Geochemistry of Crude Oils in Oman

8/19/2019 Geochemistry of Crude Oils in Oman

1/13

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1990.050.01.17p317-328.

1990, v.50;Geological Society, London, Special Publications

P. J. Grantham, G. W. M. Lijmbach and J. Posthuma Geochemistry of crude oils in Oman

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2/13

e o c h e m i s t r y o f c r u d e o i l s i n O m a n

P. J . GRAN THAM t, G. W. M. LIJMBA CH & J . POST HUM A

K o n i n k l i j k e / S h e l l E x p l o r a ti e e n P r o d u k t i e L a b o r a t o r i u m , P o s t b u s 60 ,

2 2 8 0 A B R i j s w i j k Z . H . ) , N e t h er l a n d s

1 P r es e nt a d d r e ss : N A M B . V . , P o s t b u s 2 8 0 00 , 9 4 0 0 M M A s s e n ,

T h e N e t h e r la n d s

A b s t r a c t :

The pet roleum geochemistry of Oman provides a picture of considerable variety

since crude oils and their source rocks are found both throughout the country and

throughout the stratigraphic column from the lnfra-Cambrian to the Tertiary. The oils can

be geochemically classified into five groups. Three of them can be related to good oil

source rocks found in the Precambrian Huqf Group, the Silurian Safiq and the Cretaceous

Natih Formations. Another group of oils probably originates from the Upper Jurassic

Diyab Formation whilst the fifth group of crudcs (name 'Q') cannot be correlated to a

known source rock but is inferred to have originated from an unpenetrated Huqf level.

The 'Huqf oils' are those that have been correlated to known Infra-Cambrian Iuqf

source rocks and are characterized by a strong C29 sterane predominance and very light

carbon isotope values (6 Cl3) of around -36.0 0. In contrast, the 'Q' crudes, derived from

the unknown source are characterized by a C27 sterane predominance and carbon isotope

ratios of around -30.5 0. Both the Huqf and 'Q' crudes also contain a series of

characteristic compounds referred to as the 'X' compounds (all isomers of methyl and

dimethyl alkanes). Oils thought to originate from Silurian Safiq source rocks have a weak

(329 sterane predominance, a significant content of rearranged steranes and carbon isotope

ratios of approx. -30.5 0. The oils correlatable to the Jurassic Diyab Formation have a

similar sterane distribution but heavicr carbon isotope values of around -26.5 0. Finally,

the crude oils from the mid-Cretaceous Natih Formation source rocks are characterized by

steranes with an equivalent distribution of C27, C2,~ and C29 isomers and carbon isotope

values of around -26.9 0.

Oil exploration in the Sultanate of Oman began

in 1937. The first well was spudded in 1955 by

Dhofar Cities Service Petroleum Co. and in

1956 with Marmul-1 they discovered heavy oil,

which was then unec onom ic to extract. In 1960,

exploration in the nort hern areas by Petrol eum

Development Oman ( PDO) came under the

direction of the Shell Group and in 1962 oil in

commercial quantities was discov ered at Yibal.

This was follo wed by the discovery of the Nat ih

Field in 1963 and the Fahud Field in 1964. In

1969, P DO also took over explorati on in Dhofa r

and re opene d interest in the heavy oils around

Marmul. Through the 1970s a number of other

oil fields in both the north and south were

discovered and progressively brought into

production. Presently some 50 fields have

been developed and a total production of over

500000 bbls per day has been established. A

pipeline network leads to the terminal West of

Muscat (see Fig. 1).

Oils and potenti al source rocks in Oman have

bee n the subject of geochemical study in the

Shell Exploration and Production Research

Laboratory (KSEPL) since the 1970s. Potential

oil source rocks have been identified in sev-

eral levels of the stratigraphy ranging from

Proterozoic to mid-Tertiary (Fig. 2). Oils have

been accumulated throughout the stratigraphic

column, but mostly in Cretaceous, Jurassic and

Permian reservoirs due to the positioning of the

best seals (see Fig. 2). Except in the south, most

Oma n crudes are generally low-sulphur, low-

wax light oils. Stand ard geoche mical techni ques

(Lijmbach 1975; Lijmbach et al. 1983) show

that all Oman crude oils appear to have been

generated from marine structureless or sapro-

pelic organic matter with no contribution from

land-plant material (see Figs 3 & 4). The most

significant variations are in biomarker distri-

butions and carbon isotope values which are

distinct enough to ensure a high degree of

certainty in the grouping of the crude oils de-

scribed here. Five chemically distinct types of

crude oil can be recognized. Three of them are

linked to source rocks in Proterozoic (Huqf),

Silurian (Safiq) and Mid-Cretaceous (Natih)

