Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
UQ
-CC
SG
Centre for C
oal Seam
Gas
The use of stable carbon isotope trends as a correlation tool: an exam
ple from the Surat B
asin, Australia
A. Hentschel (PhD
candidate), project title: The Walloon-B
irkheadtransition –
Changes in coal and interburden character
Advisory team: Prof J. Esterle, Prof S. G
oldingSchool of Earth Sciences, C
entre for Coal Seam
Gas
Thisstudyusesorganic
stablecarbon
isotopetrendsand
organicpetrology
dataasa
correlationtoolw
ithinthe
SuratBasin’sWalloon
Subgroupand
itsoverlyingSpringbok
Sandstone.TheJurassic
Walloon
Subgroup,aproductive
coalseamgas
source,iscom
monly
dividedinto
sub-unitsbased
ondifferentproportions
andthicknesses
ofcoaland
sandstone.How
ever,correlationacrossthe
basinischallenging
dueto
highlateralvariability
anda
lackofextensive
stratigraphicm
arkers.TheW
alloonSubgroup
isalso,inplaces,incised
bythe
overlyingSpringbok
Sandstone,som
etimes
interpretedas
fardow
nas
theTangaloom
aSandstone.N
ewage
datessuggestthatthe
Walloon
Subgroupis
Oxfordian
inage
andm
arksa
periodof
highrates
oforganic
carbonproduction
andburial,and
aninterm
ittentdecreaseofatm
osphericpC
O2.The
un-ordisconformable
baseofthe
SpringbokSandstone
coincidesw
itha
turningpointofthis
supposedlyglobalphenom
enon.Analysis
ofastratigraphic
suiteofcoalsam
plesfrom
severalwellsacrossthe
SuratBasin
showsa
gradualenrichmentin
13Cup
sectionfrom
theTaroom
toLow
erJuandahC
oalmeasures,w
iththe
mostpositive
δ¹³Cvaluesw
ithinthe
UpperJuandah
CoalMeasures.Thereafter
thereis
arapid
reversaltom
orenegative
δ¹³Cvalues
forcoalsam
plesfrom
theSpringbok
Sandstone.Theupw
ardenrichm
entoccursw
ellbeforea
shiftinm
aceralcomposition
toincreased
inertinitecontentin
thecoals,
suggestingm
oreglobalallogenic
processesarecontrolling
thecarbon
isotopetrend
asopposedto
local,environmentalfactors.
AC
KN
OW
LEDG
EMEN
TSThe authors w
ould like to thank The University of
Queensland (U
Q) and the School of Earth Sciences for
awarding a U
QI scholarship, as w
ell as UQ
’s Centre for C
oal Seam G
as (ww
w.ccsg.uq.edu.au) for providing industry research funding (from
APLN
G, A
rrow Energy,
QG
C and Santos). The authors are thankful to Santos
Ltd, SenexEnergy Ltd, A
rrow Energy Pty Ltd, and
MetroC
oalLtd for providing samples and data. A
special thank you to K
im B
aublys for conducting the stable isotope analysis at the U
Q Stable Isotope G
eochemistry
Laboratory, and Dr Fengde Zhou for his advice and help.
Analyses conducted
Well
Petrographic analysis
Organic stable carbon
isotope analysis
127 block sam
ples3
27 samples
3
227 block sam
ples2
17 samples
2
319 grain m
ounts3
20 samples
3
425 block sam
ples3
25 samples
3
542 grain m
ounts1
38 samples
1
641 grain m
ounts1
40 samples
1
The Walloon Subgroup
Stratigraphic variability in coal and interburden
character allow
s subdivision into different sub-units (Fig. 1)
Maxim
um thickness of approxim
ately 400-450 m
Walloon Subgroup underlies erosively
based unit called Springbok Sst, w
hich also contains thin coal seams
The unconform
able base is thought to represent a major
global event
Oxfordian (158-162 M
a) in age
Recent C
orrelations
Thickness and distribution of the upper coal measures
controlled by erosive base of the overlying Springbok Sstand is a function of differential erosion due to regional uplift
Fluvial incision at the base of the Springbok Ssthas been recognized from
well-log correlation
H
owever, correlation errors due to a lack of stratigraphic
marker horizons, the nature of the depositional system
, as w
ell as uncertainty regarding the extent of the incision of the Juandah
CM
Fig. 1. Surat Basin stratigraphy (after Hamilton et al. (2012), Hoffmann et al. (2009), Scott et al. (2007) and Wainman et al. (2015)). Triangles
represent base level sequences. Direction of narrowing represents increasing base level. LST=low stand systems tract, TST=transgressive systems tract, HST=high stand systems tract, LSFC=late stage falling cycle.
Theaim
ofthisstudy
isto
assistwith
am
oreconfidentcorrelation
ofsub-unitsofthe
Walloon
Subgroupand
totest
existingbasin-w
idecorrelations.
Inorder
toachieve
thisaim
,robust
stratigraphicm
arkerhorizons
arerequired.
Thesecould
come
froma
systematic
petrographicand
stableisotopic
characterisationof
theW
allooncoals;
anapproach
thathasnotbeenapplied
beforeto
theseJurassic
coals.
