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E L S E V I E R
Marine and Petroleum Geology 15 (1998) 57-72
Mar ine and
Petroleum Geology
The re lationship b e tween pe tro leum emp lacem ent and carbonate
reservoir quality exam ples from A bu D ha bi and the A m u
Darya Bas in
J o y c e E . N e i l s o n a * l, N o r m a n H . O x t o b y a 2, M i c h a e l D . S i m m o n s b 3, I v o r R . S i m p s o n b 4,
N a t a l i a K . F o r t u n a t o v a c
BP Research, Chertsev Road Sunbury-on-Thames, Middlesex. TW16 7LN, UK
bBP Exploration and Production, Uxbridge One, 1 Harefield Road, Uxbridge, Middlesex, UB8 IPD. UK
VN1GNI, 36 Shosse Entusiastov, Moscow 105819, Russia
Received 12 February 1996; revised 28 May 1997; accepted 31 May 1997
A b s t r a c t
The relative import ance of petrole um emplacement in inhibi ting diagenetic processes and preserving porosity and permeabilit y
in Lower Cretaceous, Thamama Group (Kharaib Formation) carbonate reservoirs of Abu Dhabi, UAE, and in Callovian Kim-
meridgian carbonate reservoirs of the A mu Darya Basin in Uzbekistan and Turkmenistan, has been evaluated by combining geologic,
petrophysical and geochemical data. When petrol eum emplaceme nt is synchronous with and prior to significant burial cementat ion
in carbonates, pr imary petr oleum inclusi ons are trapped in the cements. The process appears to be characterised by steep intra-field
porosity~:lepth trends within a more gradual regional decline in porosity with depth. This has profound implications for the
prediction of porosity in carbonate reservoirs.
Reservoir quality is better in grainstones and packstones compared to adjacent wackestones and lime mudstones in the Kharaib
Formation because of preserved macroporosity (intergranular, vuggy, mouldic); the pore system in the finer units is dominated by
micropores. These features indicate a primar y textural control on porosity and permeability. Within the grainstone s and packstones,
macroporosity is variably filled by late equant sparry calcite cements. Porosity and permeability variations in grainstones and
packstones at a reservoir scale are therefore controlled by the variation in amount of equant sparry calcite cement. This in turn
depends on the timi ng of the precipitat ion of this cement relative to petroleum emplacement, as shown by fluid inclusion data. Where
petroleum emplacement has occurred relatively early, at migration foci, prior to significant burial cementation by equant sparry
calcite, reservoir quality is preserved. Where it has occurred after significant burial ceme ntati on, reservoir qualit y has been destroyed.
In the A mu Darya sequences, primary macropor osity is commo nly preserved down to depths of I 1,000 ft (3.5 km) with differences
in the porosity and permeability characteristics of grainstones being controlled by variations in the amount of early, probably
freshwater, cement and the extent of associated dissolution. Small volumes of burial cements do occur, but they do not contain
petroleum incl usions. Conseque ntly, there is no firm evidence that pet roleum emplace ment has inhibit ed diagenesis in this area. This
part of the study has shown that it is not always possible to obta in conclusive evidence from the diagenesis to pin down the processes
responsible for the preservatio n of reservoir quality and t hat petroleu m filling may not always be the primary cause.
The rela tionships document ed here show that the +race for space' between diagenetic waters and petroleum is a major cont rol on
reservoir quality in the Thamama Group carbonate reservoirs, but is not so important for the Jurassic carbonates in the Amu Darya
basin. ~ 1998 Elsevier Science Ltd. All rights reserved.
Kevwords: Carbonates Reservoir quality; Diagenesis; Petroleum; Abu Dhabi; Amu Darya; Thamama Group
*Corresponding author. Tel: + 44 1224 868416; fax + 44 1224 868416
~Present address: Carbonate Reservoirs Ltd., l l Cairn Crescent,
Cults, Aberdeen, ABI5 9TW, UK.
2Present address: Department of Geology, Royal Holloway, Uni-
versity of London, Egham, Surrey, TW20 0EX, UK
Present address: Department of Geology and Petroleum Geology,
University of Aberdeen, Meston Building, King's College, Aberdeen,
AB9 2UE, UK
4present address: BP Australia Ltd.+ 1 Albert Road, Melbourne,
Victoria, 3004, Australia
S0264-8172/98/$19.00 ,~ 1998 Elsevier Science Ltd. All rights reserved.
P11:S0264 8172(97)00033 0
I I n t r o d u c t i o n
The controls on the spatial variations in porosity and
permeabili ty observed in carb onat e rocks have been
understood for many years. Special Publication Number
36
of the
Society o f Economi c Paleontologists and Min-
eralogists (Sch neide rmann & Harris , 1985) docu ments
most of the processes that can po tential ly act upon car-
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58 J.E. Neilson et al./Ma rine an d Petroleum Geolo,q3 15 (1998) 5~72
b o n a t e r o c k s d u r i n g d i a g e n es i s, b u t d e f i n in g t h e r e l a ti v e
i m p o r t a n c e o f t h e s e c o n t r o l s i n a n y s p e ci f ic a r e a h a s
p r o v e d f a r m o r e e l u s iv e . A s S c h o l l e a n d H a l l e y ( 1 9 85 )
p o i n t o u t , t h e r e i s a c r i ti c a l n e e d t o in d e p e n d e n t l y
c a s s es s h o w r a t e s o f p o r o s i t y l o s s w i t h d e p t h a r e a f f e c t e d
b y t i m e , t e m p e r a t u r e , d e p o s i t i o n a l s e t t i n g , e a r l y d i a -
g e n e ti c h i st o r y , m a t u r a t i o n h i s t o r y o f o r g a n i c m a t t e r , a n d
. . . o v e r p r e s s u r i n g , e a r l y o i l m i g r a t i o n , d o l o m i t i s a t i o n
a n d h y d r o t h e r m a l a l t e r a t i o n . T h e s e i de a s h a ve m o r e
t h a n a c a d e m i c i n t e r e s t , s i n c e t h e y h a v e p r o f o u n d i m p l i -
c a t i o n s f o r t h e m e t h o d s u s e d t o p r e d i c t t h e d i s t r i b u t i o n
o f p o r o s i t y a n d p o r e t y p e s i n p e t r o l e u m a c c u m u l a t i o n s .
In th i s pape r , we l ink geochem ica l wi th geo log ic and
p e t r o p h y s i c a l d a t a t o a d d r e s s t h is p r o b l e m a s i t is m a n i f e s t e d
in th e T h a m a m a G r o u p ( K h a r a i b F o r m a t i o n ) o f A b u
D h a b i a n d t h e J u r a s s ic r e s e r v o ir s o f th e A i n u D a r y a r e g i o n
o f U z b e k i s t a n a n d T u r k m e n i s t a n . W e h a v e o b t a i n e d f l u id
i n c l u s i o n a n d i s o t o p i c ( C , O , S r ) d a t a f r o m s p a r r y c a l c i t e
a n d p e t r o g r a p h i c a l l y l a t e d o l o m i t e c e m e n t s . T h e i s o t o p i c
d a t a w il l h e lp t o e v a l u a t e t h e e x t e n t o f s o l u t e i m p o r t , w h i l e
the f lu id inc lus ion da ta wi l l he lp to cons t ra in the t em -
pe ra tu re an d s a l in i ty o f the p rec ip i t a t ing wa te rs , a s we l l a s
t h e t e m p e r a t u r e s o f p e t r o l e u m e m p l a c e m e n t . T h e f u n -
d a m e n t a l a i m o f t h is w o r k i s t o a d d r e s s t h e q u e s t io n : t o
w h a t e x t e n t h a s p e t r o l e u m i n h i b i t e d o r s t o p p e d d i a g e n e s i s
i n A b u D h a b i a n d t h e A i n u D a r y a B a s in ? T o b e g i n t o
a n s w e r t h i s q u e s t i o n w e m u s t a s k t h e s u p p l e m e n t a r y q u e s -
t ion: Does pe troleu m inhibit or s lop d iayenes is?
i n b o t h c a r b o n a t e a n d s a n d s t o n e p e t r o l e u m r e s e r v o i r s ,
p o r o s i t y a n d p e r m e a b i l i t y m a y s h o w s y s t e m a t i c d e c l i n e s
f r o m c r e s t t o f l a n k a n d o c c a s i o n a l l y s t e p - c h a n g e s a t t h e
pe t ro leum wa te r co n tac t (e. g. F eaze l and S ch a tz inge r , 1985;
Gluyas e t a l . , 1993) . I t has o f ten been sugges ted tha t pe t -
r o l e u m e m p l a c e m e n t h a s c a u s e d t h i s , b y i n h i b i t i n g d i a -
gene t i c p roces ses , l ead ing to poros i ty p rese rva t ion in pe t -
r o l e u m c o l u m n s , a n d g e n e r a t i n g a c o n t r a s t in p o r o s i t y a n d
p e r m e a b i l i t y b e t w e e n t h e p e t r o l e u m c o l u m n s a n d t h e i r
a s soc ia ted aqu i fe rs . In the d i s cus s ion (S ec t ion 7 ) , we com -
p a r e o u r r e s u l t s w i t h s o m e c a s e s t u d i e s b y o t h e r w o r k e r s .
In the fo l lowing , we d i s cus s the m echanism s by which
p e t r o l e u m e m p l a c e m e n t m i g h t i n f l u e n c e c e m e n t a t i o n .
F e a z e l a n d S c h a t z i n g e r ( 1 9 8 5 ) s t a t e t h a t t h e d i s-
p l a c e m e n t o f p o r e w a t e r b y o i l o r g a s p r e v e n t s o r s ig -
n i f ic a n t ly r e t a r d s t h e p r e c i p i t a t i o n o f c e m e n t a s m o s t
m i n e r a l s a r e i n s o l u b l e in h y d r o c a r b o n f l u id s . I n f a c t ,
t h e s y s t e m a t ic v a r i a t i o n o f c e m e n t v o l u m e s w i t h d e p t h
m a y b e c o n t r o l l e d b y p e t r o l e u m e m p l a c e m e n t i n t h r e e
b a s i c w a y s :
2 . T h e m a s s t r a n s f e r p r o c e s s e s o f d i f fu s i o n a n d w a t e r
f l o w m a y b e s l o w e d . S c h o l l e a n d H a l l e y ( 1 9 8 5 ) i n d i c a t e
t h a t t h i s c o u l d b e i m p o r t a n t . M u r p h y e t a l. (1 9 8 9 )
s h o w t h a t d i f f u s i o n i s t h e r a t e - l i m i t i n g p r o c e s s f o r
c a l c it e d is s o l u t i o n a n d p r e c i p i t a t i o n a t t h e t e m -
p e r a t u r es o f p e tr o l e u m e m p l a c e m e n t . T h e r e f o r e
p e t r o l e u m s h o u l d a l s o i n h i b i t c a r b o n a t e d i a g e n e s i s
( e . g . p r e s s u r e d i s s o l u t i o n a n d c e m e n t a t i o n ) .
