high resolution sequence stratigraphy, reservoir analogues & 3d seismic interpretation - appea,...

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APPEA JOURNAL 2002—5

S.C. Lang1, N. Ceglar 1, S. Forder 2, G. Spencer 2

and J. Kassan3

1National Centre for Petroleum Geology and Geophysics

(NCPGG) and Australian Petroleum Cooperative

Research Centre (APCRC)

University of Adelaide

SA 50052 Santos Ltd

60 Edward St

Brisbane Queensland 4000.3 Whistler Research Pty Ltd34–36 Whistler Court

Spring Mountain Queensland 4124

[email protected]

[email protected]

[email protected]

[email protected]

[email protected]

ABSTRACT

Gas exploration and reservoir development in the

Baryulah area, Cooper Basin, southwest Queensland hasfocussed on the fluvial-lacustrine, Permian coal-bearing

Patchawarra Formation, Murteree Shale, Epsilon and

Toolachee Formations. Geological interpretation of

drilling and 3D seismic data has benefitted from

integration of sequence stratigraphic concepts with the

judicious use of reservoir analogues and seismic attribute

mapping. Initially, a coherent regional chrono-

stratigraphic framework was established, based on broad

palynological zonations, and correlating extensive

lacustrine flooding surfaces and unconformities, tied to

3D seismic reflectors. This framework was subdivided by

using local key surfaces identified on wireline logs

(usually high-gamma shaly intervals overlying coals),resulting in recognition of numerous high-resolution

genetic units. Wireline log character, calibrated by cores

from analogous fields around the Cooper Basin and

supported by analogue studies, forms the basis for a log-

facies scheme that recognises meandering fluvial

channels, crevasse splays, floodplain/basin, and peat

swamps/mires. Fluvial stacking patterns (aggradational,

retrogradational or progradational), produced by the

ratio of sediment supply to accommodation within each

genetic unit, were used to help determine depositional

HIGH RESOLUTION SEQUENCE STRATIGRAPHY, RESERVOIR

ANALOGUES, AND 3D SEISMIC INTERPRETATION—

APPLICATION TO EXPLORATION AND RESERVOIR

DEVELOPMENT IN THE BARYULAH COMPLEX,

COOPER BASIN, SOUTHWEST QUEENSLAND

systems tracts (alluvial lowstand, transgressive,

highstand) and likely reservoir connectivity. Lo

signature maps for genetic intervals form the basis

palaeogeographic mapping. Modern and ancien

depositional analogues were used to constrain like

facies distribution and fluvial channel belt widths. Sy

depositional structural features, net-to-gross trends, an

seismic attribute mapping are used to guide the scal

distribution and orientation of potential reservoir trend

When used in conjunction with structural and productio

data, the palaeogeographic maps help develo

stratigraphic trap play concepts, providing a predictiv

tool for locating exploration or appraisal drillin

opportunities.

KEYWORDS

Sequence stratigraphy, alluvial basins, intracraton

basins, fluvial, lacustrine, delta, crevasse spla

floodplain, reservoirs, petroleum geology, sedimentolog

3D seismic interpretation, stratigraphic trap

palaeogeography maps, reservoirs, Permian, Patchawar

Formation, Murteree Shale, Epsilon Formatio

Toolachee Formation, Cooper Basin, Australia, Ob Rive

Western Siberia.

INTRODUCTION

The Cooper Basin is a northeast trending, Permia

Triassic intracratonic basin covering an area of 130,00

km2 in southwest Queensland and northeast Sou

Australia (Fig. 1). The basin lies entirely in the subsurfac

unconformably overlain by the Mesozoic Eromanga Basi

and represents Australia’s most significant onsho

petroleum oil and gas producer (Gravestock et al, 1998

Exploration has been successfully focussed on structur

and combined structural-stratigraphic traps, but th

future of exploration and reservoir development in thCooper Basin lies in the recognition and exploitation

flank plays and stratigraphic traps (Apak et al, 199

Lang et al, 2001; Nakanishi and Lang, 2001a, b; Taylor

al, 1991).

In the last decade a powerful new toolkit to identi

stratigraphic traps in fluvial successions has becom

available, involving high-resolution sequenc

stratigraphy integrated with 3D seismic visualisatio

interpreted with the aid of appropriate reservo

analogues.

mailto:[email protected]










512—APPEA JOURNAL 2002

S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan

The purpose of this paper is to illustrate how these

approaches can be used for the exploration and reservoir

development of stratigraphic traps in the Cooper Basin,

using the Baryulah complex in southwest Queensland as

an example.

