High Resolution Sequence Stratigraphy, Reservoir Analogues & 3D Seismic Interpretation - APPEA, 2002
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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
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.
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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.).
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APPEA JOURNAL 2002—5
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
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514—APPEA JOURNAL 2002
S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan
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.
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APPEA JOURNAL 2002—5
High resolution sequence stratigraphy of the Baryulah Compl
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.
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516—APPEA JOURNAL 2002
S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan
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.
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APPEA JOURNAL 2002—5
High resolution sequence stratigraphy of the Baryulah Compl
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
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518—APPEA JOURNAL 2002
S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan
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.
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APPEA JOURNAL 2002—5
High resolution sequence stratigraphy of the Baryulah Compl
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
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520—APPEA JOURNAL 2002
S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan
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
S.C. Lang, N. Ceglar, S. Forder, G. Spencer and J. Kassan
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.
THE AUTHORS