t e 2 ) . W a x i s s i g n i f i c a n t l y h igher i n t h e Rock- Eva1 a n a l y s i s of a l l DSDP samples, but Rock-Eva1 Tmax i s lower than Tmax from TEA f o r two dredge samples ( P l a t e 2 ) . The Rock-Eva1 measurements f o r t h e s e two parameters are cons idered l e s s r e l i a b l e than TEA measurements and t h e r e f o r e a r e n o t cons idered f u r t h e r . Rock-Eval ana lyses y i e l d hydrogen i n d i c e s ( H I ) and oxygen i n d i c e s (01) from which a van Krevelen diagram can be cons t ruc t ed (Table 2 ) . For t h e most p a r t a l l of t h e samples l i e a long t h e

Type I11 pathway of t h i s diagram (Fig. 9 ) . Type I11 organic matter i s general ly considered t o be of t e r r e s t r i a l o r i g i n and gas-prone (T i s so t and Welte, 1978).

V i t r i n i t e r e f l ec tance (Ro) values of primary v i t r i n i t e range from 0.36 t o 0.50% (Table 2 ) . Recycled v i t r i n i t e of g rea te r Ro values i s a l s o present i n a l l samples. Appendix 2 gives d e t a i l e d desc r ip t ions of v i t r i n i t e r e f l ec tance readings. TAX evaluat ions range from 1.5 t o 2.6 (Table 2) . Both v i t r i n i t e r e f l ec tance and T A I va lues i n d i c a t e t h a t t h e organic matter of these samples is immature with r e spec t t o o i l o r gas generation and shows s t rong ind ica t ions of reworking. Visual kerogen analyses (Table 2 ) i n d i c a t e t h a t , i n addi t ion t o v i t r i n i t e and recycled v i t r i n i t e , t h e samples a l s o contain e x i n i t e , i n e r t i n i t e , recycled s p o r i n i t e , and amorphous mater ia l . The d i s t r i b u t i o n of these kerogen macerals suggests t h a t most of t h e organic matter i s t e r r e s t r i a l i n o r ig in , and t h e pos i t ion of t h e s e samples on t h e diagram ind ica tes immaturity.

Taken together our geochemical analyses show t h e following: The amount of organic carbon i n t h e samples i s small and approaches t h e lower l i m i t as a p o t e n t i a l source of hydrocarbons. The presence of i n e r t i n i t e f u r t h e r reduces t h e hydrocarbon p o t e n t i a l of t h e s e samples. The organic matter i s mainly f r o m t e r r e s t i a l sources, i s gas prone, and i s immature with r e spec t t o gas generation. Upon thermal evolut ion t h e samples are expected t o generate only small amounts of gas.

ESTIMATION OF G E O T H E W GRADIENT

To determine t h e region where gas i s generated i n t h e subduction complex of t h e Aleutian Trench requ i res some knowledge of t h e geothermal regime. Information regarding t h i s regime i n t h e Aleutian Trench i s minimal, and d i r e c t readings of geothermal temperatures have been made a t only one place. A temperature l o g from a well d r i l l e d jus t of fshore of Middleton Is land on t h e edge of t h e she l f showed an average temperature gradient of 2B0C/km.

An innovative approach t o t h e determination of regional geothermal gradients has been described by Yamano e t a l . (1982) f o r a reas where gas hydrates a r e present and a r e manifest on seismic records as an anomalous bottom-simulating r e f l e c t o r o r BSR. mom t h e depth of t h e BSR, t h e geothermal gradients are est imated using t h e phase r e l a t i o n s of t h e gas hydrate system. This method has been successful ly appl ied t o da ta from t h e Nankai Trough offshore Japan, around Central America inc luding t h e Middle America Trench, and along t h e Blake Outer Ridge. Geothermal gradients i n t he Nankai Trough ranged from 41.5 t o 65.8OC/km, and along t h e Middle America Trench they ranged from 35.1 t o 28.2OC/km. Both of these a r e a s a r e convergent margins a s i s t h e Aleutian Trench. MacLeod (19821, using t h e same methods, est imated t h e geothermal gradient i n sediment of t h e Gulf of Alaska t o be 27.8OC/km.

B S B can be seen on some seismic records from t h e upper s lope of t h e Aleutian Trench (Fig. 10). Depths t o t h e BSR were obtained us ing a seismic ve loc i ty of 2.0 lun/sec. Five depths t o BSRs were ca lcu la ted a t f i v e pos i t ions with d i f f e r e n t water depths. This information was appl ied t o the phase diagram f o r gas hydrates given by Kvenvolden and McMenamin (1980) f o r five d i f f e r e n t bottom-water temperatures t o obta in es t imates of t h e geothermal gradient (Table 3 ) . In add i t ion , on Table 3 are geothermal g rad ien t s estimated from hydrate s t a b i l i t y curves given by Macleod (1982). There a re severa l reasons f o r t h e d i f fe rence i n gradients . Kvenvolden and McMenamin assume a pure methane - pure water , gas-hydrate system whereas Macleod uses a pure methane - Arctic seawater, gas-hydrate system. Also t h e latter system

assumes bottom-water temperatures that vary with depth o f water and are all less than I0C.

Our estimates for the geothermal gradient on the upper slope o f the Aleutian Trench area, based on the depth of the BSR, range from 28 to 36OC/km (Table 3) with a best estimate for bottom-water temperatures near 1 ° C o f about 30°C/km. This number agrees reasonably well with the geothermal gradient from the Middleton Island well of 28OC/km and with the estimate of MacLeod (1982) of 27.8OC/km for sediment of the Gulf of Alaska.