BARYULAH COMPLEX

The Baryulah complex is a cluster of gas fields lyingapproximately 40 km southwest of Ballera, in ATP 259P,

Cooper Basin, southwest Queensland (Fig. 1). The area

lies over a three-pronged basement high on the southern

flank of the Cooper Basin. A 3D seismic survey was

acquired in 1999 to advance exploration and development,

encompassing 9 wells in the Baryulah, Juno, Juno North,

Hera, and Vega gas fields. These wells were targetted

over structural closures at the Permian Patchawarra

Formation to Toolachee Formation interval (Fig. 2), with

reservoirs in fluvial to lacustrine sandstones within an

overall coal measure succession.

A key driver for this study was the need for a better

understanding of reservoir connectivity in the Baryulahcomplex. Lithostratigraphic correlation of reservoir

sandstones tended to over-estimate lateral connectivity.

However, pressure data from the same reservoir interval

showed evidence of pressure compartmentalisation.

Sequence stratigraphic correlation techniques use key

surfaces to subdivide the succession into genetic

chronostratigraphic units (Galloway, 1989). This

approach tends to highlight the lateral disconnectivity

between channel sand bodies within the same genetic

interval because correlation focusses on mapping

enveloping shale packages and then locating

unconformities or erosion surfaces, often within sandy

intervals. As illustrated by Lang et al (2001), this approach

offers predictive insights into vertical connectivity and

net-to-gross trends for a given genetic interval in a fluvial-

lacustrine succession, which depending on the controls

on sedimentation, can be extrapolated across the basin.

Stratigraphic traps comprising fluvial channel bodies

do exist in the Patchawarra Formation to Toolachee

Formation interval, but what is the scale, geometry,

orientation and likely connectivity of these reservoirs?

Answers to these questions lie in the understanding of

both the depositional style and stratigraphic position ofthe fluvial sandstones determined from available cores,

log motifs and judicious use of depositional analogues.

SEQUENCE STRATIGRAPHIC CONCEPTS

The application of high-resolution sequence

stratigraphy to continental successions has changed our

approach to predicting reservoir distribution in fluvial-

lacustrine successions (Legarreta et al, 1993; Galloway,

1989; Etheridge et al, 1998; Shanley and McCabe, 1994,

139 140∞ 141∞ 142∞ 143∞

QUEENSLAND

Cooper Basin

QUEENSLAND

SOUTH AUSTRALIA

Q U E E N S L A N D

S O U T H

A U S T R A L I A

NEW SOUTH WALES

0 25 7550 100Km

26∞

27∞

28∞

29∞

LEGENDOil and Gas Field/Well

Oil Field/Well

Gas Field/Well

C o o

p e r C

r e e

kLAKE

YAMMA YAMMA

GOYDERS LAGOON

E y r e C r e e k

C o o

p e r C

r e e k

LAKE GREGORY

P o r t B

o n y t h

o n

LAKE BLANCHE

LAKE CALLABONNA

D i a m a

n t i n a

R i v

e r

S t r z

e l e

c k i C r e

e k

WASA

NSW

VIC

QLDNT

Location Map

BARYULAH

COMPLEX

Figure 1. Location of the Baryulah complex in the Cooper Basin.

Figure 2. Permian chronostratigraphic framework for the Cooper

Basin (courtesy of Santos Ltd.).




High resolution sequence stratigraphy of the Baryulah Compl

1998; Aitken and Flint, 1995, 1996; Posamentier and

Allen, 1999). These ideas were pioneered in the Cooper

and Eromanga Basins by the late George Allen (Allen et

al, 1996). Using data from the Cooper and Eromanga

Basins, a detailed account of how these ideas can be

applied to continental successions was outlined by Lang

et al (2001). The use of appropriate depositional reservoir

analogues integrated with 3D seismic visualisation has

enhanced our ability to conceptualise, and in some casesdirectly identify stratigraphic trap opportunities using

‘seismic geomorphology’ (Alexander, 1993; Lang et al,

2000; Nakanishi and Lang, 2001a, b, this volume; Strong

et al, this volume).

The essential first step in the sequence stratigraphic

analysis is the development of a chronostratigraphic

framework based initially on palynology (e.g. Strong et

al, 2002). This is related to key surfaces identified in

cores, wireline logs, and where possible, seismic reflectors

(e.g. sequence boundaries or lacustrine flooding surfaces).

In this study, an alluvial sequence is defined as a

conformable succession of genetically related alluvial

and lacustrine strata bounded by unconformities or theircorrelative conformable equivalents in the basin centre

(Posamentier and Allen, 1999). Unconformities are mainly

recognised by breaks in the palynostratigraphic record,

but seismic character (e.g. truncation), dipmeter, and

rock typing from cuttings can also help locate the likely

position of these surfaces. Each sequence is subdivided

into chronostratigraphic intervals typically using

regionally extensive lacustrine flooding surfaces which

often drowned extensive coals. These intervals represent

genetic units that can be packaged into depositional

systems tracts, where the resolution is dictated by the

scope of the study and/or the temporal resolution. A

depositional systems tract is defined as the linkage of

contemporaneous depositional environments, from

proximal to distal settings, formed during a particular

state of relative base level represented by a specific

chronostratigraphic interval.