levels, whilst the fourth is probably derived

from an Upper Jurassic (Diyab) source. The

fifth group has characteristics and occurrences

that argue strongly for a distinct, as yet uniden t-

ified, source.

From

B ~ o o K s . J . e d . ) ,

1990, Classic Petroleum Provinces, Geological Society Special Publication

No 50, pp 317-328.

317


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P . J . G R A N T H A M

E T A L .

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so

R U Q F S O U R C E R O C K S

CRUOES 0

Fig . 3 . C15 and C30 r ing d is tr ibu t ions of Om an c rude o i l s and source rock ex t rac ts .

I ; I L UQrCUDEOLp

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N A T IH C R U D E O I L

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Fig . 4 . Typica l gas chromatograms for the sa tura ted

h y d r o c a r b o n s o f th e f iv e ma in g r o u p s o f O m a n c r u d e

oils.

T h e s t r a t i g r a p h y ( F i g . 2 ) i n t h e o i l p r o d u c t i v e

a r e a s o f O m a n h a s re c e n t ly b e e n d e s c ri b e d b y

H u g h e s C l a r k e ( 1 9 8 8) . F o r th e p e t r o l e u m g e -

o l o g y o f O m a n s e e t h e s y n o ps i s i n G r a n t h a m

et

al.

( 1 9 8 8 ) . T h e m o s t s i g n i f i c a n t f e a t u r e o f

O m a n ' s h d y r o c a r b o n h a b i t a t is t h a t s e d i m e n t a r y

r o c k s o f s u c h d i v e r s e a g e s c a n a l l l i e w i t h i n t h e

h y d r o c a r b o n w i n d o w . H e n c e , o r g a n i c - r i c h

h o r i z o n s f o r m i n g p o t e n t i a l s o u r c e r o c k s i n a n y

o f t h e s e a g e i n t e r v a l s ca n h a v e g e n e r a t e d a n d

m a y s t i l l b e g e n e r a t i n g o i l a n d g a s .

C r u d e o i l c h a r a c t e r i z a t i o n a n d o i l s o u r c e

r o c k c o r r e l a t i o n s

T a b l e s 1 - 5 p r e s e n t th e g e o c h em i c a l d a t a o f

s p e ci f ic e x a m p l e s o f t h e f i ve g r o u p s o f O m a n

c r u d e o i l s a n d t h e i r s o u r c e r o c k s . R e p r e s e n t a -

t iv e g a s c h r o m a t o g r a m s o f th e s a t u r a t e d f r a c -

t io n , s t e r a n e f r a g m e n t o g r a m s a n d s te r a n e

c a r b o n n u m b e r d i s t r ib u t i o n s a r e g i v e n i n F i g s

4 - 6 .

H u q f o i ls ( t y pi f ie d b y o il f r o m A m a l

S o u t h - l )

T h e s e o i l s o c c u r i n t h e e a s t e r n f l a n k p r o v i n c e

o f S o u t h O m a n ( A l e y N a s h 1 98 4; d e l a

G r a n d v i l l e 1 9 82 ). T h e y a r e g e n e r a l l y h e a v y a n d

s u l p h u r - r i c h a n d o c c u r i n P a l a e o z o i c c l a s t i c r e s -

e r v o i r s l y i ng a b o v e t h e H u q f se d i m e n t s ( s e e

F ig . 2 ) .

T h e m a i n d i s ti n g u is h i n g g e o c h e m i c a l f e a t u r e s

o f th e s e c r u d e s a r e a s fo l l o w s :

( i) P r i s t a n e / p h y t a n e r a t i o s o f g e n e r a l l y l e ss

th a n 1 .

( i) T h e p r e s e n c e o f t h e ' X ' c o m p o u n d s ( F i g .

