Chen et al 2000

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B U LL E T INOF CANADIAN PETROLEUM GEOLOGYVOL. 48, NO. 2 JUNE, 2000), P. 150-163

Chara c t e r i z ing the spa t ia l d i s tr i bu t ion o f an und i scove red hy droca r bon r e sou rce : t he KegRive r Ree f p l ay, Wes te rn Canada Sed imen ta ry Bas in

ZHUOHENG CHEN , K IRK OSADETZ,HAIYU GAO , PETER HANNIGANAND CAMERON WATSONGeological Survey of Canada

3033- 33 Street NWCalgary AB

T2L 2A 7

ABSTRACT

Spec i f i c maps cons t ruc ted us ing in tegra ted exp lora t ion h i s to ry da ta , geo log ica l p lay charac te r i s t i c s and se i smp r o s p e c t i n g a c t i v it y a r e us e f u l f o r b o th p e t r o l e u m e x p l o r a t i o n a n d r e s o u r c e m a n a g e m e n t . G e o s c i e n c e d a t a c a n b e c ob ined w i th exp lora t ion resu l ts to p red ic t the poss ib le loca t ions and s izes o f und iscovere d poo ls in the Midd le D evonK e g R i v e r R e e f p l a y. T h e c o m b i n a t i o n o f a l l a v a il a b l e i n f o rm a t i o n g i v e s a b e t t e r e s t i m a t e o f p e t r o l e u m o c c u r r e n c e tany ind iv idua l da ta se t . A pr io r p robab i l i ty o f hydrocarb on occur renc e , e s t im ated us ing exp lora t ion resu l t s , can be upde d u s i n g k e y g e o l o g i c a l e l e m e n t s t h a t r e f l e c t f a c t o rs c o n t r o l li n g p o o l s l o c a t i o n s a n d s i z es . T h e a n a l y s i s o f g e o p h y ss u r v e y i n f o r m a t i o n a n d t h e c o n s i d e r a t io n o f e x h a u s t i o n o f u n d i s c o v e r e d p o t e n t i a l b y s u c c e s s f u l a n d u n s u c c e s s f u l e x pra t ion wel l s p rov ides a rea l i s t i c vo lumet r ic pa ramete r tha t i l lus t ra tes the space ava i l ab le fo r und iscovered poo ls . c o n s t r u c ti o n o f a f a v o r a b i l i t y m a p , u s i n g a c o m b i n a t i o n o f t h e p o s t e r i o r p r o b a b i li t y m a p o f h y d r o c a r b o n o c c u r r e n c e the vo lume t r ic va r iab le , p roduce s a r i sk -weigh ted re se rvo i r space tha t ou t l ines the poss ib le loca t ions and s izes o f unc o v e r e d p e t r o l e u m r e s o u r c e s i n a p l a y. T h e f a v o r a b i l i t y m a p , i t s e l f a us e f u l t o o l f o r b o th h y d r o c a r b o n e x p l o r a t i on ap e t r o l e u m r e s o u r c e m a n a g e m e n t , p r o v i d e s a m e a n i n g f u l m e t h o d f o r d e s c r i b i n g t he s p a t i a l d is t r ib u t i on o f u n d i s c o v ep o o l s a n d f i e ld s p r e d i c t e d u s i n g o t h e r s t o c h a s t i c r e s o u r c e a s s e s s m e n t m e t h o d s .

R~SUM~

Des ca r tes sp6c i f iques cons t ru i t es en u t i l i san t des donn4es in t4gr4es d 'h i s to i re d ' exp[0ra t ion , l e s ca rac t4 r i s t iquesc ib les d ' exp lora t ion g6o log ique e t l ' ac t iv i t4 de p rospec t ion s i smique son t u t i l e s pour l ' exp lora t ion p6 t ro l ih re e t l a gt ion de l a ressource . Les donn6es g6osc ien t i f iques peuven t 4 t re combin4es avec l es r6su l t a t s d ' exp lora t ion pour p rOdles loca l i sa t ions poss ib les e t l e s s i t e s de g i sements non-d4couver t s dans l a c ib le d ' exp lora t ion rdc i fa le de Keg River.c o m b i n a i s o n d e t o u t e l 'i n f o r m a t i o n d i s p o n i b le d o n n e u n e m e i l l e u r e 6 v a l u a t i on d e l ' o c c u r r e n c e d u p 4 t r o l e q u e t o u t j ede donn4es p r i s ind iv idue l lement . La p robab i l i td p rda lab le de l ' occur rence d 'hydrocarbures , e s t im6e en u t i l i san t r6su l t a ts d ' e xp lora t ion , peu t Ot re mis ~ jour en u t i l i san t des fac teurs g 6o log ique c l4s qu i r e f lh ten t l e s fac teurs con t r0l a l o c a l is a t i o n e t l a d i m e n s i o n d e s g i s e m e n t s . L e s a n a l y s e s d e l ' i n f o r m a t i o n d e s l e v 4 s g 4 o p h y s i q u e s e t l a c o n s i d 4 ra td e l ' 6 p u i s e m e n t d u p o t e n t i e l n o n - d 6 c o u v e r t p a r d e s p u i t s d ' e x p l o r a t i o n c o u r o n n 6 s d e s u c c h s o u n o n , f o u r n i s s e n t pa ramht re vo lum6t r ique r4a l i s t e qu i i l lus t re l ' e space d i spon ib le pour l e s g i sements non-d6couver t s . La cons t ruc t ionc a r t e s d e z o n e f a v o r a b l e s e n u t i l is a n t u n e c o m b i n a i s o n d e c a r t e s d e p r o b a b i l it 4 p o s t 4 ri e u r e d ' o c c u r r e n c e d ' h y d r o c a rr e s a v e c l a v a r i a b l e v o l u m 4 t r i q u e p r o d u i s e n t u n e s p a c e d e r 4 s e r v o i r p o n d 6r 4 p a r l e r i s qu e , q u i d r e s s e l ' 4 b a u c h e l o c a l is a t i o n s p o s s i b l e s e t d e s d i m e n s i o n s d e s r e s s o u r c e s e n p 6 t r o l e n o n - d 6 c o u v e r t e s d a n s u n e c i b l e d ' e x p l o r a t i o n .

ca r te de favorab i l i t4 , en e l l e -mOme un ou t i l u t i l e pour ~t l a fo i s l ' exp lora t ion e t l a ges t ion des ressourc es en hydroc arres , fourn i t une m6thode s ign i f i ca t ive pour d4cr i re l a d i s t r ibu t ion spa t i a le des g i sements e t champs non-d6couver t s son t p r6d i t en u t i l san t d ' au t res mdthodes s tochas t iques d '6va lua t ion des ressources .

Traduit par Lynn Gagnon

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C H A R A C T E R I Z I N G S PAT I A L D I S T R I B U T I O N O F U N D I S C O V E R E D H Y D R O C A R B O N S I N T H E W C S B 151

INTRODUCTION

K n o w l e d g e o f t h e s p a ti a l d i s tr i b u t i o n o f p e t r o l e u mresources i s essen t ia l fo r the deve lopment of corpora te andpubl ic po l ic ies for the i r e ffec t ive management , economicana lys i s, l and-use p lanning , and envi ronm enta l impac t assess -

ment . For the pe t ro leum indus t ry such knowledge i s v i ta l lyimpor tan t for r i sk avoidance and explora t ion por t fo l io man-agement . To address the dem ands f rom both indus t ry and gov-ernment , severa l modern methods and models for descr ib ingthe spa t ia l charac te r i s tics o f hydroca rbon resources hav e beenproposed . These approaches can be ca tegor ized in to two gen-era l g roups . One i s the gene t ic method , represen ted by W hi te ' sp lay mapping techniques (Whi te , 1988; Grant and Thompson,1996) . In th is approach , geo logica l fac tors cont ro l l ing hyd ro-c a r b o n a c c u m u l a t io n a r e m a p p e d a n d s u p e r i m p o s e d o n e o n t oanother to h igh l igh t a reas of h igh poten t ia l and low explo-ra t ion r i sk . The resu l t ing p lay summary map i s a geo logica lfavorab i l i ty map tha t represen ts an in tegra t ion of gene t ic geo-log ica l in format ion usefu l for explora t ion dec is ion-making .The secon d, s tochas t ic g roup of approaches uses po in t p rocessmode ls (Kaufm an and Lee , 1992) , geos ta t i s ti ca l methods (Pan ,1997) and mul t ivar ia te c lass i f ica t ion methods (Harffe t a l . ,1992) . These methods re ly pr imar i ly on the ou tcomes ofexplora t ion dr i l l ing ( see examples f rom Kaufman and Lee ,1992; Pan , 1997) . The resu l t s o f s tochas t ic methods a re d i s -p layed e i ther as a hydrocarb on occur ren ce probabi l ity map, o ras an explora t ion uncer ta in ty map. Both a re pro jec t ions o f spa-t ia l hydrocarbon occur rence pa t te rns based on explora t ionresults.