A geothermal gradient of about SO°C/km for the upper slope o f the Aleutian Trench is significantly lower than the average gradient of 513~C/km in the Nankai Trough but nearly the same as the average gradient of 32OC/km along the Middle America hrench and the 24°-320C/!un qradient for the Japan Trench (Langseth and Burch, 1980). All of these areas are convergent margins at the edge of the Pacific Plate (Fig. 1). The Nankai Trough is exceptional in that it is a convergent margin where back-arc crust of the Philippine Plate is being subducted, and thus its thermal structure is unusual. Because of the uncertainties in the use of gas hydrate BSRs to estimate geothermal gradients, the accuracy of the estimates is not high, but the values obtained are consistent with those measured by conventional means and sufficient for our purposes.

Temperature distribution across active margins is characteristically complex (cf Watanabe, et al., 1977). The uncertainties derive not only from the subduction of cool oceanic crust but also from the fluid flux and differential rates of tectonic transport as well. Most investigators model a zone of temperature reversal along the principal slip plane of a subduction zone. Our temperature information is so rudimentary that any attempts to account for such variations is ovexshadowed by the uncertainties of sparse observations. Thus, for our estimates of thermal structure, we simply assume a linear vertical gradient but are aware of the imprecision this assumption incorporates.

ASSESSMENT OF POTENTIAL FOR NATURAL GAS GENERATION

An assessment of the potential of the Aleutian Trench subduction zone for natural gas generation requires basic geologic information, some of which has been obtained for this report. We now have preliminary ideas about the amount and nature of carbon in some of the sediments involved in the subduction process. Our seismic surveys show the tectonic framework of the area from which we can estimate subducted sediment thicknesses and assume sediment volumes. Finally, we have estimated a geothermal qradient based on considerations of gas hydrates and a measurement in a well. We can now put this geochemical, geophysical, and geothermal information together to estimate the amount of natural gas generated by thermogenic processes. For our subduction model we have chosen to use average values of our data, and to extrapolate these averages for the eastern Aleutian Trench subduction complex.

Our geochemical studies show that the average organic carbon content of the samples analyzed is 0.44% and that an average of 15% of this carbon i s "live", that isr able to thermally react further to produce hydrocarbons. These average values were calculated from Table 2. Because the kerogen in these samples is Type 111, we assume an H/C ratio o f about 1. However, the W/C ratio of methane is 4; therefore, we assume that only about one fourth of this produced hydrocarbon can be methane gas, because of the deficiency of hydrogen in the kerogen. This assumption leads to maximum values of potential generation of natural gas. The actual amount of methane possible could be two

o r t h r e e o r d e r s o f magnitude lower. These average v a l u e s coupled w i t h t h e f o l l o w i n g assumptions l e d t o a c a l c u l a t e d p o t e n t i a l methane c o n t e n t o f t h e s e sediments of a b o u t 5.5 x lo8 c u b i c m e t e r s of methane p e r c u b i c k i l o m e t e r o f

3 3 sediment ( m /km ) : ( 1 ) 12 g of " l i v e carbon" e q u a l s 1 mole o f " l i v e carbon"; ( 2 ) 1 mole o f " l i v e carbon" e q u a l s 22.4 l i ters (1) of methane; and ( 3 ) t h e

8 3 3 d e n s i t y of sed iments i s 1.8 g/cc. A v a l u e of 5.5 x 10 m o f methane/km o f sediment r e p r e s e n t s t h e maximum amount o f methane d i s p e r e s e d i n t h e sediment . E s t i m a t e s o f t h e r a t i o o f d i s p e r s e d and d i s s o l v e d g a s t o r e s e r v o i r gas i n sed imenta ry b a s i n s i n g e n e r a l r a n g e from 10 - 200 t o 1 (Hunt, 1979) . Because we saw l i t t l e ev idence f o r r e s e r v o i r - t y p e sed iment , t h a t i s , sediment w i t h good p o r o s i t y and p e r m e a b i l i t y , d u r i n g o u r sampl ing (Appendix I ) , we s e l e c t e d t h e l a r g e r v a l u e o f t h i s ra t i .0 t o a p p l y i n our e s t i m a t e . Thus t h e amount o f r e s e r v o i r e d methane i n each km3 o f sediment i s e s t i m a t e d t o b e abou t

6 3 2.8 x 10 rn . Thicknesses o f sediment i n v o l v e d i n t h e subduc t ion p r o c e s s o f t h e

Aleu t ian Trench a r e a can be e s t i m a t e d from t h e seismic s e c t i o n s o f t h e two g r i d s and two l i n e s d e s c r i b e d p r e v i o u s l y .

Grid/I;ine Thickness (k) Commen t A 3.7 lower s e c t i o n o n l y

2.75 i n c l u d i n g t r e n c h and s l o p e ( ? sed iment 0 a c c r e t i o n a r y sediment o n l y 2.7 whole t r e n c h s e c t i o n

The a v e r a g e t h i c k n e s s of s u b d u c t i n g sediment i s a b o u t 3 km. If we c o n s i d e r a n e lement o f sediment measur ing 1 )an by 1 km o f a r e a l e x t e n t and 3 lan t h i c k , t h e

3 volume of t h i s e lement i s , o f c o u r s e , 3 km . If t h e r a t e o f s u b d u c t i o n i s about 60 km/m.y. ( F i g . 1 ) t h e n i n t h a t t ime p e r i o d , 6 0 of t h e s e sediment e lements would have been subduc ted e q u a l i n g a volume o f subduc ted sed iment o f 180 km3. The amount of r e s e r v o i r e d methane t o b e e x p e c t e d from t h i s volume o f

a 3 sediment i s t h e r e f o r e 180 x 2 . 8 x lo6 = 5.0 x 10 m . If t h i s number i s e x t r a p o l a t e d o v e r the l e n g t h o f t h e e a s t e r n A l e u t i a n Trench o f 600 km, t h e n t h e amount o f expec ted , t h e r m a l l y genera e d r e s e r v o i r e d methane per m i l l i o n

i n t h i s p r o v i n c e i s about 0.3 x l o f 2 A3 o r , i n 20 m i l l i o n y e a r s , abou t 6 year72 3 x 10 m . T h i s , q u m ~ e r i s a b o u t 9% o f t h e c u r r e n t wor ld r e s e r v e s o f n a t u r a l gas of 66.4 x 10 rn ( R i c e and Claypool, 1981) .