Fig 2. Thickness map of the Walloon Coal Measures interval
(Juandah Coal Measures, Tangalooma Sandstone and Taroom Coal Measures) including locations of study wells.
Table1.
Analysesconducted
andsources
ofdata.
Pacey(2011) 1,Hentschel(2013) 2andthisstudy 3.
Fig 3. Maceral composition (vol. %, mmf) and stable isotopic composition (δ¹³C in ‰
, reported against VPDB standard) for coal samples from wells 1 to 6 (west to north to the east of the Surat Basin). Correlation of sub-units according to company picks.
Fig 4. West to north to southeast trending cross-section through the
Surat Basin including wireline logs for a number of wells throughout the Surat (black labels) and wireline logs for wells 1 to 6 as shown in this study (red labels). Correlation of sub-units in study wells 1 to 6 is based on the use of organic stable isotopes (note: boundary between Springbok Sandstone and Juandah Coal Measures was shifted to a depth of 171 m in well 1). The stable isotope trends for wells 1 to 6 are schematically shown in green.
1. BACKGROUND
2. AIMS
3. METHODOLOGY
4. RESULTS
Petrographic analysis
All sam
ples dominated by vitrinite group m
acerals (purple triangles; Fig. 3)
Abundant liptinite group m
acerals (blue diamonds); how
ever, no significant trend with depth
Jum
p to increased inertinite group macerals recorded in U
. JuandahC
M and coals of the Springbok Sst
(yellow circles) suggests change to low
er base level conditions in the mire
Organic stable carbon isotope analysis
Taroom
CM
and the Tangalooma Sstcoal sam
ples show relatively negative δ¹³C values across all study w
ells
Shift to less negative compositions w
ithin samples from
the U. Juandah C
M, follow
ed by shift back to more
negative δ¹³C values for samples from
the Springbok Sst(Fig. 3)
In well 1 this shift to m
ore negative δ¹³C values occurs in U. Juandah CM
(as correlated by exploration com
pany; Fig. 3), as opposed to the Springbok Sst
ABSTRACT
Conclusions
M
aceral composition of U
. Juandah CM and coals of the Springbok Sstsuggest environm
ental change (Fig. 3)
Positive excursion in δ¹³C does not respond to change in maceral com
position, as it sets in earlier
Shift to less negative δ¹³C values in U. Juandah CM
is consistent for wells 2 to 6
Follow
s global climate trend, as recorded in M
id-Oxfordian m
arine carbonates
In well 1, the stratigraphic boundaries had to be adjusted based on positive excursion in δ¹³C
M
ore negative δ¹³C values of samples from
depth interval of 139–163m im
plies that these are Springbok coals
Samples from
a depth of 190–257m have inertinite group m
acerals only in trace amounts, w
hereas samples
from U
. Juandah CM
showed increased inertinite contents in all other w
ells
U. Juandah C
M in w
ell 1 were eroded in their entirety (Fig. 4)
Implications
Trends in isotope record of the W
alloon coals are substantial and fairly consistent, and have potential to serve as stratigraphic m
arkers, as they represent changes in global ocean-atmosphere carbon reservoir
Isotope trends can be applied as a tool to test existing correlations and especially the extent of Springbok incision
References: H
amilton, S.K
., Esterle, J.S., Golding, S.D
., 2012. Geological
interpretation of gas trends, Walloon Subgroup, eastern Surat B
asin, Queensland,
Australia. International Journal of C
oal geology, Vol. 101, pp. 21-35.
Hentschel, A
., 2013. Geochem
ical Characterisation of Surat B
asin Coal C
ores for G
as Origin A
ssessment. M
Sc thesis, RW
TH A
achen University of Technology,
Aachen, G
ermany, unpublished.
Hoffm
ann, K.L., Totterdell, J.M
., Dixon, O
., Simpson, G
.A., B
rakel, A.T., W
ells, A
.T., McK
ellar, J.L., 2009. Sequence stratigraphy of Jurassic strata in the lower
Surat Basin succession, Q
ueensland. Australian Journal of Earth Sciences 56, pp.
461–476.Pacey, D
., 2011. An investigation of m
acroscopic and microscopic vertical trends,
characteristics and cyclicityin the Jurassic age W
alloon Coals, Surat B
asin, Q
ueensland. BSc H
ons Thesis, The University of Q
ueensland, Brisbane,
Queensland, A
ustralia, unpublished.Scott, S., A
nderson, B., C
rosdale, P., Dingw
all, J., Leblang, G., 2007. C
oal petrology and coal seam
gas contents of the Walloon Subgroup –
Surat Basin, Q
ueensland, A
ustralia. International Journal of Coal G
eology, 70, pp. 209-222.W
ainman, C
.C., M
cCabe P.J., C
rowley, J.L. &
Nicoll, R
.S., 2015 -U-Pb
age of the W
alloon Coal M
easures in the Surat Basin, southeast Q
ueensland: implications for
palaeogeography and basin subsidence. In: Australian Journal of Earth Sciences.
5. CONCLUSIONS AND IMPLICATIONS
CONCLUSIONS AND IMPLICATIONS (continued)
CCSG A1 Posters 2016.indd 7 11/08/2016 4:46 PM