applications of modern & ancient geological analogues in characteriastion of fluvial-deltaic...

7/25/2019 Applications of Modern & Ancient Geological Analogues in Characteriastion of Fluvial-Deltaic Reservoirs in the Cooper Basin - APPEA, 2000.pdf

http://slidepdf.com/reader/full/applications-of-modern-ancient-geological-analogues-in-characteriastion-of 1/24A PPEA JO U RN A L 2000—393

S.C. Lang1, J. Kassan2, J.M. Benson3, C.A.Grasso3

and L.C. Avenell41N C PG G and APCRC

U niversity of A delaide Thebarton C am pus

A delaide SA 50052W histler Research Pty Ltd

34–36 W histler C ourt

Spring M ountain Q ueensland 41243Santos Lim ited

91 King W illiam Street

A delaide SA 50004Stanw ell C orporation Ltd

199 C harlotte Street

Brisbane Q ueensland 4000

slang@ ncpgg.adelaide.edu.au w histler@ uq.net.au

jim .benson@ santos.com .au

carm ine.grasso@ santos.com .au

cavenell@ stanw ell.com

ABSTRACT

Reservoir characterisation in fluvial and fluvial-

lacustrine delta successions is enhanced by the use of appropriate modern and ancient analogues to understandsubsurface reservoir architecture and to help buildappropriately scaled reservoir models. Two case studiesof reservoir characterisation in the Cooper Basin areused to illustrate the value of analogues. Firstly the LatePermian Toolachee Formation crevasse splay reservoirsof the Cooper Basin, southwest Queensland are outlined,and analogues from the Ob River in Western Siberiaillustrate the relative scale of crevasse splay depositswithin avulsion belts in a cool-temperate peat-formingenvironment. The South Blackwater coal mine in thePermian Bowen Basin is used as an analogue to quantify

the 3D geometry and reservoir architecture of crevassesplays and to highlight subsurface reservoirheterogeneity.

Secondly, the Early Permian Epsilon Formationshallow water lacustrine delta reservoirs are outlined,and analogues from the extant geometry of thedistributary channels and relict mouth bar deposits fromthe fluvial dominated Neales Delta in Lake Eyre are usedto interpret flow rate decline trends and probablereservoir architecture. The subsurface Tertiary lacustrinedeltaic complex of the Sirikit Field from the Phitsanulok Basin, central Thailand, is selected as an ancient analogue

for the multistorey reservoirs developed withinamalgamated mouth bar complexes intersected in thelower Epsilon Formation.

KEYWORDS

Reservoi r chara cteri sat ion, ana logues, f luvial , f luvial

l acustr ine, l acustr ine del ta , crevasse splay , coal-bea rin

st rata , reservoi rs , petroleum geology, sedimentology

Bowen Basin , Neales River , Lake Eyre, Cooper Basin

Toolachee Forma t ion, Epsi lon Format ion, Austral i a .

INTRODUCTION

M o d e r n a n d a n c i e n t a n a l o g u e s c a n p r o v i d e a

conceptual f ramework and useful constraints wi thin

w hi ch to i n terpre t a nd corre la t e o f t en w i de l y spaced w el

da ta . The judic ious use of a nalogues a l so helps to dea

wi th s t ra t igraphic uncerta inty in regional correlat ions a

wel l as det ai led 3D reservoi r model l ing.

What i s a meaningful dis tance of correlat ion? Can a

thin sand, for example, be expected to be correlatab le

over 5 km? I f a f luvial channel sand i s intersected in

several wel l s—wha t i s a rea sonable est imate of the sand

body w idth? How smal l do 3D-grid cel l s need to be

def i ned , t o adeq ua te l y m odel the i n terna l he terogenei t yof s tacked crevasse splay deposi ts or l acustr ine del ta

mouth bars?

Anal ogues have grea t po ten t i a l t o a i d i n answ er i ng

these quest ions , and a re powerful communicat ion tool

that can assi s t in the decis ion making process . The

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

appl icat ion of modern and ancient reservoi r analogue

wi l l help a void pi t fa l l s (such as a purely model-driven

interpreta t ion) in the use of reservoi r ana logues.

The key elements of reservoi r chara cteri sat ion (Fig. 1

are reservo i r s t ra t i g raphy , reservo i r geom et ry , and

reservoi r qua l i ty corresponding c losely to ma cro, mes

and micro-scale cha racte ri s t ics of the rock volume.In this paper, we highl ight the role analogues play in

reservoi r chara cteri sat ion, by i l lust rat ing tw o case s tudie

from the Cooper B asin (Fig. 2), highlight ing the processe

and potent ia l pi t fa l l s involved. The f i rs t ca se out l ines th

reservoir chara cteri sat ion of the La te P ermian Toolachee

F o r m a t i o n c r e v a s s e s p l a y r e s e r v o i r s i n s o u t h w e s

Queensland, including the key features of reservoi

s t ra t i g raphy a nd geom et ry . Modern ana l ogues f rom the

Ob River in Western S iberia are used to i l lust rate the

relat ive scale of crevasse splay deposi ts wi thin a vuls ion

bel ts in a cool-tempera te pea t-forming en vi ronmen

APPLICATIONS OF MODERN AND ANCIENT

GEOLOGICAL ANALOGUES IN CHARACTERISATION

OF FLUVIAL AND FLUVIAL-LACUSTRINE DELTAIC

RESERVOIRS IN THE COOPER BASIN

mailto:[email protected]











http://slidepdf.com/reader/full/applications-of-modern-ancient-geological-analogues-in-characteriastion-of 2/24394—A PPEA JO U RN A L 2000

S.C. Lang, J. Kassan, J.M. Benson, C.A. Grasso and L.C. Avenell

com parab l e w i th the La te Perm i an . An anc i ent a na l ogue

f rom the S outh B l ackw ater coa l m ine i n the La te P erm i an

Ra ngal Coa l Measures in the Bowen Ba sin is then used toqua nt i fy the 3D geometry , and reservoi r archi tecture of

crevasse splays and to highl ight subsurface reservoi r

heterogenei ty .

The second case out l ines the Ea rly P ermian E psi lon

Format ion shal low water l acustr ine del ta reservoi rs ,

including reservoi r s t ra t igraphy, geometry and qual i ty .

Analogues from the extant geometry of the dis t r ibuta ry

channels and r el ic t mouth ba r deposi ts f rom the f luvial

dom i na ted Nea l es Del t a i n Lake Eyre a re then used to

interpret f low rat e decl ine t rends and proba ble reservoir

archi tecture. An ancient ana logue from the subsurface

Tert i a ry l a custr ine del ta ic complex of the S i r iki t Field

from the P hi tsanulok Basin , central Thai la nd, i s used to

i l lust ra te how mul t i s torey reservoi rs intersected in theLow er Eps i l on Form at i on deve l oped f rom a m al gam ated

mouth ba r complexes.

Reservoir characterisation

Prior to a ny use of ana logues, the reservoi r needs to be

chara cteri sed comprehensively , ut i l i s ing a nd honouring

al l avai l ab le data . The corner s tones for any reservoi r

charac ter i sa t i on a re the ava i l ab i l i t y o f fu l l d i am etercore, directional informa tion (e.g. FMI logs), petrograph ic

da ta , core analysis results , and a sound understa nding of

Figure 1. Schematic diagram illustrating the critical areas of study in reservoir characterisation and analogue utilisation.



Characterisation of fluvial and fluvial-lacustrine deltaic reservoirs in the Cooper Basin

the genet ic s t ra t igraphy. The correlat ion of reservoi r

s a n d s i n m a n y c a s e s d o e s n o t r e p r e s e n t a g e n e t i c

subdivis ion of the reservoi r interva l ; instead t ime-sl ices

(genet ic uni ts) should b e ident i f ied w hich wi l l include

latera l fac ies varia t ions from reservoi r qual i ty sandstones

to, for example, overbank mudrocks (lat era l seals). Ideal ly

this should a nswer some key q uest ions such as :

• H o w i s th e r e se r vo ir c om p a r t me n t a l is ed ? D o es it a c t

as a l arge cont inuous tank or a complex network of

smal ler compart ments?

• Wh a t w e r e t h e d o m in a n t s e d im e n ta r y p r o ce ss es w h ic h

led to the accumulat ion of th e reservoi r rocks?

• Wh a t w a s t h e d e po si t io na l s e t t in g o f t h e r es er v oi r

rocks (environment , deposi t ional s lope, seasonal i ty ,

discharge, and s t ructura l set t ing)?

• Wh a t c o n t ro ll e d t h e d i st r i bu t io n of f a c ie s l a t e ra l l yand s t ra t igraphical ly (c l imate, fa ul t ing, base-level )?

• D o e s d e p os i t i on a l f a c i e s d i r e ct l y c on t r ol t h e

dis t r ibut ion of porous and permea ble intervals? I f not ,

i s there a feedba ck loop between fa c ies and diagenesis?

Reservoir stratigraphy

The reconstruct ion of pa laeogeogra phic set t ings for

individual reservoir horizons is not possible, without

determining genet ic , chronostrat igra phical ly meaningful

i n terva l s . These sur f aces a re se l ec ted b ased on an

approxima te combination of biostrat igra phic, l i thologica

or seismic markers , integrat ed wi t h fac ies ana lysis.

Reservoir geometry

Fa cies analysis of dri l l core provides the only means t

ident i fy deposi t ional envi ronments of reservoi r rock

(e.g . mul t i s torey f luvial channel sandstones can have

similar w irel ine log responses to s tacked creva sse splay

or even turbid i tes). As out l ined ab ove the presence of a

chronostrat igraphic f ramework, including estab l i shed

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

correlat ion between wel l s , but i t must be emphasised

tha t the reservoi r intervals are composi te bodies and thi

wi l l be ref lected in the i sopach and sa nd percent maps

Reservoir quality

Reservoi r qua l i ty includes a l l those propert ies rela t in

the de posit ional ( fac ies) characte ri s t ics of the reservoi

rocks to thei r ab i l i ty to f low commercial quant i t ies o

f luids . I t involves petrophysical interpretat ion of thereservoi r interval being integra ted wi th the petrologica

studies (special core a nalyses , diagenet ic ana lyses).

Analogue selection

The select ion of sui tab le a nalogues wi l l be constra ined

by the level of understand ing of the subsurfa ce reservoi

s tudied. For both modern and a ncient ana logues severa

key aspects should be considered when choosing an

ana l ogue .