Other s t u d i e s s u p p o r t t h e i d e a t h a t t h e r m a l l y produced g a s e s a r e p r e s e n t i n sed iments o f t h e Aleu t ian Trench a r e a , b u t none o f t h e s e s t u d i e s p r o v i d e s q u a n t i t i v e assessments of t h e amount o f gas . Fox example, Claypool , e t a l . (1973) sugges ted t h a t a c t i v e the rmal g e n e r a t i o n o f g a s e s was t a k i n g p l a c e a t d e p t h s a s sha l low a s 250 rn i n t h i s a r e a . Some of t h e gas r e p r e s e n t s e a r l y the rmal g e n e r a t i o n a s opposed t o peak g e n e r a t i o n . On t h e b a s i s o f s p o r e - c o l o r a t i o n d a t a , Grayson and L a P l a n t e (1973) i n d i c a t e d t h a t t h e l e v e l of m a t u r a t i o n o f a sample from DSDP S i t e 181 from a subbottom d e p t h o f abou t 340 m was e q u i v a l e n t t o a n overburden d e p t h i n t h e Gulf Coast o f a b o u t 2,600 m. Dow (1978) c a l c u l a t e d an e q u i v a l e n t Ro = 0.3% f o r t h i s sample. Our measured R v a l u e s f o r s h a l l o w e r sed iments a t t h e same s i t e e q u a l 0.46 and 0.47% ( T a b l e 27. The c a l c u l a t e d and measured Ro v a l u e s i n d i c a t e t h a t t h e samples c u r r e n t l y a r e a t lower t e r m p e r a t u r e s t h a n r e q u i r e d f o r gas g e n e r a t i o n ; however, as we have shown, t h e s e samples may e v e n t u a l l y be i n c l u d e d i n t h e subduc t ion complex and be exposed t o t e m p e r a t u r e s a t which t h e o r g a n i c m a t t e r w i l l b e t r ans formed t o methane.

There a r e l a r g e u n c e r t a i n t i e s i n o u r e s t i m a t e o f 0.3 x 1012 m3 of p o t e n t i a l , r e s e r v o i r e d methane g e n e r a t e d each m i l l i o n y e a r s o f s u b d u c t i o n , Our c a l c u l a t e d v a l u e r e s u l t s f r o m e x t e n s i v e e x t r a p o l a t i o n s and assumes t h a t

t h e methane becomes r e s e r v o i r e d and i s n o t l o s t from t h e system e i t h e r downward i n t o t h e subduc t ion zone o r upward i n t o t h e atmosphere. Our model r e q u i r e s t h a t t h e g e n e r a t e d methane somehow m i g r a t e s from t h e s u b d u c t i n g sediments where it formed i n t o t h e o v e r l y i n g , non-subducted sed iments where it becomes t rapped . W e have no d i r e c t e v i d e n c e r e l a t i n g t o c o n d u i t s f o r gas migra t ion o r f o r t r a p s of s u i t a b l e s i z e f o r e x p l o r a t i o n . Also o u r e s t i m a t e s r e p r e s e n t a maximum and c o u l d e a s i l y b e two o r d e r s o f magnitude s m a l l e r i f , for example, o n l y one p e r c e n t o f t h e " l i v e carbon" i s t rans formed t h e r m a l l y i n t o methane. Our l a r g e number e s t i m a t e r e s u l t s n o t from t h e r i c h n e s s o f o r g a n i c matter i n t h e s e sed iments b u t r a t h e r from t h e enormous volumes o f sediment i n v o l v e d i n t h e subduc t ion p r o c e s s . I n f a c t , t h e amount o f o r g a n i c m a t t e r i s v e r y s m a l l and, by i t s e l f , would i n d i c a t e o n l y a poor p o t e n t i a l f o r s i g n i f i c a n t g a s g e n e r a t i o n .

Our c a l c u l a t i o n s have n o t t a k e n i n t o account b i o g e n i c a l l y produced methane which was p r e s e n t i n t h e modern sed iments a s s o c i a t e d w i t h t h e A l e u t i a n Trench. Kulm, von Huene e t a l . (1973) and m e a g e r , S c h o l l , et a l . (1973) r e f e r t o t h e gassy n a t u r e o f sediment c o r e s r e c o v e r e d d u r i n g DSDP Legs 18 and 19, a l though no q u a n t i t a t i v e measure o f t h e t o t a l amount o f g a s was made. Claypool e t a l . (1973) showed t h a t t h e carbon i s o t o p i c composi t ion o f methane from DSDP S i t e 180 (one o f t h e sample si tes u s e d i n t h i s r e p o r t ) ranged f r o m -72.6 t o -80.8 p e r m i l r e l a t i v e t o t h e PBD s t a n d a r d . Th is range o f v a l u e s i s w e l l w i t h i n t h e r a n g e d i a g n o s t i c o f b i o q e n i c g a s (Fuex, 1979). Also we have n o t c o n s i d e r e d t h e g a s volume consequences of the p r e s e n c e o f g a s h y d r a t e s observed i n some o f o u r s e i s m i c r e c o r d s . I n s p i t e o f t h e low measured c o n c e n t r a t i o n s o f o r g a n i c m a t t e r i n t h e sed iments o f t h e A l e u t i a n Trench a r e a , our s t u d y s u g g e s t s t h a t v e r y large c o n c e n t r a t i o n s a£ b o t h b i o g e n i c and thermogenic methane can be formed d u r i n g s e d i m e n t a t i o n and subsequen t subduct ion. A major unknown i s where t h e g a s i s now. We s t i l l l a c k s u f f i c i e n t d a t a on p r e s e r v a t i o n , m i g r a t i o n , and t r a p p i n g t o draw f i r m c o n c l u s i o n s a t t h i s t ime. Clues from onshore geology ( F i s h e r , 1 9 8 0 ) i n d i c a t e b o t h poor r e s e r v i o r and s o u r c e p o t e n t i a l a t l e a s t f o r t h e a r e a o f t h e Kodiak s h e l f .