At leas t four types of da ta car ry in form at ion regard ing the

spa t ia l charac te r i s t ics o f hydrocarbon occur rence in a maturep lay. These inc lude 1) geo logica l da ta ; 2 ) ex plora t ion dr i l lingresu l t s ; 3 ) geophys ica l da ta ; and 4) loca t ion and da ta qua l i tyinformat ion regard ing geosc ience surveys . Each da ta se t con-ta ins un ique in format ion .

Geolo gica l in format ion i s gene t ic in charac te r. In pr inc ip le ,i t a l lows the in fe rence of pe t ro leum occu r rence spa t ial charac-te r is t ics . The a reas of g rea tes t in te res t occu r where kn owled geof pe t ro leum accum ula t ion and geologica l in format ion a reimperfec t and incomple te . In such a reas , one cannot make ade te rmin is t ic p red ic t ion of hydrocarb on occur rence . Av ai lab legeologica l in format ion provides the necessary condi t ions forhydrocarbon occur rence , bu t necessary condi t ions a t a g iven

loca t ion do not guaran tee the ex is tence of a comm erc ia l pe t ro-leum accu mula t ion . Other geologica l in format ion , such as thesecond ary migra t ion pa thwa y to , and the dra inage reg ion of , atrap, are not general ly available prior to dri l l ing. Such infor-m a t i o n i s c o m m o n l y s u b j e c t i v e ly i n f e r re d f r o m j u d i c i o u sexplora t ion exper ience .

Explora t ion dr i l ling i s se lec t ive . As a resu l t , the in forma -t ion per ta in ing to d i scovered pe t ro leu m pools , the i r s izes andspatial locat ions, is a biased sample of the spatial distr ibutionof pe t ro leum accumu la t ions . T he s ta t i s ti ca l b ias o f the d i scov-ered pool s ize sample compared to i t s na tura l popula t ion i s a

w e l l- k n o w n p h e n o m e n o n( e . g . K a u f m a ne t a l . , 1975; Lee andWan g,1985) . T he spa t ia l b ias of such a sample i s ind ica ted byse lec t ive explora t ion dr i l l ing based on fac tors tha t inc ludegeologica l favorab i l i ty and access ib i l i ty. Geologica l se t t ingand processes of hydrocarbon genera t ion , migra t ion , andent rapme nt wi th in a pe t ro leum sys tem co nt ro l the spa t ia l d i st r ibu t ion o f the pe t ro leum resou rce . In genera l , the cons iderat ion of addi t iona l geo log ica l in format ion improve s the in fe r redescr ip t ion of pe t ro leum resource spa t ia l var ia t ion . To red ucthe impact o f observa t iona l b ias on p lay concepts and geo logica l models , one m ay in tegra te gene t ic geologica l in format ionf rom explora t ion resu lt s , in to the exp lora t ion dec is ion-makinprocess . For examp le , a geo logica l model o f reservoi r fac ieprovides a usefu l cons t ra in t a ffec t ing the in fe rence of reservoth ickness be twee n wel l s .

Reflect ion seismic surveying is an established oi l and gasexplora t ion tool; the primary purpo se of which is the identif icat ion and descr ip t ion of v iab le prospec ts for eva lua t ion and

dri l l ing. Features such as acoust ic wave travel t ime, ampli tudeve loc i ty, and f requency /phase a l l conta in in format ion per t inento the spa t ia l charac te r iza t ion of subsur face hydrocarbonresources . Se ismic in format ion a l lows the in fe rence o f reservoparameters , inc lud ing poros i ty (Doy ene t a l . , 1996) and thick-ness (Wolfe t a l . , 1994) from seismic ampli tude, as well as thepossibi l i ty for indirect inference of natural gas (Houck, 1999)Seismic data provides a spat ial coverage and deep penetrat ionot ach ieved by the d i rec t and def in i t ive techniques assoc ia tewith the dri l l ing, evaluating and complet ing of wells .

The probabi l i ty of de tec t ing a geologica l anomaly as afunc t ion of i t s s ize , shape , and or ien ta t ion , as wel l as thegeom et ry of the se i smic surve y gr id has been s tud ied s ince th

1950s ( e . g . K a u f m a n , 1 9 9 4 ). T h e c o m p u t a t i o n o f a n o m a l yde tec t ion probabi l i ty as a fu nc t ion o f explora t ion dr i l l ing pa tte rn and h is tory a l so has been s tud ied( e . g . Drew, 1967; S ingerand Drew, 1976) . Both types of s tud ies we re dr iven pr imar i lby the need to op t imize search s t ra teg ies . Such an op t imiza t ioseeks to ba lan ce the increased cos t s assoc ia ted wi th h igheda ta dens i ty aga ins t the va lue of improving the probabi l i ty oprospec t iden t i f ica t ion . Informat ion regard ing the loca t ion o

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L o n g i t u d e

F i g . 2 . P l a y b o u n d a r y a n d e x p l o r a t i o n w e l l l o c a t i o n s a n d i n it ia l s t a t u s ( x : d r y w e l l ; + : g a s w e l l ; o : o il w e l l; * : o i l a n d g a s w e l l ) f o r t h eD e v o n i a n K e g R i v e r R e e f p la y u s e d i n t h is s t ud y L o c a t i o n s a r e d e s c r i b e d u s i ng l a t it u d e a n d l o n g i tu d e . S u b s e q u e n t m a p s d i s p l a y th e r e g i o n u s i ng t h e U T M c o o r d i n a t e s y s t e m w i t h a c e n t r a l m e r i d i a n o f 1 1 9 . T h e U T M c o o r d i n a t e s y s t e m i s m o r e c o n v e n i e n t f o r c o m p u tp u r p o s e s .

g e o p h y s i c a l a n d g e o l o g i c a l o b s e r v a t i o n s r e p r e se n t s t h e e x p l o -ra t ion sampl ing scheme and i t s spa t i a l cha rac te r i s t i c s . Wheni n f e r ri n g t h e s p a t i a l c h a r a c te r i s ti c s o f h y d r o c a r b o n a c c u m u l a -t i o ns u s i n g o n l y d i s c o v e r e d a c c u m u l a t i o n s , t h e a v a i l a b i li t y o fg e o s c i e n c e s u r v e y i n f o r m a t i o n a l l o w s f o r a n a n a l y s i s o f o b s e r -va t ions . Wi th su ch da ta , i t i s poss ib le to use l ine - sea rch s t r a te -g i e s a n d e x p l o r a t i o n w e l l s e a r c h m e t h o d s t o e s t i m a t e t h er e m a i n i n g u n d i s c o v e r e d h y d r o c a r b o n r e s o u r c e .

T h e o b j e c t i v e o f t h is p a p e r i s t o d e m o n s t r a t e t h e u s e f u l n e s sof in tegra t ing ava i l ab le in fo rm at ion in to an imp rove d spa t i a l

d e s c r i p ti o n o f t h e u n d i s c o v e r e d r e s o u r c e d i s t r ib u t io n . T h e p r o -pos ed m etho d i s i llus t r a ted b y the ana lys i s o f the spa t i a l va r i-a t i o n o f g e o l o g i c a l f a c t o r s a n d d i s c o v e r y p a t t er n s i n t h eM i d d l e D e v o n i a n K e g R i v e r R e e f p la y. T h e r e s u l t is a p r e -d i c t io n o f t h e s p a ti a l p a t te r n o f u n d i s c o v e r e d p e t r o l e u mr e s o u r c e s i n t h e e x a m p l e p l a y.

METHOD

T h e m e t h o d f o r c h a r a c t e r i z i n g t h e s p a t i al p a t t e r n o f u n d i s -c o v e r e d p e t r o l e u m r e s o u r c e s i n t h e M i d d l e D e v o n i a n K e gR i v e r R e e f p l a y c o n s i s t s o f t h re e s t ep s :

A . T h e e s t i m a t i o n o f t h e p r o b a b i li t y o f h y d r o c a r b o n a c cm u l a t i o n u s i n g g e o l o g i c a l a n d e x p l o r a t i o n i n f o r m a t i o

B . T h e e s t i m a t i o n o f t h e s p a c e r e m a i n i n g f o r u n d i s c o v e rp e t r o l e u m r e s o u r c e s.

C . T h e e s t i m a t i o n o f p o s s i b l e l o c a t i o n s a n d s i z es o f u n dc o v e r e d p e t r o l e u m p o o l s .