Alluvial lowstand (LST), transgressive (TST) and

highstand (HST) alluvial systems tracts are recognised.

Each systems tract represents a particular ratio of

sediment supply to fluvial accommodation (the potential

space for alluvial sediment to accumulate). Fluvial

accommodation can be formed by tectonic subsidence,

back-tilting of the fluvial profile, and/or lacustrine base

level rise. Sequence boundaries mark periods of negative

accommodation, where widespread erosion takes place,

rivers become incised into their substrate, and palaeosolsdevelop on the interfluves (Lang et al, 2001). Each systems

tract is characterised by distinctive sediment stacking

patterns: aggradational (typical of the LST, but can occur

in the late HST), retrogradational (typical of the TST)

and progradational (typical of the HST, and in the most

distal part of the LST). Readers are directed to

Posamentier and Allen (1999) and Lang et al (2001) for a

more detailed account of these concepts. The ratio of

sediment supply to fluvial accommodation should be

expected to change spatially, and in continental basins

this may be subtle or rapid, as it will be greatly influence

by differential subsidence associated with basin tectonic

especially where basement faults influence transver

vs longitudinal drainage patterns (Lang et al, 2001).

The highest net-to-gross (ratio of sandstone to tot

interval thickness) typically occurs in the lowstand an

late highstand systems tracts. The optimum connectivi

occurs in the lowstand, because it has the lowe

accommodation and hence channel belts undergsignificant reworking resulting in the accumulation of

laterally extensive sand deposit. Lowstand sands may b

locally thicker where incised valleys are developed, an

in these areas a preferred orientation, typically parall

to basement structure, is often apparent (Posamenti

and Allen, 1999). Aggradational stacking pattern

commonly topped by thick coals are typical of the la

lowstand because of the slow rate of increase

accommodation relative to sediment supply. The coa

mark the transition to the transgressive systems trac

The late highstand channel belts may have more variab

lateral connectivity, but, depending on the nature of th

overlying sequence boundary, they may be in hydraulcontinuity with the lowstand systems tract, althoug

typically flow properties on either side of the bounda

may be different.

The transgressive systems tract is typified by

retrogradational stacking pattern with decreasing ne

to-gross up section reflecting increasingly isolate

channel belts, and increasing thickness and later

continuity of coals and lacustrine deposits. The interv

of maximum lacustrine inundation includes the maximu

flooding surface, generally picked on the highest gamm

log spike, marking the beginning of a progradation

stacking pattern typical of the highstand systems trac

with an increasing connectivity of channel belt sandston

up-section (Lang et al, 2001). Highstand coals an

floodplain deposits are typically of highly variable later

connectivity.

Several low-order unconformity bounded sequenc

are recognised in the Cooper Basin with higher-ord

sequences recognised within these intervals. Sequence

systems tracts and stacking patterns are summarised

Figure 3, and are all keyed to specific chronostratigraph

intervals in the Baryulah area.

CHRONOSTRATIGRAPHY

The development of a chronostratigraphic framewor

for the Baryulah area of the southeastern Cooper Basinbased primarily upon identification of region

unconformities and secondly on widespread lacustrin

flooding surfaces and other marker horizons (e.g. coal).

revised chronostratigraphic nomenclature, develope

for the Cooper Basin, was used (Strong et al, this volume

expanding on the generic palynostratigraphic schem

originally outlined by Price et al, 1985.

The naming system for key stratigraphic surfaces

alphanumeric (outlined in Fig. 2), and illustrated

Figure 3 on a typical stratigraphic log from the Baryula





complex showing key surfaces and systems tracts. The

prefix is based on traditional seismic horizon

nomenclature conventions in the Cooper Basin. The suffix

denotes the type of the surface for example, regional

unconformities (U), coal markers or flooding surfaces

(C). A number that increases with depth denotes the

relative stratigraphic position. In the Baryulah area, the

following chronostratigraphic units are recognised:

• Toolachee (PC00–PC60; PU70 is not present).• Epsilon (TC40 –UC00; upper part eroded towards the

south).

• Murteree (UC00–VC00).

• Patchawarra (VC00–VC45; lower part present towards

the north).

The key regional unconformities are well known (Apak

et al, 1997) and include intra-Patchawarra Formation

unconformities (VU75 and VU45) and the Daralingie

unconformity at the base of the Toolachee Formation

where the Daralingie Formation, Roseneath Shale and

uppermost Epsilon Formation have been removed by

erosion. The Patchawarra Formation onlaps onto the

Baryulah high; therefore only the younger part of thesection occurs throughout the area.