If w e a c c e p t t h e i d e a t h a t s i g n i f i c a n t q u a n t i t i e s o f methane can b e t h e r m a l l y g e n e r a t e d i n t h e subduc t ion zone o f t h e A l e u t i a n Trench, we can determine, based on t h e geothermal g r a d i e n t , i n what r e g i o n o f t h e subduc t ion complex t h e g a s might b e p r e s e n t . F igure 11 shows t h e g e o l o g i c s e c t i o n f o r seismic l i n e 111 o f Gr id A. Superimposed on t h i s s e c t i o n a r e i s o t h e r m a l l i n e s f o r 50° , 150° , and 250°C based on o u r e s t i m a t e d , c o n s t a n t v e r t i c a l geothermal g r a d i e n t o f 30°C/km. The t h r e e i s o t h e r m a l t e m p e r a t u r e l i n e s r e p r e s e n t t h e t empera tu res f o r t h e g a s c e i l i n g , maximum g a s g e n e r a t i o n , and t h e g a s f l o o r , r e s p e c t i v e l y . M t h e s e t h r e e t e m p e r a t u r e s , t h e g a s f l o o r i s t h e most u n c e r t a i n and i s l i k e l y a h i g h e r t e m p e r a t u r e t h a n 250°C (Barker , 1982) . A 3 h - t h i c k s e c t i o n of marine sed iment i n v o l v e d i n s u b d u c t i o n would p a s s th rough t h e t empera tu re o f maximum g a s g e n e r a t i o n somewhere benea th upper s l o p e sediment. Thus, i f o u r i d e a s a r e c o r r e c t , t h e r e g i o n below upper s l o p e sediment would be most p r o s p e c t i v e f o r f u t u r e g a s e x p l o r a t i o n i f r e s e r v o i r s and t r a p s a r e p r e s e n t . T h i s i s t h e same r e g i o n t h a t was p r e d i c t e d by Thompson (19761, based on a t h r u s t - f a u l t - c o n t r o l l e d model, f o r p e t r o l e u m accumulat ion i n t h e A l e u t i a n Trench a r e a .

Bes ides t h e p o s s i b l e e x i s t e n c e o f g a s i n deep, upper s l o p e sed iments , gas may a l s o be p r e s e n t i n t h e deep b a s i n s a s s o c i a t e d w i t h t h e A l e u t i a n convergent margin. Except f o r t h e g r e a t d e p t h o f wa te r , t h e s e b a s i n s a r e s i m i l a r t o b a s i n s of t h e s h e l v e s and r e q u i r e c o n v e n t i o n a l s t r a t e g i e s f o r e v a l u a t i o n o f pet roleum p r o s p e c t s . Hedberg e t a l . ( 1 9 7 9 ) concluded t h a t deep ocean b a s i n s ,

i n c l u d i n g t h o s e a s s o c i a t e d w i t h convergent margins , are p o t e n t i a l sites f o r pet roleum accumulat ion. We have n o t c o n s i d e r e d these k i n d s o f b a s i n s i n o u r assessment . If i n t h e A l e u t i a n Trench a r e a b o t h t h e b a s i n s , a s sugges ted by Wedberg e t al. (1979), and t h e upper s l o p e sed iments , a s sugges ted by Thompson (1976) and o u r s e l v e s , a r e p r o s p e c t i v e for petroleum gas, t h e n t h i s v a s t a r e a may become a n impor tan t e x p l o r a t i o n t a r g e t i n t h e f u t u r e .

SUMMARY

This r e p o r t p r o v i d e s p r e l i m i n a r y data a d d r e s s e d t o t h e q u e s t i o n of the p o t e n t i a l o f subduc ted sediment w i t h i n t h e U e u t i a n Trench convergent margin t o g e n e r a t e s i g n i f i c a n t q u a n t i t i e s o f - n a t u r a l gas . Our results suggest that even though t h e c o n t e n t o f o r g a n i c ca rbon i n sed iments o f the ocean b a s i n i s low, a v e r a g i n g a b o u t 0.4%, t h e volume of sediment invo lved i n subduc t ion i s s o l a r g e , hav ing a n a v e r a g e t h i c k n e s s of 3 h, that, indeed , s i g n i f i c a n t q u a n t i t i e s o f g a s can b e t h e r m a l l y genera ted . W e c a l c u l a t e t h a t a s much a s 6 x 10 l 2 m3 of n a t u r a l g a s c o u l d have been g e n e r a t e d and s t o r e d d u r i n g t h e l a s t 20 m i l l i o n y e a r s o f subduc t ion . Th is number i s e q u a l to 9% o f t h e c u r r e n t l y known World r e s e r v e s of n a t u r a l gas . To t h i s number can a l s o be added a n a s y e t unknown, b u t p robab ly l a r g e q u a n t i t y , of b i o g e n i e a l l y d e r i v e d methane which i s p r e s e n t i n the sed iments even before they undergo subduction. I n a d d i t i o n , t h e r e i s a n unknown amount of n a t u r a l g a s i n b a s i n s a s s o c i a t e d w i t h t h i s convergen t margin. Our e s t i m a t e of p o t e n t i a l g a s g e n e r a t i o n d u r i n g subduc t ion h a s g r e a t u n c e r t a i n t i e s , but because o f t h e l a r g e volumes of sediment i n v o l v e d , a s i g n i f i c a n t amount o f g e n e r a t e d g a s i s p o s s i b l e even though t h e c o n t e n t o f o r g a n i c matter i n t h e sediment i s small. A l a r g e unknown i s where t h e g a s m i g r a t e s and i s c u r r e n t l y s t o r e d . Our model assumes t h a t t h e g a s i s n o t l o s t th rough subduc t ion o r th rough d i f f u s i o n t o t h e atmosphere. W e have n o d i r e c t e v i d e n c e f o r m i g r a t i o n pathways or f o r traps o f s u i t a b l e s i z e f o r e x p l o r a t i o n . Our r e s u l t s , based i n paxt on c o n s i d e r a t i o n of our e s t i m a t e d g e o t h e m l g r a d i e n t of 30°C/km, s u g g e s t t h a t , i f s i g n i f i c a n t q u a n t i t i e s of n a t u r a l g a s can migrate upward above t h e a r e a of maximum gas g e n e r a t i o n , t h e n t h i s g a s would l i k e l y be found i n sed iment o f t h e upper s l o p e o f t h e Aleutian Trench and n o t i n s h e l f or lower s l o p e d e p o s i t s .