MODERN

Modern a nalogues provide some unique opportuni t ie

for c lose examinat ion of deposi t ional processes and

provide the only possib i li ty to exa mine scale depend enc

of sediment body geometry in three dimensions over a

l a rge a rea , t yp i ca l l y b y us i ng a com b i na t i on o f aer i a

photography a nd f ield surveys . In add i t ion the ab i l i ty to

re-vis it t he same spot many t imes, a l lows the moni toring

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

importance of seasonal varia t ion. Consequent ly modern

analogues should be chosen that show simi lar i ty in

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

invest igat ion. This should be ref lected in grea t s imi lar i ty

of sedimenta ry s t ructures a nd g rainsizes observed. Thereis considerable scope for varia t ions between t he reservoi

and i t s ana logue regar ding the c limate, f luvial di scharg e

and tectonic set t ing, as long as this i s ful ly a ppreciated

To assess the geometry a nd modes of sediment accret ion

on f luvial braidbars , for example, does not necessari ly

requ i re tha t the anc i en t reservo i r and the m odern

analogue share the same c l imat ic or tec tonic set t ing—

unless cl imate or tectonic set t ing i s recognised t o play a

s igni f icant part in this process .

The use o f m odern a na l ogues i s no t w i thout poss i b l

p i t f a l l s . For exam pl e , a f a i l u re to recogn i se tha t sur f a c

Figure 2. Location map showing relative position of Cooper Basinshowing approximate location of study areas for the ToolacheeFormation in southwest Queensland, and the Epsilon Formation inSouth Australia. The location of the Rangal Coal Measures at SouthBlackwater Mine in the Bowen Basin is also indicated.




g e o m or p h o lo g y m a y n o t r e l a t e t o p r e s e r v e d s e d i m e n t

b ody geom et r y cou l d resu lt i n b i ased (or p la i n w r ong)

v i ew s o f reservo i r a rch i t ec ture . S i m i l a r l y th e e f fec t s of

c o mp a c t i o n a n d d i a g e n e s i s ca n n o t b e a p p r e c ia t e d i n

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

c o n d i t io n s m a y n o t e x i s t on E a r t h t o d a y t o a d e q u a t e l y

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

env i ronm ents .

ANCIENT

Ancient analogues typical ly comprise wel l exposed

and /or w el l documented examples of rocks s imi lar t o the

reservoi r under invest igat ion and can be used to provide

useful informat ion on the preserved archi tecture of

s e d i m e n t b o d i e s a n d /o r t h e r e l a t i o n s h i p b e t w e e n

s e d i m e n t a r y f a c i e s a n d r e s e r v o i r q u a l i t y . I n s o m e

environments considerable di ffe rences may exist betw een

the deposi t ional geometry a nd the preserved a rchi tecture

of the sediments . In pa rt icular coal-bea ring successions

show complex autocycl ic processes rela ted to ra pid later al

varia t ions in subsidence rates caused by compact ion ofpea t .

Because b o th the reservo i r and the ana l ogue a re

l i thi f ied, i t i s important t hat they sha re s imi lar controls

on sediment a ccumulation (in fluvial a nd f luvio-lacustrine

environments mainly the rat io of subsidence versus

sediment supply) . Often this req uires detai led knowledge

of the s t ructural set t ing of the reservoi r as wel l as the

ana logue. Only the highest qua l i ty dat a sets , including,

ideal ly , surface exposures as wel l as densely spaced

subsurface data , provide sui tab le analogues. The two-

dimensional nature of many natural exposures makes

them of only l imi ted use for study a s ana logues on thei r

own.

Analogue utilisation

The mot ivat ion for the use of ana logues l ies in thei r

grea t uti l i ty during correlat ion and simulat ion of reservoir

intervals during explorat ion and prod uct ion phases .

EXPLORATION

The use of a nalogues a l lows more conf ident ma pping

of sand t r ends in f luvial reservoi rs, by providing an order

of magni tude f or channel bel t widt hs , thickness to width

rat ios , and s imi lar geometric para meters . Correlat ion ofwirel ine logs can be constra ined b y ord er-of-magni tud e

est imates of reasonable correlat ion dis tances , derived

f rom m odern and anc i en t ana l ogues.

DEVELOPMENT

Heterogenei ty of the reservoi r as determined from

engineering and production da ta , wirel ine log correlat ions

and cores can b e com pared to m odern and anc i en t

analogues to construct meaningful reservoi r geometry

aw ay from we l l control . Analogues provide a n order-of-

magni tud e est imat e of reasona ble correlat ion dis tances

and he terogenei t y .

During the model l ing of reservoi rs , analogues are

i ns t rum enta l i n prov i d i ng num er i ca l i nput . Ob jec t

orientat ed model l ing i s l argely ba sed on the ava i l ab i l i ty

of sediment geometry dat a f rom ana logues. S imi lar ly in

grid-ba sed reservoir models the construction of variogra m

models and the select ion of grid cel l s i zes are chosen

care fu l l y t o adequa te l y re f l ec t an t i c i pa ted reservo i r

heterogenei ty ba sed on ana logue s tudies .

CASE STUDY 1: LATE PERMIAN TOOLACHEERESERVOIR, COOPER BASIN, SOUTHWEST

QUEENSLAND

The La te P ermian Toolachee Format ion in southwest

Queensla nd (ATP 259P ) is one of t he ma in reservoir s for

several ga s f ields in the r egion (Fig. 2) . The La te P ermian

Toolachee Format ion overl ies Early Permian s t ra ta of

the Cooper Basin across the Daral ingie Unconformi ty

surfa ce (Fig. 3).

Reservoir stratigraphy

S evera l l i t ho l og i ca l m arker sur f aces occur w i th i n

the Tool achee Form at i on , a l l ow i ng sub di v i s i on i n to

genet i c s t ra t i g ra ph i c un i t s . Marker sur f a ces com pri se

t h e b a s a l D a r a l i n g i e U n c o n f or m i t y , l oc a l l y d e v e l o p e d

coa l s , l oca l l y deve l oped l acus t r i ne sha l e i n terva l s , t he

reg i ona l l y s i gn i f i c an t P2 coa l , and the i ncurs i on o f

h i gh-GR m udrocks m ark i ng t he top o f t he Tool achee

Form at i on . Recogni t ion o f the Da ra l i ng i e U nconform i ty

f rom w i re l i ne l ogs , se i sm i c or core i s o f t en d i f f i cu l t .

C l o s e l y s p a c e d s a m p l e s f r o m m u d r o c k i n t e r v a l s ,h o w e v e r , a l l o w a p p r o x i m a t i o n o f t h e u n c o n f o r m i t y

u s in g p a l y n o st r a t i g r a p h y .

The Tool achee Form a t i on t yp i ca l l y con t a i ns a sa nd-

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

U nconform i ty a nd t he P 2 coa l ( l ow er Tool achee) . The

upper par t o f the Tool achee i s charac ter i sed b y f i ne-

g r a i n e d s t r a t a w i t h i s o l a t e d s a n d s t o n e s , o f t e n

exhi b i t i ng w el l deve l oped coarsen i ng-up pro f i l es on

l ogs . Reservo i r deve l opm ent occurs a l m os t exc l us ive l y

i n the sands tones o f the l ow er Tool achee Form at i on .

L o c a l l y , p r e f e r e n t i a l d e v e l o p m e n t o f h i g h q u a l i t y

r e s e r v o i r o c c u r s a t o r n e a r t h e v e r y b a s e o f t h e

T o o l a c h e e F o r m a t i o n , i m m e d i a t e l y o v e r l y i n g t h eD a r a l i n g ie U n c o n f o r m i t y s ur f a c e .

L i t t l e i s k n ow n a b o u t d e t a i l e d s e d i m en t d i s p e r s a l

a n d s a n d b o d y e l o n g a t i o n i n s o u t h w e s t Q u e e n s l a n d

dur i ng Tool achee t i m es . I t c an b e specu l a ted tha t

s e d i m e n t w a s d e r i v e d e i t h e r f r o m t h e n o r t h w e s t

( t ransverse l y i n to the b a s i n) o r f rom ax i a l t runk r i vers

f l o w i n g b r o a d l y n o r t h e a s t – s o u t h w e s t , n e a r t h e

d e p o c e n t r e s o f t h e C o o p e r B a s i n. O n l a p o n t o p a l a e o -

h i g h s ca n b e d e m o n s t r a t e d l o ca l l y , a n d i s a s s oc i a t e d

w i th a m al ga m at i on o f m i d-Tool achee coa l seam s i n to

single t hick (up to 10 m) coals .




Reservoir geometry

C ores f rom severa l f i e l d s i n sou thw e s t Queens l andw e r e l o g g e d i n d e t a i l t o d e t e r m i n e t h e f a c i e s ,depos i t i ona l env i ronm ent s , geom et ry a nd con t ro l s ondepos i t i on o f the reservo i r sec t i on .

FACIES

Li thofa cies and gen et ic fac ies assemblages ident i f ied

in core from the Toolachee Format ion in southwes

Queensland are summarised in Table 1 . The vert ica

succession of fa c ies was used to d etermine d eposit iona

environments .

Varia t ions in deposit ional s ty le and fa c ies archi tectur

occur in the lowe r (sand-rich) Toolache e Forma tion ove

the a rea . Tw o pr i nc i pa l f ac i es assem b l ages can b e

recognised:

• F a c i e s As se m b la g e 1. Co r e s a r e c h a r a c t e r is e d b y

stacked, ama lgama ted d eposi ts of f ine-gra ined, mino

channels and sa ndy crevasse splay lobes .• Fac i es Assem b l age 2. C ores exh ib i t w e l l -deve l oped

features commonly associated wi th f luvial channels

such as erosional scour surfa ces and a prevalence o

cross-bed ded sand stones.

In ei ther case there i s scant indicat ion for the presence

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

corresponding to indiv idual deposi ts are typical ly les

t h a n 1 . 5 m t h i c k . I n t h i s p a p e r w e f o c u s o n t h e

u n c o n v e n t i o n a l s a n d s t o n e d e p o s i t s o f F a c i e

Assemblage 1 , which form thick and l a t eral ly extensive

accumulat ions of potent ia l reservoi r sandstones in part

Figure 3. Stratigraphic column showing lithostratigraphic andchronostratigraphic framework for the Early and Late Permian inthe Cooper Basin.

Table 1. Abbreviations used for the main Lithofacies and GeneticFacies Assemblages.

Lithofacies Genetic facies

Associations

GS Stratified conglomerate SS Scour Surface

Sg Pebbly sandstone JCH In channel facies

high flowstage

Sl Laminated sandstone ICL in channel facieslow flowstage

Sx Low angle, cross- CF Channel fill

laminated sandstone

Sd Matrix-rich sandstone PF Proximal floodplain

Sp Planar, cross-bedded DF Distal floodplain

sandstone

St Trough cross-bedded SC Small or splay channel

sandstone

Sm Massive sandstone BC Channel plugs,

bartop drapes

Sr Ripple cross-laminated IL Inter-channel lake

sandstone

Sfl Interlaminated sands and XS Proximal splay

silt

Sf2 Interbedded sandstone MS Medial splay

Sb Bioturbated sandstone DS Distal splay

ES Mudrock with sandy

laminae

Fc Convoluted laminated

mudrock

Fl Laminated mudrock

Fb Bioturbated mudrock

Fm Massive mudrock




Figure 4a (left). Core log through Toolachee Formation in WellA, displaying composite sandstone facies comprising stacked depositsof splays and minor channels. Figure 4b (above). Legend.