3 . P e t r o l e u m e m p l a c e m e n t m a y i n h i b i t t h e r e l e a s e o t
m i n e r a l i s i n g s o l u t i o n s f r o m t h e c e m e n t s o u r c e s . F i e l d
e v i d e n c e c o l l e c te d b y n u m e r o u s w o r k e r s , f r o m D u n -
n i n g t o n ( 1 9 6 7 ) t o O s w a l d e t a l . ( 1 9 9 5 ) , h a s s u g g e s t e d
t h a t s t y l o l i t i s a t i o n c a n b e i n h i b i t e d b y p e t r o l e u m
e m p l a c e m e n t , w h i l e W o n g a n d O l d e r s h a w ( 1 98 1 ) h a v e
s h o w n t h a t s t y l o l i t e s c a n b e a s i g n i f i c a n t c e m e n t
s o u r c e . T h u s i f s t y l o l i t i s a t i o n is in h i b i t e d b y p e t r o l e u m
e m p l a c e m e n t s o m i g h t c e m e n t a t i o n . S u c h c e m e n t s
s h o u l d c o n t a i n g e o c h e m i c a l s i g n a t u r e s o f a l o ca l
s o u r c e , e .g . c a r b o n a n d s t r o n t i u m i s o t o p e s.
T h e w o r k d i s c u s s e d a b o v e s h o w s t h a t t h e r e a r e r e a s o n -
a b l e g r o u n d s f o r s u p p o s i n g t h a t p e t r o l e u m d o e s h a v e a n
i n h i b i t i n g e f f e c t o n d i a g e n e t i c p r o c e s s e s i n c a r b o n a t e s . I n
o r d e r [ k3 r t h i s t o o c c u r , i t is n e c e s s a r y f o r t h e
p r o c e s s
l e a d i n g t o c e m e n t a t i o n t o o v e r l a p p e t r o l e u m e m p l a c e -
m e n t . C o n c l u s i v e e v i d e n c e f o r t h i s s y n c h r o n e i t y m a y b e
p r o v i d e d b y t h e o c c u r r e n c e o f p r im a r y p e t r o le u m
i n c l u s i o n s i n t h e c e m e n t s ( e . g . S e l l w o o d e t a l ., 1 9 93 ). T h e
i d e n t i f i c a t i o n o f s y s t e m a t i c s p a t i a l i n c r e a s e s in c e m e n t
v o l u m e s a n d s y s t e m a t i c d e c r e a s e s i n p o r o s i t y a n d p e r -
m e a b i l i t y a c r o s s a p e t r o l e u m - f i l l e d s t r u c t u r e , a l t h o u g h
s u g g e st iv e , a r e n o t c o n c l u s i v e . L o w e r p e r m e a b i l i t y r o c k s
( e . g . l i m e - m u d s t o n e s ) g e n e r a l l y h a v e l o w e r p e t r o l e u m
s a t u r a t i o n s ( o r h i g h e r S w , s e e e . g . c h a p t e r 5 i n A r c h e r &
W a l l , 1 9 95 ). T h e r e f o r e , a p r e - e x i s t i n g c o n t r a s t i n p o r -
o p e r m c a n p r o d u c e e f f ec t s w h i c h l o o k a s t h o u g h t h e y
a r e c a u s e d b y p e t r o l e u m e m p l a c e m e n t . C o n s e q u e n t l y , t o
i s o l a t e a p e t r o l e u m e m p l a c e m e n t e f f e c t , i t i s n e c e s s a r y
t o s e l e c t r o c k u n i t s w h i c h h a v e s i m i l a r p r e - c e m e n t a t i o n
t e x t u r e s a n d e a r l y d i a g e n e t i c h i s t o r i e s o v e r a r a n g e o f
d e p t h s .
C o a r s e g r a i n e d u n i t s c o n t a i n i n g s p a r r y c a l c i t e a n d
l o ca ll y , d o l o m i t e c e m e n ts i n th e T h a m a m a G r o u p
( K h a r a i b F o r m a t i o n ) r e s e r v o i rs o f A b u D h a b i ( F i g . 1 )
a n d i n th e A i n u D a r y a a r e a o f U z b e k i s t a n a n d T u r k -
m e n i s t a n ( F i g. 2 ) p r o v i d e m a t e r i a l w h i c h c a n b e u s e d t o
e v a l u a t e t h e e f fe c t o f p e t r o l e u m e m p l a c e m e n t o n c e m e n -
t a t i o n a n d p o r o p e r m w t r i a t i o n .
I . P h y s i c a l o r c h e m i c a l c o v e r a g e o f m i n e r a l s u r f a c e s b y
p e t r o l e u m o r r e l a t e d o r g a n i c c o m p o u n d s m a y s h i e l d
p a r t s o f t h e s u r f a c e f r o m a c c e s s b y d i a g e n e t i c f l u id s .
S u e s s ( 1 9 7 0 ) s h o w e d t h a t t h i s p r o c e s s w a s s i g n i f i c a n t
f o r c a lc i t e a n d d o l o m i t e i n th e p r e s e n c e o f o r g a n i c
a c i ds a t s u r f a c e c o n d i t i o n s , a n d is r ea d i l y e x t r a p o l a t e d
t o s u b s u r f a c e c o n d i t i o n s .
2 Geolog ic s e t t ing
2.1 . Kharaib b~rmation reser~:oirs ( L o w e r C r e tac e ous ,
A b u D h a b i
T h e K h a r a i b F o r m a t i o n o f th e T h a m a m a G r o u p ( F ig .
3 ) w a s d e p o s i t e d d u r i n g t h e c o n s t r u c t i o n o f t h e b r o a d ,
7/23/2019 The Relationship Between Petroleum Emplacement and Carbonate Reservoir Quality- Examples From Abu Dhabi a…
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J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 5 ~7 2
G e n e r a l i s e d L o c a t i o n M a p S h o w i n g t h e P o s i t i o n o f A b u D h a b i F i e l d s
59
, ,, A b u . I ~ , ,
L e ge n d I , D h a b l
~ ' A s a b
( , , '
. . . . B o u n d a r y " , __ .
I . ~ r S h a h Q u s a hw i ra ;
F ie ld . . . . . . . . . . ~ - - -" _ )
& I S a u d iA r a b ia : l l b . . . . . , ~ . ~ . j M e n d e r
F i g . 1 . G e n e r a l is e d lo c a t i o n m a p s h o w i n g t he p o s i t io n o f A b u D h a b i f i el d s, U A E .
O m a n
0 5 0 k m
I I
regionally extensive carbonate shel f which covered much
of the eastern Arabian Plate during the early Cretaceous
(Murris, 1980). It is some 100-200 ft in thickness and like
much of the early Cretaceous section, it is composed of
cyclical shelf carbonate sediments. The basic cycle patte rn
is of a gradational coarsening-up cycle sharply overlain
by the succeeding cycle. This pattern can be recognised
at various length-scales, and its multiple superposition
results in variable distances between facies changes
through a vertical section. At the scale of the formation
as a whole, basal O r b i t o l i n a lime-mudstones pass
upwards into oncoidal and algal packstones and grain-
stones, while the upper half is dominated by rudist-rich
packstones and grainstones and miliolid grainstones
(Hughes-Clark, 1988). In Qatar and parts of Abu Dhabi,
the top of the Kharaib Forma tion is marked by the shaley
Hawar Member which represents a major maximum
flooding surface (Simmons, 1994). Outcrop equivalents
in Oman show two smaller scales of cyclicity (Simmons,
1994). Large-scale cycles are composed of doublets of
massive bioclastic wackestone-packstone units overlain
by argillaceous mudstone-wackestones. Smaller scale
cycles in the massive units are composed of coarsening-
upward mudstone-grainstone packets, often sharply
overlain by further mudstone. The major units can be
correlated on a regional scale, but individual grainstones
cannot.
2 . 2 . J u r a s s i c r e se r v oi r s i n t h e A m u D a r y a B a s i n T u r k -
m e n i s t a n a n d U z b e k i s t a n )
During Callovian times, a southward-facing carbonate
ramp developed on the northern margin of Tethys,
coincident with the present northern margin of the Amu
Darya Basin (Ilyn and Fortunatova, 1988). The ramp
evolved to a rimmed shelf with marginal reefs following
a major relative sea-level rise in the Oxfordian. Shelfal
facies included back-reef shoals, lagoonal and tidal
deposits, while pinnacle reefs and carbonate turbidite
fans developed basinward of the reef margin (Fig. 4).
Carbonate deposition ceased during the Late Kim-
meridgian when the Amu Darya Basin became restricted
and a succession of evaporites was deposited. VNIGNI
(the All Russia Geological Research Oil Prospecting
Institute) geologists have studied material from the Jur-
assic (Callovian-Kimmeridgian) carbonate reservoirs of
the Ainu Darya Basin (Fig. 2) for many years and have
amassed a wealth of information regarding the sedi-
mentology and reservoir quality of the region. A sum-
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60
3.E. Neilson et al./ Marine and Petroleum Geology 15 (1998) 57-72
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J.E. Nei on et al./Marine and Petroleum Geology 15 (1998) 57-72 61
W
ABU DHABI
<
C E N T R A L O M A N
M O U N T A I N S S T U D Y A R E A
Z ~ ~ . ~ . ,.,,
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S H U A I B A F M .
i
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K H A R A I B F M .
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* N A S R F M . ' 0 0
~ i HABSHA N FM
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z [ ]
n
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,-n .
] - R eserv o i rZ o n e
F i g . 3 . L o w e r C r e t a c e o u s l i t h o s t r a t i g r a p h y , N o r t h e a s t A r a b i a ( f r o m S i m m o n s , 1 9 94 ). R e s e r v o i r z o n e s ( X ) a r e m a r k e d b y t h e l e t t er s A I a n d n u m e r a l s
V V I o f l o c a l n o m e n c l a t u r e .
m a r y o f th is i n f o r m a t i o n c a n b e f o u n d i n I ly n a n d
F o r t u n a t o v a ( 1 9 8 8 ) .
3 M e t h o d s
T h e d a t a w e r e p o r t h e r e w e r e o b t a i n e d d u r i n g r e g i o n a l
s u b s u r f a c e a n d f ie ld s t u d ie s o f c a r b o n a t e r e s e r v o i r r o c k s
f r o m A b u D h a b i a n d t h e A m u D a r y a B a s i n ( T u r k -
m e n i s t a n a n d U z b e k i s t a n ) .