4 ; K l o m p 1 9 8 6) .

( ii i) A s t r o n g p r e d o m i n a n c e o f

C 9

s t e r a n e s

( F i g s 5 6 ) . C 29 s t e r a n e p r e d o m i n a n c e s

h a v e a ls o b e e n r e p o r t e d f o r S i b e r i a n c ru d e

o il s o f p r e C a m b r i a n o r ig i n ( s ee A r e f e v

et




6/13

G E O C H E M I S T R Y O F C R U D E O I L S I N O M A N 321

t ' H U Q F ' C R U D E O I L ~ 2 9 ~

29N 29N

'Q ' C R U D E O I L ~ , / ~ '

2 n 27N z g I

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27 Z

2 r R Z / 2 9

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2 7 I 5aIH), 14p(H), 17f l(H) cholestane 20R g~ S

27R 13/J (H) , 7a(H) rearranged or 'd ia ' cholestane 20R 8 S

28N 5a(H), 14a(H), 17e(H) 24-methyl-cholestane 20R 8" S

28] 5a(H), 14/3(H), 17/3(H)24 me thy l cho les tane 20R 8 - S

28R 13 ~( H ), 7aIH) rearranged or 'd ia ' 24-methyl-cholestane 20R 8" S

29N 5aIH), 14a(H), 17~(H) 24-ethyl~cholestane 20R Et S

29] 5aIH), 14f~(H), 17f3(H) 24-ethyl-cholestane 20R ~ S

29R 13/3 (H), 7a(H) rearranged or 'd ia ' 24-ethyl-cholestane 20R ~" S

Fig.

5

Typical s terane f ragm entograms

(m/z

217 +

218 + 259) for the f ive ma in groups of Om an crude

oils.

( iv)

v )

al.

1 98 0; M o l d o w a n

et al.

1 9 85 ; F o w l e r a n d

D o u g l a s 1 9 8 7 ) . F o w l e r a n d D o u g l a s a l s o

r e p o r t t h e p r e s e n c e o f t h e ' X ' c o m p o u n d s

i n t h e i r S i b e r ia n c ru d e s . M e a s u r e d c o n c e n -

t r a t i o n s o f t o t a l s t e r a n e s i n H u q f c r u d e o i ls

a p p r o a c h 6 00 p p m . S i m i la r c o n c e n t r a t io n s

o f s t e r a n e s a r e a l s o f o u n d i n t h e H u q f

s o u r c e r o c k .

T h e a b s e n c e o r v e r y lo w r e l a t iv e c o n t e n t s

( a p p r o x . 5 o r l e ss ) o f r e a r r a n g e d s t e r a n e s

( F i g . 5 ; G r a n t h a m 1 9 8 6 ) .

H i g h l y n e g a t i v e c a r b o n i s o to p e v a l ue s

o f a r o u n d - 3 6 0 . T h e s e c a r b o n i s o t o p e

C Z 8

' : -

7 O

cz? ~ 9

Fig . 6. S terane carbon nu mb er d is t ribut ion for the

f ive main groups of O ma n crude o i ls .

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

o i l v a l u e s k n o w n .

G o o d s o u r c e r o c k s f o r o i l o c c u r i n s o u t h

O m a n i n se v e r a l f o r m a t i o n s o f t h e H u q f g r o u p

( s e e F i g . 7 a n d M e y e r & N e d e r l o f 1 9 8 4 ) . T h e y

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

m a t t e r , m o s t p r o b a b l y o r i g i n a ti n g f r o m b a c-

t e r i a a n d c y a n o - b a c t e r i a . T h e o i l / s o u r c e r o c k

c o r r e l a t i o n w o r k w a s c a r r i e d o u t w i t h so u r c e

r o c k s a m p l e s f ro m w e ll s t h r o u g h o u t t h e H u q f

s e d i m e n t s .

