Thermal and Burial History

8/12/2019 Thermal and Burial History

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INTRODUCTION

This chapter draws on preliminary results from theNational Geoscience Mapping Accord (NGMA) Cooper andEromanga Basins Project, due for completion in September1999, and presents conclusions based on calculated maturityand source rock expulsion curves from 15 wells in theCooper Basin, South Australia (Fig. 9.1). These wells forma subset of the overall study which encompasses 90 wellsfrom the South Australian and Queensland portions of the

Cooper Basin, and includes a number of synthetic wells inundrilled troughs.

PREVIOUS STUDIES

Kantsler et al. (1983) noted that higher palaeo-temperatures than present were necessary to model maturitiesin the Nappamerri Trough but claimed that hydrocarbongeneration elsewhere in the Cooper Basin was likely to

post-date deposition of the Winton Formation. They did notpresent any maturity cross-plot validation for this claim.Similar conclusions were substantiated by Kantsler et al.(1986) who found that variable palaeoheatflow regimesappeared to apply in different parts of the Cooper and

Eromanga Basins. Pitt (1986) reached similar conclusionsand also identified a recent rise in geothermal gradient in thelast 510 million years. Duddy (1987) and Gallagher (1988)identified rising heatflow in the last 12 million years usingapatite fission track analysis (AFTA) and argon spectrumanalysis, respectively. Gallagheret al. (1994) and Tingateand Duddy (1996) have confirmed the apparently widespreadnature of this Plio-Pleistocene thermal event. Toupinet al.(1997) modelled these heatflow variations in terms ofchanges in aquifer flow rates in the Great Artesian Basin.Their model suggests invasion of Permian strata by hotartesian waters in the early Tertiary, locally shifting the depthof the zone of oil generation.

BURIAL HISTORY

Figure 9.2 shows the burial geohistory for Burley 2 in theNappamerri Tr ough, cons tructed using standar ddecompaction techniques, a fluctuating sea level and

palaeowaterdepths derived from sedimentological and fossilcontent. The plot indicates the main features of deposition inthe Cooper and overlying Eromanga Basins a thick(1500 m) non-marine Permian and Early Triassic sequenceof sandstone, siltstone and shale is overlain disconformably

by ~800 m of non-marine sandstone, siltstone and shale ofmid-Jurassic to Early Cretaceous age. Rapid deposition ofmarine mudstone and siltstone took place in the late Early

Cretaceous (shaded area in Fig. 9.2) and was followed by athick sequence (900 m) of non-marine siltstone in themid-Cretaceous. A minor erosional event separates thislower sequence from intermittent Tertiary deposition of thinn o n -m a ri n e s e di m e nt s a c co m p an i e d b y m i n or compressional structuring. Interpretation of erosionalevents is based on Moussavi-Harami (1996b).

TECTONIC SUBSIDENCE

Previous burial history studies of the Cooper andEromanga Basins have concentrated on explaining theapparently anomalous rapid deposition of the Winton

143

THERMAL AND

BURIAL HISTORY I. Deighton*

and A.J. Hill

Chapter 9

98-1283

Cuttapirrie 1

Tirrawarra North 1

Dullingari 1

Tinga Tingana 1

Merrimelia 7

Gidgealpa 16

Burley 2

Tirrawarra 1

Nulla 1

Moomba 27

Daralingie 5

Toolachee 1

Kobari 1

Kenny 1

Beanbush 1

Fig. 9.1 Location of wells studied; bold indicates wells discussedin detail.

* Paltech Pty Ltd, [email protected]


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Formation (Middleton, 1989; Gallagher, 1988; Zhou, 1989).In this study, the authors have modelled terrestrialcompaction based on work by Nadon and Issler (1997;near-surface porosities are lower than for marine sediments;see Fig. 9.3) and palaeo-elevation some 100 m above sealevel for deposition of the Winton Formation. The resultanttectonic subsidence during Winton deposition and erosion is

no different from the background Eromanga trend (Fig. 9.4).Cretaceous to Tertiary tectonic subsidence is smallcompared to Permian subsidence.

Figure 9.5 shows the tectonic subsidence of the wells inthis study, normalised from the start of burial. Note theuniformity of mid-Cretaceous to present subsidence and thecomparatively greater magnitude of Permian subsidence.Interpreted simply, in terms of the McKenzie (1978) crustalstretching model, Cooper subsidence at Burley 2 wouldcorrespond to a stretching factor of 1.2 (based on stretchingsubsidence of ~1 km), while Eromanga subsidence wouldcorrespond to a stretching factor of


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Palaeotemperature profiles were highest at 90100 Ma, with cooling prior to 70 Ma in the CooperBasin. Cooling may have been caused by uplift and

erosion, or heatflow decline.

Palaeotemperature profiles were low prior to 25 Mathen increased to high at present. Argon dating(granite in Moomba 1; Gallagher, 1988), AFTAdating and reflectance modelling (not on all wells)indicate a recent rise in geothermal gradient of1020C/km.

The results for four wells are shown in Figure 9.6,expressed in terms of geothermal gradient. The details forBurley 2 are shown in Figure 9.7.

This thermal scheme has been implemented in thecurrent NGMA study, applying higher heatflows from 90 to

85 Ma, declining to below background till 25 Ma (see Figs9.8, 9.9, 9.10). With the exception of suppressed vitrinite,

particularly just above the oil window (e.g. Figs 9.15, 9.21),this heatflow model successfully matches most vitrinitereflectance (VR) data.