T h e s a m p l e s e t f r o m A b u D h a b i c o m p r i s e s a p p r o x i -
m a t e l y 2 00 c o re s a m p l e s o f t h e K h a r a i b F o r m a t i o n
( T h a m a m a I I o r T h a m a m a B r e se r v o ir s o f l o ca l
n o m e n c l a t u r e ) f r o m o v e r 2 0 w e ll s p e n e t r a t i n g t h e T h a m -
a m a G r o u p ( L o w e r C r e t a c eo u s ) i n a n u m b e r o f oi lf ie ld s
a n d s e l e c t e d w e l ls in A b u D h a b i ( F i g . 1 ). T h i n s e c t i o n s
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62
J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 5~ 72
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63
a n d p o l i s h e d w a fe r s w e re a n a l y s e d u s i n g p e t ro g ra p h i c ,
f lu id inc lus ion , SEM and i so top ic (C , O and Sr) t ech -
n i q u e s t o e v a l u a t e t h e m a j o r s e d i m e n t o l o g i c a l a n d d i a -
g e n e t i c c o n t ro l s o n r e s e rv o i r q u a l i t y i n w h a t i s o n e o f
t h e m a j o r r e s e rv o i r s i n A b u D h a b i . P o ro s i t y a n d p e r -
m e a b i l it y d a t a w e r e o b t a i n e d f r o m o il c o m p a n y r e c o r d s
a n d h a d b e e n c o l l e c t e d b y s t a n d a rd p e t ro l e u m e n g i n -
eer ing t echn iques du r ing co re p lug ana lys i s .
T h e s a m p l e s e t f r o m t h e A m u D a r y a B a s in w a s m u c h
smal le r (17 samples f rom e igh t wel l s ) . Un l ike the Abu
D h a b i s t u d y , w e ll c o n s t r a i n e d d a t a s e t s f r o m i n d i v i d u a l
f ie ld s w e re n o t a v a i l a b l e b u t t h e o b s e rv a t i o n s a r e o f i n te r -
e s t w h e n c o n s i d e r in g t h e ro l e t h a t p e t ro l e u m p l a y s in t h e
bur ia l d iagenes i s o f reservo i rs . Th in sec t ions and po l i shed
wafe rs were exam ined us ing pe t rog rap h ic , f lu id inc lus ion ,
S E M a n d i s o t o p i c (C , O ) t e c h n i q u e s t o d e t e rm i n e t h e
m a j o r c o n t ro l s o n r e s e rv o i r q u a l it y .
P e t ro l e u m i n c l u s i o n s w e re r e c o g n i s e d b y t h e i r f l u o r -
escence und er inc iden t U V i l lumina t ion . Burru ss (1991)
g i v e s a d d i t i o n a l c r i t e r i a w h i c h p e rm i t p e t ro l e u m
i n c l u s i o n s t o b e d i s t i n g u i s h e d f ro m o t h e r f l u o re s c e n t
c o m p o n e n t s i n s e d i m e n t a ry ro c k s . Tw o p ro p e r t i e s o f p e t -
ro l e u m i n c l u si o n s a r e o f m a j o r i n t e re s t : t h e i r c o m p o s i t i o n
a n d t h e i r d i s t r i b u t i o n . W e h a v e c o l l e c t e d d i s t r i b u t i o n
d a t a b y t h e m e t h o d o f O x t o b y e t a l. ( 19 9 5) , a n d c o m -
p o s i t i o n a l d a t a f ro m f l u o re s c e n c e a n a l y s i s ( i n c l u d i n g
s p e c t ro m e t ry u s i n g a Ze i s s U M S P 5 0 ) , a n d m i c ro -
t h e rm o m e t ry . F l u o re s c e n c e c o l o u r s a re i n t e rp r e t e d in
t e rm s o f A P I g r a v it ie s b y t he m e t h o d o f H a g e m a n n a n d
Hol le rbach (1986) .
A m i c r o s c o p e - m o u n t e d L i n k a m T H M 6 0 0 h e a t i n g -
c o o li n g s ta g e w a s u s e d f o r m i c r o t h e r m o m e t r y o f a q u e o u s
a n d p e t ro l e u m i n c lu s i on s . Th e s t a g e g i v es a m e a s u re m e n t
p re c i s io n o f _ 0 .1 ° C , a n d a n a c c u ra c y o f b e t t e r t h a n
1° C t h ro u g h o u t t h e t e m p e ra t u r e r a n g e o f th e i n c lu s i o n s
s t u d i e d . B u l k s t a n d a rd s a n d s y n t h e t i c i n c l u s i o n s w e re
u s e d fo r s ta g e c a li b r a t io n . A l l te m p e ra t u r e s m e a s u re d o n
f l ui d i n c lu s i o n s w e re o b t a i n e d b y t e m p e ra t u r e c y c l in g t o
o b t a i n m a x i m u m p re c i s i o n (R o e d d e r , 1 9 8 4 ) . H o m o -
gen iza t ion , a nd f ina l i ce mel t ing we re rou t ine ly measur ed ,
b u t a f e w g a s h y d ra t e d i s s o c i a ti o n t e m p e ra t u r e s a n d f i r s t
m e l t i n g t e m p e ra t u r e s w e re a l s o o b t a i n e d . F i n a l i c e m e l t -
i n g t e m p e ra t u r e s w e re o b t a i n e d f ro m a q u e o u s i n c l u s i o n s
i n t h e p r e s e n c e o f a v a p o u r b u b b l e , s o t h a t a s a l i ni t y
c o u l d b e c a l c u l a t e d f ro m t h e e q u a t i o n o f O a k e s e t a l.
( 1 9 9 0 ) , w h i c h i s d e r i v e d f ro m v a p o u r - s a t u r a t e d e x p e r -
i m e n ta l d a t a . H o m o g e n i z a t i o n t e m p e r a t u r e s w e r e m e a -
su red in s t r i c t ly increas ing o rder and befo re any f reez ing
e x p e r i m e n t s t o m i n i m i s e t h e p o t e n t i a l f o r l a b o ra t o ry
damage to the inc lus ions .
S t a b l e (C , O ) i s o t o p e d a t a w e re o b t a i n e d f ro m l a s e r
a b l a t i o n -m a s s s p e c t ro m e t ry o f 2 0 m i c ro n d i a m e t e r a r e a s
o f d o u b l y p o l i s h e d w a fe r s f ro m w h i c h a l l in t e rg r a n u l a r
o rg a n i c m a t e r i a l ( i n c l u d in g t h e m o u n t i n g r e si n ) h a d b e e n
re m o v e d . D e t a i l s o f t h e t e c h n i q u e a r e g i v e n b y S m a l l e y
et al. (1992).
R a d i o g e n i c (S r ) i s o t o p e s w e re m e a s u re d u s i n g a VG
3 5 4 m a s s s p e c t ro m e t e r . S m a l l p o w d e r s a m p l e s f ro m a r e a s
o f m ic r i ti c m a t r i x a n d s p a r ry c e m e n t s w e re o b t a i n e d f ro m
uncovered , s l igh t ly th icker , th in -sec t ions us ing a Went -
w o r t h La b o ra t o r i e s u l t r a s o n i c m i c ro d ri l l.
4 Diagen esis and pore systems
4 1 Kharaib Formation
Tra n s m i t t e d l i g h t a n d b a c k s c a t t e r e d e l e c t ro n
m i c ro s c o p y s h o w t h a t t h e d e p o s i t i o n a l l i t h o l o g y h a s
e x e r t e d a s i g n i fi c a nt c o n t ro l o n t h e t y p e s o f p o re s y s t e m
w h i c h a r e d e v e l o p e d a n d p r e s e rv e d . P r i m a ry i n t e r -
g r a n u l a r m a c ro p o ro s i t y w h i c h fo rm e d d u r i n g d e p o s i t i o n
of the g ra ins ton e-pac ks ton e fac ies has bee n f il led by ca l -
c i t e cemen ts to vary ing degrees (F ig . 5 ) . Other types o f
m a c ro p o re w h i c h a r e p r e s e rv e d i n t h e r e s e rv o i rs a r e s e c-
o n d a ry v u g s a n d m o u l d s (F i g . 5 b , c ) . Th e s e a l s o c o n t a i n
v a r i a b l e a m o u n t s o f e q u a n t s p a r ry c a l ci t e c e m e n t s . P e r -
m e a b i l i t y d a t a i n d i c a t e t h a t t h i s m a c ro p o re s y s t e m h a s
var iab le co nnec t iv i ty , th oug h i t i s o f ten su f f ic ien t to c rea te
s ign i fi can t reservo i r po ten t ia l . M icro por os i ty wi th in the
s e d i m e n t g r a i n s (F ig . 5 ) is a ls o c o m m o n t h ro u g h o u t . Te x -
t u r e s s h o w t h a t i n t e rg r a n u l a r c o m p a c t i o n h a s n o t b e e n
s ign i fi can t in reduc ing reservo i r qua l i ty in g ra ins tone and
p a c k s t o n e u n i t s e x c e p t w h e re c e m e n t a t i o n h a s b e e n v e ry
l im i t e d. I n c o n t r a s t , t h e p o re s y s t e m in w a c k e s t o n e s a n d
l i m e m u d s t o n e s , m a i n l y c o m p r i s e s p o o r l y c o n n e c t e d
microporos i ty (see Budd (1989) and Mosh ier (1989) fo r a
fu l l d i scuss ion on the o r ig ins ) . In add i t ion , l aminar t ex -
tu res in these f ine g ra ined un i t s ind ica te tha t s ign i f i can t
c o m p a c t i o n h a s o c c u r r e d . Lo c a l l y , t h e s e f a c i e s h a v e
been par t i a l ly do lom i t i sed (e .g . Alsharhan , 1990) , l ead ing
to the developm ent of som e intercrystal line porosi ty . N ever-
t h e le s s, d o l o m i t i s a t i o n d o e s n o t a p p e a r t o h a v e i n c r e a s e d
p e rm e a b i l i t y s i g n i f i c a n t l y i n t h e w a c k e s t o n e -m u d s t o n e
u n i ts ; b o t h d o l o m i t i s e d a n d u n d o l o m i t i s e d e x a m p l e s t y p i-
ca l ly have perm eab i l i t i es be low 10 mD . T hese un i t s ra re ly
ac t as s ign i f i can t reservo i rs . Li tho fac ies var ia t ions a re
t h e r e fo re c r i t ic a l i n c o n t ro l l i n g t h e t y p e s o f p o ro s i t y e n -
c o u n t e r e d w i t h t h e b e s t r e s e rv o i r q u a l it y b e i n g fo u n d i n
t h e g r a i n s t o n e -p a c k s t o n e u n i ts .
4 2 Amu Darya Jurassic
Th e m a i n r e s e rv o i r f a c i e s e n c o u n t e r e d a r e t h e c o a r s e
s h e l f g r a i n s t o n e s a n d r e e fs ( f r a m e s t o n e s a n d a s s o c i a t e d
gra ins tones ) , wh ich genera l ly confo rms to the c las s ic
m o d e l s o f s u c h e n v i ro n m e n t s (W a l k e r a n d J a m e s , 1 9 92 ).