An intra-Toolachee Formation sequence boundary

(above PC50) is recognised, but this is coincident with

the Daralingie unconformity in the Baryulah area. The

widespread lacustrine flooding surfaces are indicated by

maximum gamma-ray log motifs, many immediately

overlie extensive coal markers (e.g. VC30, UC00, TC50,

PC60, PC20 or PC10; Fig. 3).

Significant flooding events are correlatable in much

of the Baryulah area. Significant events are VC00, UC00,

TC80 and PC00 which represent periods where the rate

of creation of fluvial accommodation rapidly exceeded

the rate of sediment supply, resulting in lacustrine

inundation, or, in some intervals, extensive floodplain

lakes, peat mires and marshes. Flooding surfaces are

identified on the basis of wireline character, and are

assumed to approximate timelines at the scale of this

investigation. It should be emphasised that these flooding

surfaces are diachronous events. We assume, however,

that the initiation of the lacustrine flooding events were

rapid (hundreds or a few thousands of years) responding

to tectonic or climatic events in the basin, and for all

practical purposes can be considered geologically

instantaneous. A key criterion for selecting the key

surfaces is that they are regional, preferably basin-wide,

although it is also useful to pair these with more local

markers (coal) that can help identify areas of localsediment supply variation (i.e. low sediment input favours

coal, high local input precludes coal).

Subdividing wireline log motifs into chrono-

stratigraphic intervals is necessary to produce log motif

facies maps of discrete time intervals. This enables the

preparation of palaeogeography maps, using isopach and

sand distributions, and interpretations of depositional

environments for each systems tract.

Figure 3. Type log for the Baryulah complex showing sequence

stratigraphic key surfaces used to define mappable genetic units.





RESERVOIR SEDIMENTOLOGY

Few cores are available from the Baryulah area; and

therefore facies interpretation was based on comparison

with log motifs from other fields in the Cooper Basin

where cores were available in comparable strata.

Dullingari field in South Australia was used as a field

analogue dataset for the development of a facies scheme

(an example is shown in Fig. 4).A generic suite of reservoir facies is recognised as

follows:

• Channels: fine-medium-grained sandstone;

occasionally pebbly; cross-bedded or rippled;

sometimes heterolithic; usually a few metres thick;

fining-upward to mudstone or coal; may be stacked,

amalgamated or isolated; usually confined to channel

belts up to a few kilometres wide, possibly of

meandering, braided or anastomosing origin; fining-

upward gamma-log motifs common, but may be

aggradational.

• Channel margin/levee: stacked; typically heterolithic;

fine-grained sandstone and mudstone, with commonripples, laminations and rootlets; variable log motifs.

• Crevasse splay/distributary channels: fine-medium-

grained sandstone; typically laminated or massive;

generally lack cross-bedding; climbing ripples and

floating rip-up-clasts eroded from the channel margin/

levee; can be stacked and amalgamated similar to

fluvial channels, but with much narrower aspect ratios

(<50 width/thickness); typically a more spiky gamma-

log motif, and smaller scale than channels except

where amalgamated.

• Medial to distal crevasse splays: fine- to very fine-

grained sandstone; can be uniform in texture though

commonly silty; laminated and common climbing

ripples; typically a few tens of centimetres thick, but

can be amalgamated into several metres thick with a

sheet like distribution; log motifs may fine- or coarsen-

upward; smaller scale than channels.

• Delta mouthbar: typically a coarsening-upward

succession a few tens of centimetres or metres thick;

very fine- to fine-, sometimes medium-grained

sandstone; lamination, ripples and small scale cros

bedding at top; sometimes wave-rippled; often weak

bioturbated; topped by coal and lacustrine mudston

log motifs typically progradational; especially we

developed in the Epsilon Formation.

Non-reservoir facies include:

• Coal: bright to dull with low or high gamma-r

response depending on siliciclastic content; can be

situ or allochthonous; often very extensive and thic(up to several metres, occasionally much thicker); co

splits common; may be associated with any oth

reservoir facies; found either within abandone

channels or filling entire channel belts, but the thicke

developments are typically in areas away from clast

influx (raised mires over interfluves).

• Floodplain: mudstones with rootlets, plant debris an

weakly developed sideritic palaeosols.

• Abandoned channel fill: carbonaceous shale gradin

to coal, laterally restricted.

• Floodbasin lake: mudstone; delicately laminated wi

finely-divided plant debris; often overlies coa

generally local; not easily correlated between wellcommonly associated with distal crevasse splays.

• Lake: laterally extensive mudrock intervals up

several metres thick; often associated with relative

high gamma-log spikes; sometimes with scattered ou

sized clasts (ice-rafted dropstones?).