ACKNOWLEDGEMENTS

Samples from DSDP Legs 18 and 19 were supplied by the Deep Sea Drilling Project through the assistance of the U.S. N a t i o n a l Science Foundation. Geo- chemical analyses were provided by GeoChem Research Laboratories of Houston, Texas, and by Clark Geological Services of Fremont, California. We greatly a p p r e c i a t e t h e he lp of John M i l l e r in processing the s e i s m i c data. This work was partially funded by the Morgantown Energy Technology Cente r under U.S. Geolog ica l Survey - Department of Energy Interagency Agreememt No. DE-A121-83MC20422.

REFERENCES

Aubouin, J., von Huene, R . , e t a l . , 1982, I n i t i a l Repor ts o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 67: Washington (U.S. Gov't. P r i n t i n g Office), 799

P. Aubouin, J., von Huene, R. , et a l . , 1983, Leg 84 of t h e Deep Sea D r i l l i n g

P r o j e c t , Subduction w i t h o u t a c c r e t i o n : Middle America Trench o f f Guatemala: Nature , V. 297, p. 458-460.

Barker , C. , 1982, Methane g e n e r a t i o n and s u r v i v a l i n t h e deep subsur face : 2 W i l l i a m , W . J . , ed . , Deep Source Gas Workshop Technica l P roceed ings , DOE/METC/82-50, UC-92a, p. 86-107.

Bode, G. W . , 1973a, Carbon and c a r b o n a t e a n a l y s e s , Leg 18: I n Kulm, L.D. , von Huene, R., e t a l . , I n i t i a l Reports o f t h e Deep Sea D r i l l i n g h - o j e c t , v. 18: Washington (U. S. Gov't. P r i n t i n g O f f i c e ) , p. 1069-1076

Bode, G.W., 1973b, Carbon - carbona te : I n Creager , J.S., S c h o l l , D.W., e t al., I n i t i a l Repor t s o f t h e Deep SZ D r i l l i n g P r o j e c t , v. 19: Washington ( U . S . Gov't. P r i n t i n g O f f i c e ) , p. 663-665.

Claypool, G.E., and Reed, P. R . , 1976, Thermal a n a l y s i s t e c h n i q u e f o r source- rock e v a l u a t i o n : q u a n t i t a t i v e e s t i m a t e o f o r g a n i c r i c h n e s s and e f f e c t s of l i t h o l o g i c v a r i a t i o n : Am. Assoc. Petroleum G e o l o g i s t s Bull., v. 60, p. 608-626.

Claypool, G.E., P r e s l e y , B.J., and Kaplan, I R . , 1973, Gas a n a l y s e s i n sediment samples from Legs 10 , 11, 13, 14, 15, 18, and 19: In Creager , J. S. , S c h o l l , D.W., et a l . , I n i t i a l Repor t s o f t h e Deep SZ D r i l l i n g P r o j e c t V. 19: Washington (U. S. Gov' t. P r i n t i n g Off i c e ) , p. 879-884.

Creager , J.S., S c h o l l , D.W., e t a l . , 1973, I n i t i a l R e p o r t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 19: Washington (U.S. Gov' t P r i n t i n g O f f i c e ) , 91 3 P

Delong, S.E., and P.J. Fox, 1977, G e o l o g i c a l Consequences o f r i d g e subduc t ion : - I n Talwani, M . , and P i t tman , W.C., e d s . , I s l a n d Arc, Deep Sea Trenches , and Back-Arc Basins : Maurice Ewing S e r i e s 1, Am. Geophysical Union, Washington, D.C., p. 221-228.

Dow, W.D., 1978, Petroleum s o u r c e beds on c o n t i n e n t a l s l o p e s and rises: Am. Assoc. Petroleum G e o l o g i s t s B u l l . , v. 62 , p. 1584-1606.

F i s h e r , M.A., 1980, Petroleum geology o f Kodiak s h e l f , Alaska: Am. Assoc. Petroleum G e o l o g i s t s B u l l . , v. 64, p. 1140-1157.

Fuex, N, 1977, The u s e of s t a b l e i s o t o p e s i n hydrocarbon e x p l o r a t i o n . J o u r . Geochemical E x p l o r a t i o n , v. 7 , p. 155-188.

Grayson, J., and U P l a n t e , R.E., 1973, Es t imated t e m p e r a t u r e history i n the lower p a r t o f h o l e 181 from c a r b o n i z a t i o n measurements: 2 Kulm, L.D., von Huene, R . , e t a l . , I n i t i a l Repor t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 18: Washington ( U . S . Gov't . P r i n t i n g O f f i c e ) , p. 1077.