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S C

X S

M S

M S

D F

P S

P F

M S

D S

D F

M S

D F

C

C

C

C

C

C

p y

C

p y

C

C C

C

C

C

C

C

C

F

F

F

F

F

F

F

C

F

p y

p y

C

F

L e n t i c u l a r l a m s .

F l a s e r l a m i n a e

R i p - u p c l a s t s

C o n g l o m e r a t i c

P e b b l e l a g

R i p p l e c r o s s - l a m .

C l i m b i n g r i p p l e c r o s s - l a m .

P l a n t d e b r i s

R o o t l e t s

L o w - a n g l e

c r o s s - b e d d i n g

C a r b o n a c e o u s l a m i n a e

S t y l o l i t e s

P y r i t e c o n c r e t i o n s

S i d e r i t e c e m e n t a t i o n

D i s h f r a c t u r e s

L e g e n d

of the b asin . Figure 4a provides a ty pical example (Wel l

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

assemblages , which i s discussed in det ai l below. A legend

is provided in Figure 4b .

Core description—Well A

Sand stones are f ine to medium-grained , modera tely

sorted a nd domina ted b y smal l scale cross-lamina t ion

(mostly ta bula r: facie s Sp), ripple cross-lamin a tion (Sr)

and a pparen tly massive intervals (Sm). Occasional pebbly

lag deposi t s (Sg), an d sma ll (<1 m) fining -up cycles occur.

In many of the w el l s , sandy fa c ies account for ca . 90% of

the basal Toolachee Formation sandstone. Thin (<50

mm) mud-dra pes a re common, a nd 50–100 mm t hick

interva ls r ich in carbona ceous l amina e occur a t the ba ses

of f ining a nd coarsening-up cycles .

Muddy a nd carbona ceous f ine-grained pa rt ings and

drapes of ten are incl ined to the core a t 5–15°. The

over l y i ng b ed s o f c ross-s t ra t i f i ed and r i pp l e c ross-

laminated sandstone are f l a t l y ing. This suggests that

i n c l i n a t i o n o b s e r v e d o n m u d d y d r a p e s r e f l e c t sdeposi t ional topography, perhaps from a levee, or pre-

exis t ing splay d eposits .

Deposi t ional processes interpreted fr om sedimentary

st ructures are mainly moderate to low energy t rac t ion

currents lead ing to the forma t ion of low-rel ief tw o-

dimensional subaqueous dunes and (more commonly)

ripples. The presen ce of cl imbin g ripple cr oss-lamin a tion

is characteri s t ic of rapidly d ecelerat ing, sediment-laden

currents , wi th a s igni f icant proport ion of sand carr ied as

suspended loa d. The common preserva tion of f ine-gra ined

drape deposi ts in the sandstones suggests only minor

reworking b y successive deposi t ional events .

The deposi t ional environment is cha ract erised by sand -dominated sedimentat ion rapidly grading into coal swamp

development towards the top of the basal Toolachee

Forma tion sand stone. Fining-up and coarsening-up cycles

occur, but ra rely a t ta in more tha n 1 m in thickness . At

lea st one we ll-def ined coa rsening-up succession occurs

in Wel l A, which gra des from lamina ted a nd r ipple cross-

laminated very f ine s tone and s i l t s tone, to medium-




gra ined ma ssive to crudely s t ra t i f ied sa ndstone. This i s

i n terpreted t o re f l ec t prograda t i on o f a sp lay deposi t ,

proximal to the main crevasse splay cha nnel .

The b a sa l Tool achee F orm at i on sand s tone i n Wel l A

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

s a n d y s p la y l o b e s a n d s m a l l s t r e a m s , w h i c h c o a l e s ce d

t o f o r m a s h e e t s a n d s t o n e b o d y w i t h i n t e r n a l

a r c h i t e c t u r e d e t e r m i n e d b y t h e e x t e n t a n d g e o m e t r y

o f f i n e g r a i n e d d r a p e d e p o si t s a c c u mu l a t e d b e t w e e n

f l ood even t s .

In some wel l s , deposi ts of modera te s ized channels are

m ore prom i nent , charac ter i sed b y a grea ter a b undance

of cross-bedd ed fa c ies and numerous erosional scour

surfaces . Considera ble varia t ions in deposi t ional energy

are ev i den t a nd m ay sugges t tha t channel s w ere on ly

short-l ived, perhaps as part of ana bra nching or splay

cha nnel system s. Where complet e fining-up chann el fi l l

successions are preserved , the bankf ull depths of channels

can be est ima ted t o not have exceeded 1.5m.

RESERVOIR ARCHITECTURE

Large i nd i v i dua l f l uv i a l channel s have no t b een

recognised in deposi ts of Fa cies Assemblage 1 . Instea d,

sands tones accum ul a ted i n sm al l t o m odera te s i z ed

channels , and more commonly in splay d eposi ts, which

are o f t en am al gam ated to fo rm th i ck and reg i ona l l y

e x t e n s i v e s u c c e s s i o n s o f s a n d s t o n e . W i t h i n t h e s e

composi te sandbodies , channel fac ies ( including splay

channels) exhib i t bet ter reservoi r development than

splay lobes . Development of higher permea bi l i ty in s t ra ta

immediately overly ing the Dara l ingie Unconformi ty may

resul t f rom the combinat ion of deposi t ional fac ies and

specif ic dia genet ic processes rela ted to the unconformity

surface, but i s not rela ted t o the presence of a residual ,or l ag de posit .

La tera l con t i nu it y o f a m al gam ated sands tone b odi es

i s s i gn i f i c an t (1 0 ’ s o f km ) . Ind i v i dua l a rch i t ec tura l

elements of the sandstones are only of moderate s ize ,

resul t ing in complex s tacking pat terns a t a scale of

hundreds of metres . Splay part ings , channel plugs and

abandonment drapes may form local baf f les . Where

complete channel f i l l successions are preserved these d o

not exceed 1 .5 m in thickness and a re encased in f lood

basin fac ies . Equal ly , indiv idual splay deposi ts rarely

exceed 0.75m in thickness, al though thick successions

i n t e r p r e t e d a s a m a l g a m a t e d s p l a y s a n d s t o n e s w e r e

intersected in several w el l s .

CONTROLS ON SEDIMENTATION

Isopach t rends of Facies Assemblage 1 in the s tudy

are a show only subt le va ria t ions in thickness , which i s

consis tent wi th a ccumulat ion in a low rel ief f lood basin

d u r i n g a t i m e o f s t r u c t u r a l q u i e s c e n c e . R e g i o n a l

sub s i dence , ra ther than l oca l depocent res , w as the

primary control on the dis t r ibut ion of fac ies during

T o o l a c h e e t i m e s , w i t h t h i c k e n i n g o f t h e i n t e r v a l

coinciding wi t h an increa se in f ine member ( f lood ba sin)

deposi ts and a decrease in the thickness of indiv idua

sandbodies .

T h e d i s t r i b u t i o n o f t h e t w o m a j o r s a n d s t o n e

deposi t ional envi ronments (splay and smal l channels) in

F a c i e s A s s e m b l a g e 1 d o e s n o t e x h i b i t s y s t e m a t i c

varia t ions and i s most l ikely the resul t o f auto-cycl i

processes , such a s channel avuls ion.

Within sandstone intervals reservoi r qual i ty show

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

d e p o s i t i o n a l s e t t i n g ( s p l a y , c h a n n e l , e t c ) , d u e t o

diag enet ic overprint ing. Consequent ly regional ana lysi

of sand t rends a nd thei r a rchi tecture may not be suf f ic ien

to es t i m ate reservoi r deve l opm ent i n a rea s aw ay f rom

immediate w el l control .

Analogue selection criteria

The use of ana logues is indicated to constrain :

• t h e m e d iu m a n d l on g -r a n g e f a c ie s va r i a t i on s t o be

e x p e c t e d i n t h e s e a m a l g a m a t e d s a n d s t o n e s ( 3 D

reservoi r archi tecture) ;

• t h e l i k el y e xt e n t of a r e a s c om p r is in g F a c ieAssemblage 1; an d

• t h e l ik e l ih o od o f e n co u nt e r i n g s ig n i f ic a n t f l u vi a

fa i rwa ys of Facies Assemblage 2.

To select an a ppropriate a nalogue for the Lat e Permia n

f luvial reservoi rs in the Cooper Basin several cr i teria

need to be met . Ideal ly the t ectonic set t ing should be a

large int ra cratonic basin , as we know the Cooper Ba sin

had n o proven di rect connect ion to the sea , and w as a

least 500 km southwest of the Late Permian coast l ine

(Gra vestock et al , 1998).

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

depos i t i ona l b as i n ( the ex ten t o f the C ooper Bas i n

exceed ed 300 x 510 km). Based on t he sedime ntologicainterpretat ion of numerous cores (Stuart , 1976; Mackie

et a l , 1995) , a range of f luvial systems needs to be

present . These include low s inuosi ty sa ndy a nd gra vel l

c h a n n e l s , s o m e m i x e d l o a d , p r o b a b l y m e a n d e r i n g

channels , and extensive f loodplains wi t h crevasse splays

f loodplain l akes ( f loodbasins) , and most important ly

peat forming envi ronments . The Toolachee Forma t ion

has extensive coal deposi ts , which were dominated by

p t e r i d o s p e r m f o r e s t s w a m p a n d r a i s e d m i r e s

(Gangamopteris -Glossopteris f lora ; Gould , 1982) in a vas

wet la nd system, which extended into the Bowen Ba sin to

the eas t .

The c l imate of the a nalogue idea l ly should ref lect theLat e Per mian c l imate, which i s thought to be humid coo

tempera te to t emperat e (Gra vestock et a l , 1998). The

Cooper Basin l ay between 65 and 70˚ south (Veevers

1984), based on palaeogeographic reconstructions fo

eas tern Aus t ra l ia , w h i ch w as then par t o f the G ondw ana n

supercont inent . The proximal reaches of the f luvia

system w ere l i ke l y t o have em ana ted f rom e l eva ted , a nd

poss ib l y i ce-covered h i n t er l a nds i n the i n ter i o r o

Gondw ana , b u t i m por t an t l y there i s no ev i dence o

permaf rost-relat ed s t ructures in the ba sin (Gra vestock

et a l , 1998) . S tacked f lood-generated crevasse splay




deposi ts are a bunda nt in the Toolachee Format ion, so i t

i s likely that f looding was common, indicat ing varia b le

f low condi t ions typical of a seasonal c l imate. Ice raf t ing

in the r ivers and f loodplain l akes during winter to ea rly

spring i s indicated by the presence of ra re dropstones in

shaley interva ls in the Toolachee Format ion.