T h e c r u d e o i ls c o r r e l a t e v e r y w e l l w i t h e x t r a c t s

o f H u q f s o u r c e r o c k s ( s e e T a b l e 1 ) . T h e ' X ' -

b r a n c h e d h y d r o c a r b o n s a r e a l w a ys p re s e n t i n

H u q f - d e r i v e d c r u d e o i l s b u t n o t a l w a y s i n a l l

s o u r c e r o c k e x t r a c ts . T h e v a r i a t i o n in c a r b o n

i s o t o p e r a t i o s o f s o u r c e r o c k e x t r a c t s a n d k e r o -

g e n i s s o m e w h a t l a r g e r t h a n t h a t i n t h e o i l s

a t tr i b u te d t o t h em . H o w e v e r , t h is p h e n o m e n o n

i s f r e q u e n t l y o b s e r v e d i n o t h e r o i l / s o u r c e r o c k

p a i r s .

I n t e r e s ti n g l y , t h e s a m e g e o c h e m i c a l f e a t u r e s

a r e f o u n d i n a t a r f r o m w e l l K a r a m p u r - 1 i n

P a k i s t a n ( s e e T a b l e 1 ) . T h e P a k i s t a n a n d s o u t h

O m a n s a lt b a s in s w e r e i n c lo s e p r o x i m i t y a n d

p o s si b ly c o n t i n u o u s d u r i n g t h e i n f r a - C a m b r i a n

( G o r i n et al. 1982) .

T h e Q c r u d e oi ls ( t y p i fi e d b y o i l f r o m

S a y y a l a - 1 )

I n C e n t r a l O m a n a g r o u p o f l i g h t o i ls is f o u n d i n

U p p e r P a l a e o z o i c r e s e r v o i r s (s e e T a b l e 2 ) w h i c h

a r e c h a r a c t e r i z e d b y t h e f o l l o w i n g .

( i) T h e p r e s e n c e o f t h e ' X ' c o m p o u n d s ( a s

i n t h e H u q f o i l s ) .

( ii ) P r i s t a n e / p h y t a n e r a t i o s g r e a t e r t h a n 1 .

( ii i) S t r o n g p r e d o m i n a n c e s o f C 27 s t e r a n e s a n d




7/13

322 P . J . G R A N T H A M

E T A L .

H U Q F S O U R C E R O C K

o t 5 o z . o l

z 9

E

. .

. .

i.i

- ~

. .

Fig. 7. Oman source rocks.

S F I Q S O U R C E R O C K

N T I H S O U R C E R O C K

u

L i t h o h

o

i o o

C y r ~ -

g

Y

re l a t i ve ly h igh concen t ra t i ons o f t r i cyc l i c

t e r p a n e s ( s e e F i g s 5 & 6 a n d G r a n t h a m

1986) . The unusua l s t e rane d i s t r i bu t ion o f

these o i l s i s d i scussed i n m ore de t a i l by

G r a n t h a m & W a k e f i e l d ( 1 9 8 8 ) . S t e r a n e

c o n c e n t r a t i o n s i n th e ' Q ' c r u d e s a r e r o u g h l y

e q u i v a l e n t t o t h o s e f o u n d i n t h e H u q f

c rudes .

( iv ) M o d e r a t e l e ve l s o f r e a r r a n g e d s t e r a n e s

( - 2 0 , s e e F i g. 5 ) .

( v ) C a r b o n i s o to p e ra t i o s o f a r o u n d - 3 0 . 5 0 .

N o n e o f th e k n o w n s o u r c e r o c k s c a n h a v e

sou rced t h i s unusua l o i l . The 'Q ' o i l s con t a in

t h e ' X ' - b r a n c h e d h y d r o c a r b o n s b u t , r e l a t i v e t o

t h e n - a l k a n e s , i n l o w e r c o n c e n t r a t i o n s t h a n i n

the Huqf c rude o i l s ( see F ig . 4 ) . S o fa r , w i th in

O m a n , t h e ' X ' - b r a n c h e d h y d r o c a r b o n s h a v e

b e e n f o u n d o n l y i n t h e H u q f a n d ' Q ' c r u d e s ,

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

iden t i f i ed i n Eas t S ibe r i an c rude o i l s (F owle r &

Doug las 1987) and t en t a t i ve ly i n t he wor ld s

o l d e s t k n o w n c r u d e o i l f r o m t h e P r o t e r o z o i c o f

A u s t r a l i a ( J a c k s o n et al. 1986). I f t hese com -

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

C a m b r i a n , t h e n a s o u r c e f o r t h e ' Q ' c r u d e s a l s o

in t he Huqf g roup i s l i ke ly , a sugges t i on sup -

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

w h e r e H u q f l e v e l s a r e d e e p a n d u n p e n e t r a t e d

by the dri l l and the o i l s are l ight and wel l

m a t u r e d , s u g g e s t i n g a d e e p s o u r c e . H e n c e , a n

undr i l l ed P ro t e rozo i c Huqf l eve l m igh t be t he

sou rce fo r t h i s d i s t i nc t i ve o i l t ype found in

rese rvo i r s i n upper P a l aezo i c c l a s t i c s .