W i l l i a m , W . J . , 1982, Subducted o r g a n i c o r i g i n gas h y p o t h e s i s f o r deep s o u r c e methane: 2 Gwilliam, W.J., ed . , Deep Source Gas Workshop Technica l P roceed ings , DOE/METC/82-50, UC-92a, p. 140-161.

Hedberg, H.D., Moody, J . D . , and Hedberg, R.M., 1979, Petroleum p r o s p e c t s o f t h e deep offshore: Am. Assoc. Petroleum G e o l o g i s t s B u l l . , v. 63, p. 286-300.

Hunt, J.M., 1979, Petroleum Geochemistry and Geology: San k-anc i sco ( W . H . Freeman), 617 p.

Hussong, D.M., and Uyeda, S. , 1981, T e c t o n i c s i n t h e Mariana Arc: r e s u l t s of r e c e n t s t u d i e s i n c l u d i n g DSDP Leg 60: Oceanologica Acta, No. SP of v. 4, p. 203-212.

Karig, D.E., Kagami, H . , e t a l . , i n p r e s s , I n i t i a l Repor t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 87: Washinqton ( U . S . Gov't . h r i n t i n g Office).

K e l l e r , G . , von Huene, R . , McDouyall, K . , and Bruns, T.R., i n p x e s s , Paleoc l i rna t i c ev idence f o r Cenozoic m i g r a t i o n o f Alaskan t e r r a n c e s : T e c t o n i c s .

Kulm, L. D. , von Huene, R. , e t a l . , 1973, I n i t i a l R e p o r t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 18: Washington (U.S. Gov't . P r i n t i n g O f f i c e ) , 1077

P . Kvenvolden, K.A. , and McMenamin, M.A., 1980, Hydra tes o f n a t u r a l gas : a

rev iew o f t h e i r g e o l o g i c occur rence : U. S. Geol. Survey C i r c u l a r 825, 1 1 P

Langseth , M . , and Burch, T., 1980, Geethermal o b s e r v a t i o n s o f t h e Japan Trench t r a n s e c t : 2 S c i e n t i f i c P a r t y , I n i t i a l Repor t s of t h e Deep Sea D r i l l i n g P r o j e c t , v. 56, 57, p t 2, Washinqton (U.S. Gov't . P r i n t i n g O f f i c e ) , p. 1207-1210.

Macleod, M.K., 1982, Gas h y d r a t e s i n ocean bot tom sediments : Am. Assoe. Petroleum G e o l o g i s t s B u l l . , v. 66, p. 2649-2662.

Magoon, L. B. , and Claypool , G. E., 1981, Petroleum geology o f Cook I n l e t Basin , Alaska - An e x p l o r a t i o n model: Am. Assoc. Petroleum G e o l o g i s t s B u l l . , V. 65, p. 355-374.

Moore, G.W., 1982 , P l a t e - t e c t o n i c map o f t h e Circum-Pacif ic r e g i o n -- e x p l a n a t o r y n o t e s : Am. Assoc. Petroleum G e o l o g i s t s , Tu l sa , OK., 14 p.

Moore, J . C . , B i j o u - Duval, B . , 1981, Near Barbados Ridge s c r a p i n g o f f subduc t ion s c r u t i n i z e d : Geotirnes, v. 26, n. 10, p. 24-26.

P l a f k e r , G. , Bruns, T. R., Winkler, G. R., and Tysdal , R.G. , 1982, Cross - sec t ion of t h e e a s t e r n A l e u t i a n a r c , from Mount Spurr t o t h e A l e u t i a n Trench n e a r Middleton I s l a n d , Alaska: Geolog ica l S o c i e t y o f America, map and c h a r t s e r i e s MC-28-P.

Rau, W.W., P l a f k e r , G . , a n d Winkler, G.R., 1977, P r e l i m i n a r y f o r a m i n i f e r a 1 b i o s t r a t i q r a p h y and c o r r e l a t i o n of s e l e c t e d s t r a t i g r a p h i c s e c t i o n s and w e l l s i n t h e Gulf o f Alaska T e r t i a r y Province: U.S. Geolog ica l Survey, Open-File Report 77-747, 54 p.

Rice, D.D. , and Claypool, G.W., 1981, Genera t ion , accumulat ion, and r e s o u r c e p o t e n t i a l , of b i o g e n i c gas: Am. Assoc. Petroleum G e o l o g i s t s Bul l . , v. 65, p. 5-25.

Rober t s , D.G., 1981, G e o l o g i c a l i s s u e s i n o f f s h o r e hydrocarbon e x p l o r a t i o n : I n t h e Fu ture o f Of fshore Petroleum, New York (McGraw H i l l ) , p. 27-79.

s c i e n t E i c P a r t y , 1980, I n i t i a l Repor t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 56 and 57: Washington ( U . S . Gov't . P r i n t i n g O f f i c e ) , 1417 p.

Thompson, T.L., 1976, P l a t e t e c t o n i c s i n o i l and g a s e x p l o r a t i o n o f c o n t i n e n t a l margins: Am* Assoc. Petroleum G e o l o g i s t s B u l l . , v. 6 0 , p . 1463-1 50 1.