DEPOSITIONAL ANALOGUES

The facies interpretation is supported by a range

appropriate modern and ancient depositional analogu

that can be related to wireline log motifs and seism

attribute maps. Of particular use have been the moder

cool-temperate peat-forming fluvial systems

Saskatchewan and Western Siberia, and the Late Permia

coal measures of the Bowen Basin described by Lang

al (2000; 2001), and Strong et al (2002). The rationale f

selecting these modern, high-latitude, cool-temperat

peat forming fluvial systems is based mainly o

similarities with the palaeolatitude and palaeoclimat

situation of the Cooper Basin in the Permian, which w

part of large peat-forming alluvial basin lying at hig

palaeolatitudes (Veevers, 2000). Satellite image

especially Synthetic Aperture Radar (SAR) imagery fro

Smith and Alsdorf (1998) were particularly useful

obtain cloud- and snow-free images depicting chann

belt scale, relative orientation and relationship

channels and splays to floodplain, floodbasin lakes aninterfluve peat lands, and Taiga coniferous forest.

To test some of these ideas, reconnaissance fieldwo

was undertaken in Western Siberia in late 200

confirming the view that the region provided usef

reservoir analogues. Fieldwork was focussed on th

entrenched meander belts of the Ob River and Wac

River tributary near Nizhnevartovsk, and the interfluv

peat mires and smaller rivers in the Noyabrsk area sou

of the Arctic Circle (Fig. 5). Using satellite images initial

as a guide (Strong et al., this volume), oblique aeriFigure 4. Example of a log motif facies scheme used for the fluvial

facies in the Baryulah area.





Figure 5. Location of modern fluvial analogues from the Noyabrsk

and Nizhnevartovsk areas in the cool-temperate, peat-forming Ob

River basin of Western Siberia, and comparison with seismic

horizon amplitude slice map from the upper part of the Toolachee

Formation in the Baryulah 3D survey. The seismic slice is through

a low net/gross sand interval showing both low-amplitude sandy

channel fills in the southeastern part of the image as well as high-

amplitude coal prone cut-off meanders in the north. Inset A. Peats

and organic silts accumulating in the abandoned channels of the Ob

River near Nizhnevartovsk. Note the distinct edge along the

channel belt, similar in shape and scale to the seismic amplitude

image from Baryulah. Inset B. Highly-sinuous meandering channel

and meander cut-off in the early stages of being filled with peat, nearNoyabrsk. Inset C. Active sandy meandering channel (200 m wide)

in tributary of the Ob River near Nizhnevartovsk, showing well

developed laterally accreting scroll bars with peat filling chute

channels.





images were taken of the meander belts and peatmires

and compared directly with 3D seismic amplitude maps

from the Baryulah area (Figs. 5, 6). From the available

core, wireline log motifs, and the 3D seismic

interpretation, meandering fluvial systems are known to

be common in both the Toolachee and Patchawarra

Formations (Figs. 5, 6). Based on the modern analogues,

we can expect to see high sinuosity sandy channel point

bars of up to several kilometres in diameter, covered byfloodplain fines or coal, and confined within channel

belts from a few hundred metres to 5 kilometres wide.

Individual channels can range from tens of metres in the

interfluve region to several kilometres in the major

fluvial axes, orientated parallel to the structural grain of

the underlying basement, especially during the alluvial

lowstand.

Crevasse splays are likely to form complex, lobate,

distributary and anastomosing networks in the low relief

areas adjacent to the channel belts (Avenell, 1998; Smith

and Perez-Arlucea, 1994). In Western Siberia, the splay

belt is particularly dominant in the lower reaches near

Salekhard, where extremely low relief, flooding, and icejams promote abundant avulsions (Smith and Alsdorf,

1998; Lang et al, 2000). Interestingly, the middle reaches

of the Ob River near Nizhnevartovsk are apparently

lacking in crevasse splays, presumably because of the

structural entrenchment of the modern river during the

last Ice Age, constraining the channel belt to a structurally

controlled corridor (Sergey Vasiliev, personal

communication). By analogy, this may mean that during

an alluvial lowstand, the amalgamated channel belts

may lie on the flanks if not completely off the palaeo-

highs. This would result in these ideal reservoir fairways

commonly lying off-structure unless there was significant

basin inversion. Evidence for this has been presented for

the Toolachee Formation at Pondrinie and Moorari fields

in the Patchawarra Trough by Nakanishi and Lang, (2001a,

b, this volume), and this may occur in the Baryulah area

within the older parts of the Patchawarra Formation.

Palaeo-interfluve areas between the main channel

belts are likely to be more coal prone, lying slightly

higher and away from sediment influx where raised

mires can flourish in the raised water tables, as occurs in

the Vasuganye and Noyabrsk peatland systems (Vasiliev,

2001).