INTEGRATION OF GEOLOGICAL INFORMATION WITH

EXPLORATION OUTCOMES

To d e t e r m i n e t h e s p at i al p a t te r n o f u n d i s c o v e r e d p e t r o l e u

r e s o u r c e s i n a p l a y, w e n e e d t o k n o w t h e m o s t l i k e l y l o c a t iof o r h y d r o c a r b o n a c c u m u l a t i o n s , a s w e l l a s i ts p o s s i b l e ps ize. Sup pose tha t the a rea o f in te res t can be d iv ided in to tc a t e g o r ie s , A a n d 7 k; t h e r e g i o n s o c c u p i e d b y h y d r o c a r bp o o l s v e r s u s t h e b a r r e n r e g i o n s , r e s p e c t iv e l y. We u s e ind ica to r I to desc r ibe the c lass i f i ca t ion . I f the a rea undi n v e s t i g a t i o n b e l o n g s t o A , I = 1 ; o t h e r w i s e I = 0 . S u p p o s e tw e h a v e p r i o r i n f o r m a t i o n o n t h e c l a s s i fi c a t io n . F o r e x a m pa s s u m e t h a t w e k n o w t h e s u c c e s s r a t i o f r o m e x p l o r a t id r i l l ing and add i t iona l geo log ica l in fo rmat ion pe r t inen t to c l a s si f i ca t i on . W e c a n u s e t h e B a y e s i a n t h e o r e m t o u p d a t e

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F i g . 3 . Tw o s e i s m i c s u r v e y m a p s i ll u st ra t in g g e o p h y s i c a l p r o s p e c t i n g e f f o r ts i n th e M i d d le D e v o n i a n K e g R i v e r r e e f p la y o v e r a n a r e a o f ai m a t e l y 1 2 , 0 0 0 k m 2 , p r i o r t o 1 9 7 5 ( F ig . 3 a ) , a n d b e t w e e n 1 9 7 5 - 1 9 8 6 ( F ig . 3 b ) .

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p r i o r i n fo r m a t i o n t o r e f i n e o u r s t a te o f k n o w l e d g e ( H a r b a u g het al., 1 9 9 5 ; D o y e n et al . , 1996; Gao et al . , 1997; Houck ,1999). Th is i s expre ssed in t e rms o f a cond i t iona l p robab i l i ty.The condi t iona l p robab i l i ty tha t the a rea be longs to A, g ivengeo log ica l ev idence , can be wr i t t en as :

p o p [ o b s e r v a t i o n a l e v i d e n c e I =1]P P = p [ o b s e r v a t i o n a l e v i d e n c e ]

W h e r e Po i s t h e p r i o r p r o b a b i l i t y o f h y d r o c a r b o n o c c u r -rence , a s ind ica ted by pas t exp lora t ion resu l t s ,p[observat ion-a l evidence]i s the p robab i l i ty tha t geo log ic a l ev idenc e occurs ,a n d p[observa t ional evidence I I=1]i s the cond i t iona l p roba-b i l i ty tha t the g iven a rea be longs to ca tegory A ( I= l ) and tha tthe geo log ica l ev idence occurs . Th is cond i t iona l p robab i l i typpi s a l so ca l l ed the pos te r io r p robab i l i ty, because i t i s the p r io rprobab i l i ty upda ted by add i t iona l in format ion .

SPATIAL PATTERN OF REMAININGAREAS AVAILABLEFOR

HYDROCARBON ACCUMULATIONS i n c e u n d i s c o v e r e d p e t r o l e u m a c c u m u l a t i o n s o c c u p y

space , i t is poss ib le to charac te r ize the i r spa t i a l d i s t r ibu t ion byi d e n t if y i n g t h e u n t e s te d p l a y v o l u m e a n d e x c l u d i n g f r o m t h a t

v o l u m e b o t h t h o s e r e g i o n s t h a t g e o l o g i c a l i n f o r m a t i o n i n d ia re unfavorab le and those reg ions where exp lora t ion ac t ivh a v e e x h a u s t e d t h e p o t e n t i a l f o r u n d i s c o v e r e d r e s o u r c e ss i m p l e m e t h o d f o r d e t e r m i n i n g t h e u n t e s t e d f a v o r a b l e a r e at o e x c l u d e a l l p r e v i o u s l y t e s t e d a r e a s . T h e e x c l u d e d r e g iinc lude d i scovered poo ls , d ry wel l loca t ions and the i r ine n c e r a n g e , a s w e l l a s r e g i o n s s u r v e y e d b y s e i s m i c p r o s pi n g. G e o l o g i c a l f e a t u r e s n e c e s s a r y f o r h y d r o c a r b o n a c c u mt ion a re a l so iden t i f i ed wi th in the un tes ted a rea , fu r the r e lna t ing reg ions f rom cons idera t ion .

I f w e k n o w b o t h t h e g e o m e t r y o f s e i s m i c s h o t li n e s, a n dcharac te r i s t i c s i zes , shapes and loca t ions o f iden t i f i ed geopi c a l a n o m a l ie s , t h e n w e c a n e s t i m a t e t h e t o t a l n u m b e r o f gphys ica l an oma l ies and the i r d is t r ibu tion w i th in the inves t ied a rea us ing a p robab i l i s t i c mode l (Kaufman , 1994) . Suppt h a t w e h a v e c o n d u c t e d a r e c t a n g u l a r s e i s m i c s u r v e y c o n s i sof four se i smic l ines wi th A and B as the d i s tances be twee np a i r s o f p a ra l l e l li n es . We a p p r o x i m a t e t h e m a p b o u n d a r y po ten t ia l geophys ica l p rospe c t by curve C , us ing an e l l ipse major ax i s a and minor ax i s b (F ig . 1 ) . C has an o r ien ta tdesc r ibed by an ang le , % be tween a f ixed line , L , d rawn th rothe in te r io r o f the anom aly and the nor th ings /l a t i tudes , o r x -aof the map (F ig . 1 ) . Le t Lx andL y denote the p ro jec t ions o

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2 3

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

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i - . * * i

0 00 2 3 0 1 2 3

D i s t a n c e 4 D i s t a n c e 4x 1 0 x 1 0

Fig . 4 . S e m i - v a r i o g r a m o f t h e t h ic k n e s s o f t h e u p p e r K e g R i v e r F o r m a t i o n a s a f u n c t i o n o f f o u r s e l e c t e d a z i m u t h s . T h e h o r i z o n t a l r e s e n t s t h e d i s t a n c e b e t w e e n w e l ls . T h e v e r t i c a l a x i s i s t h e a v e r a g e s q u a r e d d i f f e r e n c e o f t h e t h i c kn e s s b e t w e e n a l l d a t a p a i r s a g i vt a n c e a p a r t .

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x l o 4

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T h i c k n e s s , K e g R i v e r a x 1 0 4

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T h i c k n e s s , M u s k e g C

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F i g . 5 . a . I s o p a c h o f K e g R i v e r F o r m a t i o n ( u n i t : m e t r e s ) , b . K r i g i n g s t a n d a r di n g t h e u n c e rt a i n t y i n t h i c k n e s s i n t e r p o l a t i o n , c . I s o p a c h o f M u s k e g F o r m a t i o nt h i c k n e s s ( un i t : m e t re s ) i n d i c a t i n g t h e u n c e r t a i n t y i n t h i c k n e s s i n t e r p o l a t i o n .

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F i g . 6 . a . P r i n ci p a l C o m p o n e n t I r e p re s e n t i n g g e o l o g i c a l i n f o r m a t i o n m a i n ly i n d i c at i v e o f f o r m a t i o n t h i c k n e s s v a ri a t i o n s , b . P r i n c i p aI II , c o n t a i n i n g t h e m o s t i n f o r m a t i o n t h a t s e p a r a t e s d i s c o v e r i e s a n d d r y w el ls . T h i s i s r e g a r d e d a s a n i n d e x o f g e o l o g i c a l f a v o r ab i li t y, c.g a s o c c u r r e n ce e s t i m a t e d f r o m e x p l o r a t i o n w el ls . T h i s i s r e g a r d ed a s t h e p r i o r p r o b a bi l i t y, d . P o s t e r i o r p r o b a b i l i t y o f t h e o c c u r r e n c e u p d a t e d b y g e o l o g i c a l f a v o r a bi l i t y.

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156 Z. CHEN K. OSADETZ H. GAO P. HANN IGAN and C. WATS ON

anomaly C on to nor th ings and eas t ings (x and y -ax i s ) r espec-t ive ly, o f the map . I f any se i sm ic l ine c rosses the anom aly weassum e i t is iden ti f i ed. Th e de tec t ion p robab i l i ty o f an anom alyo f b o u n d a r y c u r v e C , o r i en t e d % g i v e n t h e s e i s m i c c o n f i g u r a -t ion # (A, B), can be wri t ten as (Kaufman, 1994):

p(d (A,B),g)= 1 ; [A-~()][B-LY(AB0 ] i f A > a B > bo t h e r w i s e

T h e H o r v i t z - T h o m p s o n e s ti m a t o r, a m o m e n t e s t im a t o r, c a nb e u s e d t o e s t i m a t e t h e p o p u l a t io n p r o p e r t i e s o f t h e g e o p h y s i -ca l anomal ies (K aufm an , 1994; Chen and Fu , 1996) . Inreg ions o f 3D se i smic surveys , i t i s a ssum ed tha t a l l geo-p h y s i c a l a n o m a l i e s w i t h d i m e n s i o n s l a rg e r t h a n t h e r e s o l ut i o nof the s u rvey a re iden t i f i ed .