With these condi t ions in mind our search f or modern

and anc i en t ana l ogues w as d i rec ted to the vas t m odern

Ob River system of the West S iberian plain , and the

c l ose ly com parab l e La te Perm i an R anga l C oa l Measures

in the Bowen Basin of eastern Austra l i a .

MODERN ANALOGUE: THE OB RIVER SYSTEM,

ARCTIC SIBERIA

The majori ty of l arge modern peat forming f luvial

systems in non-tropica l regions l ie in the cool-tempe ra te

and a rc t i c reg i ons o f C ana da , A l aska , and R uss i a . Based

on both the palaeogeographical and sedimentological

evidence, we deduce tha t the Toolachee Format ion l ay in

a c o o l-t e m p e r a t e , h u m i d c l i m a t i c z on e c a p a b l e o f

suppor t i ng ab undan t o rgan i c grow th i n a vas t pea t -

forming wet la nd system. The largest ent i rely cont inenta l

non-tropica l peat -forming d eposi t iona l system on the

planet t oday i s the West S iberian plain in the Ob R iver

basin (Fig . 5a) . The Ob R iver i s a vast r iver system, wi th

numerous t r ibuta ries that a re major r ivers in thei r own

right (e.g. the Irty sh Rive r). The Ob str etche s over 5,600

km from the elevate d hinterland in the Al tay Mountains

( the hinterland of the H imala ya n col l is ion zone), to the

Gul f of Ob in the Arct ic , and i s the fourth l argest r iver

system on the planet dra ining over 2 mi l l ion km2. The

main chann el near Surgut i s over three ki lometres wide,

widening to over 40 km near i t s mouth north of Sa lekhard

(Fig. 5a) . The Ob River has n umerous rela t ively s t ra ightsub-para l lel ana bra nches tha t form an ana stomosing

network for most of the lowla nds, wi th c lear s t ructural

control on river a l ignment (Fig. 5b).

Al though t here i s some sporadic evide nce of possib le

deposi ts of very l arge indiv idual channels wi thin the

eastern Austral i an basins (Falkner and Fielding 1993;

Field ing et al , 1995), our focus on th e Ob R iver syste m is

on the envi ronments peripheral to the giant f luvial

channels .

Two-third s of the system t ra verses the West Siber ian

plains (500,000 km2), most of which is swamp. The

Vasyugaskoye Swamp, located betw een the Ob and Irtysh

Rivers occupies an area of approximately 52,000 km2 ofuninterr upted pe at land s (Diessel, 1992), and this is about a

third of the swa mp system. The Vasyugaskoye Swa mp is

compara ble with the scale of va rious coal deposi ts in the

Late Permian Toolachee Formation of the Cooper Basin

that covers an area of about 130,000km2. The whole Ob

River drainage system is comparable in scale with the

combined Permia n coal measures of the Cooper-Ga li lee-

Bowen-Gun eda h-Sydn ey ba sin of eastern Austra lia. The

swamps a re d ominated by gymnosperms (conifers) forming

a classic Taiga forest. Although Taiga is a di fferent flora to

the extinct pteridosperms, we consider i t a reasonable

floral ana logue given that vast forest swamp peat d eposi ts

are indicated by coal maceral studies (Hunt and Smythe,

1986). The presence of significant arboreal vegetation in

the P ermian ba sins of Ea stern Austral ia is documented by

common Glossopteris sp and Vertebraria sp plant fossils.

During the winter months the r ivers and f loodbasin

l a k e s f r e e z e a s t e m p e r a t u r e s f a l l t o b e t w e e n

–16 an d – 40˚C. How ever, a s spring develops th ese def rost ,

a l though the lowe r rea ches ly ing wi thin the Arct ic Ci rc le

remain frozen, resul t ing in the r iver ba cking-up and

f looding most of the swa mp land, forming a v i r tual inland

sea of f reshw at er . As the summer passes the inunda ted

swamps dry out lea ving a myria d of f loodba sin l akes . The

f looding inevi tab ly r esul ts in numerous breaching of the

b anks and the deve l opm ent o f c revasse sp l ays and

lacust rine d elta s (Fig. 5c). The SAR ima ges (Figs 5b a nd

c, courtesy of Larry Smi th , UCLA) i l lust rate spring

f looding in the lower rea ches of the Ob R iver and highl ight

both scale and plani form geometry of f luvial system

elements . In part icular , note the elongat e crevasse splay

channel s tha t appear t o have f l ooded up r i ver t o the

southeast due to the r iver breaching i t s banks as wa terlevels r ise . Most of the area i s except ional ly low gra dient ,

and th i s m ay b e a good ana l ogue fo r the f l oodpl a i n-

dominate d basa l Toolachee Format ion in many area s of

southwest Queensland .

ANCIENT ANALOGUE: RANGAL COAL MEASURES,

SOUTH BLACKWATER MINE

The South Bla ckwat er Coa l Mine (Fig. 2) i s located in

the Bowen B asin , Queensland. Coal i s mined from several

seams in the Late P ermian R anga l Coal Measures . This

success i on i s an approx i m ate age equ i va l en t o f the

Toolachee Forma t ion (Fig. 3) , that a l so conta ins severalcoal seams and rela ted coal spl i t s of s imi lar type and

thickness (Gra vestock et a l , 1998 ).The coa l mine include s

>30 km of hig hwa ll exposures, 336 m of core an d 3,370

geophysical logs through potent ia l reservoi r interseam

rocks ly ing betwe en three maj or coal seams (C = Pol lux

seam, B = Castor , A = Aries) . A study ar ea wi th 230

boreholes in an a rea 4 x 2.75 km covering the K enmare

underground mine was selected by Avenell (1998) to

analyse in detai l the 3D reservoi r geometry . Dri l lhole

densi ty a verages be tween 50 and 100m.

Using cores to cal ibrate with ga mma-ray wirel ine log

motifs, Avenell (1998) ident ified eight fa cies associations

similar to those recognised in th e Toolachee Forma tion inthe Cooper Ba sin of southwe st Queensland. They include:

A. Sheetlike, ama lgama ted cha nnel deposits; B. Heterolithic

distributary channel fi l l , C. Minor crevasse splay cha nnel

fi l l ; D. Channel margin; E. Medial f loodbasin; F. Distal

floodbasin; G. Shal low floodbasin lakes; and H. Pea t mire.

Fa cies A-D represent poten tia l reservoir facies, wherea s

the others are ba ffles and seals. A typical example of a core

interval from the B-A interseam a nd a ssociated ga mma ra y

well log motif s in drillhole 7906 are provided for compar ison

with a GR log from the basal Toolachee in southwest

Queensland (Fig. 6).




The success i on w as d i v i ded i n to four m appab l e

interva ls ut i l is ing l acustr ine f looding surfaces for high

resolut ion s t ra t igra phic correlat ion. Maps of t he C-B

lower , C-B mid dle a nd C -B upper sub-inter va ls, plus the

B-A interseam interva l were prepa red f or cumulat ive

clean sand percent (<90 API), cumulative si l ty sand

percent (<110 API) , interval i sopach and sedimentar y

fac ies .

Figure 5. (a) Location of ERS–1 SAR images from the Ob River in Western Siberia south of the Arctic Circle. (b) A coherency imagproduced by interferometric processing of SAR scene of the Ob River south of Salekard showing the structural control on river alignmenand the development of crevasse splays and lacustrine deltas. (c) Change detection image from ERS-1 SAR images showing detail omeandering and anastomosing anabranches of the Ob River, and associated elongate crevasse splays and lacustrine deltas and floodbasilakes between 1992 and 1993. SAR images courtesy of Smith and Alsdorf (1998), and the European Space Agency, http://lena.sscnet.ucla.eduarctic_russia.html.




Sedimenta ry fa c ies maps were a l so prepared using acombinat ion of log mot i fs , sand percent and i sopach

maps.

These m aps , and a ssoc i a t e d com puter v i sua l i sa t i ons

(Fi gs 7 a–c) , revea l a de t a i l ed p i c ture o f cyc l i ca l f i ne-

g r a i n e d f l o od b a s i n se d i m e n t a t i o n b e t w e e n l a c u s t r i n e

f l o od i n g s u r f a c e s (i n t r a f o r m a t i o n a l s e a l s a n d b a f f l e s

ran g i ng f rom 5 00 x 50 0 m to b l anket s ac ross the a rea ) .

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

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

over the C sea m ) , fo l l ow ed b y progress i ve i n f i l l i ng o f

t h e f l o o d b a s i n l a k e s w i t h c r e v a s s e s p l a y s a n d s h a l l ow

Figure 6. Comparison of a typical example of stacked crevasse splays from South Blackwater Mine (borehole 7906) though the A-Binterseam interval, with a typical wireline log through basal Toolachee Formation illustrating the similarity in coal seam thickness andinterseam clastics.

l a c u s t r i ne d e l t a s f r o m a l l s i d e s ( ce n t r i p e t a l d r a i n a g e ) .In the C -B i n terva l the evo l u t i on o f c revasse sp l ays

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

sha l l ow f l oodb a s i n l akes (e . g . C -B l ow e r sub -i n terva l )

m a t u r i n g t o m o r e e l o n g a t e , a n a s t o m o si n g p l a n f o r m s

(C -B m i dd l e sub -i n terva l ) w as ev i den t . D i f feren t i a l

s u b si d e n c e a n d a s h i f t i n t h e l o c u s of s e d i m e n t a t i o n

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

f l oodb a s i n l a kes (e . g . C -B upper sub -i n terva l ) , t hen

f o l l ow e d b y e x t e n s i v e p e a t m i r e a g g r a d a t i o n (e . g . B

seam ) . A si m i l a r p i c ture em erges f o r the B-A i n terva l

b u t the de pos i t i ona l sys tem i s re l a t i ve l y d i s t a l (m ore




f l o o d b a s i n d e p o s i t s ) a l t h o u g h n a r r o w , e l o n g a t e

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

reso l vab l e .

A key f inding of the South Blackwa ter Mine analogue

i s the sm al l sca l e o f the i nd i v i dua l and am al gam ated

reservoi r elements. Crevasse splay d is t r ibutary cha nnel

are t yp i ca l l y narrow and e l onga te , e i ther b i furca t i ng o

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

f loodbasin l a kes . In the C-B interva l , indiv idual channel

are t yp i ca l l y 2 5 m w i de , b u t a m al gam ated channel w i d th

range from 50–340 m, averaging 150 m based on 50%

cumulat ive s i l ty sand. Narrower crevasse splay channe l

ar e indica ted for t he B-A interva l (25-100m wide a nd

avera ging 60 m). This i s supported by da ta f rom a bove

the A-sea m in highw a ll exposures (e.g. Ra mp 11, Figure

8) where a s ta ck of amalga mate d decimetre-scale sandy

crevasse splays s imi lar to those described from the

Toolachee Format ion, thin out and sha le out rapidly ove

200 m.