T i s s o t , B., and Welte, D . , 1978, Petroleum Formation and Occurrence: N e w York ( Springer-Verlag) 538 p.

von Huene, R . , 1972, S t r u c t u r e o f t h e c o n t i n e n t a l margin and t e c t o n i s m a t t h e e a s t e r n A l e u t i a n Trench: Geol. Soc. Am. B u l l . , v. 83 , p. 3613-3626.

von Ruene, R., 1979, S t r u c t u r e of t h e o u t e r c o n t i n e n t a l margin o f f Kodiak I s l a n d , Alaska, from mul t i channe l s e i s m i c r e c o r d s ; fn Watkins, J . , and Montader t , L., eds. , Geolog ica l I n v e s t i g a t i o n s o f C o n t i n e n e t a l Margins: Am. Assoc. Petroleum G e o l o g i s t s , Memoir 29, p. 261-272.

von Huene, R., and Kulm, L.D., 1973, T e c t o n i c summary o f Leg 18: I n Kulm, L.D., von Huene, R . , e t a l . , I n i t i a l Repor ts of t h e Deep sea- i l l ing P r o j e c t , v. 18: Washinqton (U. S. Gov ' t P r i n t i n g O f f i c e ) , p. , 1069-1 076

von Huene, R., M i l l e r , J . , F i s h e r , M . , and Smith, G . , 1983, An e a s t e r n Aleu t ian Wench s e i s m i c r e c o r d : In B a l l y , A.W., ed . , Seismic Express ion o f S t r u c t u r a l Styles - A Picture a n d Work A t l a s : Am. Assoc. Petroleum G e o l o g i s t s , S t u d i e s i n Geology, n. 15, v. 3.

Watanabe, T . , Langseth , M.G., and Anderson, R.N. , 1977, Heat f low i n back-arc b a s i n s of the western P a c i f i c : 5 Talwani, M . , and P i t tman , W.C., eds., I s l a n d Arcs , Deep Sea Trenches , and Back-Arc Basins : Maurice Ewing Series 1, Am. Geophysical Union, Washington, D.C., p. 137-161.

Watkins, J . S . , Moore, J .C. , e t a l . , 1981, I n i t i a l Repor t s o f t h e Deep Sea D r i l l i n g P r o j e c t , v. 66: Washington ( U . S . Gov' t . P r i n t i n g O f f i c e ) , 864

P Yamano, M. Uyeda, S . , Aoki, Y. , and S h i p l e y , T.H., 1982, E s t i m a t e s o f h e a t

f l o w derived from gas h y d r a t e s : Geology, v. 10, p. 339-343.

Table 1 . Summary of Geochemical Regults by Sample Sites

Site Number of OC Hydrocarbon "Live Carbon" Volatile Hydrocarbons 'Ihlax Samples ( % ) Yield ( % I ( % 1 ( PW) ("C)

Table 2. Results of Analyses of Kerogen

Recycled Recycled Other Sample HI 01 Ro TAI Exinite Vitr inite Inertinite Vitr inite Sporinite Amorphous N o . I rngHc/gOC) (mgC02/gOC 1 ( % 1 ( % 1 ( % ) ( % 1 ( % ) ( % 1 c % 1

Table 3. Geothermal Gradients in *C/krn Determined from Base of Gas Hydrate Reflector ( B S R ) on Marine Seismic Records

Water Depth ( m l

Sediment Depth (m 1

Total Depth (m)

~ o t t o m water Temperatures (OC) Temperature Gradient from

1.0 1.5 2.0 2.5 3 0 Macleod (1982)

32.5 31.8 31.1 30.3 29.6 29 35.9 35.0 34.0 33.1 32.2 32

33.9 33.2 32.4 31.7 3 0 . 9 30 30.8 30.1 29.3 28.6 27.8 28 31.5 30.7 30.0 29.2 28.4 28

Figure 1. P l a t e t e c t o n i c map of t h e Circum-Pacific r eg ion showing major p l a t e boundaries inc lud inq the Aleu t i an Thrus t , t h e s i t e of t h e Aleut ian Trench subduction zone. Numbers and arrows i n d i c a t e r a t e s ( cen t ime te r s pe r yea r ) and d i r e c t i o n s of p l a t e movements. The Cocos P l a t e i s t h e s i t e of t h e Middle America Trench; t h e Japan Thrus t i s t h e s i t e of t h e Japan T r e n c l ~ . (Modified a£ t e r Moore, 1982. )

OSDP dralling snfps A USGS dredge slfrs

Figure 2 . Locations of seismic-survey grids A and B with positions of multichannel seismic lines in the q r i d s shown at expanded scale. Locations of sinsle seismic lines C and D are also shown. In addition, DSDP sarnplc sites 178, 180, 181, 183. and 192 (*I and USGS

d r c d q e sites 2 and 4 ( ~ 1 3 arc i nd i ca t c .d .

ALBATROSS UPPER SLOPE LOWER SLOPE ALEUTIAN TRENCH

15- 3rd thrust

v - 2nd thrust packet ez!iZ2%4&-~ ! lot thrust packet 1 1aI thruato -

Depth of ignaou8 cruml / Cr\'.\ /

Irom refrrcilon -C - Wedoe and folded -

/- aedlment

16

F i g u r e 4 . Depth section of se i smic l i n e 111 from g r i d A ( F i q . 2 ) (after von Hucnc, et al., 1983).

LlNE 7 0

LlNE 6 1 " - - . . . -. , --- ,." ~ . . , . .- , . . .- - - . .. .. -- - --

LINE 7 1 -. -- ..

DSDP 181

- 4 -

I - ; 6 -

i

LlNE 72/62

r-.

." 0 5 10 K ILOMETERS Tuma m c I + o n har 2 6 varlncsl slaggeralbon r l ocean lloor U lh. doplh l e c l l o n I S w11houI e n a p ~ e r a l t o n

Figure 5 . T i m e and depth sections of seismic l i n e s 61, 70, 71, and 72/62 from seismic grid B (Fig. 2 ) .

Figure 6. (Top) Time and depth sections of seismic line 13 which is t h e same as line C (F ig . 2 ) . (Bottom) Depth and t i m e sections of seismic line D (Fig. 2 ) from Plafker, et al. (1982) .

Figure 7. Pyrograms from TEA/FID analyses of 40 samples used in this report. The curves show the profile of the release of volatile material as temperature is increased during pyrolysis. Temperature increases to the right on the horizontal axis; amount of released volatiles increases upward on the vertical axis.

