example from the Surat Basin, Australia A1...Surat Basin’s Walloon Subgroup and its overlying...

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)

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