Interconnectedness ma ps for the C-B lower-middle

an d midd le-upper sub-inter va ls show a complex pictur e

of worm holes where seals ar e l a cking or thin (0 .1 to 1.m) betw een sands, and t his is due to the overal l low net

gross. Where sa nd per cent excee ds 50% in the en tire C

B i n t e r v a l , i n t e r c o n n e c t e d n e s s w i l l b e g o o d . T h e

strat igraphic amalgamat ion of crevasse splays in the

b a s a l T o o l a c h e e F o r m a t i o n a b o v e t h e D a r a l i n g i e

unconformi ty in some areas of southwest Queensland

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

connec tedness than the S outh B l ackw ater ana l ogue

indicates .

C r e v a s s e c h a n n e l t h i c k n e ss e s a r e u p t o a m a x i m u m

of 10 m, though m ost ly 6–7 m, wi th wid th/thickn es

ra t ios ve ry low (5:1 – 15:1 and ma ximum 25:1) . This i

i n accord w i th dr i l l i ng i n the Tool achee Form at i onw h e r e r a p i d f a c i e s c h a n g e s f r o m r e s e r v o i r t o n o n

r e s e r v oi r c a n o c cu r l o c a l l y w i t h i n d i s t a n c e s o f a f e w

h u n d r e d m e t r e s . C r e v a s s e sp l a y d i s t r i b u t a r y c h a n n e l

are re l a t i ve l y s t ra i gh t t o s i nuous ( s i nuos i t y range

f rom 1 : 1. 02 to 1 .1 8) and channe l l eng t hs ra nge f rom

3 00 m up to 3 , 00 0 m a cross the s tudy a r ea . B i furca t i ng

c r e v a s se s p l a y d i s t r i b u t a r y c h a n n e l w i d t h s g e n e r a l l y

d e c l i n e w i t h c h a n n e l l e n g t h i n a l i n e a r o r e x p o n e nt i a

f ash i on (e . g . approx i m ate l y 5 0 % reduc t i on i n w i d th

over 1 –1 . 5 km dow ns t ream ) . Of the few b i furca t i ng

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

b i furca t i on f requency ranges t yp i ca l l y f rom 1 0 0 –3 7

m i n the C -B sub -i n terva l , w herea s i t i s m ore var i a b l ein th e B-A inter va l (35 to 675m). As a g ener a l i sat ion

the th i cker i n terva l i sopachs approx i m ates the z one

w h e r e c r e v a s s e s p la y d i s t r i b u t a r y c h a n n e l s b i f u r c a t e

(Fi g . 7 b ) , h i gh l i gh ted b y v i sua l i s i ng the a rea w her

c l e a n s a n d i s g r e a t e r t h a n 5 0% a n d t h e i n t e r v a l i s o p a c h

i s th i ckes t (F i g . 7c) . In ter-sp l ay f l oodb a s i n a re as up t

500 x 500 m in area comprise mudstone-dominated

l a t e r a l s e a l s a n d b a f f l e s r e s ul t i n g i n

c o mp a r t m e n t a l i sa t i o n o f t h e r e s e r v o i r i n t e r v a l .

Figure 7. Computer visualisation of the lower C-B interseam sub-interval with 230 boreholes in a 4 x 2.75km area at SouthBlackwater Mine, Bowen Basin. (a) Small, bifurcating crevassesplays using clean sand percent (<90 API gamma units) projected aspseudo-topography. (b) Sub-interval isopach draped over cleansand percent. (c) All the area with greater than 50%cumulative siltysand percent that is also thick (light blues to red), compared withmuddy areas (floodbasin lakes forming lateral seals and baffles).




POTENTIAL PITFALLS

The South Blackwater Mine analogue in the Rangal

Coal Measures l ies in a foreland basin set t ing whereas

t h e T o o l a c h e e F o r m a t i o n i s e n t i r e l y c r a t o n i c a n d

there fore ra t es o f sub s idence a t S ou th B l ackw ater w ere

higher and sediment supply was a l so high. This may

present a problem, but i t i s considered a minor i ssue.

Furthermore, a l though the Ob R iver i s a useful modern

analogue i t i s probably a much larger system than the

Toolachee Format ion and sediment supply (volume and

type) may provide di f ferences that ha ve not been ful ly

assessed. Also, a l though t he SAR imag es are f rom just

sou th o f the perm af ros t z one today , the Tool achee

Form at i on c l i m ate m ay have b een m ore hum i d and

possibly wa rmer, based on t he lack of cryogenic structures.

Analogue utilisation

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

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

com puter m odel s fo r f i e l ds i n sou thw es t Queens l and

b y S an tos L td . The d i m ens i ons o f sp l ay and channel

d e p o si t s , st u d i e d a t S o u t h B l a c k w a t e r M i n e g u i d e d t h e

choice of grid cel l dimensions to ref lect t he high medium

r a n g e v a r i a b i l i t y a n t i c i p a t e d . S i mi l a r l y t h e se d a t a w e r e

u t i l i sed i n the cons t ruc t i on o f var i ogra m m odel s .

T h e a n a l o g y w i t h t h e m o d e r n O b R i v e r s y s t e mh i g h l ig h t e d t h a t v a s t a r e a s o f t h e C o o p e r B a s in m a y

have b een dom i na ted b y f l oodb a s i n and coo l -tem pera te

p e a t s w a m p e n v i r o n m e n t s w i t h o ut t h e d e v e l o p me n t o f

m a j o r , i n d i v i d u a l f l u v i a l c h a n n e l s . L o w g r a d i e n t s i n

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

an as tom os i ng r i vu l e t s and sp l ay com pl exes , w h i ch can

ea si ly occupy a rea s excess of 50,000 km2. The a ppar en t

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

l a rge a rea s o f Fa c i es Assoc i a t i on 1 i s en t i re l y cons i st en t

w i t h t h e s e f i n d i n g s .

The an t i c i pa ted sm al l s i z e o f i nd i v i dua l sed i m ent

b o d i e s w i t h in t h e a m a l g a m a t e d s h e e t s a n d o f t h e l ow e r

To o la c h e e F o r m a t i o n s u g g e st s t h a t a t t h e c u r r e n t w e l lspac i ng i t i s i m poss i b l e t o corre l a t e i n de t a i l i nd i v i dua l

sp l ay/channe l depos i t s . Ins tead the f ocus has sh i f t ed

to s tochas t i c m odel l i ng o f the reservo i r i n terva l s .

Figure 8. (a) Panorama of highwall exposure of amalgamated, decimetre scale, stacked crevasse splays shaling–out along strike over 200m,South Blackwater Mine. (b) Detail inset. Individual crevasse splay with climbing ripple lamination.




CASE STUDY 2: EARLY PERMIAN EPSILONFORMATION FLUVIO-DELTAIC LACUSTRINE

MOUTH BARS– MOOMBA-BIG LAKECOMPLEX (COOPER BASIN, SOUTH

AUSTRALIA)

Reservoir characterisationThe evaluat ion of the reservoi r s i ze , geometry and

interna l archi tecture of f luvio-del ta ic sandstones wi thin

the Early Permian Epsi lon Format ion of the Coope

Basin i s of s t ra tegic importance in the evaluat ion o

hydrocarb on reserves and the p l ann i ng o f e f f i c i en

development s t ra tegies . The E psi lon Format ion in the

Moomba -Big La ke Field of t he Cooper Ba sin (Figs 2 an d 3

consis ts of an aggradat ional l acustr ine del ta sequence

deposi ted ad jacent t o the Dara l ingie Shel f in response to

di f fere nt ia l subsidence along the t rend of the Big Lake

Moomba Fault complex (Taylor et al ; 1991, Fairburn

1992; Leckie 1992). The a bsence of q uan ti ta t ive d a ta on

the w idth : thickness ra t io of these non-marine mouth ba

sands and dis t r ibutary cha nnel sand s signi f icant ly a f fect

the certa inty of reservoi r dis t r ibut ion wi thin the Epsi lo

Format ion. Mul t i -s torey chan nel mouth ba r geometrie

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

contribut ing to improved reserves est imat es and bet t e

posit ioning of inf i l l wel l s .

In the Moom b a-Bi g Lake a rea t he Eps i l on Form a t i on

i s d i v i ded i n to e l even de l t a i c cyc l es b ased on l og

s i gna tur es (F i gure 9 ). C orre l a t i on o f the d e l t a i c cyc l e

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

pa l yno l og i ca l z ones der i ved f rom Pr i ce e t a l , (1 9 8 5

(Fi gure 3 ) . For de t a i l ed reservo i r corre l a t i on each

cyc l e i s sub -di v i ded i n to genet i c i nc rem ents us i ng t hree

m aj or even t hor i z ons descr i b ed b y Al l en an d Merc i e

( 1 9 8 8 ) : t h e d e l t a p r o g r a d a t i o n s u r f a c e , t h e d e l t a

e m e r g e n c e su r f a c e a n d t h e d e l t a f l o od i n g s u r f a c e .

A m a j o r f l o o d i n g s u r f a c e ( t h e T C 5 0

c h r o n o s t r a t i g r a p h i c m a r k e r ) d i v i d e s t h e E p s i l o n

Form a t i on i n the Moom b a-Bi g Lake F i e l d i n t o l ow e

a n d u p p e r u n i t s w h i ch d i s p l a y d i f f e r e n t l o g s ig n a t u r e

(Fi g . 9) . Three d e l t a i c cyc l es w i th i n the l ow er E ps i l o

F o r m a t i o n c o n t a i n c o a l i n t e r b e d s i n d i ca t i n g

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

c o n t r a s t o n l y o n e o f t h e f i v e d e l t a i c c y c le s w i t h i n t h e

u p p e r E p s i l o n F o r m a t i o n c o n t a i n a c o a l b e d

Never the l ess , ex tens i ve coa l s a re deve l oped w i th i nt h e E a r l y P e r m i a n E p s il o n F o rm a t i o n o n t h e D a r a l i n g i

S h e l f a d j a c e n t t o t h e c h a n n e l b e l t o f t h e M o o mb a -B i g

L a k e F i e l d .

The intra crat onic lacustrine delt a sett ing of the Epsi lon

F o r m a t i o n w i t h i n t h e B i g L a k e -M o o m b a F i e l d i

interpreted as a rela t ively shal low, mixed load, f luvia

dominat ed system with variab le flow regime which formed

a complex of high construct ive, lobate del ta s developed

a t the t erm i nus of a f l uv ia l channel b e l t .Figure 9. Chronostratigraphic units and lithostratigraphic unitsrecognised within the Lower and Upper Epsilon Formation in theMoomba-Big Lake Field.