PRESENT HEATFLOW

Areas of present heatflow maxima are generallycoincident with distribution of granites and/or highconductivity basement. Heatflow ranges from 70 to 120milliwatts per square metre (mW/m

2), with the higher values

occurring over granite bodies and Warburton Basin ridges.

The higher heatflows can be explained by 5 km thick, 50 kmwide granite bodies which provide an additional 3540mW/m

2above a background of 6070 mW/m

2(Gallagher,

1988, fig. 8.3.5). An additional 15 mW/m2

can be producedby conductivity contrast in the basement (Gallagher, 1988,fig. 8.3.3). Minor heatflow maxima may be associated withaquifer discharge near basin depocentres.

PALAEOHEATFLOW MODEL

The palaeoheatflow curves shown in Figure 9.8, andwhich result in a valid palaeotemperature model, arecommented on below.

Cooper eventGranite cooling may provide a method for overall

declining heatflow in the Cooper Basin. Vitorello and

Pollack (1980) proposed a three-component model ofdecreasing heatflow with tectonic age as observed incontinental granitic areas (Fig. 9.11). This model proposesthat the main heatflow decline results from the decay of asub-lithosphere transient thermal perturbation associatedwith tectogenesis of granite. In the case of the Cooper Basin,the age of the granite is ~300 Ma (i.e. only 510 millionyears prior to onset of Cooper Basin sedimentation). Thus,the majority of cooling proposed in the Vitorello and Pollack

model took place during sedimentation of the Cooper andEromanga Basins (Fig. 9.12). Most subsidence resultingfrom this cooling took place early, which explains the rapidsubsidence of the Nappamerri Trough, over granite(represented by Burley 2) compared to the PatchawarraTrough (represented by Cuttapirrie 1 and Tirrawarra

North 1). The deep-seated nature of the event means thatlateral heat transfer will result in higher heatflow away fromthe granite basement areas and that similar (though lower)

palaeoheatflow patterns should apply to areas withoutgranite basement.

Eromanga event

A high heatflow peak has been modelled from 90 to85 Ma to provide the high temperatures evidenced by AFTAand Ro data in the mid-Cretaceous, although the origin ofthis high temperature event is unknown. At least part of therise may be due to thermal blanketing by very lowconductivity smectitic Allaru Mudstone and WallumbillaFormation sediments, combined with a smaller crustalheatflow increase. The decline in this temperature eventmay be as late as 60 to 70 Ma (Fig. 9.7).

Post-Eromanga event

The low temperature phase from 85 to ~5 Ma is alsoevidenced by AFTA and Roanalysis. Previous studies have

proposed that the present heatflow is anomalous and the lowtemperature phase is normal heatflow. However, there isno easy mechanism to create a rise in heatflow over the last

145

Petroleum geology of South Australia. Vol. 4: Cooper Basin.

0100200300

Time (Ma)

TERTIARYCRETACEOUS

Dep

th(km)

0

1

1.6

Gidgealpa 16

Tinga Tingana 1Cuttapirrie 1

Merrimelia 7Burley 2

OtherDullingari 1

Tirrawarra North 1

98-1077

PERMIAN

TRIASSIC JURASSIC

Fig. 9.5 Tectonic subsidence for selected Cooper Basin wells.

98-1415

Tirrawarra 1 Burley 2

Toolachee 1

Beanbush 1

Beanbush 1

Present-day geothermal gradient (C/km)

Solid symbols: Paleogeothermal gradient in mid-Cretaceous (~90 Ma)

Open symbols: Paleogeothermal gradient immediately prior to ~2 Ma

Paleogeothermalgr

adient(C/km)

Fig. 9.6Palaeogeothermal gradients determined from AFTA andVR palaeotemperature results versus present-day gradient for fourCooperEromanga wells (after Geotrack, 1997).


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146


98-1416

Temperature (C)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

20 100 200 300 140

Depth(kmbelowKB)

f

C/km

p

t

i

p

p

Corrected bottom hole temperature measurement: gradient supplied by PIRSAMaximum steady-state present temperature allowed by AFTA

Maximum paleotemperature from AFTAMaximum paleotemperature from VR

Fluid inclusion homogenisation temperature

Namba Fm

Eyre Fm

Winton Fm

Macunda Fm

Alluru Mdst.

Wallumbilla Fm

Cadna-owie FmMurta Fm

Namur Sst.Westbourne Fm

Adori Sst.Birkhead FmHutton Sst.

NAPPAMERRIGROUP

Toolachee Fm

Daralingie toPatchawarraFms

Tirrawarra Sst.& Merrimelia Fm

Granite

3706 m

Toolebuc Fm

-

Fig. 9.7 Plot of palaeotemperatures derived from AFTA and VR data in Burley 2, against sample depth and the estimated presenttemperature profile for this well (after Geotrack, 1997).

98-1079

0100200300

Time (Ma)

TERTIARYCRETACEOUSJURASSICTRIASSICPERMIAN

Heatflow(mW/m

)2

0

20

40

60

80

100

120

140

Gidgealpa 16



OtherDullingari 1

Tirrawarra North 1

Fig. 9.8 Modelled heatflow versus time for selected Cooper Basinwells.

98-1080

0100200300

Time (Ma)

TERTIARYCRETACEOUSJURASSICTRIASSIC

PERMIAN

Bedtemperature(C)100

200

150

250

50

0

Gidgealpa 16



OtherDullingari 1

Tirrawarra North 1

Fig. 9.9 Modelled temperature versus time, top PatchawarraFormation.