By exc lud ing a l so those a reas t e s ted p rev ious ly by wel l s ,

t h e g e o m e t r y o f d r y p r o s p e c t s c a n b e u s e d d i r e c tl y i n t h e e s t i-m a t i o n o f t h e r e m a i n i n g a v a i l a b l e s p a c e f o r u n d i s c o v e r e dr e s o u r c e s . A s w e d o n o t h a v e i n f o r m a t i o n r e g a r d i n g t h e s i z ea n d s h a p e o f p r e v i o u s l y d r il l e d p r o s p e c ts , a n e x i s t i n g m o d e l o fexp lora to ry wel l exhaus t ion i s used (Drew, 1967) , a ssuming acons tan t exhaus t ion rad ius .

ESTIMATING POSSIBLE LOCATIONS AND SIZES OFUNDISCOVERED RESOURCES

T h e p o s s i b l e l o c a t i o n s a n d a s s o c i a t e d s i z e s o f u n d i s c o v e r e dr e s o u r c e s a r e e x p r e s s e d i n t e r m s o f a p o r e v o l u m e w i t h i n aspec i f i c ce l l . The quan t i ty o f hydroc arbon s in th i s spec i f i c ce l lcan be wr i t t en as :

V(x) = p(x)w(X)

w h e r e x r e p r e s e n t s t h e c e n t e r o f a c e l l ;p ( x ) i s t he h y d r o c a r -b o n o c c u r r e n c e p r o b a b i l i t y w i t h i n t h e c e l l, a n dw ( x ) i s a vo l -u m e t r i c v a r i a b l e r e p r e s e n t i n g t h e p o r e v o l u m e a v a i l a b l e f o rh y d r o c a r b o n a c c u m u l a t i o n w i t h i n t h e c e l l . T h e m e a s u r e o ft h e q u a n ti t y o f h y d r o c a r b o n a c c u m u l a t i o n ,V ( x ) ,and the vo l -u m e t r i c v a r i a b l e ,w ( x ) , c a n b e d e s c r i b e d a s k n o w n f u n c t i o n so f p o r e v o l u m e . F o r e x a m p l e ,w ( x ) a n d V ( x ) m a y b e m e a -s u r e d a s p o o l a r e a i f w e k n o w t h e r e l a t i o n s h i p b e t w e e n t h ep o r e v o l u m e a n d p o o l a r e a .

CASE STUDY

GEOLOGICALSETTING

We i l lus t ra te th i s method by p red ic t ing poss ib le loca t ionsa n d a s s o c i a t e d s i z e s o f u n d i s c o v e r e d p e t r o l e u m r e s o u r c e s i nK e g R i v e r r e e f s o f t h e R a i n b o w s u b - b a s i n o f th e We s t e r nC a n a d ia n S e d i m e n t a ry B a s in ( W C S B ) . T h e K e g R i v e r R e e fp lay i s a mature hydrocarbon p lay loca ted in no l r thwes te rnA l b e r t a b e t w e e n 5 8 - 5 9 N a n d 11 8 - 1 2 0 W ( To w n s h i p s 1 0 6 -111 , R a n g e s 4 - 1 1 w r ) . M a j o r g e o l o g i c a l c o n t r o ls o n t h is p l a ya n d i t s p e t r o l e u m s y s t e m h a v e b e e n d e s c r i b e d ( B a r s se t a l . ,1970 ; Re insone t a l . , 1993; Li e t a l . , 1999) . The o rgan ic - r i ch

K e g R i v e r F o r m a t i o n i s th e m a j o r s o u r c e f o r p e t r o l e u m ( Be t a l . ,1970; Lie t a l . , 1 9 99 ) . K e g R i v e r r e e f s , t he p r i m a r y r e s ev o i r, a r e d e v e l o p e d o n t h e l o w e r K e g R i v e r r a m p - p l a t f o r m b o n a t e s . T h e r e e f s , d o m i n a n t l y d o l o m i t i z e d c o r a l - s t r o m aporo id f loa t s tones /b inds tones , a re capped by a lga l g ra ins top a c k s t o n e s h o a l d e p o s i t s ( R e i n s o ne t a l . , 1993) . MuskegF o r m a t i o n e v a p o r i t e s f o r m a t o p s e a l o v e r l y i n g p o r o u s rbu i ldups .

F i g u r e 2 s h o w s t h e p l a y b o u n d a r y a n d t h e e x p l o r a t i o n w edr i l l ed p r io r to 1994 . Pr io r to 1994 ,407 exp lo ra to ry wel l s wdr i l l ed in to the Keg River Format ion wi th in the p lay a rThese t es t s r esu l t ed in 22 gas d i scover ies , 87 o i l d i scoveand 77 o i l and gas d i scover ie s wi th a to ta l e s t imated res e rv269 .064x106 m 3 o i l equ iva len t in p lace .

DATA ANALYSIS

T h r e e t y p e s o f d a t a a r e a v a i l a b l e f r o m t h i s m a t u r e p l a y :1 . Geo log ica l in fo rm at ion tha t charac te r iz es bo th pe t ro

um pools and geo log ica l se t t ing , inc lud ing dep ths to K e g R i v e r, M u s k e g a n d S u l p h u r P o i n t f o r m a t i o n s , tt h i c k n e ss o f t h e s e f o r m a t i o n s , a n d p e t r o l e u m p o o l r e sv o i r p a r a m e t e r s .

2 . Explora t ion wel l s loca t ions , s t a tus (186 d i sco very weand 221 dry ho les ) a s we l l a s poo l s i ze and a rea fo r mdiscovery wel l s .

3 . T h r e e s e i s m i c s u r v e y m a p s d e s c r i b i n g g e o p h y s ip r o s p e c t i n g e f f o r t s i n a r e g i o n o f a p p r o x i m a t e12 ,000 km 2, p r io r to 1975 , be twe en 1975-1986 ab e t w e e n 1 9 8 8 - 1 9 9 7 . P r i o r to 1 9 7 5 , m o r e t h a n 1 9s e i s m i c l i n e s w e r e s h o t w i t h a t o t a l l i n e l e n g t h abo u t 1 .5 x l04 km . Dur ing 1975-1986 , the mo s t in tes i v e s e is m i c a c q u i s i ti o n p e r i od , m o r e t h a n 9 6 0 0 s e i s ml i n e s w e r e s h o t w i t h a t o t a l l e n g t h o f 6 .2 x l 0 4 kBetween 1988 and 1997 about 1400 l ines wi th a tol e n g t h o f a b o u t 7 50 0 k m w e r e a c q u i r e d . F i g u r e s 3 a a3 b s h o w t h e s e i s m i c s u r v e y m a p s s h o t in t h e t i m e p eods p re -1975 and 1975-1986 . Se i smic l ines acqu i redthe eas t -wes t d i rec t ion a re denser than those in tnor th - sou th d i rec t ion (F igs . 3a and 3b) , a f ea tu re usw h e n c a l c u l a t i n g s p a c e a v a i l a b l e f o r u n d i s c o v e rresources .

S p a t i a l C h a r a c t e r i s ti c s o f G e o l o g i c a l F a c t o r s

We a re ce r ta in tha t a l l r eg iona l geo log ica l cond i t ions ne s s a r y f o r h y d r o c a r b o n a c c u m u l a t i o n e x i s t b e c a u s e t h i s i

m a t u r e p e t r o l e u m p l a y. T h e s p a t i al d i s t ri b u ti o n o f p e t r o l er e s o u r c e s i s c o n t r o l l e d p r i m a r i l y b y l o c a l v a r i a t io n s i n g e o lica l f ac to rs , such as the ex i s tence o f t r aps , second ary migt i on p a t h w a y s , a n d t h e a v a i l a b i l i ty o f a h y d r o c a r b o n c h a rH o w e v e r, s e c o n d a r y m i g r a t i o n p a t h w a y s a n d h y d r o c a r ba v a i l a b i l i t y f o r a s p e c i f i c p r o s p e c t a r e u n k n o w n p r i o r, aeven somet imes subsequen t , to d r i l l ing . In con t ras t , the extence o f a t r ap o r p rospec t , in th i s case a ree f , may be in fe rf rom o ther ind ica to rs .