Figure 10. Location map of the Neales Delta. Satellite image (courtesy of NASA, Space Shuttle Mission 41, STS 041) illustrates thedevelopment of a shallow water deltaic prism within Lewis Bay at the mouth of the Neales River fluvial channel belt on the northwesternshoreline of Lake Eyre, South Australia.




Analogue selection criteria

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

sub sur face reservo i rs i n de l t a i c se t t i ng s i s one o f the

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

m odel s o f these hydrocarb on sys tem s . Thus recen t

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

M a h a k a m D e l t a i n I n d o n e s ia p r o v e d t o b e o f s t r a t e g i c

v a l u e i n f u r n i sh i n g d a t a o n m a r i n e d e l t a i c m o u t h b a rs a n d s w h i c h f o r m t h e m a i n r e s e r v oi r t y p e s i n t h e g i a n t

M i oc e n e g a s f i e l d s o f t h e K u t e i B a s i n , E a s t K a l i m a n t a n ,

Indones i a (Al l en e t a l , 1 97 6) . In con t ras t , ve ry l i t t l e i s

k n o w n o f t h e g e o m e t r y , s i ze a n d a r c h i t e ct u r e o f n o n -

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

reservo i rs i n the Eps i l on Form at i on . Moreover , the

l a t e r a l e x t e n t o f s t r e a m m o ut h b a r s a n d s w i t h i n n o n -

m a r i n e H o l o c e n e l a c u s t r i n e d e l t a i c c y c l e s r e m a i n s

l a r g e l y u n q u a n t i f i e d a n d m a y b e s i g n i f ic a n t l y d i f f e r e n t

w h e n c o m pa r e d w i t h m a r i n e d e l t a s w h e r e t i d a l e f f e c t s

a n d w a v e e n e r g y r e g i m e s a l s o co n t r o l t h e g e o m e t r y o f

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

a n d d e l t a f r o n t .

S e l ect i ng m odern and a nc i en t a na l ogues fo r the Ea r l y

Permia n f luvial-lacustr ine del ta complexes of t he E psi lo

Format ion in the Moomba-Big La ke Field of t he Coope

Basin req uires several cr i teria to be met .

Ba sed on the generic descript ion of del ta ic cycles by

Allen and Mercier (1988) the inter va l thickness observedon wirel ine logs between the emergent surface and

prograda t i on sur f ace o f de l t a i c parasequences w i th i n

the l ow er Eps i l on Form at i on dem ons t ra tes tha t the

pa l aeow ater dep th o f the rece iv i ng b as i n shoul d vary

betw een 5 and 10 m.

The c l imate of t he Ea rly Permia n wa s st rongly seasona

ba sed on the close spacing of a nnual growt h rings recorded

from the pa laeof lora of the cool-tempera te coal swa mp

(Diessel , 1992) indicat ive of d ormant cold winte r grow th

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

Consequent ly , the f luvio-del ta ic systems selected a

Figure 11. Aerial view (looking upstream) showing present daylowstand lake level exposure of relict mouth bars and extant braidbars deposited by the east-west distributary channel system of theNeales Delta.

Figure 12. Aerial view of bifurcating rectilinear distributarychannel system and exposed side bars/lateral bars developed othe Neales Delta plain.

Figure 13. Relict mouth bar geometries within bifurcatingdistributary channel system of Neales Delta plain.




ana logues should display highly varia b le r iver discharges

resul t ing from seasonal c l imat ic regimes.

Presumably , the highly variab le r iver discharge of

Ear l y Perm i an f l uv i a l sys tem s i n the C ooper Bas i n

tra nsported a high volume, mixed sediment load into a

shal low, f resh water l acustr ine receiv ing basin in the

Moomba -Big La ke study a rea . The modern f luvio-delt a ic

ana logue should display an overa l l lobat e geometry wi th

coalescent shoal-wa ter lobes . Ba sed on log s ignat ure

maps of the Epsi lon Format ion a modern ana logue should

also contain wel l def ined f luvial channel bel ts up to1,000 m wide with individual channels up to 200 m in

w i d th .

MODERN ANALOGUE: NEALES DELTA, SOUTH

AUSTRALIA

Satel l i te sphotographs of the extant Neales Del ta

complex in South Austral ia (Fig. 10) suggest that i ts

overal l lobate morphology and the shal low receiv ing

b a s i n /v a r i a b l e f l o w r e g i m e h y d r o l o g y m a y o f f e r a

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

deve l oped w i th i n the anc i en t sha l l ow w a t er l acus t r i ne

del ta systems of the upper Epsi lon Format ion intersected

in the Moomba-Big La ke Field. As publ i shed a ccounts of

the sedimentology of extant f luvia l dominated l acustr ine

systems contain few deta i led descript ions of sand fa c ies

geom et ry d eve l oped w i th i n m outh b a rs and d i s t r ib u t a ry

channel s a f i e l d s tudy o f the Nea l es Del t a w a s used to

qua n t i f y the re l a t i ve sca l e and a rch it ec ture o f poten t i a l

subsurface Ea rly P ermian f luvio-del ta ic reservoi rs.

C ondi t i ons preva i l i ng dur i ng cons t ruc t i on o f the

Middle Holocene Neales Del ta are inferr ed to include al a rge sha l l ow l acus t r ine b a s in ; a m i xed l oad dra i nage

com pl ex ; var i ab l e r i ver d i scharge ra t es and f l uv i a l

dom i na ted rec t i li near d i s t r ib u t a ry channel s.

The Neales Del ta i s developed at the mouth of the

Neales River in Lewis Bay on the northwestern shorel ine

of La ke Ey re (Fig. 10). The de lta consists of a fa n-sha ped,

f luvial dominat ed, shal low wa ter , h igh construct ive lobe

appr oximat ely 25 km2 i n a rea w i th tw o m ajor d i s t r i b ut a ry

c h a n n e l s . D u r i n g h i g h s t a n d c o n d i t i o n s t h e N e a l e s

distributa ry channel system discharg es sand into a shal low

s t andi ng b ody o f w a ter a t the de l t a f ron t fo rmi ng a th i n

Figure 14. Reservoir heterogeneity and longitudinal facies variation developed within the east-west distributary channel of the Neales Delta.




but l a teral ly extensive mouth bar complex. However,

during episodes of f luvial di scharge when the l ake i s

empty , sediment out f low from the dis t r ibuta ry channels

form s a sub aer i a l b ra i d d e l t a on the exposed de l t a f ron t

(Fig. 11).

When Lake Eyre i s bankful l the Neales Del ta pla in

const i tutes the subaeria l part of the del ta ic pri sm and

extends in a 5 km arc from the mouth of the Neales f luvial

channel bel t . The del ta pla in i s incised by rela t ively

rect i l inear dis t r ibutary channels wi th widths varying

between 30 m and 160 m. The dis t r ibutary channels

b i furca te dow ns t ream and even tua l l y t e rm ina te w i th i n

dis t r ibutary mouth bar complexes on the del ta f ront

(Figs 12, 13 and 14). The channel thalweg meanders

w i t h i n t h e d i s t r i b u t a r y c h a n n e l s w h i c h d i s p l a y a n

asymmet ric profi le as a result of the deposi t ion of elongat e

l a t era l a cc re t i on b ars .

PVC push cores were used to evaluate the three-

dimensional geometry and reservoi r characteri s t ics of

po in t b a r a nd l a t era l b ar system s deve loped w i th i n the

east–west dis t r ibuta ry . As shown in Figure 14 the overa l l

geometry of a dis t r ibuta ry channel deposi t i s tha t of a low

sinuosi ty sand body elongated paral lel to the channe

axis . Mud plugs accumulated at t he end of f lood s tage

l oca l l y com par tm enta l i se the reservo i r sands . S and

th i cknesses decrease rap i d l y aw ay f rom the channe

ax i s . Para l l e l t o the d i s t r i b u t a ry channel , t he sand

thickness decreases over a dis tance of 5 km from a

maximum of 5 m, a t the f luvial cha nnel mouth, to 25 cm

wi thin coalescent braid ba r deposi ts on the del ta f ront

Current ly , during lowstan d f lood episodes , when th e

Neales River i s bankful l , sand i s t ransported in the

di s t r i b u t ary channel s t o the de l t a f ron t w here i t i

deposi t ed as a sub aer i a l b ra i d d e l t a on the exposed p l aya

lake margin . However, during a highstand, a t the l akeside

mouth of the distributary cha nnels, a decrea se in sedimen

transport veloci ty occurs due to decelerat ion of the

unconfined f luvial current f low as i t enters a s tanding

body of water . This resul ts in deposi t ion of sand as a

complex of subaq ueous, tria ngular shaped t erminal mouth

bars or mid ground bars (Wright, 1977) (Fig. 15). The

l a tera l am a l gam at i on o f the adja cen t sandy m outh b a

Figure 15. Sand facies geometry of relict mouth bar complexes illustrating the development of multi-storey reservoirs at distributarybifurcations on the Neales Delta; (a) plan view of mouth bar, (b) architecture, (c) development of amalgamated mouth bar/lateral bar (fromBenson and Glasso, 1989).




Figure 16. Genetic linkage of reservoir heterogeneity (apparent from typical gas flow rate – time plot of a well drilled in a lacustrine deltaicsetting) and sand facies architecture developed at a distributary channel bifurcation. Extant channel geometry of the Neales Delta is usedto illustrate potential reservoir heterogeneity over a distance of 1 km within a lower delta plain setting.




deposi ts on the del ta f ront forms a crescent ic f r inge of

sand. Subseq uent lowsta nd condi t ions resul t in exposure

of the amalgamated mouth bar complex and dissect ion

by f luvial di scharge from the dis t r ibutary channel mouths

form ing a b ra i d p l a i n .

The Neales Delta is unique in tha t al l of the de posi t ional

elements are exposed due to the ephemeral nature of

wa ter levels within Lake E yre. Consequent ly, i t is possible

to examine the sedimentology of reservoi r sand fac ies

tha t a re norm al l y sub m erged and to q uan t i f y the re l a t i ve

scale of l a tera l bars , point bar s and mouth ba rs . Width to

thickness ra t ios were calculated for f luvial channels ,

d i s t r i b u t a ry channel s and b ra i d b ars ( i n terpre ted as

lowstand equivalents of mouthbars) using the vert ical

and l a t era l ex ten t o f the sand b odi es in tersec ted b y the

f ield survey grids and subsurface sampl ing of the del ta

plain a nd a dja cent del ta f ront . The reservoi r/sand f ac ies

geom et ry a t ea ch survey t ra nsect d i sp l ays a dow ns t ream

decrease i n the w i d th to th i ckness (W: T) ra t i o o f

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

However the W:T rat io of braid bar sands increases

rap i d l y a t the de l t a f ron t due to am al gam at i on of a d jacen tmid-ground ba rs . Consequent ly t he bra id-bar complex

on the de l t a f ron t d i sp l ays a grea ter l a t era l ( s t r i ke)

d i s t r ib u t i on tha n the de l t a p l a i n d i s t r ib u t a ry channel

sands. A more deta i led account of the sedimentology of

the Neales Del ta i s current ly being developed from a

fol low up field study by H icks (1998).