In the reefa l mater ia l , s ign i f i can t vuggy poros i ty i s com-
mon in co re samples and in th in -sec t ion and i s l ined by
d ru s y c a l c i te c e m e n t (F i g . 5 e ) . Th e p e t ro g ra p h i c c h a ra c -
te r i s t i cs sugges t tha t an ear ly d iagene t ic model i s b road ly
a p p l i c a b le , w i t h t h e d e v e l o p m e n t o f f r e s h w a t e r l e n s e s o n
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64 J . E . N e i l s o n e l a l . / M a r i n e a n d P e t r o l e u m G e o l o g y 1 5 1 9 9 8 ) 5 7 7 2
a n d a r o u n d t h e s u b a e r i a l ly e x p o s e d p a r t s o f t h e r e e fs a n d
g r a i n s t o n e s . O n l y s m a l l v o l u m e s o f e q u a n t s p a r r y c a l c it e
p r e c i p i t a t e d d u r i n g .b u r ia l ( F ig , 5 f ) d e s p i t e p r e s e n t d a y
b u r i a l d e p t h s r a n g i n g u p t o 11 0 0 0 f t ( c a 3 .5 k i n ) . C o n s e -
q u e n t l y d i a g e n e s is h a s f o l l o w e d a d i f f e r en t p a t h w a y t o t h e
K h a r a i b F o r m a t i o n . P o r o s i t y is d o m i n a t e d b y p r im a r y
i n t e r g r a n u l a r a n d s e c o n d a r y v u g g y a n d m o u l d i c m a c r o -
p o r e s ( F i g . 5 e , f ) . M i c r o p o r o s i t y w i t h i n s e d i m e n t g r a i n s
i s a l so p resen t .
5 F lu id inc lus ion s
D a t a s e t s w e r e o b t a i n e d f r o m p e t r o g r a p h i c a l l y s i m i l a r
e q u a n t s p a r r y c a l c it e s a n d d o l o m i t e s in b o t h t h e K h a r a i b
F o r m a t i o n a n d p e t r o g r a p h i c a l l y l a t e c a l ci t es in o n e w e ll
f r o m t h e A m u D a r y a a r e a .
5 1 A q u e o u s i n c l u s io n s
I n e a c h s a m p l e s o m e a q u e o u s i n c l u s i o n s d e m o n s t r a b l y
c o n t a i n e d d i s s o lv e d g a s , a c c o r d i n g t o t h e c r i t e r ia o f C o l -
l i n s ( 1 9 7 9 ) . T h u s i t i s c o n s i d e r e d u n n e c e s s a r y t o a p p l y a
p r e s su r e c o r r e c t i o n t o a q u e o u s i n c lu s io n h o m o g e n i z a t i o n
t e m p e r a t u r e d a t a .
5 1 1 K h a r a i b F o r m a t i o n
T h e w i d e s p r e ad o c c u r r e n c e o f m o n o p h a s e a q u e o u s
i n c l u si o n s ( G o l d s t e i n , 1 9 8 6 ) a n d m i c r o t h e r m o m e t r y o f 2 -
p h a s e a q u e o u s i n c l u si o n s s h o w t h a t r e l a ti v e l y l o w t e m -
p e r a t u r e b u r ia l c e m e n t s ar e c o m m o n i n th e K h a r a i b F o r -
m a t i o n , a n d t h e i r o c c u r r e n c e is n o t c o r r e l a t e d w i t h d e p t h
( F i g . 6 ) . E v i d e n t l y , b u r i a l c e m e n t a t i o n b e g a n a t a n e a r l y
s t a g e i n t h i s u n i t i n m a n y f i e l d s . T h e e x c e p t i o n s t o t h i s
m a y b e i n t h e d e e p e s t t r a p s w h e r e d a t a a r e r e s t r i c t e d t o
h i g h t e m p e r a t u r e s , s u g g e s t in g t h a t t h e o n s e t o f c e m e n -
t a t i o n m a y h a v e b e e n l a t e r . H o w e v e r , n o t t o o m u c h
w e i g h t s h o u l d b e p l a c e d o n t h i s o b s e r v a t i o n b e c a u s e o f
t h e p o s s i b i l i t y t h a t a l o w t e m p e r a t u r e p o p u l a t i o n m a y
h a v e r e - e q u i l i b r a t e d ( G o l d s t e i n , 1 9 8 6) . P r i m a r y p e t -
r o l e u m i n c l u si o n s a r e o n l y f o u n d i n t h e p e t r o g r a p h i c a l l y
l a t e r p a r t s o f e q u a n t s p a r r y c a l c i te c r y st a ls , a n d t e m -
p e r a t u r e s o f c o e x i st i n g a q u e o u s i n c lu s i o n s a r e a l w a y s i n
t h e u p p e r p a r t o f t h e r a n g e r e c o r d e d f r o m a s a m p l e.
S o m e r e l a t i v e l y s h a l l o w f i e l d s s h o w a n o m a l o u s l y h i g h
t e m p e r a t u r e s - u p t o 4 0 C a b o v e p r e s en t d a y f o r m a t i o n
t e m p e r a t u r e s . S o m e o f th e s p a r r y c a l c i te s h e r e m a y h a v e
b e e n p r e c i p i t a t e d u n d e r t h e r m a l l y a n o m a l o u s ( ? h y d r o -
t h e r m a l ) c o n d i t i o n s .
T h e r a n g es o f a q u e o u s i n cl u si o n h o m o g e n i z a t i o n t e m -
p e r a t u r e s i n d o l o m i t e a r e n a r r o w , a n d c l u s t e r a r o u n d
p r e s e n t d a y f o r m a t i o n t e m p e r a t u r e s ( F i g . 6 ) . T h i s , a n d
t h e c o m m o n t r a p p i n g o f a b u n d a n t p e t r o l e u m i n c lu s io n s ,
i m p l i e s t h a t t h e d o l o m i t e s a r e b u r i a l c e m e n t s p r e -
c i p i t a t i n g l a t e r t h a n t h e s p a r r y c a l c i t e s i n t h e s a m e
s a m p l e , a n d p r o v i d e f u r t h e r c o n s t r a i n t s o n t h e l ik e l y t e m -
p e r a t u r e r a n g e o f c al c i te p r e c i p i t a t i o n i n s o m e s a m p l es .
I n t h e K h a r a i b F o r m a t i o n t h e r e is n o s y st e m a t i c v a r i -
a t i o n o f s a l i n i ty w i t h d e p t h , a n d t h e i n c l u s i o n s c o n t a i n a
v e r y w i d e r a n g e o f w a t e r t y p e s (F i g . 7) . T h e r e m a y h o w -
e v e r b e ( p a l a e o ) g e o g r a p h i c v a r i a t i o n . I n t h e n o r t h a n d
n o r t h e a s t o f th e a r e a s t u d i e d , t h e l o w e s t s a l i n it i e s g e n -
e r a l ly a p p r o a c h s e a w a t e r c o m p o s i t i o n ; i n th e c e n t r a l p a r t
o f t h e a r e a , t h e y h a v e t h e c o m p o s i t i o n o f f r e sh w a t e r ;
a n d t o t h e s o u t h e v e n t h e l o w e s t s a l i n i t i e s a r e g r e a t e r
t h a n s e a w a t e r . T h e h i g h e s t s a l i n i t i e s r e c o r d e d a r e o f t e n
v e r y h i g h i n d e e d ( 4 7 t i m e s m o r e s a l i n e t h a n s e a w a t e r ) .
T h e s a l i n i t y d a t a a r e c o n s i s t e n t w i t h t h e h o m o -
g e n i z a t i o n d a ta . C e m e n t a t i o n s t a r t e d a t l o w t e m p e r a t u r e s
i n t h e K h a r a i b f r o m l o w s a l i n i t y f l u i d s . A t h i g h e r t e m -
p e r a t u r e s i n t h e K h a r a i b , b u r i a l f l u i d s w h i c h h a v e
a c q u i r e d h i g h c h l o r i d e c o n t e n t s t h r o u g h i n t e r a c t i o n w i t h
e v a p o r i te s b e c o m e d o m i n a n t .
F i g . 5 . ( a ) T h a m a m a r e s e r v o i r ( o i l l e g , F i e l d A ) i n w h i c h b u r i a l c e m e n t a t i o n p r e c e d e d p e t r o l e u m e m p l a c e m e n t , A b u D h a b i . N o t e e x t e n s i v e
c e m e n t a t i o n o f i n t e r g r a n u l a r p o r o s i t y . N o p e t r o l e u m i n c l u s i o n s o c c u r i n t h e s p a r r y ca l c it e c e m e n t s . R e s i d u a l m i c r o p o r o s i t y o c c u r s w i t h i n th e s e d i m e n t
g r a i n s ( b lu e ) . P P L . S c a l e b a r - 5 0 0
I t m .