E x p l o r a t i o n e x p e r i e n c e i n d i c a t e s t h e p r e s e n c e o f a p i n n ar e e f t r a p is b o t h t h e m a j o r c o n t r o l o n h y d r o c a r b o n o c c u r r e

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in the p lay, and the key c r i t e r ion fo r exp lora t ion success andu n d i s c o v e r e d p o t e n ti a l. D r y w e l l s c o m m o n l y r e s u l t w h e r e t h et rap i s insuff i c ien t o r incor rec t ly loca ted . P innac le ree f s a rei n d i ca t e d b y a m a r k e d t h i c k e n in g i n t h e K e g R i v e r F o r m a t i o n ,o r b y d r a p e o r th i n n in g o f o v e r l y in g M u s k e g F o r m a t i o n a b o v ea ree f . I sopach maps , s t ruc tu ra l maps , and assoc ia ted res idua lm a p s o f K e g R i v e r a n d M u s k e g f o r m a t i o n s w e r e a n a l y z e d t od e t e r m i n e c o r r e l a t i o n s b e t w e e n g e o l o g i c a l v a r i a b l e s a n dp e t r o l e u m p o o l s .

T h e s p a t i a l v a r i a t i o n s o f t h e se g e o l o g i c a l v a r i a b l e s w e r ei n t e r p o l a t e d u s i n g k r i g i n g , a g e o s t a t i s t i c a l t e c h n i q u e .K r i g i n g w a s e m p l o y e d d u r in g m a p - m a k i n g b e c a u s e i t t a k e sa d v a n t a g e o f s p a t i a l c o r r e l a t i o n i n f o r m a t i o n t o o p t i m a l l yw e i g h e s t i m a t e s o f gr i d v a l u e s c o n s i d e r i n g n e i g h b o r i n g d a t ap o i n t s . K r i g i n g a l s o i n d i c a t e s t h e r e l i a b i l i t y o f t h e r e s u l t i n gm a p , s i n c e k r i g i n g v a r i a n c e i n d i c a t e s t h e u n c e r t a i n t y i n t h ee s t i m a t e a t g r i d n o d e s a s a f u n c t i o n o f d a t a p o i n t p r o x i m i t y.K r i g i n g i n t e r p o l a t i o n h a s b e e n u s e d i n m a n y g e o l o g i c a l

m a p p i n g a p p l i c a t i o n s ( O l e a , 1 99 2 , 1 9 94 ; H a r b a u g he t a l . ,1 9 9 5 ) . R e a d e r s a r e r e f e r r e d e l s e w h e r e f o r t h e m a t h e m a t i c a ld e t a i l s o f th i s t e c h n i q u e ( I s a a k s a n d S r i v a s t a v a , 1 9 89 ;D e u t s c h a n d J o u r n e l , 1 9 9 2 ) .

Da ta f rom 407 exp lora t ion wel l s a re used in th i s ana lys i s .O f t h e s e , 2 5 8 d r i l l e d c o m p l e t e l y t h r o u g h t h e K e g R i v e rF o r m a t i o n . T h e s e 2 5 8 w e l l s a r e u s e d t o m a p K e g R i v e r t h i c k-n e s s. T h e t h i ck n e s s o f K e g R i v e r F o r m a t i o n h a s a d i s c e r n ib l ean i so t ropy i l lus t ra ted by var iograms (F ig . 4 ) . A var iogramgraph ica l ly por t rays the spa t i a l con t inu i ty o f a mappab le va r i -ab le on a c ross p lo t . The c ross p lo t o f a se lec ted var iab le i sc o m p o s e d o f t h e a v e r a g e s q u a r e d d i f f e r e n c e b e t w e e n t h e v a l -

ues o f a l l da ta pa i r s , a g iven d i s tance apar t , p lo t t ed aga ins t thed i s t a n c e b e t w e e n d a t a p o i n t s ( Wo l fe t a l . , 1994) . Th isan i so t ropy show s tha t con t inu i ty o f Keg R iver th ickness i s be t -t e r in the nor theas te r ly d i rec t ion compared to the nor thwes te r-ly d i rec t ion . Th is i s ind ica ted by a l a rger range o f va lues ,s h o w n i n v a r i o g r a m s w i t h a z i m u t h s o f 4 5 a n d 9 0 , w h e r ethere i s a g rea te r d i s tance f rom the o r ig in o f the va r iogra m towher e the va r iogr am curve f l a t t ens (F ig . 4 ) .

T h e c o n t o u r e d K e g R i v e r th i c k n e s s m a p s h o w s a g e n e r a lnor theas te r ly t r end in the cen t ra l p lay a rea wi th nor thwes te r lyt rends appear ing more obv ious in the nor theas te rn p lay a rea(F ig . 5a ) . The pa t t e rn o f Keg River th ickness va r ia t ions inF i g u r e 5 a m a y r e f l e c t a c o m p l i c a t e d g e o l o g i c a l c o n t r o l o nreg iona l depos i t iona l pa t t e rns , inc lud ing ree f fo rmin g process -es . Uncer ta in t i es in the Keg River th ickness map (F ig . 5a ) a repor t rayed by the map of the k r ig ing s tandard dev ia t ion (F ig .5b) . F igure 5b ind ica tes tha t the uncer ta in ty in F igure 5a i sg rea te r in the sou thwes te rn p lay a rea where da ta a re sparse .

T h e t h i c k n e s s o f o v e r l y i n g M u s k e g F o r m a t i o n e x h i b i t s av e r y d i f f e r e n t m a p p a t t e r n ( F i g . 5 c ) . I n c o n t r a s t t o t h e K e gR i v e r t h i c k n e s s , th e a n i s o t r o p y o f M u s k e g t h i c k n e s s i s m o r ep r o n o u n c e d a l o n g n o r t h w e s t e r l y t r e n ds . T h e k r i g i n g s t a n d a r ddev ia t io n i l lus t ra tes the uncer ta in ty in the in te rpo la t ion o f the

M u s k e g i s o p a c h m a p ( F i g . 5d ) . W h e n t h i c k n e s s m a p s o f th eK e g R i v e r a n d M u s k e g f o r m a t i o n s a r e c o m p a r e d , t h e r e i s ag e n e r a l c o r r e l a t i o n o f th i c k K e g R i v e r F o r m a t i o n w i t h t h i n si n o v e r l y i n g M u s k e g F o r m a t i o n ( F i g s . 5 a a n d 5 c ). T h i s q u a n -t i t a t i v e r e l a t i o n s h i p i s c o n s i s t e n t w i t h g e o l o g i c a l o b s e r v a -t ion.

Fur ther ana lys i s shows a d i rec t cor re la t ion be tween d i scove r e d p e t r o l e u m p o o l l o c a t i o n s a n d t h i c k n e s s e s o f K e g R i v e ra n d M u s k e g f o r m a t i o n s . K e g R i v e r F o r m a t i o n is a l w a y s m o r ethan 100 m th ick , and commonly g rea te r than 200 m th ick ,wi th cor respon ding th inn ing o f Muske g Form at ion , a t a ll d isc o v e r e d p o o l l o c a t i o n s . H o w e v e r, t h e p r e d i c t i v e p o w e r o fthese th ickness cor re la t ions i s r educed when fo rmat ion th ickness va r ia t ion a t d ry wel l s i s compared to th ickness va r ia t ionw h e r e p o o l s w e r e d i s c o v e r e d . T h i c k n e s s m a p s c o n t a i n i ns u f f ic i e n t i n fo r m a t i o n t o c o m p l e t e l y s e p a r a t e d i s c o v e r y w e l l s f r o mdry ho les . The ana lys i s o f s t ruc tu ra l r es idua l maps o f KegR i v e r a n d M u s k e g f o r m a t i o n t o p s g i v e s s i m i l a r r e su l ts .

In o rder to ob ta in geo log ica l charac te r i s t i c s and spa t i a l pa tt e rns o f hydrocarbon poo ls tha t d i s t ingu ish p roduc t ive loca-t i o ns f r o m d r y w e l l s , w e c o n s i d e r e d d i f f e r e nt c o m b i n a t i o n s om a p s . C a l c u l a t e d m a p s i n c l u d e d c o m b i n a t i o n s o f b a s ic t h i c kness and s t ruc tu ra l da ta , such as ra t ios o f fo rmat ion th icknesse s a n d h i g h f r e q u e n c y c o m p o n e n t s o f f o r m a t i o n t h i c k n e s sm a p s . P r e l i m i n a r y s t u d i e s o f s u c h c a l c u l a te d m a p s s h o w t h as o m e w e r e r e d u n d a n t , w h i l e o t h e r s w e r e o n l y w e a k l y c o r r e l a ted wi th poo l loca t ions . To ex t rac t map in format ion d i rec t lyre la ted to pe t ro leum occur rence and i t s spa t i a l charac te r i s t i c sw e u s e P r i n c i p a l C o m p o n e n t A n a l y s i s ( P C A ) . P C A p r o v i d e so r t h o g o n a l p r o j e c t i o n s o f m a p d a t a t h a t b e t t e r d i s c r i m i n a t e s

the geo log ica l charac te r i s t i c s o f d i scovery wel l s . PCA i s s imp l y a m a t h e m a t i c a l t r a n s f o r m a t i o n o f t h e o r i g i n a l v a r i a b l e sP C A i n v o l v e s t h e s e l e c ti o n o f u n c o r r e la t e d l i n e a r c o m b i n at ions o f o r ig ina l va r iab les in such a manner tha t each success i v e l i n e a r c o m b i n a t i o n o f v a r i a b l e s h a s a s m a l l e r v a r i a n c eT h e r e a r e m a n y a p p l i c a t i o n s o f P C A t o g e o l o g i c a l p r o b l e m sinc lud ing da ta mat r ix reduc t ion , mul t iva r ia te cor re la t ion , anc l a s s i fi c a t io n ( D a v i s , 1 9 8 6 ; Ya m a m o t o , 1 9 76 ) . T h e m a t h e m a tica l fo rmu la t ion o f PC A i s d i scussed e l sewhe re (Davis , 1986M a r d i a e t a l . , 1989).