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25 million years without some other geophysical evidence.For example, mantle hot spots or any other lower lithosphereheating event should cause some few hundred metres bulge,which is not observed.

Other studies have proposed that the present heatflow isnormal and that the low temperature phase is anomalous.This would suggest major changes in the aquifer flowregime of the Great Artesian Basin as the mechanism. Thesemodels propose that the low temperature phase results fromincreased artesian flow in the past, which removed largeamounts of heat laterally, and that a slower rate in the last

25 million years allowed the thermal regime to equilibrate

to normal. Habermehl (1986), however, suggested that the

hydrologic flow rate is at its peak now. Gallagher (1988) has

shown that constant background heatflow is unlikely to be

perturbed significantly by fluid flow except at the margins(or up faults) where vertical movement may be significant

(Gallagher, 1988, p.94). Over the centre of the granite the

interval heatflow is lower above a 500 m thick aquifer (i.e.

temperature gradient is lower). Gradients below are

constant, but temperatures are lower for higher flow rates.

For low flow rates, the gradient above the aquifer is the same

as the gradient below. The main effects of high aquifer flow

rates are to reduce temperatures over the granite and shift the

temperature peak laterally downflow by about the half width

of the granite. Toupinet al.(1997) proposed major uplift in

the centre of the basin at the end of the Cretaceous as the

origin of the cooling event, causing a depression of

isotherms because of local influx of meteoric water.Although there was certainly uplift (though probably in the

Late Cretaceous), it is difficult to see how this could provide

the regional effect evidenced by AFTA and VR data. Not all

areas were uplifted by the same amount (Moussavi-Harami,

1996b), with some experiencing no uplift at all.

RESULTS

Using the above heatflow model to produce the

palaeotemperature paths evidenced by AFTA and Ro data,

kerogen generation and expulsion of the Cooper Basin

sequence was modelled in version 2.4 of Winbury using

the standard industry model of Tissot and Welte (1984) andothers. A summary of geohistories for major structural areas

within the Cooper Basin using representative wells follows.

147


98-1081

100 0200300

Time (Ma)


R

(logscale)

o

0.2

0.5

0.6

0.70.8

1.0

2.0

4.0

2.5

3.2

1.3

1.6dry gas maturity zone

PERMIAN

wet gas maturity zone

oil maturity zone

Gidgealpa 16



OtherDullingari 1

Tirrawarra North 1

Fig. 9.10 Modelled maturity versus time, top PatchawarraFormation.

0 500

Tectonic age (Ma)

100

50

0

Heatflow(mW/m

)2

27

54

87

1000 1500 2000 2500

1

0

(hfu)

2

I

I

II

III

Sub-crustal heatflow

Transient thermal perturbation

Crustal radiogenic heat

(Deep seated, due to orogeny which resulted in graniteemplacement. NB: This component is responsible formost of the heatflow decline.)

(Long-term radioactive decay, also diminished by erosionof granite roof.)

II III

98-1082

Fig. 9.11 Theoretical (and observed) heatflow decline in graniteterranes (after Vitorello and Pollack, 1980). Total heatflow (greencurve) is the sum of components I to III.

Tectonic age (Ma)

100

50

0

Heat

flow(mW/m

)2

300

27

II

I

III54

87

0

98-1083

I

II

III

Sub-crustal heatflow

Transient thermal perturbation

Crustal radiogenic heat

(Deep seated, due to orogeny which resulted in graniteemplacement. NB: This component is responsible formost of the heatflow decline.)

(Long-term radioactive decay, also diminished by erosionof granite roof.)

Fig. 9.12 Theoretical heatflow decline in young granite terranes(extracted from Fig. 9.11, with ages reversed. Cooper Basin

granites are 290300 million years old).


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NAPPAMERRI TROUGH

Figure 9.13 shows the burial geohistory for Burley 2,indicating that Cooper Basin sediments passed through theoil and gas maturity windows very early, due to highheatflow in the Permian. The palaeotemperature paths foreach layer are shown in Figure 9.14 whilst depths to maturitywindows are summarised in Table 9.1.

Figure 9.15 shows the cross-plot of observed andcomputed reflectance, which indicates a good fit except forsome shallower data here presumed to be suppressed.

Figure 9.16 shows oil and gas generation through timefor source rocks in Burley 2, calculated using standardtechniques and compositional kerogen kinetics discussed in

Chapter 8. These traces indicate that while some gas wasgenerated in the Late Permian and Early Triassic fromPatchawarra Formation coal and shale, most was generatedfrom the Patchawarra and Toolachee Formations in themid-Cretaceous. Patchawarra Formation shale expelled atotal of 29 bbl equivalent/m

2of gas compared to only 7 bbl

148


0100200300Time (Ma)

TERTIARYCRETACEOUSJURASSICPERMIANTRIASSIC

-0.5

Sub-seadepth

(km)

0

1.0

2.0

3.0

4.0

WintonFormation

Allaru Mdst.

WallumbillaFormation

Hutton Sst.

NAPPAMERRIGROUP

Toolachee Fm

Epsilon Fm

PatchawarraFormation

dry gas

wet gas

oil

WestbourneFormation

98-1084

Sea level

Maturity method: easy R %o

Sediment interface

R contouro

Fig. 9.13 Burial and maturity geohistory plot, Burley 2.