S a f iq o il s t y p i fi e d b y o i l f r o m t h e S a h m a h -

1 w e l l )

In sou thwes t Om an a d i s t i nc t c rude o i l t ype i s

found ( see Tab le 3 ) , i n Upper P a l aeozo ic c l a s -

t i c s , w i th t he fo l l owing geochem ica l fea tu res .

( i ) L o w s u l p h u r c o n t e n t s a n d m e d i u m A P I

gravi t ies .

( ii ) P r i s t a n e / p h y t a n e r a t i o s o f 1 . 6 - 1 . 7

(Tab le 3 ) .

( ii i) S t e r a n e c a r b o n n u m b e r d i s tr i b u ti o n s

w h i c h s h o w a r e d u c e d p r e d o m i n a n c e o f

C 29 s t e r a n e s c o m p a r e d t o H u q f c r u d e s

( a p p r o x i m a t e l y 5 0 , F i g . 5 ) .

( i v) Re l a t i ve ly h igh conce n t ra t i ons o f rea r -

r a n g e d - s t e r a n e s ( 3 0 - 3 9 , F ig . 5 ) .

( v ) C a r b o n i s o t o p e v a l u e s o c c u r in t h e r a n g e

- 2 9 . 9 t o - 3 0. 5 0 .

M o s t g e o c h e m i c a l p a r a m e t e r s s u p p o r t t h e

c o r r e l a t i o n o f t h e o i l f r o m E l f O m a n ' s S a h m a h




8/13

G E O C H E M I S T R Y O F C R U D E O IL S IN O M A N 3 3

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10/13

G E O C H E M I S T R Y O F C R U D E O I LS I N O M A N 3 25

f ie l d w i t h t h e e x t r a ct s o f c o m b i n e d s i d e w a l l

s a m p l e s o f t h e S i l u r i a n S a fi q F m . f r o m w e l l

H a s i r a h - 2 ( s e e T a b l e 3 ) . T h i s F o r m a t i o n h a s a

h i g h o r g a n i c c o n t e n t i n i t s b a s a l s h a l e w h i c h

c o n s t i t u t e s a s o u r c e r o c k ( F i g . 7 , M e y e r &

N e d e r l o f 1 9 84 ).

T h e c o r r e l a t i o n o f o il s a n d e x t r a c t s i s n o t

p e r f e c t ; t h e r e a r e d i f f e r e n c e s i n t h e C ~5 d i s t ri -

b u t i o n s ( F ig . 3 ) . H y d r o u s p y r o l y s i s o f t h e n o t

f u l ly m a t u r e H a s i r a h - 2 s a m p l e a l s o r e s u l te d i n

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

( s e e T a b l e 3 ) . H o w e v e r , t h e m o s t d i a g n o s t i c

p a r a m e t e r s ( s t e r a n e s a n d c a r b o n i s o t o p e s ) c o r -

r e l a t e v e r y w e l l . W e t h e r e f o r e c o n c l u d e t h a t t h e

S a h m a h o il s m o s t p r o b a b l y o r i g i n a t e fr o m t h e

S i l u r i a n S a f i q s o u r c e r o c k .

T h e o i l c h a r a c t e r ( n a p h t h e n i c , l o w s u l p h u r )

i n d i c a t e s a s o u r c e e n v i r o n m e n t i n a m a r i n e

s e t t i n g b u t w i t h t e r r i g e n o u s c l a s t i c i n f l u x . T h i s

i s i n k e e p i n g w i t h t h e s e t t i n g o f t h e g l o b a l

t r a n s g re s s i v e c y c le s o f l a t e O r d o v i c i a n a n d e a r l y

S i l u r i a n a g e c o v e r i n g t h e n o r t h e r n a n d e a s t e r n

s h e l v e s o f t h e A f r o - A r a b i a n c r a t o n s .