For a given chronostratigraphic interval, estimating

channel belt widths from preserved channel-fills can be

attempted using a methodology originally proposed by

Fielding and Crane (1987). A detailed outline of thismethodology can be found in Strong et al (2002). In

summary, the process requires a complete fluvial channel-

fill to be identified from cores or wireline logs. The

compacted thickness of the whole channel-fill (sandstone

and any mudstone fill) is then converted to a minimum

estimate of channel bankfull depth, which is then plotted

against all the published ranges of channel belt widths

for meandering streams (both modern and ancient). For

each systems tract, channel belt widths were calculated,

and range from 100 m to 3.2 km. As pointed out by Bridge

and Tye (2001), the reliability of these estimates

dependent on careful picking of complete channel-fill

usually the last in a stack of channel sands, includin

both the thickness of the sand and the abandoned channe

fill. In the Baryulah area, estimates of channel be

widths for several intervals were independently checke

against the 3D seismic attribute maps. Channel be

width estimates typically lie within the lower range

widths measured directly from meander belts imaged othe seismic attribute maps (Figs. 5 and 6).

SEISMIC INTERPRETATION

The 1999 Baryulah 3D seismic survey used in th

study covers 319 km2 from the Juno North gas field to ju

north of the Winninia gas field (Fig. 6). All wells with

the 3D survey area were used to identify the seism

reflectors and to develop the depth conversion veloci

field. Data quality is generally good, particularly in th

Permian Toolachee Formation interval. Short perio

multiples generated by the Permian coals result in som

difficulty in interpreting the basal PatchawarrFormation units and pre-Permian basement.

The key interpreted seismic markers, approxima

equivalents of chronostratigraphic markers, include

PC00 (Top Toolachee Formation), PC40, VC00 (To

Patchawarra Formation), VC30, VC35, VC45; and Z* (to

pre-Permian basement). Horizon seismic amplitude slic

were examined and time-thickness maps were generate

Although experience has shown that the prevalence

inter-bedded coal in the section dominates the reflectivi

and that the majority of the sandstones are below th

seismic tuning thickness (20 m; ~65 feet), horizon slic

through the Baryulah 3D volume show spatial amplitud

variations that represent stratigraphic features. The

patterns are particularly pronounced within th

Toolachee Formation. Figure 5 illustrates an examp

horizon slice that shows a number of features interprete

to be meandering fluvial channels and cut-off meander

which can be directly compared with fluvial analogu

from the Ob River (Fig. 5a, b, c).

Due to the thin-bedded nature of the clastics and coa

it is difficult to isolate the response of a particular laye

In fact, amplitude slices of adjacent reflectors (pea

trough pairs) are commonly very similar (albeit wi

opposite reflection polarities). For this reason, the ro

body causing the amplitude pattern that is exhibited b

an event can only be clearly related to a relatively gro

interval. The reflection amplitude patterns, howevemake geologic sense and are related to rock properties

the inter-bedded clastics as well as the coals. The seism

data was also flattened on regional seismic markers an

amplitude slices were generated at 2 millisecond interva

Velocities and densities of many of the Toolache

Formation sandstones in the greater Baryulah area a

somewhat lower than the background shales, suggestin

that reflection amplitude is a useful tool for studyin

their distribution. Wireline data show that the velociti

of the Patchawarra Formation sandstones in the area a





similar to those of the shales; this may explain why

similar features are scarce in the Patchawarra Formation

interval.

Several iterations of neural network classification

were conducted using stratigraphic software. Using the

PC40 peak reflection as a reference, several different

windows were used in an attempt to delineate

stratigraphic features. These seismic facies maps give a

different perspective, but all of the features seen are also

evident on the amplitude slices. Figure 6 illustrates an

example of one of the seismic facies maps for a mid-

Toolachee Formation interval. Drilling results confirm

the interpretation of many of these features as fluvial in

origin.

PALAEOGEOGRAPHIC MAPPING

Once the sequence stratigraphic and sedimentologic

analysis was completed, palaeogeographic maps of each

systems tract were then drawn. This was achieved by

plotting on a base map all the gamma-log motifs for each

well between the relevant key surfaces, and then

interpolating the predominant fluvial-lacustrine

depositional environment represented at each well, based

on basic sedimentological concepts (Miall, 1996). Critical

to the success of this approach is the reliability of the

depositional facies interpretation based on relevant

analogues, cores, cuttings and wireline logs.

Overlays from the 3D seismic amplitude and other

attribute maps are then used to guide the mapping

between well control. For example, these maps show

structural grain that may have influenced the trend of

Figure 6. Seismic facies classification map from Stratimagic for a mid Toolachee Formation interval (PC40–PC50) from the 3D seismic

survey over Baryulah, and associated palaeogeographic map for the same chronostratigraphic interval.





fluvial axes, including basement faults (typically

reactivated depending on the orientation), but cross-

cutting post-depositional faults were ignored. High

amplitude, coal-prone areas often fill the outline of

abandoned channel belts (Figs. 5, 6), or occur in floodplain

areas. Most useful for gaining confidence in the style and

scale of the fluvial systems are the obvious abandoned

channel meander loops and neck-cutoffs visible on

seismic. Coal that has filled an abandoned channelproduces impedence contrast (within the confines of the

channel) resulting in the amplitude variations imaged on

3D seismic (Fig. 5). Note that older, palimpsest sandy

channel deposits beneath the obvious abandoned

channels may not be easily imaged by seismic, but their

presence is identified by the available wells and the

‘bite-in-the-apple’ shapes that are clearly evident on

some amplitude maps (Figs. 5, 6).