98-1085

0100200300

Time (Ma)

TERTIARYJURASSICTRIASSIC

PERMIAN

Temperature(C)

Gradienttobase

100

200

300

150

250

50

00

20

40

60

80

100

Temperature

Sediment surface temperature

Gradient

CRETACEOUS

Fig. 9.14Bed temperature versus time, Burley 2.

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (log scale)

-0.5

0

1.0

2.0

3.0

3.5

Sub-seadepth

(km)

Winton

Formation

Allaru Mdst.

Wallumbilla

Formation

Westbourne Fm

Hutton Sst.

NAPPAMERRI

GROUP

Toolachee Fm

Epsilon Fm


oil wet gas dry gas

oil at 1144 m

wet gas at 1513 m

dry gas at 2021 m

Maturity method: easy R %o

Keiraville, 1991

ANALYST

B and S (B#1)

98-1086

Fig. 9.15 Maturity versus depth plot, Burley 2.

Table 9.1 Hydrocarbon maturity, Burley 2.

Unit Ro Maturit y Depth(%) window (m subsea)

Allaru, upper Wallumbilla 0.65 oil ~1144

Lower Wallumbilla, Murta, 1.0 wet gas ~1513Namur, Westbourne,

Adori, upper Birkhead

Lower Birkhead, Hutton, 1.6 dry gas ~2021Poolowanna, Nappamerri,Toolachee, Epsilon, Murteree,upper Patchawarra,lower Patchawarra


7/14

equivalent/m2

of gas from coal*. Significantly, any oilexpulsion from the Patchawarra Formation in Burley 2 took

place in the Late Permian. Conversely, a total of 27 bblequivalent/m

2of waxy oil was expelled from Toolachee

Formation coal in the mid-Cretaceous. There appears to beconsiderable scope for Toolachee Formation sourced oil tomigrate up into Early Cretaceous reservoirs depending uponseal integrity of the intervening units.

PATCHAWARRA TROUGH

Burial and thermal histories of the western and centralPatchawarra Trough are represented by Tirrawarra North 1and Cuttapirrie 1.

Cuttapirrie 1

The geohistory plot of Cuttapirrie 1 indicates that theEarly Permian to Late Jurassic succession entered the oilwindow at ~95 Ma, with Early to Late Permian rocksentering the wet gas window at ~87 Ma until present day(Fig. 9.17; Table 9.2).

Figure 9.18 plots maturity with depth and clearly showssignificant suppression of vitrinite up to 0.1% at the onset ofoil generation over the Westbourne and Birkhead

Formations.

Figure 9.19 shows oil and gas generation through time

for source rocks in Cuttapirrie 1, calculated using standardtechniques and compositional kerogen kinetics discussed inChapter 8. These traces indicate that some oil and minor gaswere expelled during the Late Cretaceous at ~90 Ma,

principally from Patchawarra Formation (coal, 4 bblequivalent/m

2of oil; shale, 0 bbl equivalent/m

2of oil) and

Toolachee Formation (coal, 4 bbl equivalent/m2

of oil)

source horizons. Significantly, little or no oil appears tohave been expelled from the Poolowanna Formation, whichhosts commercial oil reserves. This is consistent withwhole-oil gas chromatography data (Appendix 2) thatsupport a Permian source.

Tirrawarra North 1

The geohistory plot of Tirrawarra North 1 indicates thatthe Early Permian to Late Jurassic succession entered the oilwindow at ~100 Ma. The Early Permian rocks entered thewet gas window at ~90 Ma and have remained there until the

present day (Fig. 9.20; Table 9.3). It is worth noting that

depths to the relative oil and wet gas windows for TirrawarraNorth 1 are very close to those for Cuttapirrie 1 (Tables 9.2,9.3).

149


equivalent/m2

of gas from coal*. Significantly, any oilexpulsion from the Patchawarra Formation in Burley 2 took

place in the Late Permian. Conversely, a total of 27 bblequivalent/m

2of waxy oil was expelled from Toolachee

Formation coal in the mid-Cretaceous. There appears to beconsiderable scope for Toolachee Formation sourced oil tomigrate up into Early Cretaceous reservoirs depending uponseal integrity of the intervening units.

PATCHAWARRA TROUGH

Burial and thermal histories of the western and centralPatchawarra Trough are represented by Tirrawarra North 1and Cuttapirrie 1.

Cuttapirrie 1

The geohistory plot of Cuttapirrie 1 indicates that theEarly Permian to Late Jurassic succession entered the oilwindow at ~95 Ma, with Early to Late Permian rocksentering the wet gas window at ~87 Ma until present day(Fig. 9.17; Table 9.2).

Figure 9.18 plots maturity with depth and clearly showssignificant suppression of vitrinite up to 0.1% at the onset ofoil generation over the Westbourne and Birkhead

Formations.

Figure 9.19 shows oil and gas generation through time

for source rocks in Cuttapirrie 1, calculated using standardtechniques and compositional kerogen kinetics discussed inChapter 8. These traces indicate that some oil and minor gaswere expelled during the Late Cretaceous at ~90 Ma,

principally from Patchawarra Formation (coal, 4 bblequivalent/m

2of oil; shale, 0 bbl equivalent/m

2of oil) and

Toolachee Formation (coal, 4 bbl equivalent/m2

of oil)

source horizons. Significantly, little or no oil appears tohave been expelled from the Poolowanna Formation, whichhosts commercial oil reserves. This is consistent withwhole-oil gas chromatography data (Appendix 2) thatsupport a Permian source.

