Geochemistry of Crude Oils in Oman
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Transcript of Geochemistry of Crude Oils in Oman
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8/19/2019 Geochemistry of Crude Oils in Oman
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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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Notes
© The Geological Society of London 2013
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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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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 .
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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
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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
9 \ ZZN 2/
z e R
] S A F I Q C R U D E O I L ~ / ~ ,
27 Z
2 r R Z / 2 9
/ \ 2 8 2 /Y 2 ~ ~, ~?
I ' 2 s R ~ ~ 2 B . 2 a .
f . .
I
J ~ . l / < ?
. ~ . . . .1 1 t
T
27~
q ' N A T I H ' C R U D E O I L i / I ~ ~ 2 9~
~ 7 . [ / /
Z7N / 28N 29N 9 N
X 20N
/
27N 5a(H), 14cdH), 17a(H) cholestane 20R Et S
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
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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
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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
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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
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A PI Isoprenoids ' X ' Sterane
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Rearr. Carbon
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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
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