IMPLICATIONS FOR EXPLORATION ANDRESERVOIR DEVELOPMENT

The study has provided a detailed depositional andstratigraphic model that can be applied to the exploration

and exploitation of this stratigraphic play. This model

helps with interpretation of the seismic attribute analysis

to build a broad-scale picture of reservoir geometry and

heterogeneity within the gas accumulation. Recognition

of geologic patterns relating to meander loops, point

bars, channels and floodplain deposits, for a given

chronostratigraphic interval, allows the overall geometry

of the reservoir system to be defined and the probable

location of lateral stratigraphic baffles and seals to be

identified. To undertake this work, 3D seismic coverage

is essential, as the attribute analysis required to define

the reservoir units cannot be accomplished with 2Dseismic data.

Effective exploitation of the play requires definition

of the internal reservoir geometry, including potential

flow barriers and reservoir sweet spots. These reservoir

units are composed of a complex intercalation of

depositional units dominated by point bars and channel

sands within a broad channel belt. Underlying basement

structure and re-activation of major pre-existing fault

trends contemporaneous with deposition, broadly

influence the orientation of these channel belts.

CONCLUSIONS

New opportunities for exploration and reservoir

development of stratigraphic traps in the Cooper Basin

can be identified by employing an integrated approach

to palaeogeographic mapping of key reservoir intervals.

Essential steps include reservoir facies analysis,

supported by meaningful reservoir analogues, development

of a chronostratigraphic framework underpinned by

palynology and development of a geological model

interpreted using sequence stratigraphic concepts.

Log motif facies and palaeogeographic maps should

be constructed with the focus on genetically meaningful

intervals that have predictable reservoir connectivi

trends (i.e. systems tracts not lithostratigraphic units

3D seismic interpretation and attribute analysis f

each chronostratigraphic interval should then b

attempted, ideally with amplitude slice and oth

attribute maps picked as close approximations

particular systems tracts. Where possible, the seism

maps need to be integrated with the palaeogeograph

maps. Other data, including structure, pressurcommunication, seal capacity and production data ca

then be employed to develop play concepts for potenti

stratigraphic traps.

In the Baryulah complex, this methodology resulted

the development of a wireline log fluvial-lacustrine faci

scheme calibrated against cores from adjacent Coop

Basin fields with similar stratigraphy and log characte

and included meandering channel fill, crevasse splay

floodplain and peat mire facies. Modern analogues fro

the Siberian Ob River and adjacent cool-tempera

peatlands were used to guide interpretations for reservo

scale, geometry, relative orientation and likely reservo

connectivity as well as the potential complex architectuof baffles and seals. A chronostratigraphic framewo

was developed for the Patchawarra to Toolachee interva

and interpreted in terms of high-resolution alluvi

sequence stratigraphy based on stacking patterns an

key surfaces. Optimum lateral and vertical connectivi

was shown to occur in alluvial lowstand systems tract

with isolated stratigraphic traps more common in th

transgressive and highstand systems tracts. 3D seism

interpretations picked to represent systems tracts great

enhanced palaeogeographic mapping, giving confiden

to the interpretation of depositional environments (larg

meandering rivers in broad channel belts up to

kilometres wide, flanked by floodplains and peat miresChannel belt width estimates based on wireline log da

were cross-checked directly against the seismic amplitud

slice maps and results were generally in the lower rang

as estimated from 3D seismic amplitude maps.

The implications for exploration and production ar

that significant potential for stratigraphic plays remain

within the Baryulah complex and similar settin

elsewhere in the Cooper Basin. However, effectiv

exploitation requires definition of “sweet spots” usin

the integrated approach outlined in this paper.

ACKNOWLEDGEMENTS

The authors would like to thank Santos Limited f

permission to publish this paper, and financial suppo

for reservoir analogue studies, including recent fie

reconnaissance studies in western Siberia. The view

presented in this paper are those of the authors and d

not necessarily reflect those of the joint ventur

participants. We wish to thank Rob Heath, Peter Havor

Dan Fearfield, Olaf Kloss at Santos QNTBU for the

support, useful technical reviews and encouragemen

and Julian Evanochko, Gerry Carne and Geoff Wood f

support accessing Santos SABU datasets. We also than





the staff and students of the NCPGG for valuable

discussions and assistance over several years pertaining

to the Cooper Basin, in particular Nick Lemon, Ghazi

Kraishan, Adam Hill and Robert Root. We thank Joan

Esterle (CSIRO, Brisbane), Larry Smith and Karen Frey

(UCLA), Valentina Vasiliev and the late Sergey Vasiliev

for their assistance with field work in west Siberia. We

thank the referees, John Draper of the Queensland

Department of Natural Resources and Minerals, andRichard Suttil of Origin Energy Resources Ltd.