Tirrawarra North 1

The geohistory plot of Tirrawarra North 1 indicates thatthe Early Permian to Late Jurassic succession entered the oilwindow at ~100 Ma. The Early Permian rocks entered thewet gas window at ~90 Ma and have remained there until the

present day (Fig. 9.20; Table 9.3). It is worth noting that

depths to the relative oil and wet gas windows for TirrawarraNorth 1 are very close to those for Cuttapirrie 1 (Tables 9.2,9.3).

98-1087

Volume(bblequiv/m

):rate/Ma

2

0

0

0

0

3

3

3

3

300 0200 100

0

1

2

3

Winton

Formation

Mackunda Fm

Allaru Mdst.

Wallumbilla Fm

Cadna-owie Fm

Westbourne Fm

Hutton Sst.

NAPPAMERRIGROUP

ToolacheeFormation

Daralingie Fm

Epsilon Fm


Source rocks Time (Ma)Sub-sea

depth(km)

TERTIARY

CRETACEOUS

JURASSIC

TRIASSICPERMIAN

Gas ( )in situ Gas (expelled) Oil ( )in situ Oil (expelled)

Toolachee Fm mud: gas (ins: 1) (exp: 3) - bbl equiv./m2

Toolachee Fm coal: gas (ins: 0) (exp: 19); oil_waxy (exp: 27) - bbl equiv./m2

Patchawarra Fm mud: gas (ins: 4) (exp: 29) - bbl equiv./m2

Patchawarra Fm coal: gas (ins: 0) (exp: 7); oil_waxy (exp: 11) - bbl equiv./m2

Fig. 9.16Hydrocarbon generation and expulsion, Burley 2.

98-1088

TERTIARYCRETACEOUSJURASSICPERMIAN

TRIASSIC

0100200250

Time (Ma)

Sub-sea

depth(km)

-0.5

0

1.0

2.0

3.0

Namba Fm

WintonFormation

Mackunda Fm

Allaru Mdst.


Westbourne Fm

Birkhead Fm

Hutton Sst.

NAPPAMERRI

GROUP

Patchawarra Fm

wet gas

oil

Sea level

Maturity method: easy R (%)o

Sediment interface

R contouro

Fig. 9.17 Burial and maturity geohistory plot, Cuttapirrie 1.

* bbl equivalent/m2 refers to yield/m2 of kitchen area.

Table 9.2 Hydrocarbon maturity, Cuttapirrie 1.

Unit Ro Maturity Dep th(%) window (m subsea)

Westbourne, Adori, 0.65 oil ~1896Birkhead, Hutton,

Poolowanna, Nappamerri,upper Toolachee

Lower Toolachee, Epsilon, 1.0 wet gas ~2760Patchawarra, Tirrawarra


8/14

Figure 9.21 plots maturity with depth and also showssignificant suppression of vitrinite up to 0.15% Ro at theonset of oil generation over the Westbourne to PoolowannaFormations.

A significant volume of oil was expelled during the Late

Cretaceous at ~90 Ma principally from PatchawarraFormation coal (99 bbl equivalent/m2) with a further 7 bbl

equivalent/m2

of oil remaining in situ. Shale of thePatchawarra Formation contributes only minor amounts ofexpelled oil (1 bbl equivalent/m

2).

It appears that Toolachee Formation source rocks haveexpelled wet gas in the order of 1 bbl equivalent/m

2which is

consistent with a minor gas accumulation in basal ToolacheeFormation sand in crestal wells of the Tirrawarra Field (Fig.9.22).

Tirrawarra Sandstone of the Tirrawarra Field hosts thelargest Permian oil accumulation of the Cooper Basin; theoil is overlain by large gas reserves within Patchawarra

Formation reservoirs. The kinetic model adopted in theNGMA study assumes that both oil and gas are generatedsimultaneously but with gas expulsion preceding oil

150


Figure 9.21 plots maturity with depth and also showssignificant suppression of vitrinite up to 0.15% Ro at theonset of oil generation over the Westbourne to PoolowannaFormations.

A significant volume of oil was expelled during the Late

Cretaceous at ~90 Ma principally from PatchawarraFormation coal (99 bbl equivalent/m2) with a further 7 bbl

equivalent/m2

of oil remaining in situ. Shale of thePatchawarra Formation contributes only minor amounts ofexpelled oil (1 bbl equivalent/m

2).

It appears that Toolachee Formation source rocks haveexpelled wet gas in the order of 1 bbl equivalent/m

2which is

consistent with a minor gas accumulation in basal ToolacheeFormation sand in crestal wells of the Tirrawarra Field (Fig.9.22).

Tirrawarra Sandstone of the Tirrawarra Field hosts thelargest Permian oil accumulation of the Cooper Basin; theoil is overlain by large gas reserves within Patchawarra

Formation reservoirs. The kinetic model adopted in theNGMA study assumes that both oil and gas are generatedsimultaneously but with gas expulsion preceding oil

98-1090

250 0200 100Source rocks

Time (Ma)

Sub-seadepth(km)

TERTIARY

CRETACEOUSJURASSIC

TRIASSIC

PERMIANNamba Fm

WintonFormation

Mackunda Fm

Allaru Mdst.

Wallumbilla

Formation

Cadna-owie Fm

Westbourne Fm

Birkhead Fm

Hutton Sst.