Diyab oi ls typif ied by oi l fro m

Lekhwair -8 )

I n t h e n o rt h w e s t o f O m a n a c h a r a c te r i st i c t y p e

o f c r u d e o i l i s f o u n d i n t h e e a s t L e k h w a i r f i e ld

( F ig . 1 ) . T h i s o i l i s p r o d u c e d f r o m B a r r e m i a n -

A p t i a n ( S h u a i b a - K h a r a i b ) r e s e rv o i r s w h i l st

t w o n o n - c o m m e r c i a l a c c u m u l a t i o n s a r e f o u n d

i n U p p e r J u r a s s i c T u w a i q M o u n t a i n F m .

r e s e r v o i r s .

T h i s o i l t y p e h a s t h e f o l l o w i n g c h a r a c t e r i s t i c s

( s e e T a b l e 4 ) .

( i) L o w s u l p h u r a n d m e d i u m A P I g r a v i t ie s .

( ii ) P r i s t a n e / p h y t a n e r a t i o s a b o v e I (e x c e p t i n

t h e l e s s m a t u r e T u w a i q c r u d e s ) .

( i i i ) C 2 9 s t e r a n e s w h i c h a r e j u s t p r e d o m i n a n t

w i t h i n t o t a l s t e r a n e s ( a p p r o x i m a t e l y 5 0 ,

F i g . 5 , T a b l e 4 ) .

( iv ) R e l a t i v e l y h i g h c o n c e n t r a t i o n s o f r e -

a r r a n g e d s t e r a n e s ( 3 1 - 3 9 , F i g . 5 ) .

( v ) C a r b o n i s o t o p e v a l u e s a r o u n d - 2 6 .5 0 .

T h e o i ls a r e g e o c h e m i c a l l y i d e n t i c a l w i t h t h e

o i l f o u n d i n t h e U p p e r J u r a s s ic ( A r a b z o n e ) a n d

L o w e r C r e t a c e o u s ( T h a m a m a ) r e s e r v o ir s i n t h e

U n i t e d A r a b E m i r a t e s a n d Q a t a r ( s e e F r e i

1 9 8 4 ) . T h e r e i s n o g o o d s o u r c e r o c k d e v e l o p e d

i n O m a n t h a t m a t c h e s t h e s e o i l s. S t u d i e s o f t h e

o i ls a n d s o u r c e r o c k s i n A b u D h a b i ( H a s s a n &

A z e r 1 98 5; A l s h a r a n 1 98 5 ) c o n c l u d e t h a t t h e o i l

i n t h e J u r a s s i c - L o w e r C r e t a c e o u s r e s e rv o i r s

o r i g i n a t e s f r o m U p p e r J u r a s si c s o u r c e r o c k s

( H a n i f a o r D i y a b F o r m a t i o n s ) . T h e e a s t

L e k h w a i r o il s a r e t h e r e f o r e i n t e r p r e t e d t o h a v e

m i g r a t e d l a t e r a l l y f r o m s u c h a s o u r c e o u t s i d e

O m a n . T h e o c c u r r e n c e o f a n o n - c o m m e r c i a l

a c c u m u l a t i o n o f t h i s o i l i n a n U p p e r J u r a s s i c

r e s e rv o i r ( T u w a i q M o u n t a i n F o r m a t i o n ) i n w e ll

D h u l a i m a - 1 c l o s e t o L e k h w a i r s u p p o r t s t h e

p o s t u l a t e t h a t t h e o i l i s f r o m a J u r a s s i c r a t h e r

t h a n a C r e t a c e o u s s o u r c e . T h e r e i s a l s o t h e

p o s s i b il i t y t h a t s o m e o f th e o i l m a y h a v e b e e n

s u p p l i e d b y o r g a n i c - r i c h A p t i a n m a r i n e s e d i -

m e n t s ( B a b f a c ie s o f t h e S h u a i b a F o r m a t i o n )

b u t n o s a t i s f a c t o r y o i l / s o u r c e r o c k c o r r e l a t i o n s

f o r t h e S h u a i b a c o u l d b e e s t a b l i s h e d .