( b ) T h a m a m a r e s e r v o i r ( o i l l e g , F i e l d B ) i n w h i c h b u r i a l c e m e n t a t i o n a n d p e t r o l e u m e m p l a c e m e n t o c c u r r e d
a t t h e s a m e t i m e , A b u D h a b i . B u r i a l c e m e n t a t i o n b y c a l c it e h a s b e e n v e r y l i m it e d . S i g n i fi c a n t i n t e r g r a n u l a r p o r o s i t y r e m a i n s u n f il l ed ( b l ue ) . S p a r r y
c a l c it e c e m e n t s ( w h i t e ) c o n t a i n p e t r o l e u m i n c l u s i o n s . P P L . S c a le b a r = 5 0 0 t ~m . ( c) T h a m a m a r e s e r v o i r (o il l e g , F i e l d B ) i n w h i c h b u r i a l c e m e n t a t i o n
a n d p e t r o l e u m e m p l a c e m e n t o c c u r r e d a t t h e s a m e t i m e , A b u D h a b i . B u r i a l c e m e n t a t i o n b y c a l c it e h a s b e e n v e ry l im i t e d . S i g n if i c a n t i n t e r g r a n u l a r
p o r o s i t y r e m a i n s u n f il l e d ( b l ue ) . S p a r ry c a l c it e c e m e n t s (w h i t e ) c o n t a i n p e t r o l e u m i n c l u s i o n s . P P L . S c a l e b a r - 5 0 0 / n n . ( d ) T h a m a m a r e s e rv o i r (w a t e r
l eg , F ie l d B ) i n w h i c h b u r i a l c e m e n t a t i o n a n d p e t r o l e u m e m p l a c e m e n t o c c u r r e d a t t h e s a m e t i m e , A b u D h a b i . S i g n i f ic a n t c e m e n t a t i o n o f i n t e r g r a n u l a r
p o r o s i t y b y c a l ci t e h a s o c c u r r e d . M i c r o p o r o s i t y o c c u r s w i t h i n s e d i m e n t g r a i n s ( b l u e - g r e e n h u e ) . P P L . S c a le b a r = 1 25 t i m . (e ) O x f o r d i a n r e s e rv o i r
( F i e l d C ) , A m u D a r y a B a s i n . P e l o i d a l -b i o c l a s t ic g r a i n s t o n e f r o m t h e r ee l t a l u s f a c i e s. S i g n i fi c a n t i n t e r g r a n u l a r p o r o s i t y o c c u r s , m u c h o f w h i c h i s
s o l u t i o n e n h a n c e d ( c e n t r e t o p ). M i n o r c e m e n t a t i o n b y e q u a n t c a l c i te m i c r o s p a r p a r t i al l y s u r r o u n d s s e d i m e n t g r a i n s a n d fi ll s p o r o s i t y . A t h i n b i t u m e n
l a y e r ( b la c k ) l i n e s s o m e p o r o s i t y a s a f in a l p h a s e . P P L . S c a l e b a r = 5 0 0 # m . ( f ) O x f o r d i a n r e s e r v o ir ( F i e l d D ) , A i n u D a r y a B a s i n . P e l o i d a l- b i o c l as t i c
g r a i n s t o n e f i ' o m t h e c l o s e d s h e l f f a c ie s . S i g n if i c a n t i n t e r g r a n u l a r p o r o s i t y r e m a i n s b u t o c c a s i o n a l b u r i a l c a l c i te s p a r i s o b s e rv e d . T h e l a c k o f e ar l y
c a l c it e c e m e n t s i s e v i d e n c e d b y c o m p a c t i o n o f t h e s e d i m e n t g r a i n s . N o t e t h e a b s e n c e o f b i t u m e n . P P L . S c a le b a r = 1 25 ~ tm . ( g) I n t e r g r a n u l a r s p a r r y
c a lc i te , T h a m a m a G r o u p , A b u D h a b i . L a r g e, p r o b a b l y p r i m a r y , 2 - p h a s e p e t r o le u m i n c lu s i o n, c o n t a i n i n g o r a n g e - b r o w n l i q ui d a n d v a p o u r b u b b l e .
T h e s e h o m o g e n i z e t o l i q ui d a t t e m p e r a t u r e s m u c h h i g h e r t h a n m a x i m u m b u r i a l t e m p e r a t u r e s , i n d i ca t i n g h e t e r o g e n e o u s t r ap p i n g , c o n s i s t e n t w i t h a n
e a r ly , p h a s e - s e p a r a t e d c h a r g e t o a r e l at i v e ly l o w p r e s s u r e - t e m p e r a t u r e r e s e rv o i r . S c a le b a r = 4 0 / L m . ( h ) I n t e r g r a n u l a r s p a r r y c a l c i te . J u r a ss i c , A i n u
D a r y a B a s i n . A r r a y s o f 2 - p h a s e , p r o b a b l y p r i m a r y , a q u e o u s i n c l u s i o n s a r o u n d a r h o m b f o r m g r o w t h s u rl : ac e . C o n s i s t e n t v a p o u r - l i q u i d r a t i o s ( a n d
h o m o g e n i z a t i o n t e m p e r a t u r e s ) i n i n c l u s i o n s o f v a r i a b le s i z e s u g g e s t r e s e tt i n g h a s n o t o c c u r r e d . D a r k p a r a ll e l l i n e s a r e t w i n b o u n d a r i e s . O n e l i n e a r
a r r a y o f d e n s e l y p a c k e d s e c o n d a r y i n c l u s i o n s c r o s s e s t h e p r i m a r i e s a n d t w i n s a t a n a n g l e . S c al e b a r = 3 0 / a n .
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J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 5 ~7 2
65
F ig . 5 . C ap t io n s ee f ac in g p ag e .
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66
1 ~ 0
K h ~ a i b F m .
- 1 5 0 0 .
- 2 0 0 0
m
- 2 5 ~
I
~
3 0 0 0 m I
- 3 5 0 0
I I O
- 4 0 0 0
- 4 5 0 0 " I
0
p r e s e n t d a y t e m p . ]
J.E. Neilson et al./Marine and Petroleum Geology 15 (1998) 5~72
AinuDarya times more saline than seawater) throughout this range
(Fig. 7).
p r i m a r y o i l i n c l u s i o n s
[] v e r ti c al b a r = s e o o n d a d e s
-5 00 02 0 "4 0 ' ' ' , • i • = • , ' , . , ' , • , • , • , • , • , •
6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 8 0 1 8 0
Hom ogenization temp erature (°C)
Fig, 6. Plots of homogenization emperature ranges of primary aque-
ous inclusions against depth for carbonate cements in Kharaib For-
mation reservoirs in Abu Dhabi and Jurassic reservoir from Ainu
Darya. Data from sparry calcite cements represented by thinner black
lines; dolomites by thicker grey lines. The occurrence of common
monophase inclusions is noted by an 'm' .
g
£
CI
- 1 0 0 0
- 1 5 0 0
- 2 0 0 0
- 2 5 0 0
- 3 0 0 0
2
- 3 5 0 0 '
4 0 0 0 "
- 4 5 0 0 '
- 5 0 0 C
K h a r a i b
F m .
" " " U
I
I
A m u D a ~ a
5 1 0 1 5 2 0 2 5 3 0 5 1 0 1 5 2 0 2 5 3 0
Salinity (wt. % NaCl equiv.)
Fig. 7. Plots of salinity ranges of primary aqueous inclusions against
depth for carbonate cements in Kharaib Formation reservoirs in Abu
Dhabi and Jurassic reservoir from Amu Darya. Data from sparry calcite
cements represented by thinner black lines; dolomites by thicker grey
lines. First melting temperatures lie between -40cC and -60°C indi-
cating solutes dominated by NaCI and CaCI2, Note the wide range in
salinities throughout the depth range, suggesting that fluid mixing is an
important cause of cementation.
5.1.2. Am u Darya Jurassic
Because of a general scarcity of coarsely crystalline
burial calcites suitable for microthermometric analysis,
only one data set was obtained from the Ainu Darya
sequence. Homogenization temperatures ranged from
45':C to over 100°C (Fig. 6), though individual crystals
showed a narrower range. Salinities were largely con-
centrate d between 20 and 25 wt% NaC1 equivalent (6-7
5.2. Petroleum inclusions
We now focus on petrole um inclusion distributions and
compositi ons. At a sample scale, the simplest measura ble
characteristic of petroleum inclusions is whether they are
present or absent. If petroleum inclusions are present,
then we can be sure that the rock has been in contact
with petroleum at some point in its history.
The population of petroleum inclusions is composed
of three sub-populations: primary, which are distributed
along growth zones and growth sector boundaries in
cementing minerals; secondary, which occupy healed
microfractures that formed after the component they are
found in; and pseudosecondary, which occupy healed
microfractures that formed during cementation. Vari-
ations in this population are discussed below.
5.2.1. Kharaib Formation
Prima ry petr oleum inclusions were foun d locally in the
equant sparry calcite cements, and in dolomite cements.
In this paper we will not present a detailed description of
the compositions of primary petroleum inclusions found
in the Kharaib Formation, but we note that, in general,
the petroleum in the inclusions bears a strong resem-
blance to the reservoir oils, with most inclusions flu-
orescing in the yellow to yellow-green range un der 365 nm
UV il lumi nati on with a 390 nm barr ier filter. Ther e are,
however, indications that the sparry calcite cements have
trapped charges of varying maturity. In the shallower
fields, there is a minor population of primary petroleum
inclusions which are dark brown in transmitted light and
fluoresce in the orange range; in contrast, the majority of
petroleum inclusions are straw-yellow or clear in trans-
mitted light. This indicates a heavier oil in the brown
inclusions, consistent with the properties expected of a
less mature charge. In deeper fields, the proportion of
green and blue fluorescent primary inclusions is greater.
This indicates that lighter and presumably more mature
charges were being trapped as inclusions. Within indi-
vidual fields, petroleum inclusions also increase in
maturity with depth and suggest gradual filling of the
structures from the top downwards. No primary pet-
roleum inclusions were foun d in the deepest fields. Hom o-
genization data sets from petroleum inclusions were
unhelpful in terms of the aims of this study. They had
extremely wide ranges, and decrepitation before homo-
genization was common. This is attributed to phase sep-
aration and heterogeneous trapping (Roedder, 1984).
5.2.2. Ainu Darya Jurassic
In the Amu Darya sequences although petroleum
inclusions occur, they were not found in several of the
petroleum columns examined. Primary petroleum
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J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 57-72 6 7
i n c l u s i o n s w e r e n o t r e c o r d e d a t a l l. T h e a b se n c e o f p r i -
m a r y p e t r o l e u m i n c lu s i o n s i s r e a d il y e x p l a in e d i f c e m e n -
t a t i o n o c c u r r e d w h o l l y b ef o r e p e t r o le u m e m p l a c e m e n t - -
b u t t h e a b s e n c e o f s e c o n d a r y p e t r o l e u m i n c lu s i o n s is
m o r e d i f f ic u l t t o a c c o u n t f o r , p a r t i c u l a r l y s i n c e b i t u m e n ,
i .e . r e s id u a l p e t r o l e u m , i s p r e se n t i n so m e r e se r v o i rs , a n d
s e c o n d a r y a q u e o u s i n c l u s io n s h a v e b e e n t ra p p e d . I t m a y
b e s i g n if i ca n t t h a t t h e A m u D a r y a a c c u m u l a t i o n s a re
d o m i n a t e d b y g a s e o u s p e t r o l eu m , a n d , b y a n a l o g y w i t h
p h a s e - s e p a r a t e d a q u e o u s f l u id s i n m e t a m o r p h i c a n d
i g n e o u s sy s t e m s (e .g . Ro e d d e r , 1 9 84 ), i t m a y b e m o r e
d i ff ic u lt f o r p e t r o l e u m v a p o u r p h a s e s t o b e t r a p p e d a s
f l u i d i n c l u s i o n s . H o w e v e r , f u r t h e r r e se a r c h i s r e q u i r e d o n
t h i s p r o b l e m .