Poolowanna Fm

NAPPAMERRI

GROUP

Toolachee Fm

Patchawarra Fm

1

2

0

Volume(bblequiv/m

):Rate/Ma

2

0

0

0

0

2

2

2

2

Toolachee Fm mud: gas (ins: 1) (exp: 1); oil_waxy (ins: 2) - bbl equiv./m2

Toolachee Fm coal: gas (ins: 0) (exp: 1); oil_waxy (ins: 1) (exp: 4) - bbl equiv./m2

Patchawarra Fm mud: gas (ins: 1) (exp: 1); oil_waxy (ins: 6) - bbl equiv./m2

Patchawarra Fm coal: gas (ins: 0) (exp: 1); oil_waxy (ins: 0) (exp: 4) - bbl equiv./m2


Fig. 9.19 Hydrocarbon generation and expulsion, Cuttapirrie 1.

0100200250Time (Ma)

TERTIARYCRETACEOUSJURASSIC

TRIASSIC

PERMIAN -0.5

Sub-sea

depth(km)

0

1.0

2.0

3.0

4.0

WintonFormation

Mackunda Fm

Allaru Mdst

Wallumbilla

Formation

Westbourne Fm

Hutton Sst

NAPPAMERRIGROUP


wet gas

oil

98-1091

Sea level


Sediment interface

R contouro

Fig. 9.20 Burial and maturity geohistory plot, Tirrawarra North 1.

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (log scale)

-0.5

0

1.0

2.0

2.5

Sub-seadepth(km)

Namba Fm

Winton

Formation

Mackunda Fm

Allaru Mdst.

Wallumbilla

Formation

Westbourne Fm

Birkhead Fm

Hutton Sst.

NAPPAMERRIGROUP

Patchawarra Fm

oil wet gas dry gas

oil at 1896 m

wet gas at 2760 m


98-1089

Kantsler, 1980

ANALYST

Fig. 9.18 Maturity versus depth, Cuttapirrie 1.

Table 9.3 Hydrocarbon maturity, Tirrawarra North 1.

Unit Ro Mat urity Depth(%) window (m subsea)

Westbourne, Adori, 0.65 oil ~1911Birkhead, Hutton,Poolowanna, Nappamerri,Toolachee, Epsilon,Murteree, upper Patchawarra

Lower Patchawarra, Tirrawarra 1.0 wet gas ~2761


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expulsion. This would account for gas occurringstratigraphically above the oil in the Tirrawarra Field.

METTIKA EMBAYMENT

Dullingari 1

Burial and thermal history of the Mettika Embayment ofthe Tenappera Trough is represented by Dullingari 1 (Fig.9.23). Source rocks in Dullingari 1 entered the oil windowclose to the Late Permian Early Triassic boundary,

reflecting a higher geothermal gradient than the thermallycooler Patchawarra Trough wells discussed previously.With the exception of the lower Patchawarra Formation,which entered the dry gas window in the mid-Cretaceous(~90 Ma), the remainder of the Early and Late Permiansuccession has been in the wet gas window since ~108 Ma,whilst the Triassic to Early Cretaceous succession iscurrently within the oil window (Fig. 9.23). This is inagreement with hydrocarbon discoveries in the DullingariField with proven oil reserves in the Murta Formation, wetgas in the Toolachee and Daralingie Formations, and dry gasin Patchawarra Formation reservoirs. Depth to present daymaturity windows are summarised in Figure 9.24 and Table

9.4.

151


98-1093

Volume(bblequiv/m

):Rate/Ma

2

250 0200 100Source rocks

Time (Ma)

Sub-seadepth(km)

TERTIARY

CRETACEOUSJURASSIC

TRIASSICPERMIAN

0

1

2

3

10

10

10

0

0

0

Namba Fm

Eyre Fm

WintonFormation

Mackunda Fm

Allaru Mdst.


Cadna-owie Fm

Namur Sst.

Westbourne Fm

Birkhead Fm

Hutton Sst.

NAPPAMERRI

GROUP


Toolachee Fm coal: gas (ins: 0) (exp: 1); oil_waxy (ins: 1) (exp: 2) - bbl equiv./m2

Patchawarra Fm mud: gas (ins: 5) (exp: 7); oil_waxy (ins: 34) (exp: 1) - bbl equiv./m2

Patchawarra Fm coal: gas (ins: 1) (exp: 34); oil_waxy (ins: 7) (exp: 99) - bbl equiv./m2


Fig. 9.22 Hydrocarbon generation and expulsion, TirrawarraNorth 1.

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (log scale)

-0.5

0

1.0

2.0

3.0

Sub-sea

depth(km)

Namba Fm

WintonFormation

Mackunda Fm

Allaru Mdst


Westbourne Fm

Birkhead Fm

Hutton Sst

NAPPAMERRIGROUP


oil wet gas dry gas

oil at 1911 m

wet gas at 2761 m


98-1092

Keiraville, 1984

ANALYST

Fig. 9.21 Maturity versus depth plot, Tirrawarra North 1.

0100200250Time (Ma)


PERMIAN -0.5

Sub-seadepth

(km)

0

1.0

2.0

3.0

Winton

Formation

Mackunda Fm

Oodnadatta

Formation

Bulldog Shale

Cadna-owie Fm

WestbourneFormation

Hutton Sst

NAPPAMERRIGROUP

Toolachee Fm

Patchawarra

Formationdry gas

wet gas

oil

98-1094

Sea level


Sediment interface

R contouro

Fig. 9.23 Burial and maturity geohistory plot, Dullingari 1.