A f t e r t h e t r a n s f o r m a t i o n o f o r i g i n a l v a r i a b l e s , P C A g i v ethe fo l lowing resu l t s . P r inc ipa l component I (PC I , F ig . 6a )r e p r e s e n t in g 4 2 % o f v a r i an c e , c o n t a i n s i n f o r m a t i o n p r e d o m inan t ly pe r t inen t to fo rmat ion th ickness va r ia t ions , pa r t i cu la r lthe th ickness o f M uske g Form at ion (F ig . 5c ). P r inc ipa l comp o n e n t I I I ( P C I I I ) , r e p r e s e n t in g a b o u t 1 6 % o f v a r i a n c e , c o nta ins the mos t in format ion d i sc r imina t ing d i scovery wel l sf r o m d r y h o l e s . We r e g a r d P C I I I as t h e c o m p r e h e n s i v e i n d ica to r o f geo log ica l f avorab i l i ty in th i s s tudy. The resu l t ingg e o l o g i c a l f a v o r a b i li t y m a p w a s c o m p u t e d d i r e c t l y f r o m a l i ne a r c o m b i n a t i o n o f g e o l o g i c a l m a p s , i n c l u d i n g f o r m a t i o nt h i c kn e s s , h i g h f r e q u e n c y c o m p o n e n t s o f f o r m a t i o n t h i ck n e s sand s t ruc tu ra l r es idua l s on spec i f i c fo rmat ion tops (F ig . 6b)

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x 1 0 4 X > = 0 . 0 5 a x 1 0 4 x > = 0 . 3 b

-6

-4

1

0 . 5

0

- 0 . 5

- 4

- 6

-8 -8

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x 105 x 105x 1 0 4 x > = 2 x 1 0 4 x > = 4

-6

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1

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0

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- 4

- 4

- 6

-8

0 . 5

0

- 0 . 5

-1

0 . 5

0

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-1 -14 . 6 4 . 8 5 5 . 2 4 . 6 4 . 8 5 5 . 2

x l 0 5 x l 0 5

Fig. 7. Geograp hic distributions of discoveries and dry we lls for different pool size thresholds. Red repres ents poo ls equal to or largerspecif ied s izes ; yel low indicates pools sm al ler than the specif ied s izes . Pool s ize in x106 m3;i e the s ize of e ach po ol i s indicated by i t s pool are

x 1 0 4 x > = 0 . 0 5 a x 1 0 4 x > = 0 . 3

-4

-6

b

0.5

1 1

-4

-6 0.5

-8 -8

04 . 6 4 . 8 5 5 . 2 4 . 6 4 . 8 5 5 . 2

x l 0 5 x l 0 5X 1 0 4 X > = 2 . 0 X10 4 x>=4.0

]

- 4 - 4

-6.5- 6

-8 -8

04 . 6 4 . 8 5 5 . 2 4 . 6 4 . 8 5 5 . 2

x l 0 5 x l 0 5

0

0 . 5

Fig. 8 . Probabi li t ies of hydrocarbon occurrences es t ima ted f rom explorat ion wel ls for d i fferent specified poo l s ize thresholds . Pool sm3; i e the s ize of e ach po ol i s indicated by i t s pool area .

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C H A R A C T E R IZ I N G S PAT I A L D I ST R I BU T I O N O F U N D I S C O V E R E D H Y D R O C A R B O N S I N T H E W C S B 159

X 1 0 4

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-4

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X 1 0 4

4 . 6 4 . 8 5 5 . 2 4 . 6

x 10 ~

F i g . 9 . S p a t i a l p a t t e r n o f t h e a r e a s a v a i l a b l e f o r p o t e n t i a l u n d i s c o v e r e dp e t r o l e u m a c c u m u l a t i o n s ( a r e a u n i t: s q u a r e k m ) .

Figure 6b shows tha t the re a re two para l l e l , nor theas t - s t r ik ingzones o f g rea te r geo log ica l f avorab i l i ty.

R e s u l t s o f E x p l o r a t i o n D r i l li n g

T h e p r o b l e m o f e s t i m a t in g h y d r o c a r b o n o c c u r r e n c e p r o b -a b i l i t y a t a n u n te s t e d l o c a t i o n i s e q u i v a l e n t t o e s t i m a t i n g t h el o c a l c o n d i t i o n a l c u m u l a t i v e p r o b a b i l i t y d i s t r i b u t i o n f u n c -t i o n f o r h y d r o c a r b o n o c c u r r e n c e a t t h e s a m e l o c a t i o n , u s i n gn e a r b y o b s e r v a t i o n s ( S o l o w, 1 9 8 6 ) . We b e g i n b y r e g a r d i n g

e x p l o r a t i o n d r i l l i n g r e s u l t s a s a b i n a r y r a n d o m v a r i a b l e ,i n d i c a t i v e o f e i t h e r s u c c e s s o r f a i lu r e . T h i s a l l o w s t h e s p a t i a l

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F i g . 1 0 . P r o b a b i l i t y t h a t a g e o p h y s i c a l a n o m a l y w i th a s i z e o f 0 .5k m 2 a n d c i r c u la r s h a p e i n 2 D p l a n e c o u l d h a v e b e e n m i s s e d b y d i f fe r -e n t r e c t a n g u l a r s e i s m i c l i n e s e a r c h i n g . S e i s m i c g r i d i s f i x e d a t al e n g t h / w i d t h r a t i o = 3 2 / 2 6 .

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F i g . 11 . P r o b a b i li t y t h a t a r e a s w i th a g e o l o g i c a l a n o m a l y o f s i ze e q u ato o r l a rge r than 0 .1 km 2 con ta ins hydro carbon s . The p robab i l i ty o f ex it e n c e o f g e o l o g i c a l a n o m a l y w i t h s i z e e q u a l t o o r l a rg e r t h a n 0 . 1 k m 2 if i x e d a t 7 5 % . S e e t e x t f o r m o r e e x p l a n a t i o n .

d i s t r i b u t i o n o f K e g R i v e r p o o l s t o b e e x p r e s s e d i n t e r m s oa h y d r o c a r b o n o c c u r r e n c e p r o b a b i l i t y e s t i m a t e d u s i n g i n d ic a t o r k r i g i n g . F i g u r e 6 c i s t h e r e s u l t a n t h y d r o c a r b o n o c c u r -r e n c e p r o b a b i l it y m a p . H o w e v e r , t h e k r i g e d p r o b a b i l i ty m a pd e r i v e d f r o m e x p l o r a t i o n r e s u l t s m a y b e s t a t i s t i c a l l y b i a s eb e c a u s e e x p l o r a t i o n w a s g u i d e d b y a g e o l o g i c a l m o d e l .

To inves t iga te the major geo log ica l con t ro l s on spa t i a

charac te r i s t i c s o f pe t ro le um poo ls as a func t ion o f s i ze , the 18d iscover ies and 221 dry wel l s were conver ted in to ind ica tovar iab les accord ing to the i r poo l s i zes . Four poo l s i ze th resholds of_> 0.05 106 m 3, _> 0.3 xl0 6 m 3, _> 2.0 xl0 6 m 3, and _>4xl06 m 3 o i l equ iva len t w ere chosen . Th ese th resho lds cor responding to 97 .74%, 95 .49%, 8 1 .26% an d 62 .39% o f the to tap lay rese rve , r espec t ive ly, o r to 90%, 50%, 20% and 10% ot h e t o t a l n u m b e r o f d i s c o v e r e d h y d r o c a r b o n p o o l s , r e s p e c t i v ely. F igure 7 show s the geogra ph ic d i s t r ibu t ions o f the b inaryv a r i a b l e s f o r e a c h o f t h e t h r e s h o ld v a l u e s o f m i n i m u m p o os i z e a n d F i g u r e 8 s h o w s t h e p e t r o l e u m o c c u r r e n c e p r o b a b i l it i e s fo r t he s a m e f o u r th r e s h o ld s . F r o m t h e s e m a p s w e o b s e r vthe fo l lowing :1 . L a rg e n u m b e r s o f s m a l l a n d m e d i u m p o o l s o c c u r i n t h

nor theas te rn pa r t o f the p lay, whi le l a rge poo ls oc cur inthe sou th-cen t ra l p lay a rea .

