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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-



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 ,



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-



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 ~ ~ . ~ . ,.,,

,<

0 .. I A I S H U A I B A F M .

< ~ L . . .

H A W A R M E M B E R

S H U A I B A F M .

i

] K H A R A I B F M .

K H A R A I B F M .

m i

; ~ " ~1 L E K H WA I R F M .

* N A S R F M . ' 0 0

~ i HABSHA N FM

~

Z H A B S H A N F

>'< SA LIL FM.

z [ ]

n

LU R A Y D A F M .

,-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



62

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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



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 .



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 .



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



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 .



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



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 .



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 )



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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