Table 9.4Hydrocarbon maturity, Dullingari 1.


Murta, Namur, 0.65 oil ~1345Westbourne, Adori,Birkhead, Hutton,Poolowanna, Nappamerri

Toolachee, Epsilon, 1.0 wet gas ~1982Murteree, upper Patchawarra

Lower Patchawarra 1.6 dry gas ~2439


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TINGA TINGANA RIDGE(adjacent to the Weena Trough)

Tinga Tingana 1

Tinga Tingana 1 is located on the Tinga Tingana Ridge(Fig. 5.5) and has been included in this review as a guide tothe prospectivity of the Weena Trough which lies to thewest. In the final report for the NGMA project, a syntheticwell extrapolating thermal and burial history results fromTinga Tingana 1 to the central Weena Trough will provide amore accurate assessment of the regions prospectivity.

When considering the geohistory plot of TingaTingana 1 (Fig. 9.25) it is important to note that the well haslimited maturity data (Fig. 9.26), resulting in a poorlyconstrained heatflow model. The majority of the sedimentsin the well are immature whilst the basal MerrimeliaFormation has remained within the oil window since themid-Cretaceous (Fig. 9.26; Table 9.5). Increased depth of

burial and total coal thickness in the Patchawarra Formation,possibly in excess of 40 m (Fig. 6.6), suggests that theprospectivity of the Weena Trough has been underestimated.

GIDGEALPA AND MERRIMELIA RIDGES

The thermal and burial history of the Gidgealpa andMerrimelia Ridges is represented by Merrimelia 7 andGidgealpa 16 (Fig. 9.1).

Merrimelia 7

Figure 9.27 shows that the Early Triassic to Late Jurassicsuccession in Merrimelia 7 entered the oil window between80 and 90 Ma, and has remained there to the present day.

This is consistent with oil production from the NappamerriGroup in Merrimelia 7. Depth to present day maturitywindows are summarised in Figure 9.28 and Table 9.6.

152


98-1096

0100200300Time (Ma)


PERMIAN -0.5

Sub-sea

depth(km)

0

1.0

2.0

3.0

Namba Fm

WintonFormation

Allaru Mdst.

Namur Sst.


MerrimeliaFormationoil

Sea level


Sediment interface

R contouro

Fig. 9.25 Burial and maturity geohistory plot, Tinga Tingana 1.

-0.5

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (log scale)

0

1.0

2.0

Sub-sea

depth(km)

Namba Fm

WintonFormation

Allaru Mdst

Murta Fm

Namur Sst


Tirrawarra Sst

MerrimeliaFormation

oil wet gas dry gas

oil at 2051 m


98-1097

Shell, 1984

ANALYST

Fig. 9.26Maturity versus depth, Tinga Tingana 1.

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (log scale)

-0.5

0

1.0

2.0

2.5

Sub-seadepth(km)

Winton

Formation

Mackunda Fm

OodnadattaFormation

Bulldog Shale

Cadna-owie Fm

WestbourneFormation

Hutton Sst

NAPPAMERRIGROUP

Toolachee Fm


oil wet gas dry gas

oil at 1345 m

wet gas at 1982 m

dry gas at 2439 m

Maturity method : easy R (%)o

98-1095

Kantsler, 1978

ANALYST

Shell, 1984

Fig. 9.24Maturity versus depth, Dullingari 1.

Table 9.5 Hydrocarbon maturity, Tinga Tingana 1.

Unit Ro Mat urity Depth(%) window (m subsea)

Murta, Namur,


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Gidgealpa 16

Figures 9.29 and 9.30 show that Early Permian to MiddleJurassic source rocks are capable of generating oil andentered the oil window in the mid-Cretaceous (~90 Ma) inGidgealpa 16. Computed maturity values are less thanobserved values towards the base of the well (Fig. 9.30),although the latter still lie within the oil window, albeit close

to the wet gas threshold. This may help to explain drillstemtest (DST) results over the Toolachee and PatchawarraFormations and Tirrawarra Sandstone, where commercialrates of wet gas were tested. Significantly, DST 2 tested gasat a rate of 0.048 x 10

6m

3(1.7 mmcf) per day and recovered

1.65 kL (10.4 bbl) of oil over the interval 2115.92123.5 min the Toolachee Formation, whilst the deeper formationswere only gas bearing. Gidgealpa 16 is structurally

positioned to receive gas from gas-bearing source rocks onthe flanks of the Gidgealpa Field. Depth to present daymaturity windows is summarised in Table 9.7. The depth tothe top of the oil window for Gidgealpa 16 closely matchesthat for Merrimelia 7.

SUMMARY

Combining the results of the 15 wells studied provides acomparison across the Cooper Basin of the generationpotential, and oil and gas expulsion with time. These aresummarised below.

153


0100200300

Time (Ma)


TRIASSIC

-0.5

Sub-sea

depth(km)

0

1.0

2.0

3.0

Namba Fm

WintonFormation

Mackunda FmAllaru Mdst

Oodnadatta Fm

Bulldog Shale

Adori Sst

Hutton Sstoil

98-1100

Sea level


Sediment interface

R contouro

Fig. 9.29 Burial and maturity geohistory plot, Gidgealpa 16.

98-1098

0100200300

Time (Ma)


TRIASSIC

-0.5

Sub-seadepth

(km)

0

1.0

2.0

3.0

Namba Fm

WintonFormation

Allaru Mdst.