Natih oi ls typi fied by oi l fro m Shibkah-1)

T h e N a t i h F o r m a t i o n f o r m s t h e r e s e r v o i r f o r

m a n y o f t h e n o r t h e r n O m a n o i l f ie l ds b u t a l s o

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

s o u r c e r o c k s ( F ig . 7 ) . T h e r e i s n o g e o c h e m i c a l

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

u p p e r l e v e l ( N a t i h ' b ' ) is p r o b a b l y t h e m o r e

i m p o r t a n t s o u rc e r o c k , w h e r e a s i n S a ud i A r a b i a

t h e l o w e r l ev e l ( N a t i h ' e ' , S a f a n i y a so u r c e r o c k )

i s t h e b e t t e r d e v e l o p e d p o t e n t i a l s o u r c e ( N e w e l l

& H e n n i n g t o n , 1 9 8 3 ; L e h n e r 1 9 8 4 ) .

T h e N a t i h o i ls h a v e t h e f o l l o w i n g c h a r a c t e r -

i s t i c s ( s e e T a b l e 5 ) .

( i) L o w A P I g r a v it y a n d m o d e r a t e l y h i g h

s u l p h u r .

( i i) P r i s t a n e / p h y t a n e r a t i o s o f c . 1 . 0.

( ii i) R e a r r a n g e d s t e r a n e s a r e r e l a t i v e l y l o w

(5 , F ig . 5 ) .

( i v ) S t e r a n e s w i t h a n e q u i v a l e n t d i s t r i b u t i o n

o f t h e C27 C28 an d C 29 i s om er s (F ig . 6 ) .

( v ) C a r b o n is o t o p e v a l u e s o f a r o u n d - 2 6 .9 0 .

T h e s t e r a n e d i s t r i b u t i o n s a re v e r y s i m i l a r t o

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

f r o m C r e t a c e o u s c a r b o n a t e s ( G r a n t h a m &

W a k e f i e l d 1 9 8 8 ) .

T a b l e 5 g i ve s g e o c h e m i c a l d a t a o n a n u m b e r

o f e x tr a c t s o f t h e N a t i h s o u r c e r o c k . T h e e x t r a c t s

c o r r e l a t e w e l l w i t h t h r e e N a t i h c r u d e o i ls f r o m

w e l l s S h i b k a h - 1 , S u q t a n - 1 a n d N a t i h W e s t - 5 7

( s a m e t a b l e ) .

o n c l u s i o n s

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

a p i c t u r e o f c o n s i d e r a b l e v a r i e t y . O i l a c c u m u -

l a t i o n s a n d s o u r c e r o c k o c c u r r e n c e s a r e f o u n d

g e o g r a p h i c a ll y s p r e a d t h r o u g h o u t t h e c o u n t r y

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

I n f r a - C a m b r i a n ( c . 6 50 M a ) t o t h e T e r t i a r y .

T h e m a i n l e v e l s o f so u r c e r o c k s h a v e b e e n

i d e n ti f ie d in t h e I n f r a - C a m b r i a n H u q f , t h e

S i l u r i a n S a f i q , t h e U p p e r J u r a s s i c D i y a b a n d

t h e C r e t a c e o u s B a b a n d N a t i h f o r m a t i o n s .

F i v e t y p e s o f c r u d e o i l a r e r e c o g n i z e d , f o u r o f

w h i c h c a n b e c o r r e l a t e d t o o n e o f t h e s o u r c e




11/13

3 2 6 P . J . G R A N T H A M E T A L

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t r t r t r

t ' ~ t ' r t ' ~ t " - .l

t ' x l t ' -, I t "q t " - I

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t " , l t ' - I t ' x l ~ - I