Potential Pitfalls

Obviously , the Late H olocene playa l ake deposi t ional

envi ronment of Lake E yre (3 ka to present ; Magee et a l ,

1995; Croke et a l, 1996, 1998) is dissimila r to t he pe ren nia l

l acustr ine envi ronments of the Early Permian Epsi lonForm at i on . Never the l ess the ex t an t Nea l es Del t a i s

interpreted as a pal impsest feature recording former

highstands of l ake level wi thin the Holocene as rel ic t

shorel ines a l igned wi th dis t r ibutar y channel b i furcat ions

on the d elta plain (Fig. 14).

S tudy of t he Quaterna ry chronology and sedimentology

of the f luvio-lacustr ine cat chment of t he lower Nea les

R iver (Cr oke et a l, 1996, 1998; Mage e et a l, 1995) indi ca tes

the development of a series of P leis tocene and Early-

Middle Holocene del ta systems during periods of high

l ake l eve l s assoc i a t ed w i th p l uv i a l i n terg l ac i a l and

interstadial s tages . The earl iest l acustr ine phase dates

from the Late P leis tocene (150 ka, Last Interglac ia lOxygen i sotope S tage 5) and i s associated wi th the

development of bea ch r idges a t + 10m AHD a nd deposi t ion

o f the l acus t r ine Ghos t Ya rd Beds w i th i n a w a ter co l um n

of 25 m.

I t i s postulated tha t the rel ic t highstand pha se of the

Neales Del ta wa s emplaced d uring the Cl imat ic Opt imum

of the Middle Holocene (8–5 ka) when average a i r

t em pera tures w ere w arm er a nd condi t ions w ere w et t er

in the Lake E yre cat chment area . This is evidenced by

high l ake levels a t La ke Frome and expansion of t ree and

shrub cover resul t ing from increased ra infal l l inked to

incursion of the summer monsoon bel t f rom the north

(Singh 1981, P i t tock and Sal inger 1983) . Fol lowing

development of the present playa lake extend ing from 3 ka

to the present (Croke et al , 1998) an ama lgama ted complex

o f sub aer i a l b ra i d de l t as w ere depos it ed a djacen t t o the

rel ic t del ta f ront deposi ts of the Middle Holocene Neale

Del t a .

What did it tell us?

Comparison of subsurface log s ignatures of the E psi lon

Format ion from the Moomba-Big Lake Field w i th plan

views of the lobate geometry of the Holocene Neale

Del t a show s tha t the overa l l s iz e and geom et ry o f the t w o

systems are s imi lar . Consequent ly , t he superbly exposed

deposi t ional elements of the Neales Del ta provide a

three d i m ens i ona l t em pl a te fo r v i sua l i sa t i on o f the

potent ia l reservoi r geometry of subsurfa ce del ta ic sand

b o d i e s d e p o s i t e d w i t h i n t h e E p s i l o n F o r m a t i o n a

dis t r ibuta ry channels , crevasse splays an d mouth bars .

The east–west dis t r ibutary of the Neales Del ta i

curren t l y i n a s t a t e o f ab a ndonm ent a s a resu l t o f channeavuls ion through a major crevasse splay , which i s creat ing

a separa te deposit ional lobe wi thin the interdis t r ibutar

f loodplain to the northeast of the dis t r ibutary channe

(Hicks, 1998). Clay plugs are now beginning to form

l oca l i sed perm eab i l it y b a rr i ers b e tw een sandy l a t era

b ars w i th i n the rec t i li near segm ents of the ab a ndoned

di s t r ib u t ary channel . In the sub surface the l a t era l b ar

would form d iscont inuous compart menta l i sed reservoi r

wi th very good permea bi l i ty . However those segments o

the channel which erode mouth bar deposi ts wi l l a l so

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

extensive, but lower permeab i l i ty f ine grained san ds o

the del ta f ront a s shown in Figure 16.U s i n g N e a l e s D e l t a d i s t r i b u t a r y c h a n n e l s a n d

w i dth : th i ckness ra t i os , deve l opm ent geo l og i s t s and

reservoi r engineers can ba ck-calculate the length o

del ta ic sand b odies required to produce a given volum

of hyd rocarbons. In Figure 16 we ha ve superimposed a

rectangle represent ing the area of compartmental i sed

high permeabi l i ty reservoi r required to produce 3BCF

OGIP from a 13f t thick sand onto an obl ique aeria

pho tograph o f a f i e l d surveyed l a t era l b ar w i th i n a

rect i l inear dis t r ibutary channel segment of the Neale

Del t a p l a i n .

Overlay of scaled templat es of Neales Del ta geometrie

onto 3D t ime s l ices f rom the Moomba-Big La ke Field wi lass i s t in the v isual i sat ion and id ent i f i cat ion of l acustr in

del ta ic systems wi thin the E psi lon Format ion. Geometri

pat t erns v isib le on ampl i tude a t t r ibute d isplay s can be

compared aga inst the surface expression of a nastomosing

f luvial channel bel ts , rect i l inear dis t r ibutary channel

o n t h e d e l t a p l a i n a n d t h e l o b a t e g e o m e t r i e s o

am al ga m ated m outh b ars deve l oped a l ong the edge o f a

de l t a f ron t .

Observed width to thickness ra t ios for dis t r ibutary

channels and mouth bar systems developed wi thin the

Holocene Neales Del ta provide a sc ient i f i c base fo




assessment of the l a teral cont inui ty of reservoi r sands

deposi ted wi thin shal low wat er high construct ive loba te

lacustr ine del ta s intersected in the Ea rly Permia n Epsi lon

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

dis t r ibuta ry channels updip of the del ta f ront wi l l form

discret e pods of sa nd w ith W:T ra tios of 130:1 para l lel to

the tha lweg; compartment al i sed by c lay plugs a long the

length of the channel . However, the mouth bar sands

deposi ted at the mouth of dis t r ibutar y channels downdip

w i l l am al gam ate and fo rm a th i n shee t sand reservo i rwit h a W:T ra tio of 1000:1 pa ra l lel to the d el ta front .

Sand fa c ies dis t r ibut ion pat te rns on the Neales Del ta

p l a i n dem onst ra te tha t i sol a t ed l a t era l b a rs w i l l occur

w i th i n the rec t i l i near channel segm ents deve l oped

b etw een b i furca t i on nodes o f the d i st r i b u t a ry channel

network. These compartment al i sed reservoi rs wi l l ha ve

h i gh produc t i on ra t es due t o the b e t t er poro-perm

characteri s t ics associated wi th the channel sands. Careful

placement of subsequent appraisal and development

wel l s a t di s t r ibuta ry b i furcat ions (as imaged by 3D t ime

sl ices) of the del ta pla in wi l l opt imise intersect ion of

m u l t i s t o r e y r e s e r v o i r s . A t t h e s e l o c a t i o n s h i g h

perm eab i li t y d i s t r i b u t a ry channel sands w i l l b e dra i ned

v i a w el l s com pl e ted i n the l a t era l l y ex tens i ve , l ow

permeab i l i ty mouth b ar sandstones (Figs 15 and 16) .

ANCIENT ANALOGUE: SIRIKIT DELTA THAILAND

Based on s imi lar log s ignature chara cter , interpreted

deposi t ional envi ronment a nd product ion decl ine t rends,

the subsurface S i r iki t l acustr ine del ta complex from theTert i a ry P hi tsanulok Ba sin in centra l Thai la nd (400 km

north of Bangkok) was selected a s an ancient an alogue of

t h e E a r l y P e r m i a n b a s a l E p s i l o n F o r m a t i o n i n t h e

Moomba –Big La ke Field , nota bly t he 81-1 sand (Figs 3

and 9) . Dimensions and morphology of t he S i r iki t Del ta

are derived from a 3D seismic grid, 45 product ion wel l s

and integra ted core a nd log da ta (Fl int et a l , 1988; Fl int

et a l, 1989; Ainswor th e t a l, 1999).

The S i r iki t Del ta sequence consis ts of the Miocene

f luvio-del ta ic Lan Kra bu Format ion which i s interbed ded

wi th the l acustr ine Chum Saeng Format ion, developed

Figure 17. Depositional model for high constructive lobate deltas of the Miocene Lan Krabau Formation subsurface Sirikit Delta complexPhitsanulok Basin, Thailand (Flint et al, 1988) with analogous log signatures from the 81-1 deltaic cycle of the basal Epsilon Formation.




ad jacent to the int ersect ion of two regiona l s t r ike-sl ip

fa ul t zones wi thin the ha l f-grab en ba sin . Deposi t ional

envi ronments interpreted for the S i r iki t include open

lacustrine claystones, del ta f ront lower mouth bars, mouth

bar /sheet sands, crevasse splays and dis t r ibutary channel

fill s (Flint et a l, 1989) (Fig. 17).

The large scale coarsening upwards cycles (4–8 m in

thickness) of the Lan Kra bu Format ion were f ormed by

del t a i c prograda t i on i n to a l a rge , re l a t i ve l y sha l l ow ,

f reshw ater l ake (5 –1 5 m w a ter dep th) . S hee t sand

d e v e l o p m e n t w i t h i n t h e L a n K r a b u F o r m a t i o n i s

i n terpre ted as an am al gam ated m outh b ar com pl ex

form ed b y seasona l d i scharge o f m i xed l oad f l oodw aters

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

claystones of the Chum Saen g Format ion are interpret ed

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

gast ropods, f resh-wa ter a lga e, non-marine terrest r i a l ly

dom i na ted pa l yno fac i es and geochem ica l da t a (F li n t e t

a l, 1988).

What did it tell us?

The th in-bedd ed r eservoi r sandstone geometr y of the

Sir iki t Del ta i s loba te in shape wi th sheet l ike sand f ac ies

p a r a l l e l t o t h e d e l t a f r o n t f o r m e d b y t h e l a t e r a l

ama lgama t ion of ad jacent mouth ba rs . The deposit ional

model proposed for the shal low wa ter , construct ive lobate

del ta s of the Miocene Lan Kra bau Forma t ion (Fl int et a l ,

1988) is used to r econstruct t he de posi t ional en vironment

and dem onst ra tes a shee t li ke sand f a c i es geom et ry fo r

the 81-1 del ta ic cycle of the b asa l Epsi lon Format ion

ba sed on a select ion of log signat ures from the Moomba-

Big La ke Field (Figur e 17).