6 . I s o t o p e s
6 1 K h a r a i b F o r m a t i o n
T h e s p a r r y c a l ci te ce m e n t s o f th e K h a r a i b F o r m a t i o n
h a v e ~ 8 0 v a l u es w h i ch g e n e ra l ly l ie b e t w e e n - 5 a n d
- 1 5 0 ( F i g . 8 ) su g g e s t i n g t h a t p r e c i p i t a t i o n o c c u r r e d
f r o m a v a r i e t y o f w a te r s a n d / o r u p t o r e l a ti v e ly h ig h
t e m p e r a t u r e . F o r t h e m o s t i s o t o p i ca l l y d e p l e t ed c e m e n t ,
e v e n a s s u m i n g a n i s o t o p ic a l l y d e p l e t ed w a t e r c o m -
p o s i t i o n o f - 5 % 0 , p r e c i p i t a t i o n t e m p e r a t u r e s a b o v e
a b o u t 7 0 ° C a r e c a l c u l a t e d f r o m t h e f r a c t i o n a t i o n e q u a -
t i o n g i v e n b y F r i e d m a n a n d O ' N e i l ( 1 9 7 7 ) , w i t h p r e -
c i p i t a t i o n a b o v e a b o u t 1 2 0 °C f o r a 0 %0 o r h e a v i e r w a t e r .
T h e l a t t e r v a l u e i s c o n s i s t e n t w i t h t h e f l u i d i n c l u s i o n d a t a .
U s i n g t h e f l u i d i n c l u s i o n t e m p e r a t u r e s t o c a l c u l a t e 6 ~ 8 0
r a n g e s f o r t h e p r e c i p i t a t i n g w a t e r g i v e s w a t e r c o m -
p o s i t i o n s t y p i c a l l y a b o v e 0% 0 t o o v e r + 1 5% 0 , su g g e s t i n g
a h i g h d e g r e e o f w a t e r - r o c k i n t e r a c t io n a t l o w w a t e r-
r o c k r a t i o s p r i o r t o c a l c i t e p r e c i p i t a t i o n . T h e s a m e
c e m e n t s h a v e
87/865r
v a l u e s ( F i g . 9 ) w h i c h r a n g e f r o m
s l i g h t l y a b o v e e a r l y C r e t a c e o u s s e a w a t e r t o c o n s i d e r a b l y
a b o v e a n y p o s t - K h a r a i b s e a w a t e r c o m p o s i t i o n ( S m a l l e y
e t a l . , 1994) , i nd ica t ing va r i ab le in f lux of an exot i c rad i -
o g e n i c f l u id , c o n s i s t e n t w i t h t h e f l u id i n c l u s i o n a n d s t a b l e
i so t o p e d a t a . T h e 6 1 3C v a l u e s g e n e r a l l y l ie b e t w e e n 0 t o
+ 4 % 0 , a n d t h i s i n d i c a t e s t h e i n v o l v e m e n t o f c a r b o n f r o m
m a r i n e sk e l e t a l c a r b o n a t e s . M a t r i x c a r b o n a t e s ( e .g . m i c -
r i ti c p e lo i d s a n d l i m e m u d ) h a v e s i m i l a r i so t o p i c c h a r a c -
t e r i s t i c s , t h o u g h t h e r e a r e t w o o u t l i e r s o n t h e
613C vs.
6 ~ 80 p l o t , a t ( + l , 0 % 0) a n d ( - 1 4 , - 9 % 0 ) r e sp e c t i v el y .
T h e f i rs t i s a n u n a l t e r e d f o s s i l f r a g m e n t a n d t h e s e c o n d i s
p r o b a b l y a p o r t i o n c o n t a i n i n g s h a l l o w c e m e n t s f o r m e d
a s a r e su l t o f m i c r o b i a l a c t i v i t y ( C o l e m a n , 1 9 85 ).
D e p l e t i o n i n b o t h c a r b o n a n d o x y g e n is o f t en o b s e r v e d
in such ce me nt s (Sass e t a l . , 1991).
6 2 A m u D a r y a J u r a s si c
E q u a n t s p a r r y c a l c i t e c e m e n t s h a v e ~180 v a l u e s
b e t w e e n - 0 % 0 a n d - 8 % 0 ( F i g . 10 ) su g g e s t i n g p r e -
c i p i t a t i o n i n e i t h e r a r e l a t i v e l y sh a l l o w b u r i a l e n v i r o n -
m e n t f r o m w a t e r s w i t h i s o t o p ic c o m p o s i t i o n s c lo s e t o , o r
s l i g h t ly d e p l e t e d c o m p a r e d w i t h , s e a w a t e r , o r a t h i g h e r
t e m p e r a t u r e f r o m i s o t o p ic a l ly e n r ic h e d w a t e r s . U n f o r -
t u n a t e l y , t h e w i d e r a n g e i n f l u i d i n c l u s i o n h o m o -
g e n i z a t i o n d a t a p r o v i d e n o c o n s t r a i n t s o n t h i s r a n g e o f
poss ib i l i t i e s , and 87/865r d a t a w e r e n o t a v a i l a b le . T h e 6 1 3 C
v a l u e s g e n e r a l l y l ie b e t w e e n 0 t o + 4 % 0 , c o n s i s t e n t w i t h
t h e d e r i v a t io n o f c a r b o n f r o m m a r i n e s k e le t al c a r b o n a te s .
7 . D i s c u s s i o n
7 1 B u r i a l c e m e n t a t i o n
T h r e e l i ne s o f e v i d e n c e su g g e s t t h a t t h e c o a r se e q u a n t
s p a r r y c a l c i t e c e m e n t s i n b o t h t h e K h a r a i b F o r m a t i o n
lO
C) - lO
-20 •
C,AJ..CrTEC E M E N T
• M A T R I X
1 ~ P R E C I P I T T E D
I N E Q U I L I B R I U M
Y'd 'g ' l C~qETAGEOUS
SE A W A T E R
o
-30
i
-2 0 -1 0 0 1 0
a13C
F i g . 8 . S t a b le i s o t o p i c c o m p o s i t i o n o f m i c r it i c c a r b o n a t e g r a i n s a n d
b u r i al c a lc i te c e m e n t s i n th e K h a r a i b F o r m a t i o n , T h a m a m a G r o u p ,
A b u D h a b i .
0.7095- -
0 .7090- i
0.7085---
0.7080"~
E
0.7075-_-
07o70-:
E
0 . 7 0 6 5 :
0
0.714 (fluori te)
0 .7 1 3 ( d o lo m i te )
0.712 (calci te)
\
calci t
I m a t , ,:
Sma~etal.(1994)
t l i F i t l l ,
1 0 0 2 0 0 3 0 0 4 0 0
A g e (M a )
F i g . 9 . S r - i s o t o p e c u r v e o f S m a l l e y et a l . ( 1 9 9 4 ) fo r s e a w a t e r t h r o u g h -
o u t g e o l o g i c a l t i m e . T h e v a l u e s o b t a i n e d f r o m s e d i m e n t g r a i n s a n d
c e m e n t s f r o m K h a r a i b F o r m a t i o n s a m p l e s a r e s h o w n b y t h e a rr o w .
N o t e h o w t h e y a re m o r e r a d i o g e n i c t h a n t h e o r i g in a l L o w e r C r e t a c e o u s
s e d i m e n t s w o u l d h a v e b e e n a n d i n s o m e c a s e s a r e s i g n i fi c an t l y m o r e
r a d i o g e n i c .
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20
1
5
10
5 0 5
10
a13c
Fig. 10. Stable isotopic composition of calcite cements, Amu Darya
reservoirs.
and the Amu Darya are largely burial cements. First,
they are petrographically late, post-dating compaction,
dissolution, and phases of finer-crystalline calcite pre-
cipitation. Secondly, fluid inclusion data indicate that
they formed at relatively high temperature from evolved
basinal brines. The majority of primary aqueous
inclusions trapped in the sparry calcite contain Na-Ca-
Cl bearing water with salinities up to 7 times that of
seawater (Fig. 7), and they sometimes contain primary
petroleum inclusions (Fig. 5g). Thirdly, where available.
isotopic data again suggest the cements formed at rela-
tively high temperature (b”O data) from evolved basinal
brines (
” “Sr data).
7.2.
Relationship betM,een petroleum emplacement und
diqqenesis
7.2.1. Kharaib Formation
Grainstones and packstones show considerable vari-
ation in their poroperm characteristics (Fig. 11). The data
show that grainstones which contain microporosity only
(i.e. where cementation by equant sparry calcite has been
extensive) have low permeabilities (< 10 mD) despite hav-
ing relatively high porosities (up to 20%). Samples in
which cementation has been less extensive and which
have unfilled or partially filled macroporosity, have sig-
nificantly higher permeabilities (Fig. 11).
There appears to be a very close relationship between
reservoir quality and the occurrence of primary pet-
roleum inclusions. Where reservoir quality is almost or
completely destroyed by extensive intergranular calcite
cements (Fig. 5a), no primary petroleum inclusions are
observed in those cements at the crests of fields although
the reservoir rocks are stained with petroleum. In
contrast, where unfilled macroporosity (intergranular,
vuggy and mouldic porosity) is present because of less
1000
0
o”
100 -
0
00 O
E
0 00 0”
o 0
00
_E_ IO-
0 0
0
E
t
0
l-
@ 8.
n
00
z
6 .l- 0” 0
8 .o” O
.Ol -cqQQD
(3
,001
I I
I I
I
0
10 20
.
Porosity (%)
. MICROPOROSITY
0 MICROPOROSITY AND VISUALPOROSITY
Fig. I I Porosity-permeability data for Thamama reservoirs showmg
how pore type controls permeability. Where samples contain micro-
porosity only, permeability does not exceed IOmD.
burial cementation (Fig. 5b,c). the calcite cements do
contain primary petroleum inclusions (Fig. 5g), indi-
cating that cementation has overlapped with petroleum
emplacement. In the water leg of such reservoirs (no
petroleum inclusions), cementation has been extensive
(Fig. 5d) and porosities are lower than in the oil leg. These
data suggest that an early petroleum charge, relative to
burial cementation, is required for the preservation of
economically producible reservoirs in the Kharaib For-
mation.
Despite an overall regional decline in porosity with
ddepth which also corresponds with the distribution of
primary petroleum inclusions, average oil leg porosities
are higher at any given depth than dry hole and water leg
data (Fig. 12a). Primary petroleum inclusions are only
found in the shallower reservoirs along the major struc-
tural highs. Typically, porosity varies by about 10% in
individual fields, and the largest part of the variation is
correlated with depth, from crest to flank. Integration
of the porosity, permeability and petrographic data has
allowed two end member porosityydepth curves to be
established, a water-leg and late oil charge curve and
an early oil charge (crestal) curve with individual field
porosity-depth curves running between these as shown
in Fig. 12b. Such a rapid decline in porosity across a field
within a larger overall trend has also been observed in
sandstones (Gluyas et al.. 1993: Emery et al., 1993) and
appears to be typical of reservoirs in which cementation
has overlapped with petroleum emplacement. A decrease
in porosity with depth within the oil leg reflects gradual
filling of the reservoir from the crest downwards. This
may be accompanied by an increase in stylolitisation with
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J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 5~72 6 9
0o
r-
-4000
-5000
-6000
-7000
-8000
-9000
-10000
-11000
-12000
-13000
-14000
I:IBlac~
Q
1 ' 0
d
A v e r a g e p o r o s i t y ( % )
40
El OIL WELLS
a Ii DRY WELLS
09
09
I -
LL
c-
Q.