2 . Rega rd less o f poo l s i ze , the re a re two pre fe r red l ineaa l ignm ents o f poo ls , one in a nor theas te r ly d i rec t ion andthe o ther in a nor thwes te r ly d i rec t ion .

3 . T h e p r o b a b i l i t y o f a l a rg e p o o l o c c u r r i n g (> _2 .0 x l 0 6m 3 o i l e q u i v a l e n t ) d e c r e a s e s f r o m s o u t h w e s t t o n o r t he a s t , i n d i c a t in g e s s e n t i a l g e o l o g i c a l f a c t o r s c o n t r o l l i n gp l a y r i c h n es s a r e m o r e f a v o r a b l e t o t h e s o u t h w e s

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(F igs . 8c and 8d) . Th is i s p r imar i ly due to the depos i -t i o n a l s e t t in g , w h e r e a b e t t e r c o n n e c t i o n t o o p e nm a r i n e c o n d i t i o n s e x i s t e d t o t h e s o u t h r e s u l t in g i n b e t -t e r r e s e r v o i r c h a r a c t e r i s t ic s . I t m a y a l s o b e d u e t o a u g -m e n t e d h y d r o c a r b o n c h a r g e e n t e r i n g t h e d o w n d i pl i m i t o f th e p l a y f r o m s o u r c e r o c k s l y i n g o u t s i d e t h ep l a y b o u n d a r y.

4 . I n F i g u r e 8 , t h r e e h i g h o c c u r r e n c e p r o b a b i l i t y a r e a sa r e c o n s i s t e n t l y s e p a r a t e d b y n o r t h w e s t - s t r i k i n g l o w -p r o b a b i l i t y b e l t s . T h e t h r e e h i g h - p r o b a b i l i t y a r e a s; h o w i n t e r n a l n o r t h w e s t e r l y t r e n d s o b s e r v a b l e i n t h ep o o l d i s t r i b u ti o n m a p s ( F i g . 7 ) . T h e s e d o m a i n s o f h i g ha n d l o w - p r o b a b i l i t y m a y i n d i c a t e u n s p e c i f i e d n o r t h -w e s t e r l y t r e n d i n g g e o l o g i c a l e l e m e n t s a f f e c t i n g p e t r o -l e u m m i g r a t io n a n d a c c u m u l a t i o n p r o c e s s e s .

I t i s r easonab le to ass ume tha t a lmos t a l l the l a rge poo ls a red i scovered in th i s p lay (Re insonet a l . , 1993) . There fore , wer e g a r d t h e h y d r o c a r b o n o c c u r r e n c e p r o b a b i l i t y m a p f o r p o o l s

>2.0 X106 m 3 o i l equ iva len t a s a rea l i s t i c spa t i a l desc r ip t ion o fthe l a rge pe t ro leum pools in th i s p lay.

SPATIAL CHARACTERISTICS OF UNDISCOVEREDHYDROCARBON RESOURCE

T h e P r o b a b i l i ty o f H y d r o c a r b o n O c c u r r en c e

T h e k r i g e d p r o b a b i l i t y m a p d e r i v e d f r o m e x p l o r a t i o n o u t -c o m e s i s u s e d t o i n d i c a t e t h e p r i o r p r o b a b i l i t y o f p e t r o l e u mp o o l o c c u r r e n c e i n th e K e g R i v e r R e e f p l a y ( F i g . 6c ) . T h em a p o f g e o l o g i c a l f a v o r a b i l i t y ( F i g . 6 b ) c o n t a i n s a d d i t i o n a li n f o r m a t i o n r e g a r d i n g t h e s p a t i a l c h a r a c t e r i s t i c s o f p o o l st h a t is u s e d t o u p d a t e t h e p r i o r p r o b a b i l i t y m a p . F i g u r e 6 d i s

t h e r e s u l t i n g p o s t e r i o r p r o b a b i l i t y m a p t h a t r e f l e c t s h y d r o -c a r b o n o c c u r r e n c e a n d w h i c h c o n s i d e r s b o t h e x p l o r a t i o nr e s u l t s a n d g e o l o g i c a l f a v o r a b i l i t y. C o m p a r i s o n o f t h e p o s -t e r i o r p r o b a b i l i t y m a p w i t h t h e p r i o r p r o b a b i l i t y m a p s h o w ss i g n if i c a n t i m p r o v e m e n t s i n t h e p o s t e r io r p r o b a b i l i t y m a p .T h e t w o N E - S W f a v o r a b l e b a n d s o n t h e g e o l o g i c a l f a v o r a -b i l i t y m a p a r e p r e s e n t i n t h e p o s t e r i o r p r o b a b i l i t y m a p ( F i g .6 b ) . We s u g g e s t t h a t t h e p o s t e r i o r p r o b a b i l i t y m a p p r o v i d e st h e b e s t p o s s i b le e s t i m a t e o f h y d r o c a r b o n o c c u r r e n c e u s i n ga v a i l a b l e d a t a .

Poss ib l e Loca t ions and S i ze s o f Und i scove red Resou rces

U n t e s t e d a r e a s a r e o b v i o u s r e g i o n s t o c o n s i d e r w h e nass ign ing loca t ions and po ten t ia l poo l s i zes to und iscoveredresources . Exc luded a reas , where no add i t iona l r esources a reexpec ted , inc lude a reas occup ied by ex i s t ing d i scover ies , a reaso f e x h a u s t e d p o t e n t i a l(i.e. a r e a s c o m p r i s i n g d r y h o l e s a n dt h e i r a re a s o f i n f l ue n c e ) , a n d a r e a s e x c l u d e d b y s e i s m i c s u r v e yl ine dens i ti e s . I t i s a l so necessa ry to ensure tha t a l l geo log ica le l e m e n t s n e c e s s a r y f o r h y d r o c a r b o n a c c u m u l a t i o n a r e p r e s e n tin the a reas o f concern .

In th i s s tudy we idea l i ze the resu l t s o f p rev ious d r i l l ing bya s s u m i n g t h a t t h e y a r e d i a g n o s t i c . We a s s u m e t h a t e a c h d r y

exp lor a to ry w el l exhaus t s a c i rcu la r a rea o f 1 .4 km 2 . W e aa s s u m e t h a t a l l 2 D s e i s m i c l i n e s h a v e e q u a l d e t e c t i o n p o wand tha t any su i t ab le anomaly c rossed by a s ing le se i smic lwas d iagnos t i ca l ly de tec ted and d r i l l ed . Exper ience shot h e s e a s s u m p t i o n s t o b e s i m p l i f i e d . H o w e v e r, t h e y d o nreduce the ge nera l i ty o f th is ana lys i s . I f we knew the quaand reso lv ing power o f ind iv idua l d ry wel l s and se i smic l int h e n w e m i g h t c o n s i d e r e a c h i n d i v i d u a l l y i n a m o r e d e t a ie s t i m a t i o n o f r e m a i n i n g o p p o r t u n it y.

For th i s exam ple we cons ider on ly the da ta f rom se i sm ic sveys p r io r to 1975 and tha t loca t ing se i smic surveys be tw1975-1986 . Th is was done because the da ta f rom se i smic sveys dur ing the in te rva l 1988-1993 , con tem porane ous wi th dcovery h i s to ry da ta , cannot be ex t rac ted as a subse t f rom tda ta o f se i sm ic surveys dur ing the in te rva l 1988-1997 .

We u s e a r e c t a n g u l a r m o v i n g - w i n d o w m e t h o d t o c a l cl a t e a v a i l a b l e s p a c e f o r u n d i s c o v e r e d r e s o u r c e s . A s i nm o v i n g - w i n d o w a p p r o a c h g i v e s o n l y t h e t o t a l e m p t y s p

w i t h i n e a c h w i n d o w w i t h o u t i n d i c a t i n g w h e t h e r t h e a v a i l aspace i s con t inuous ly l inked o r sca t t e red th roughout the wd o w. A s s u c h i t r e p r e s e n t s t he m a x i m u m p o s s i b l e s p a c e a va b l e w i t h i n e a c h w i n d o w. B y s e r i a l l y a p p l y i n g m o v i n g - wdows of d i ffe ren t s i zes , i t i s poss ib le to de te rmine the con tu o u s a v a i l a b l e s p a c e . T h e a r e a s e x c l u d e d f r o m a d d i t i op o t e n t i a l d u e t o d i s c o v e r e d p o o l s a n d d r y h o l e e x h a u s t ia n a l y s i s a r e t h e n d e l e t e d f r o m t h e r e m a i n i n g a v a i l a b l e s pm a p , t o p r o d u c e a n u n d i s c o v e r e d h y d r o c a r b o n a c c u m u l a tava i l ab le a rea map (F ig . 9 ) .