Given that the l acustr ine deposi t ional envi ronments

of the S i r iki t Field a nd Moomba-Big Lake Field a resimi lar w e would expect to f ind s imi lar product ion t rend s

wi thin E psi lon mouth bar sa nds. For example product ion

d a t a f r o m t h e S i r i k i t F i e l d d e m o n s t r a t e s l a t e r a l

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

forming dra inage a reas of up to 20 km2 (Flint et a l, 1989).

Multi-storey re servoirs ar e well-deve loped in both t he

Sir ikit a nd Epsi lon del ta ic cycles wi th narrow dis t r ibutary

channel-f i l l sand s eroding into ama lgama ted mouth bar

sand s with in a sha l low w at er column (5–15 m) (Figure 17).

Product ion rates recorded from individual mul t i s torey

reservoi rs in the S i r iki t Field a nd Moomba-Big La ke

Field consis t o f two f low ra te decl ine t rends which can be

explained in terms of the di f ferences in permeabi l i ty a nde f f e c t i v e d r a i n a g e a r e a s w i t h i n m o u t h b a r s a n d

dist r ibutary channel sands. Ini t i a l ly , h igh product ion

rates are recorded from the thicker, high permeabi l i ty

d i s t r ib u t ary channel sands b u t ra p i d l y dec l ine due to

deplet ion of the compartment al i sed ga s volumes wi thin

the c lay -plugged dis t r ibuta ry chan nels . However, the

decl ining f low ra te t r end fr om the mul t i -s torey reservoi r

s tab i l i ses as the ga s i s sourced from the low permeab i l i ty

sands wi thin the thin but l a tera l ly extensive ama lgamat ed

mouth bars .

P roduc t i on da t a f rom the S i r i k it F i e l d d em onst ra tes

tha t l acustr ine c laystones form f ield-wide ba rriers t

vert ical f luid communicat ion betw een the mouth-ba

sandstones in each del ta ic cycle . Each of the mouthbar

sands a c t a s indiv idual f low uni ts (Ainsworth et a l , 1999)

C o n s e q u e n t l y l o c a t i o n o f i n f i l l d e v e l o p m e n t w e l l

intersect ing analogous mouth bar deposi ts wi thin the

Eps i l on Form at i on needs to b e b ased on reservo i

archi tectural models which correlate indiv idual del ta i

cycles or parasequences wi thin a chronostrat igraphic

f ram ew ork .

F i n i ng upw ard aggra da t i ona l f l ood-p l a i n un i t s o

aba ndonment channe l f i l l c l ay plugs developed on top o

mouth-ba r and channel deposi ts wi thin the S i r iki t Del ta

com pl ex a re i n terpre ted to deve l op as a resu l t o

d i s t r ib u t ary channel ab and onm ent l i nked to the var i a b l

discharge of the f luvial system.

Potential Pitfalls

The t ropical monsoonal c l imat ic regime inferred fo

the Miocene S i r iki t del ta di f fers f rom the cool temperate

cl imate of the Ea rly Permia n. Nevertheless , the varia b led i scharge o f t he S i r i ki t De l t a m ay b e ana l ogous to the

highly seasonal pat tern of sedimenta t ion inferred wi thin

the Ea r l y Perm i an E ps il on Format i on .

ANALOGUE UTILISATION

The width to thickness ra t io of both ancient and

modern mouth ba r complexes wi thin the Holocene Neale

D e l t a a n d M i o c e n e S i r i k i t D e l t a i l l u s t r a t e s t h e

considerable downdip reservoir potent ia l o f a malgama ted

m outh b a rs and l a t era l b a rs i n sub sur face de l t a f ron

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

lacustr ine del ta systems.Both modern and a ncient ana logue models of f luvio

l acus t r i ne de l t as dem ons t ra te tha t pa t t erns o f sand

dist r ibut ion can be predic ted wi thin subsurface del ta i

systems. Sheet sandstones wi l l be characteri s t ic of the

del ta f ront in contra st to the r ibbon-l ike development o

rec t i li near d i s t r i b u t a ry channel s and l a t era l b ars on the

del ta pla in . S t i l l s tands wi l l enhance development o

mouth bar sheet sands which should be a l igned pa ral le

to the rel ic tual shorel ine coincident w i th b i furcat ion o

the dis t r ibutar y channel mouths .

Modern and anc i en t ana l ogues o f l ob a te l acus t r ine

del tas i l lust rate the importance of variab le discharge

rat es wi thin shal low receiv ing ba sins in the construct ioo f sand prone am a l gam ated m outh b a rs . Seasona l , r ap i d

inf lux of sediment l ad en f lood wat er resul ts in deepenin

o f the tha l w eg pro f i l e a t the m ouths o f d i s t r i b u t a ry

channel b i furcat ions on the del ta f ront , resul t ing in

plugging and aba ndonment of some of the cha nnel mouth

and a m al gam at i on o f l a t era l b ars and m outh b ars w i th i n

the act ive cha nnels (Allen et a l , 1976; Fl int et a l , 1989)

P a l a e o g e o g r a p h i c d a t a a n d r e l a t i v e s c a l e

considera t ions are s t ra tegica l ly importa nt for model l in

sand dis t r ibut ion models . The a rchi tecture of t he shal low

wa ter , lobate Neales l acustr ine del ta ic pri sm is probab l




more appropriate for model l ing sand dis t r ibut ion wi thin

the Epsi lon Format ion than the use of marine, deep

water digi tate Mississippi models (Taylor et al , 1991).

Coalescence of shoal-wa ter lobes creates a sand sheet

para l l e l t o the de l t a f ron t w hi ch i s much grea ter i n l a t era l

ex ten t than rec t i l i near b ar f i nger sands depos i t ed i n

deeper wa ter , b i rdsfoot del ta systems.

Channel b i furcat ions on del ta pla ins ident i f ied from

ampl i tude t ime s l ices may be useful in locat ing mul t i -

s to rey reservo i rs w here l a t era l b ars a re deve l oped

adja cen t t o s t ream m outh b ars .

ACKNOWLEDGEMENTS

This paper could not have bee n produced wi thout t he

suppor t o f S a n tos L td and i t s J o i nt Venture par t i es w ho

have funded most of the s tudies on the modern and

ancient a nalogues. In part icular w e thank S teve Tay lor

and Barry Go l ds te i n o f S an tos L td fo r the i r use fu l

constructive comme nts on the te xt . Tha nks also to Nick

Ha l l of Sant os Ltd for guida nce and enthusiast ic support

w i th the S outh B l ackw ater s tudy , and S i m on Brady o fSouth Blackwa ter Mine for a l lowing access to the mine

and assi s t ing wi th use of the da ta . The work on the Neales

Del ta w as inspi red by Dr George Al len (deceased) who

wa s pioneering the use of modern ana logues for use wi th

reservoi r s tudies . Thanks a l so to PIR SA and Dr Nevi l le

Alley for support wi th the Neales Del ta f ield work a nd

logist ics . Many t hanks to P aul G rech for ass i s t ing us wi th

edi t ing and compilat ion of the paper. Mike Mad ey, Ra lph

Pa nai t a nd Ron McNab (Sant os Dra wing Off ice) ski ll ful ly

rendered the f igures . Thanks a l so to Mark B arry for help

wi th the computer v isual i sat ion.

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Authors' biographies over page.


http://slidepdf.com/reader/full/applications-of-modern-ancient-geological-analogues-in-characteriastion-of 24/24


Simon Lang graduated from theUniversity of Queensland in 1985

with a BSc (Hons) in Geology andMineralogy, and later obtainedhis PhD (part-time) also fromUniversity of Queensland in 1994.From 1979 to 1992 he worked asa geological technician andgeologist for the GSQ in thePalaeontology, and RegionalMapping Sections mainly in

central and northern Queensland. Simon joined QueenslandUniversity of Technology in 1992 as a lecturer in sedimentologyand stratigraphy during which time he supervised petroleumand mineral related postgraduate projects in a range of basins

in Australia, Indonesia, PNG, and Venezuela. In addition hedeveloped a research program on modern sedimentology andseismic/sequence stratigraphy of Moreton Bay and the SEQueensland continental shelf as well as other moderndepositional environments in Lake Eyre and Hervey Bay. Simon joined the National Centre for Petroleum Geology andGeophysics as Associate Professor in sedimentology andsequence stratigraphy in 1999, and is currently working onreservoir characterisation for the APCRC program onGeological Disposal of CO

2 (GEODISC). Member: PESA, GSA,

AAPG, SEPM, IAS and IPA.

Jochen Kassan obtained a MSc

in petroleum geology from theUniversity of Aberdeen, Scotlandin 1987 after completingundergraduate studies in geologyand mineralogy at the Universityof Kiel, Germany. In 1988 hemoved to Brisbane to commencehis PhD at the University of Queensland on the Triassic of the Bowen Basin which was

awarded in 1993. Jochen has been working as a consultantsedimentologist to the resource industry since 1992 andfounded Whistler Research Pty Ltd in 1997. Much of the work

has been carried out in the fluvial and lacustrine strata of theQueensland onshore basins (Bowen, Surat, Clarence-Moretonand Cooper Basins). Previous employment includes RobertsonResearch in Llandudno Wales and Fern Consultants in Brisbaneand Port Moresby.

Jim Benson graduated from theUniversity of New South Wales

in 1977 with a BSc (Hons) inApplied Geology followed bythree years postgraduateresearch on the palynostrat-igraphy of the Upper CretaceousSherbrook Group in the OtwayBasin. Between 1980 and 1992he worked on the Mesozoicpalynology and sequence

stratigraphy of South African and Namibian offshore basinswhile employed with Soekor in Johannesburg and Cape Town.In 1993 he joined Santos in Adelaide as a palynologist inCorporate Technical Services and has worked on exploration

projects in the Palaeozoic and Mesozoic of the Eromanga,Cooper, Carnarvon, Browse and Bonaparte Basins. Member:PESA and AASP.

Carmine Grasso graduatedwith First Class Honours inPetroleum Geology andGeophysics from the Universityof Adelaide (NCPGG) in 1986. In1987 he joined Santos Ltd. as apetroleum geologist working inboth exploration anddevelopment of the Cooper/

Eromanga Basin fields. Recentwork has involved reservoir

characterisation of the Epsilon Formation in both a regional andfield scale and interpretation of the recent Barina/Farina 3Dsurvey. Member: SPE and PESA.

Claire Avenell graduated fromQueensland University of Technology with First ClassHonours in Applied Geology. Herwork focussed on the reservoirarchitecture of interseamsediments at South Blackwater

Mine. She is currently working asa geologist for StanwellCorporation, Brisbane.

THE AUTHORS

applications of modern & ancient geological analogues in characteriastion of fluvial-deltaic...

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