O
E
Q.
O
- 4 0 0 0
- 5 0 0 0
- 6 0 0 0
- 7 0 0 0
- 8 0 0 0
- 9 0 0 0
- 1 0 0 0 0
-11000
- 1 2 0 0 0
- 1 3 0 0 0
- 1 4 0 0 0
WATER LEG AND I -
LATEOIL CHARGE L ~
I c o R v E J " > " - ~ ' C - - -- - - - ~ -
/ / / / /
/
0 1 0 2 0 3 0 4 0
A v e r a g e p o r o s i t y ( % )
Fig. 12. (a) Average Kharaib Forma tion porosity per well versus depth,
Abu Dhabi from oil fields and dry wells. (b) Crestal, water leg and
schematic field trends for the Kharaib Formati on, Abu Dhabi
depth (Dunnington, 1967). The possible links between oil
filling and stylolitisa tion are discussed later.
7 2 2 Amu Darya Jurassic
In the Amu Darya Basin, petroleum generation began
during the Late Cretaceous (VNIGNI, unpublished data)
when the Jurassic reservoirs were buried to depths of
around 6000-8000ft (2-2.5km). Secondary petroleum
inclusions were observed in the matrix of some samples
but neither primary nor secondary inclusions were
observed in burial calcite cements from the gas/
condensate legs (Fig. 5h). Bitumen pore linings are
observed in several of the peloidal/bioclastic grainstones
(Fig. 5e). The absence of petroleum inclusions in the
sparry calcite cements and the presence of bitumen pore
linings suggests that petroleum emplacement was sub-
sequent to the main diagenetic phases.
Regional porosity~lepth data (petroleum and water
legs) for reef talus grainstones (Fig. 13) show that an
overall decrease in porosity with depth does not occur.
Indeed many shallow grainstones have lower porosity
than those buried at depth, although it should be noted
that very high porosities (>25%) are only observed in
the oil and/o r gas legs. This apparent increase in porosity
with depth may be associated with changes in the thick-
EXTENSIVE
D I A G E N E S I S
Xo X
E X T E N S I V E
D I A G E N E S I S
x
x ×
x x
x X X
x
x
X x x x
x x x Xx (~
x x
x eo •
×~
x
o o
• = 1 - = ' J o . .
00 O~
0
•
o X O x : : x
•
o • oXxoQeooX
o •
" _ o e • • | n n
n O O • m •
: . - .
•
(~)
0 0 0 0 0
I
1 2 0 3 0 4 0 %
POROSITY
1 0 0 0
1 5 0 0
2 0 0 0
2 5 0 0
3 0 0 0
g
- r
I -
o .
i J
• Samples from oil and/or gas intervals
O Samples from water intervals
x Samples from intervals where data about oil, gas or water are absent
13 Porosity determined from logs only
~
Overlying evapodtes 30-100m thick on average
Overlying evaporites 100-400to thick on average
(~)Overlying evaporites 400-650m thick on average
F i g . 1 3 . P o r o s i t y ~ l e p t h p l o t f o r r e e f t a l u s g r a i n s t o n e s i n th e A m u
D a r y a B a s i n .
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70 J.E. Neilson et al./Marine and Petroleum Geology 15 1998) 57-72
n e ss a n d c h a r a c t e r o f t h e o v e r l y i n g K i m m e r i d g i a n -
T i t h o n i a n d e p o s it s . W h e r e t h e o v e r l y i n g K i m m e r i d g i a n -
T i t h o n i a n e v a p o r i t e s a r e t h i c k e r , t h e u n d e r l y i n g r e s -
e r v o ir s m a y b e o v e r p r e s s u r e d . R e g i o n a l o v e r p r e ss u r e c a n
r e d u c e t h e e f f ec t s o f b u r i a l d i a g e n e s i s t h r o u g h r e t a r d a t i o n
o f p r e s su r e s o l u t i o n ( e . g . F e a z e l a n d S c h a t z i n g e r , 1 9 85 ).
V e r y h i g h p o r o s i t i e s ( > 2 5 % ) i n t h e o il a n d / o r g a s le g s
s u g g e st t h a t p e t r o l e u m e m p l a c e m e n t m a y h a v e p l a y e d a
p a r t i n r e t a r d i n g b u r i a l d i a g e n e s i s a n d a l l o w i n g p r i m a r y
m a c r o p o r o s i t y t o b e p r e se r v e d u p t o 3 . 5 k m b u r i a l ( F i g .
1 3 ) , b u t a s n o p r i m a r y p e t r o l e u m i n c l u s i o n s w e r e
o b se r v e d i n t h e c a l c i t e c e m e n t , c o n c l u s i v e e v i d e n c e f o r
t h i s c o u l d n o t b e o b t a i n e d f r o m o u r s a m p l e s e t .
7 3 Comp ar i son w i th s im i lar s tud ies
A c l o se r e la t i o n sh i p b e t w e e n t h e t i m i n g o f p e t r o l e u m
e m p l a c e m e n t a n d c e m e n t a t i o n h a s b e e n r e c o r d e d e l s e -
w h e r e i n t h e w o r l d , i n c l u d i n g o t h e r p a r t s o f t h e M i d d l e
E a s t , w h e r e f l u i d i n c l u s i o n d a t a h a v e b e e n a v a i l a b l e ,
t h o u g h f e w s t u d i e s h a v e p r e se n t e d t h e se f i n d i n g s i n
r e l a ti o n t o q u a n t i t a t i v e p o r o s i t y - d e p t h d a t a .
Bur russ e t a l . (1985) used pe t ro l eum inc lus ions in ca l -
c i t e c e m e n t s a s a t o o l f o r u n d e r s t a n d i n g t h e t i m i n g o f
p e t r o l e u m m i g r a t i o n i n t h e O m a n - D u b a i a r e a . T h e y
s h o w e d th a t p e t ro l e u m c h a r gi n g o f T h a m a m a G r o u p
r e se r v o i r s i n t h e a r e a h a d o c c u r r e d o v e r a w i d e r a n g e o f
t i m e f r o m L a t e C r e t a c e o u s t h r o u g h t o t h e E a r l y T e r t ia r y
( O l i g o c en e ) . T h e i r w o r k su g g e s t s t h a t p e t r o l e u m c h a r g i n g
i n so m e r e se r v o i r s b e g a n p r i o r t o s t y l o l i t i s a t io n a n d b u r i a l
c e m e n t a t i o n . A l sh a r h a n ( 19 8 9) a n d O sw a l d e t a l . (1 9 95 )
h a v e l i k e w i se su g g e s t e d t h a t p e t r o l e u m g e n e r a t i o n a n d
c h a r g i n g b e g a n d u r i n g t h e L a t e C r e t a c eo u s . T h i s w o u l d
a g r e e w i t h t h e p e t r o g r a p h i c r e l a t i o n sh i p s w e h a v e
d e sc r i b e d w h i c h su g g e s t t h a t p e t r o l e u m c h a r g i n g w a s
e a r l y r e l a ti v e to b u r i a l c e m e n t a t i o n i n m a n y o f t h e h i g h e s t
p o r o s i t y K h a r a i b F o r m a t i o n r e s e r v o i r s i n A b u D h a b i ,
p r e se r v i n g r e se r v o i r q u a l i t y ( F i g . 5 b , c ) , T h e e x t e n s i v e
b u r i a l c e m e n t a t i o n o b se r v e d i n o t h e r r e se r v o i r s i n A b u
D h a b i , w h i c h o c c u r r e d p r i o r t o p e t r o l e u m e m p l a c e m e n t
( F i g . 5 a ) c o u l d b e a c c o u n t e d f o r b y r e l a t i v e ly l a te c h a r g -
i n g ( e. g . d u r i n g t h e E a r l y T e r t i a r y ) .
M c L i m a n s a n d V i d e t i c h ( 19 8 9) c l e a r l y n o t e t h a t t h e
m o r e p o r o u s M i d d l e Ju r a s s i c G r e a t O o l i t e r ese r v o i r s ( e. g .
S t o r r i n g t o n ) i n t h e w e s t e r n p a r t o f th e W e a l d B a s i n ,
E n g l a n d , c o n t a i n p e t r o l e u m i n c l u s i o n s i n c a l c i t e c e m e n t s
w h i l e l e s s p o r o u s , e x t e n s i v e l y c e m e n t e d d r y w e l l s ( e . g .
G o d l e y B r i d g e ) d o n o t . T h e y n o t e t h a t t h e p o r e s y s t e m
i n t h e G r e a t O o l i t e i s d o m i n a t e d b y p r i m a r y p o r o s i t y
w i t h s e c o n d a r y p o r o s i t y b e i n g o f l i t t l e i m p o r t a n c e . T h i s
i s s i m i l a r t o t h e b e t t e r q u a l i t y r e se r v o i r s i n A b u D h a b i .
P r i m a r y p o r o s i t y , a n d h e n c e r e s er v o ir q u a l i t y i n t h e G r e a t
O o l i t e , h a v e b e e n p r e se r v e d w h e r e l i m i t e d b u r i a l c e m e n -
t a t i o n o c c u r r e d p r i o r t o t h e m a i n p h a s e o f p e t r o l e u m
gen era t io n . Se l lw ood e t a l . (1989 , 1993) prov ide
a d d i t i o n a l d a t a i n s u p p o r t o f t hi s i n t e rp r e t a t io n .
P e r h a p s t h e e f f e c t o f o il e m p l a c e m e n t c a n b e m o s t
c l ea r ly seen in cha lk s (Schol l e , 1 977; Feaze1 a nd Scha t -
z inge r , 1985) where in i t i a l va r i a t ions in depos i t i ona l t ex-
t u r e a n d d i a g e n e s i s a r e le s s t h a n i n sh e l f l i m e s t o n e s . I n
a m a j o r r e v i e w o f c h a l k s f r o m a v a r i e t y o f l o c a t i o n s
w o r l d w i d e , S c h o l l e ( 1 9 7 7 ) n o t e s t h a t c h a l k c e m e n t a t i o n
i s a f u n c t i o n o f p r e ss u r e s o l u t i o n a n d c e m e n t a t i o n . H e
su g g e s t s t h a t a l t h o u g h sm a l l i n i ti a l s e d i m e n t o l o g i c a l v a ri -
a t i o n s c o u l d l e a d t o b e d - t o - b e d v a r i a t i o n , o v e r a l l p a t t e r n s
o f c h a l k d i a g e n e s i s a r e r e l a t e d t o t w o m a i n f a c t o r s : f i r st l y ,
m a x i m u m b u r i a l d e p t h a n d s e c o n d l y , p o r e - w a t e r c h e m -
i s t r y . P o r o s i t y ( a n d p e r m e a b i l i t y ) d e c r e a se a s a d i r e c t
f u n c t i o n o f d e p t h e x c e p t w h e r e o i l e n t e re d t h e r o c k r e l a -
t i v e ly e a r ly , o v e r p r e s su r i n g o c c u r r e d o r t e c t o n i c s t re s se s
i n c r e a s e d s o l u t i o n a n d c e m e n t a t i o n . T w o f a c t o r s a r e
the re fore c r i t i ca l i n p rese rv ing re se rvo i r qua l i t y in the
c h a l k , p e t r o l e u m e m p l a c e m e n t a n d o v e r p r es s u r in g .