F i g u r e 9 r e p r e s e n t s t h e a r e a a v a i l a b l e f o r a c c o m m o d a tu n d i s c o v e r e d h y d r o c a r b o n a c c u m u l a t i o n s . I t a l s o i l l u s t r a

the p rev ious exp lora t ion focus on the sou theas te rn and nore r n p a r t s o f t h e p l a y a r e a . I n t e n s i v e s e i s m i c p r o s p e c t i n g d r i l l ing in those a reas has e ffec t ive ly exha us ted the poss ib if o r l a rg e u n t e s te d g e o p h y s i c a l a n o m a l i e s .

In the absence o f p rospe c t - spec i f i c pa ramete rs , i t i s d i ffito es t imate the to ta l number o f p rospec t s and the i r d i s t r ibu tu s i n g K a u f m a n ' s p r o b a b i l i t y m o d e l . To o v e r c o m e t hi s d icu l ty we assum e, qu i te reasonab ly, tha t a p innac le re e fa p p r o x i m a t e d b y a c i r c u l a r m a p a n o m a l y. Wi t h t h i s a s s u mt ion i t i s poss ib le to ca lcu la te the p robab i l i ty tha t a geophyc a l a n o m a l y o f g i v e n s i z e a n d c i rc u l a r s h a p e w a s m i s s e d bspec i f i ed se i smic sea rch g r id . F igure 10 shows the p robab io f m i s s i n g a c i r c u la r g e o p h y s i c a l a n o m a l y o f 0 . 5 k m 2 w i trec tangu la r se i smic sea rch g r id hav ing a l eng th to wid th rao f 1 6 /1 3 . T h i s c a l c u l a t i o n i n d i c a t e s t h a t a 1 .3 3 k( 1 . 2 8 x l . 0 4 ) r e c t a n g u l a r s e i s m i c g r i d h a s o n l y a 1 0 % c h a n c em i s s i n g a g e o p h y s i c a l a n o m a l y w i t h a n a r e a o f 0 .5 k m 2. Id i f f i c u l t to f i n d a 1 .3 3 k m 2 u n t e s t e d a r e a o n F i g u r e 9 , i m pi n g t h a t t h e c h a n c e o f f i n d i n g a n u n d i s c o v e r e d p o o l 0 . 5 ki n a r e a i s s m a l l . H o w e v e r, t h e r e a r e s t i l l m a n y p l a c e s w hc i r c u l a r g e o p h y s i c a l a n o m a l i e s 0 .1 k m 2 i n a r e a c o u l d m i s s e d b y p e r f e c t l y d i a g n o s t i c s e i s m i c p r o s p e c t i n g , w i t h3 0 % p r o b a b i l i t y. F r o m t h is w e c o n c l u d e t h a t m o s t o f

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f u t u r e d i s c o v e r i e s w i l l b e s m a l l , s p e c i f i c a l l y t h a t t h e y s h a l l b e

0.1 k i n 2 o r l e s s i n a r e a , a s s u m i n g b o t h p e r f e c t l y d i a g n o s t i c

s e i s m i c p r o s p e c t i n g a n d d r i l l in g e x h a u s t i o n o f 1 . 4kln 2 a r o u n d

d r y w e l l l o c a t io n s , A s s u m i n g t h a t th e s m a l l e s t v i a b l e p o o l s i z e

h a s a f o o t p r i n t o f 0. 1 k m 2 , w e c a n c o n v e r t F i g u r e 9 i n t o a

m a p i n d i c a t in g a r e a s w h e r e s i g n i f i c a n t o p p o r t u n i t i e s w e r em i s s e d b y p r e v i o u s e x p l o r a t i o n a n d p r o s p e c t i n g a c t i v i t i e s ,

w i t h a g i v e n p r o b a b il i ty . T h i s i n d i c a t o r m a p c a n b e c o m b i n e d

w i t h t he h y d r o c a r b o n o c c u r r e n c e m a p ( F i g. 8 ). T h e r e s u l ti n g

m a p s h o w s t h e p r o b a b i l i t y t h a t ar e a s w i t h p r o s p e c t s e q u a l t o

o r l a rg e r t h a n 0 . 1 0 k iT l2 c o n t a i n h y d r o c a r b o n r e s o u r c e s ( F i g . 11 ) .

C o m p a r i s o n o f t h e u n d i s c o v e r e d h y d r o c a r b o n r e s o u r c e m a p

( F i g . 11 ) w i t h th e m a p i n d i c a t i n g t h e p r o b a b i l i t y f o r th e o c c u r -

r e n c e o f h y d r o c a r b o n p o o l s g r e a t e r t h a n o r e q u a l to 0 . 3 0 x l 0 6

m 3 o i l e q u i v a l e n t , s u g g e s t s t h a t t h e t w o m a p s a r e s i m i l a r. F r o m

t h i s s i m i l a r i t y w e i n f e r t h a t t h o s e p a r t s o f t h e p l a y a l r e a d y c o n -

t a i n in g t h e h i g h e s t d e n s i t y o f l a r g e a n d m e d i u m p o o l s a r e a l s o

t h e a r e a s w h e r e i t i s m o s t l i k e l y t o f in d t h e m a j o r i t y o f t h eu n d i s c o v e r e d p e t r o l e u m r e s o u r c e s .

CONCLUSIONS

T h e u s e o f g e o l o g i c a l a n d g e o p h y s i c a l d a t a c o m b i n e d

w i t h e x p l o r a t i o n d r i l l i n g r e s u lt s c a n b e u s e d t o p r e d i c t t h e

p o s s i b l e l o c a ti o n s a n d s i z e s o f u n d i s c o v e r e d p e t r o l e u m

p o o l s i n t h e K e g R i v e r R e e f p l ay . T h e c a s e s t u d y s h o w s t h a t

t h e c o m b i n a t i o n o f b o t h e x p l o r a t i o n r e s u l ts a n d g e o l o g i c a l

c h a r a c t e r i s t i c s g i v e s a b e t t e r e s t i m a t e o f t h e p r o b a b i l i t y o f

p e t r o l e u m o c c u r r e n c e t h a n t he c o n s i d e r a t i o n o f e i th e r d a t a

s e t i n d i v i d u a ll y . T h e i n c o r p o r a t i o n o f g e o p h y s i c a l p r o s p e c t -i n g i n f o r m a t i o n g i v e s a r e a l i s ti c i l l u s t r a t i o n o f t h e s p a t i a l

d i s t r ib u t i o n o f u n d i s c o v e r e d p e t r o l e u m p o t e n ti a l . T h e c o n -

s t r u ct i o n o f a f a v o r a b i l i t y m a p f o l l o w i n g t h e m e t h o d p r o -

p o s e d h e r e i n p r o v i d e s a m e a n i n g f u l m e t h o d f o r d e s c r i b i n g

t h e s p a ti a l d i s t r i b u t i o n o f u n d i s c o v e r e d p o o l a n d f i e l d s i z e s

o b t a i n e d f r o m o t h e r r e s o u r c e a s s e s s m e n t m e t h o d s u s i n g a

s t o c h a s t i c s i m u l a t i o n . T h e f a v o r a b i l i t y m a p i t s e l f i s a u s e f u l

t o o l f o r b o t h h y d r o c a r b o n e x p l o r a t i o n a n d p e t r o l e u m

r e s o u r c e m a n a g e m e n t ( F ig . 1 1 ).

ACKNOWLEDGMENTS

T h i s w o r k w a s s u p p o r t ed b y G e o l o g i c a l S u r v e y o f C a n a d a

P r o j e c t # 9 5 0 0 0 3 a n d t he P a n e l f o r E n e r g y R e s e a r c h a n d

D e v e l o p m e n t ( N a t ur a l R e s o u r c e s C a n a d a ). T h e a u t h or s w o u l d

l i k e t o th a n k D r s . D a l e I s s l e r , M a r t i n F o w l e r , G l e n S t o c k m a l

a n d M a o w e n L i o f G S C - C a l g a r y a n d D r s. P e t e r A u k e s a n d

Te r r y G o r d o n o f t h e U n i v e r s i t y o f C a l g a r y f o r t h e ir h e l p f u l

d i s c u s s i o n s a n d s u g g e s t i o n s . T h a n k s a l s o t o M s . P i n g T z e n g

a n d D r. D a v i d L e p a r d f o r t h e i r e f f o r t s t o a s s is t t h e s t u d y w i t h

d a t a r e c o v e r y a n d g e o g r a p h i c c o o r d i n a t e c o n v e r s i o n s .

G e o l o g i c a l S u r v e y o f C a n a d a C o n t r i b u t io n N u m b e r 2 0 0 0 0 33 .

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