Hyrogen Index / Oxygen Index Diagram

Figure 9. van Krevclcn diagram showing the hydrogen index (HI) in mg H C / ~ OC and the oxygen index (01) in mg CO /g OC for nine selected samples (Table 21. R e s u l t s f a f l in the region of type 111 organic matter.

LINE 5 1

Figure 10. Example of a BSR (arrow) on seismic record 5 1 from g r i d B (Fig. 2).

ALBATROSS - BANK UPPER SLOPE LOWER SLOPE - -- ALEUTIAN TRENCH

C h a i r n ~ l -

2nd thrusl peckel 1 st thrust packel

5 .*.*Base ot mx ,lope deposals

GENERA TlOH tlssement

, ,GAS FLOOR r -----I--/ /

Oeplh of tgneous cru5t _- Wedge and folded km from reiracl lon - basement -

/ / _-- sed~ment

Figure 11. Isotherms a t t e m p e r a t u r e s of 5 0 ° C (gas c e i l i n g , i . e . , t h e lowest t e m ~ ~ e r a t u r e fo r gas q c n e r a t i o n ) , 1 5 0 ° C ( tcmfwratur t? of maximum qas g e n e r a t i o n ) , a n d 2 5 0 ° C (gas floor, i . o . , t h e h i q h e s t tern1,crature f o r q a s gene ra t i on ) superimr>osed

o n seismic l i n e 111 F i q , 4) bascd o n a c o n s t a n t geothermal g r a d i e n t of 30°C/km.

APPENDIX 1. Lithologic Logs of DSDP Cores Showing Samples (m) Used in This Report. Logs Were Reproduced from The Initial Reports of the D e e p Sea Drilling Pro jec t , Volumes 18 and 19.

r u m - - . /I

5 mod I I 1 -

I m

I ~ I A wet -%*to w t > r d nrgl

LATE PLElSfOCLWf - mOCOL HZ2123

SECT I(Y

SrEm

M d l m dark gray to o l t w black SlLTV U A Y . f l m to met. Vnlfom, flint M d l q . tin f r o c t m . mdsrat. 4 l r tor t lon .

Sltr lll Uol* Cwr 70 Comd l n t r h a l 117.0-lW.5 FOSSIL

M R K T L R

I I

- - - - - Eow -_-------.

b t c k r - = - - - - - - B B B U - - - -

Ltlll*#lC M X R I P T I O *

CRY blfitk ml m l s h block slLn u r v . 4-w .nd krd. h t a l n s fln IhlU tllt I m l ~ e d t h 70. dlps. Ms f t n f ra t turn wtmm.

11- l a y r i q and mttllq

I R 2-120 S l i d . 1-114 H I mr. 1 W I c lay in rtz. n p i l p .

10I *!La I chlor .

Sl lTV W. ol lue qray (5V 3 l Z J . wifk C U V inUrb.ds up t o J m thick

SlLTV wD (as

krrrwta l n r y bminp COW tatchr:

APPENDIX 2. V i t r i n i t e Reflectance of Nine Selected Samples

JOB NhHE: USGS SfiHPLE M F E R : KSG14/CGS8322.289 SOURCE: KVENVOLDEN

. DEPTH: U R D E W E D REFIDINGS

MEAN: 0.4 STANDARD DEV.: 0.11 NUMBER: 49 MODE: 0.3 RhNGE: 0.22-0.7 1 $ D a t a not considered i n statistical a n a l y s i s .

LKk- I mrsa I oaow I -- 3Loo-JQ +--.-------.-----..+ ----.-..----.-...

i I I t 1

I I t I I I I t I I I I + I I I

ill1 1 t

fill I '111:11! !

JOB NME: USGS SRHPLE NUBPER: KSG 17 iCGS8322.29 1 SOURCE: KVENVOLDEN DEPTH:

1 2 3 4 REFLECTFINCE <%Ro>

HECIN: 0-36 STANDARD DEV. : 0.08 NUMBER: 5 1 RUDE: 0.3

ORDERED REFIDINGS

0.25

0.28

0.31

0.32

0.35

0.38

0.4

0.42

0.44

0 . a

*1.27

Si. 35

*1.47

81-52

t1 .54

81.58

41.64

$1.72

$1.79

$2.06

RfiNGE: 0.21-0.57 * Data not considered i n statistical a n a l y s i s .

JOE N W : USGS SAWLE WHFER: KSG19/CGSB322.292 SWRCE: KVENVOtMN DEPTH:

0 1 2 3 4 5 REFLECTANCE < %Ro)

ORDERED READINGS

HECIN: 0.52 STCSNDClRD DEV.: 0.11 NUMBER: 70 MODE: 0.5 RANGE: 0.34-0.78 8 Data not considered i n statistical analys is .

VITRINITE REFLECT-NCE JOB NME: USGS S#bHPLE NUMBER: KSGZi/CGS8322.293 SOURCE: KVENVW-DEN DEPTH:

HECIN: 0.56 STANDfiRD DEV.: 0.14 NUMBER: 52 MODE: 0.5 RANGE: 0.3-0.88 I D a t a nut considered in statistical ana lys i s .

LiY .a g m E % 2 5 L I) p*

awl;;*. Z J U X L a b

OrrZ=l& o u o w 5 m l l l C I

i + t I I f I

I I + I I I I + I

I I + I I

liil I I

Ill+

ill1 11 l I

JOE NAHE: USGS SfiMPLE NUMBER: KSG39/CGSB322.2?6 SOURCE: KVENVOLDEN DEPTH:

0 t 2 3 4 5 REFLECT-NCE < %Rm)

ORDERED R E F C D Z N G S

HEAN: 0.38 SThNDARD DEV. : 0.09 NUMBER: 86 HODE: 0.3 RANGE: 0.23-0.64

'"1

m 8 D a t a n o t considered i n s t a t i s t i c a l a n a l y s i s .