Westbourne Fm

Hutton Sst.

NAPPAMERRI GP

oil

Sea level


Sediment interface

R contouro

Fig. 9.27Burial and maturity geohistory plot, Merrimelia 7.

0.50.2 0.6 0.8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

0.7

VR (Log Scale)

-0.5

0

1.0

2.0

Sub-sea

depth(km)

Namba Fm

WintonFormation

Mackunda Fm

Allaru Mdst.

Cadna-owie Fm

Namur Sst.Westbourne Fm

Birkhead Fm

Hutton Sst.

NAPPAMERRI GP

oil wet gas dry gas

oil at 1731 m

Maturity method : Easy R %o

98-1099

M15 (KK85)

ANALYST

Amdel, 1983

B and R

Fig. 9.28 Maturity versus depth, Merrimelia 7.

Table 9.6Hydrocarbon maturity, Merrimelia 7.

Unit Ro Maturity D epth

(%) window (m subsea)

Westbourne, Adori, 0.65 oil ~1731Birkhead, Hutton, Nappamerri

Table 9.7Hydrocarbon maturity, Gidgealpa 16.


Adori, Birkhead, 0.65 oil ~1713Hutton, Poolowanna,Nappamerri, Toolachee,Patchawarra, Tirrawarra,Merrimelia


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Total (residual) generation potential through time for allwells in the study is shown in Figure 9.31, which indicatest h a t m o s t h y d ro c a rb o n s w e r e g e n er a te d i n t h emid-Cretaceous. Minor amounts were generated during thePermian in the Nappamerri Trough.

Figure 9.32 shows total oil expulsion through time for allwells indicating major expulsion in the mid-Cretaceous,with minor amounts in the late Tertiary. Minor oil wasexpelled at Burley 2 in the Late Permian. The late Tertiary

event suggests that if sufficient residual kerogen remains,increased temperature as a result of the combined effect ofTertiary deposition and elevated temperatures in the lateTertiary may lead to late-stage oil expulsion in favourable

parts of the basin.

Figure 9.33 demonstrates that gas expulsion isubiquitous and mainly occurred in the mid-Cretaceous.

154


Total (residual) generation potential through time for allwells in the study is shown in Figure 9.31, which indicatest h a t m o s t h y d ro c a rb o n s w e r e g e n er a te d i n t h emid-Cretaceous. Minor amounts were generated during thePermian in the Nappamerri Trough.

Figure 9.32 shows total oil expulsion through time for allwells indicating major expulsion in the mid-Cretaceous,with minor amounts in the late Tertiary. Minor oil wasexpelled at Burley 2 in the Late Permian. The late Tertiary

event suggests that if sufficient residual kerogen remains,increased temperature as a result of the combined effect ofTertiary deposition and elevated temperatures in the lateTertiary may lead to late-stage oil expulsion in favourable

parts of the basin.

Figure 9.33 demonstrates that gas expulsion isubiquitous and mainly occurred in the mid-Cretaceous.

Volume(bblequivalent/m

):rate/Ma

2

Time (Ma)0

0

5

10

15

20

100200300


98-1104

Gidgealpa 16

Cuttapirr ie 1 Beanbush 1

Burley 2

OtherDullingari 1

Tirrawarra North 1

Fig. 9.32 Oil expulsion versus time, Cooper Basin.

Volume(bblequivalent/m

):rate/Ma

2

Time (Ma)0100200300


0

5

10

98-1105

Gidgealpa 16

Cuttapirrie 1 Merrimelia 7

Burley 2 Beanbush 1

OtherDullingari 1

Tirrawarra North 1

Fig. 9.33 Gas expulsion versus time, Cooper Basin.

98-1103

Time (Ma)0100200300


Volume(bbleq

uivalent/m

)2

0

150

100

50

200

250

300

Gidgealpa 16

Tinga Tingana 1Cuttapirrie 1Merrimelia 7Burley 2

Other

Dullingari 1Tirrawarra North 1

Fig. 9.31Generation potential versus time, Cooper Basin.

0.7

0.50.2 0.6 0 .8 1.0 1.3 1.6 2.0 2.5 3.2 4.0

VR (log scale)

-0.5

0

1.0

2.0

Sub-seadepth(km)

Namba Fm

Winton Fm

Mackunda Fm

Allaru Mdst.

Oodnadatta Fm

Bulldog Shale

Cadna-owie Fm

Westbourne FmAdori Sst.

Hutton Sst.

Patchawarra Fm

o il w et g as dry gas

oil at 1713 m

Maturity method: Easy R (%)o

98-1101

Kantsler, 1979

ANALYST

Fig. 9.30 Maturity versus depth, Gidgealpa 16.


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The foregoing conclusions are based on the 15 wellsused for this preliminary study. A more rigorous computedr egional m odel ( 90 wells in S outh Aus tr alia andQueensland) will be used in the final NGMA report.

Previous studies based on consideration of the oilmaturity window have implied significant hydrocarbongeneration in the Late Cretaceous and early Tertiary

(Kantsleret al., 1986; Pitt, 1986). However, results of thepresent study indicate that the major generation occurred inthe mid-Cretaceous, which provides explorers with a new

perspective for understanding the timing of expulsion ofhydrocarbons in the Cooper and Eromanga Basins.Accordingly, a reassessment of the changing migration

pathways during Late Cretaceous or Tertiary structuring iswarranted.

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