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522 APPEA JOURNAL 2002


Simon Lang graduated from the

University of Queensland in 1985

with a BSc (Hons) in Geology andMineralogy, and later obtained

his PhD (part-time) also from

UQ in 1994. From 1979–1992 he

worked as a geological technician

and geologist for the GSQ in the

Palaeontology and Regional

Mapping Sections. Simon joined

Queensland University of

Technology in 1992 as a lecturer in sedimentology and

stratigraphy during which time he supervised petroleum and

mineral related postgraduate projects in a range of basins in

Australia, Indonesia, PNG, and Venezuela. He developed a

research program on modern sedimentology and seismic/sequence stratigraphy of Moreton Bay and the SE Queensland

continental shelf as well as other modern depositional

environments in Lake Eyre and Hervey Bay. In 1999 he joined

the NCPGG as Associate Professor in sedimentology and

sequence stratigraphy. He leads the sedimentology and sequence

stratigraphy research program. He is project leader on reservoir

characterisation for the APCRC GEODISC program. Member:

PESA, GSA, AAPG, SEPM, IAS and IPA.

Nathan Ceglar graduated from

the NCPGG, University of

Adelaide, with a BSc (Hons) in

petroleum geology and geo-

physics (2000), working on the

sequence stratigraphy of the

Nancar Trough for Woodside

Energy Ltd. He is completing his

MSc on the sequence stratigraphy

and reservoir sedimentology of

the Baryulah area in the Cooper

Basin, sponsored by Santos Ltd.

Stephen Forder graduated from

Victoria University, Wellington

(NZ) with a BSc (Hons) in geology

in 1976. He initially worked in the

minerals exploration and goldmining industries in New Zealand

and South Africa before joining

Marathon Petroleum in London to

work as a wellsite geologist in the

North Sea in 1980. In 1981 he

moved to Sydney, working initially

for Elf Aquitaine and then Ampol Exploration on Bonaparte Gulf,

Timor Sea, Gippsland and Galilee basin acreage. Stephen returned

to New Zealand in 1986 to take up the position of Chief Geologist

with New Zealand Oil & Gas (NZOG) until 1994 when NZOGs’

exploration activities were relocated to Sydney. He remained in

NZ as a consultant working with Petrocorp Exploration onMiocene oil development projects in the Taranaki Basin until late

1996 when he obtained a contract as operations geologist with

Vaalco Energy (India) based in Chennai (Madras). After successful

appraisal and development of an offshore oil field in the south-

western Bay of Bengal, in February 1998 Stephen accepted the

Brisbane-based position of staff geologist with Santos Ltd, where

his work to date has focussed on exploration and appraisal of

ATP259P, with particular accent on the Baryulah area.

Gregg Spencer received BSc

degrees in geological engineering

and geophysical engineering from

the Colorado School of Mines in1981. He was as a geophysicist in

Rocky Mountain basins with

ARCO Exploration in Denver

from 1981–85. He completed his

MSc (geology, Colorado School

of Mines, 1985) and worked for

Mobil Oil that year. From 1985

to 2000 he worked on Exploration and Development projects

in offshore California, north Alaska (Beaufort and Chukchi

Seas), Norway (Haltenbanken, Barents Sea), Russia (Sakhalin

Island), Black Sea (Russia and Ukraine) and The Netherlands. In

March 2000, he joined Santos Ltd in Brisbane as a senior staff

geophysicist and has been involved in exploration and

development in the Baryulah and Central Fields areas of

ATP259P. Member: AAPG, SEG, ASEG and PESA.

Jochen Kassan obtained a MSc in

petroleum geology from the

University of Aberdeen, Scotland,

in 1987 after completing

undergraduate studies in geology

and mineralogy at the University of

Kiel, Germany. In 1988 he moved

to Brisbane to commence his PhD

at the University of Queensland on

the Triassic of the Bowen Basin

which was awarded in 1993. Jochenhas been working as a consultant sedimentologist to the resource

industry since 1992 and founded Whistler Research Pty Ltd in

1997. Much of the work has been carried out in the fluvial and

lacustrine strata of the Queensland onshore basins (Bowen, Surat,

Clarence-Moreton and Cooper Basins), mainly focussed on

reservoir sedimentology and building reservoir models. Previous

employment includes Robertson Research in Llandudno, Wales,

and Fern Consultants in Brisbane and Port Moresby. Jochen is a

Research Associate of the NCPGG.

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