e r t ' M

t r " ~ " t " - ~

t " - t ~ t ' ~ e r

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12/13

G E O C H E M I S T R Y O F C R U D E O IL S IN O M A N

T a b l e 6 . Main geochemical characteristics of Oman crude oils

327

A PI Isoprenoids ' X ' Sterane

grav compounds carbon

number

Rearr. Carbon

sterane isotope

content ratio

0

H u q f 2 5 - 3 0 P h p r e d o m p r e se n t strong C29

o v e r P r p r e d o m i n a n c e

' Q ' 4 0 - 5 0 P r p r e d o m p r e s e n t s t ro n g C 27

o v e r P h p r e d o m i n a n c e

S a f iq 3 7 - 3 9 d i t to a b s e n t w e a k C 29

p r e d o m i n a n c e

D i y a b - 3 5 d i t t o a b s e n t w e a k C 29

p r e d o m i n a n c e

N a t i h ~ 3 0 d i t t o a b s e n t C27 C28 C29

e q u i v a l e n t

l o w - - 3 6 . 0

l o w - 3 0 . 5

h i g h - 3 0 . 0

h i g h - 2 6 . 5

l o w - 2 6 . 5

P h = p h y t a n e ; P r = p r i s ta n e

r o c k s d e s c r i b e d a b o v e . F o r o n e t y p e o f o il ( 'Q '

o i ls ) th e s o u r c e is a s y e t u n k n o w n b u t t h e r e a r e

i m p l i c a t i o n s th a t i t i s a l so f r o m a H u q f G r o u p

s o u r c e r o c k . T a b l e 6 p r o v i d e s a s u m m a r y o f t h e

s i g n i fi c a n t g e o c h e m i c a l f e a t u r e s o f t h e f i ve

O m a n o i l g r o u p s .

T h i s i n t e r p r e t a t i o n o f t h e o i l h a b i t a t o f O m a n i s t h e

r e s u lt o f t h e w o r k o f m a n y S h e l l g e o l o g is t s a n d g e o -

c h e m i s t s w h o c a n n o t a l l b e m e n t i o n e d h e r e . T h e i r

c o n t r i b u t i o n s a r e a p p r e c i a t e d . I n p a r t i c u l a r w e a r e

i n d e b t e d t o D r . M . W . H u g h e s C l a r k e a n d D r . R . J .

W i l l i n k f o r t h e i r h e l p f u l d i s c u s s i o n s i n i n t e g r a t i n g t h e

g e o c h e m i s t r y i n t o t h e g e o l o g i c a l c o n t e x t o f O m a n .

T h i s p u b l ic a t i o n is s u p p o r t e d a n d a u t h o r i z e d b y S h e l l

I n t e r n a t i o n a l P e t r o l e u m M a a t s c h a p p i j N . V . , S h e l l

R e s e ar c h B . V ., P e t r o l e u m D e v e l o p m e n t O m a n L L C

a n d t h e M i n i s t r y o f P e t r o l e u m a n d M i n e r a l s i n t h e

S u l t a n a t e o f O m a n . W e t h a n k t h e s e a u t h o r it i e s f o r

t h e i r p e r m i s s i o n t o p u b l i s h t h i s a r t i c l e . A d d i t i o n a l l y ,

E l f - A q u i t a n e O m a n h a s k i n d l y a g r e e d t h a t w e m a y

i n c l u d e o u r a n a l y s e s o f o il s f r o m t h e i r S a h m a h - 1 f i e l d.

M e m b e r s o f th e G e o c h e m i st r y D e p a r t m e n t o f K S E P L

f r o m 1 9 7 5 t o 1 9 8 5 a r e t h a n k e d f o r t h e i r a n a l y t i c a l

a s s i s t a n c e .

e f e r e n c e s

A L E Y , A . A . N A S H , D . F . 1 98 4. A s u m m a r y o f t h e

g e o l o g y a n d o il h a b i t a t o f t h e e a s t e r n f l a n k h y -

d r o c a r b o n p r o v i n c e o f S o ut h O m a n . Proceedings

of OAPEC Seminar Source and Habitat of Petro-

leum in the Arab Countries Kuwait, October,

1984.

ALSHARAN A. S. 1 9 8 5 . D e p o s i t i o n a l e n v i r o n m e n t ,

r e s e r v o i r u n it s e v o l u t i o n a n d h y d r o c a r b o n h a b i t a t

o f S h u a i b a F o r m a t i o n , L o w e r

Cretaceous, Abu

D h a b i , U n i t e d A r a b E m i r a te s .

American As-

sociation of Petroleum Geologists Bulletin.,

6 9 ( 6 ) , 8 9 9 - 9 1 2 .

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Geochemistry of Crude Oils in Oman