7 4 S ty lo l i t i sa t ion
I n c r e a se s i n s t y l o l it e d e n s i t y a w a y f r o m s t r u c t u r a l c r e -
s ts is a n a p p a r e n t l y o b v i o u s e x a m p l e o f d i re c t s t r u c t u r a l
c o n t r o l b u t D u n n i n g t o n ( 19 6 7) a n d O sw a l d e t a l. (1 99 5 )
h a v e a l s o s h o w n t h a t p e t r o l e u m e m p l a c e m e n t p r o b a b l y
inh ib i t s s ty lo l i t i sa t ion . S ty lo l i t e s can p rovid e a s ign i f i can t
s o u r c e o f m a t e r i a l f o r b u r i a l c e m e n t a t i o n t h r o u g h p r es -
su r e d i s so l u t i o n ( S c h o ll e a n d H a l l e y , 1 9 8 5 ) a n d d a t a f r o m
A b u D h a b i su g g e s t t h a t t h i s m a y b e t h e c a se i n so m e
f i e lds (Oswald e t a l . , 1995). O ur 87/S6Sr d a t a ( F i g . 9 ) a n d
m i n o r a m o u n t s o f ' e x o t i c ' c e m e n t s su c h a s fl u o r i te , h o w -
e v e r i n d i c a t e t h a t s t y l o l i t es a re n o t t h e so l e so u r c e o f
b u r i a l c e m e n t s i n t h e f i e l d s w e h a v e s t u d i e d . V e r y h i g h
ST/S6Sr r a t i o s h a v e a l so b e e n o b se r v e d i n b u r i a l c a l c i t e
c e m e n t s f r o m t h e S m a c k o v e r F o r m a t i o n ( S t u b e r a n d
P u sh k a r , 1 9 8 3 ) a n d t h e G r e a t O o l i t e ( S e l l w o o d e t a l . ,
1989) . In bo th cases , i t was sugges t ed tha t po ten t i a l
so u r c e s o f r a d i o g e n i c S r w e r e n e a r b y b a s i n a l sh a l e s
( N o r p h l e t F o r m a t i o n a n d t h e L i a s sh a l e s r e sp e c t i v e l y ) .
8 C o n c l u s io n s
P e t r o l e u m e m p l a c e m e n t i s a m a j o r c o n t r o l o n r e se r v o ir
q u a l i t y i n m a n y c a r b o n a t e s ( e .g . S c h o l le , 1 9 7 7 ; S c h o l le
a n d H a l l e y , 1 9 8 5 ; F e a z e l a n d S c h a t z i n g e r , 1 9 8 5 ) a n d t h e
d a t a p r e se n t e d h e r e f r o m A b u D h a b i h i g h l i g h t t h i s . P e t -
r o l e u m e m p l a c e m e n t a p p e a r s t o b e p a r t i c u l a r l y i m p o r -
t a n t w h e r e i t o c c u r s r e l a t i v e ly e a r ly i n t h e b u r i a l h i s t o r y
o f a p o r o u s a n d p e r m e a b l e l i m e s t o n e , p r i o r t o b u r i a l
c e m e n t a t i o n ( e. g. m a n y K h a r a i b F o r m a t i o n r e se r vo i rs ) .
W her e i t occu rs re l a t ive ly l a t e , it s e ffec t w i l l dep end on
h o w f a r b u r i a l d i a g e n e s i s h a s p r o g r e s se d ( a f u n c t i o n o f
f a c t o r s su c h a s d e p t h o f b u r i a l , e x t e n t o f p r e s su r e so l u -
t ion , ove rpressur ing , e t c . ) .
D e m o n s t r a ti n g t h a t p e t r o le u m e m p l a c e m e n t h a s h a d
an e f fec t r equi re s seve ra l c r i t i ca l p i eces o f ev idence : (a )
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J.E. Neilson et al./Marine and Petroleum G eology 15 (1998) 57- 72
71
t h e o c c u r r e n c e o f p o r o s i t y - d e p t h t r e n d s w h i c h a r e f ir s tl y ,
s t e e p e r t h a n t h o s e d u e t o c o m p a c t i o n , s e c o n d l y , c o r -
r e l a t e d w i t h s t r u c t u r a l e l e v a t i o n a n d t h i r d l y , c o r r e l a t e d
w i t h t h e d e g r e e o f f i ll in g o f m a c r o p o r e s b y c e m e n t s ; ( b ) t h e
o c c u r r e n c e o f p e t r o l e u m i n c l u si o n s in m o r e p o r o u s p a r t s
o f t h e r e s e r v o i r a n d t h e i r a b s e n c e i n l es s p o r o u s p a r t s .
I n t h e li m e s to n e s o f th e K h a r a i b F o r m a t i o n , w e h a v e
f o u n d t h e p e t r o l e u m i n c lu s i o n e v id e n c e a n d h a v e d e m -
o n s t r a t e d t h a t p o r o s i t y ~ l e p t h t r e n d s i n f i el d s i n w h i c h
t h e r e i s a s y n c h r o n o u s r e l a t i o n s h i p b e t w e e n p e t r o l e u m
e m p l a c e m e n t a n d c e m e n t a t i o n , a r e s t e e p e r t h a n r e g i on a l
d r y h o l e o r w a t e r l eg t re n d s . N o g l o b a l c o m p a c t i o n t r e n d s
e x i st s f o r 'l i m e s t o n e s ' a s f o r s a n d s t o n e s b u t t h e d r y h o l e
a n d w a t e r l e g t r e n d s m a y b e a m o r e u s e fu l w a y o f r e p -
r e s e n t i n g s i m p l e b u r i a l p r o c e s s e s . A n y d e v i a t i o n f r o m
t h is i m p l i es t h a t o t h e r f a c t o r s h a v e b e e n i n v o l v e d .
I n t h e li m e s t o n e s o f t h e A m u D a r y a J u r a s s ic , b u r i a l
d i a g e n e si s h a s b e e n l i m i t e d . T h e a b s e n c e o f p r i m a r y p e t -
r o l e u m i n c l u s i o n s i n th e c a l c i te m a y s u g g e s t t h a t p e t -
r o l e u m e m p l a c e m e n t o c c u r r e d a f t e r b u r i a l c e m e n t a t i o n
c e a s e d a l t h o u g h v e r y h i g h p o r o s i t i e s o n l y o c c u r i n th e o i l
a n d / o r g a s l eg s . T h e b u l k o f t h e b u r i a l d i a g e n e si s t h e r e -
f o r e a p p e a r s t o h a v e b e e n a r r e s t e d b e f o r e p e t r o l e u m e v e r
e n t e r e d t h e s y s t e m . T h e r e a s o n s f o r t h i s a r e u n c l e a r b u t
m a y b e d u e t o o v e r p r e s s u r i n g a s h i g h e r p o r o s i t i e s o c c u r
w h e r e t h e o v e r l y i n g K i m m e r i d g i a n - T i t h o n i a n s a lt i s
t h i c k e s t ( u p t o 6 5 0 m ) .
T h e r e la t iv e t im i n g o f p e t r o l e u m e m p l a c e m e n t a n d b u r -
i a l c e m e n t a t i o n c a n t h e r e f o r e b e a c r u c i a l e l e m e n t i n t h e
s u c c e s s f u l p r e d i c t i o n o f r e s e r v o i r q u a l i t y i n l i m e s t o n e s .
E c o n o m i c a l l y , t h e r e a r e t w o i d e a l s it u a t io n s :
1 . P e t r o l e u m e m p l a c e m e n t o c c u r s p r i o r t o s i g n i f i c a n t
b u r i a l c e m e n t a t i o n . M a c r o p o r o s i t y w i l l b e p r e s e r v e d
a n d t h e s p a t i a l d i s t r i b u t i o n o f it w il l b e a f u n c t i o n o f
l i th o f a c i e s a n d s t r u c t u r a l c o n f i g u r a t i o n ( e .g . K h a r a i b
F o r m a t i o n r e s e rv o i r s i n A b u D h a b i w h e r e r e se r v o i r
q u a l i t y is g o o d a n d t h e G r e a t O o l i t e ) .
2 . P e t r o l e u m e m p l a c e m e n t is g e o l o g i c a l l y l a te b u t b u r i a l
c e m e n t a t i o n d o e s n o t o c c u r f o r o t h e r r e a s o n s ( e . g .
o v e r p r e s s u r i n g ) . P e t r o l e u m e m p l a c e m e n t m a y s t i l l b e
o f s e c o n d a r y i m p o r t a n c e , h e l p i n g t o m a i n t a i n r e s -
e r v o i r q u a l i t y a s i s t h e c a s e f o r t h e N o r t h S e a c h a lk .
T h i s m a y a l s o b e t h e c a se i n t h e Ju r a s s i c r e s e r v o i r s o f
t he A m u D a r y a .
W h e r e p e t r o l e u m e m p l a c e m e n t p o s t - d a t e s c e m e n t a t i o n ,
t h e r e m a y b e n o s i m p l e r e l a t i o n s h i p b e t w e e n l i t h o f a c i e s
a n d s t r u c t u re o n t h e o n e h a n d , a n d p o r o s i t y d i st r ib u t i o n
o n t h e o th e r . T h a m a m a r e s er v o i rs i n A b u D h a b i w h e r e
r e s e r v o i r q u a l i t y i s p o o r , f a ll i n t o t h i s c a t e g o r y .
cknowledgements
T h e a u th o r s a re gr a te f ul t o A D N O C , V N I G N I a n d
B P f o r a l l o w i n g t h e m t o p u b l i s h t h i s m a t e r i a l . T h e a s s is t -
a n c e o f M r G a l a l L o u t f i a n d M r . S h e r i e f B i s h la w i ,
A D N O C d u r i n g K h a r a i b s t u d y a n d c ol l ea g u e s a t B P h a s
b e e n i n v a l u a b l e . T h e a u t h o r s w o u l d a l s o l i k e t o t h a n k
D . G . R o b e r t s , E . P u r d y a n d t w o a n o n y m o u s r e v ie w e r s
f o r t h e i r h e l p fu l c o m m e n t s .
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