Exhumation in the Cooper-Eromanga Basins, Australia

Energy Sources, Part A, 29:631648, 2007Copyright Taylor & Francis Group, LLCISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/009083190957775

Exhumation Study in the Cooper-Eromanga Basins,Australia and the Implications for

Hydrocarbon Exploration

A. MAVROMATIDISPetroleum Development OmanMuscat, Sultanate of Oman

Abstract The Cooper-Eromanga Basins of South Australia and Queensland are notat their maximum burial-depth due to Late Cretaceous-Tertiary and Late Triassic-Early Jurassic exhumation. The main tool used for estimating the exhumation is theavailable vitrinite reflectance data. Exhumation studies using compaction analysishave also been compiled in order that exhumation is better constrained. The re-sults suggest that Late Cretaceous-Tertiary exhumation increases eastwards from theSouth Australia to the Queensland sector of the basins. This study has major im-plications for hydrocarbon exploration. Predicted maturation of source rocks will begreater for any given geothermal history if exhumation is incorporated in maturationmodeling.

Keywords compaction analysis, source rock maturity, vitrinite reflectance

IntroductionThe Cooper and Eromanga Basins are Australias largest onshore petroleum province,and are located in central and eastern Australia (Figure 1). The sediments of the CooperBasin were deposited during Late Carboniferous-Permian and Triassic times in predom-inantly fluvial and lacustrine environments (Thornton, 1979). After the deposition of theCooper Basin, in Late Triassic-Early Jurassic times, an exhumational event took placethat resulted in the basin wide Nappamerri unconformity (Thorton, 1979; Kuang, 1985)(Figure 1). Subsequently, the Eromanga Basin sediments were deposited in Jurassic andCretaceous times in mainly fluvial-lacustrine and shallow marine environments (Bow-ering, 1982). The Eromanga Basin is the larger of the two and completely overlies theCooper Basin. After the deposition of the Eromanga Basin, major sedimentation ceasedand over the last 90 Myr, the basin has been characterized by periods of exhumation andminor sedimentation (Moore and Pitt, 1984; Shaw, 1991; Mavromatidis 1997; Mavroma-tidis and Hillis, 2005) (Figure 1). The aims of this study are to:

Determine the magnitude of Late Cretaceous-Tertiary and Late Triassic-EarlyJurassic exhumation, using vitrinite reflectance data from 21 released wells;

Address correspondence to Angelos Mavromatidis, Petroleum Development Oman LLC, P.O.Box 81, Muscat 113, Sultanate of Oman. E-mail: [email protected]

631

632 A. Mavromatidis

Figure 1. (a) Location map for the Cooper-Eromanga Basins; (b) Cooper-Eromanga Basins strati-graphic nomenclature (FM = formation, GRP = group, MBR = member, SST = sandstone)(modified after Moore, 1986); and (c) location of wells used in vitrinite reflectance modeling,major tectonic elements and fields are also shown (GMI = Gidgealpa-Merrimelia-Innamincka, J =Jackson Field, M = Moomba Field, NM = Nappacoongee-Murteree, Patch = Patchawarra, PNJ =Pepita-Naccowlah-Jackson South, RW = Roseneath-Wolgolla, S = Strzelecki).

Present previous studies of exhumation based on compaction analysis and comparethe exhumation estimates of these studies with the exhumation estimates in thisstudy; and

Discuss the implications of exhumation results with respect to thermal maturityof source rocks.

Exhumation in the Cooper-Eromanga Basins, Australia 633

The term exhumation (as opposed to erosion or uplift) is used here in the sense of Englandand Molnar (1990) to describe displacement of rocks with respect to the surface.

Method

Quantifying Exhumation Using Vitrinite ReflectanceVitrinite reflectance-depth data from 21 wells were taken from well completion reports.The selected wells provide regional coverage (Figure 1) and the vitrinite reflectance-depth profile of each well is representative of the area in which it is located. Presentgeothermal gradients were calculated using bottom hole temperatures (corrected for timesince circulation of drilling muds) and temperatures determined from drill stem tests.Cao et al. (1988) and Russell and Baillie (1989) suggest that such values provide themost reliable formation temperatures. It is argued that the baseline temperature to whichthe most meaningful overall thermal gradient is drawn is not the temperature at groundsurface, which is subject to diurnal and seasonal variations, but the rock temperaturesome meters or tens of meters below the ground. This temperature is probably 1820C in the region under discussion (Pitt, 1986). A surface temperature of 20C wasused in this study. In modeling, the available software MATOILs default thermal con-ductivities and compaction/decompaction parameters for given lithologies were used.The two most important variables that govern the modeling of vitrinite reflectance are:(a) burial/exhumation history and (b) palaeogeothermal gradients. Hence, three differ-ent assumed palaeogeothermal histories were used and the palaeogeothermal gradientsduring the Late Triassic-Early Jurassic unconformity, and that during the Late Cretaceous-Tertiary unconformity were varied in order that the modeled vitrinite reflectance gave thebest fit to observed vitrinite reflectance.

Following the principle of Occams razor (i.e., the simplest hypothesis that fits thedata is the best hypothesis), vitrinite reflectance was first modeled assuming that palaeo-geothermal gradients were constant and equal to the present geothermal gradients (Fig-ure 2). Exhumation values associated with the Late Triassic-Early Jurassic unconformitywere extremely high and exhumation for the late Cretaceous-Tertiary unconformity couldnot be estimated since reflectances predicted by this geothermal history are higher thanthose observed for the Eromanga Basin. This indicates that the present high geother-mal gradients are a relatively recent phenomenon with which vitrinite reflectance hasnot fully equilibrated. Since the first geothermal history proved unsatisfactory, a secondgeothermal history was used. In this, the palaeogeothermal gradients increase over thelast 90 Ma from lower values to the present gradients (Figure 2). The assumed increasesin geothermal gradients the last 90 Ma were as follows:

Present Geothermal GradientIncrease in Geothermal Gradient

from 90 Ma to Present Day35C/km 5C/km

3644C/km 10C/km3644C/km 15C/km

The second geothermal history implies extremely high, similar to the first geothermalhistory, in the order of thousands, and probably unreasonable, exhumation values associ-ated with the Late Triassic-Early Jurassic unconformity for the majority of the examinedwells. As a consequence, vitrinite reflectance modeling was undertaken using a third

634 A. Mavromatidis

Figure 2. Summary of geothermal gradient histories in the three types of maturity modeling.Gradients value at any point in time reflects average gradient of existing basinal section. Detailsfor each well are given in Table 1 (Gt = geothermal gradient).

and preferred geothermal history. In an attempt to match the vitrinite reflectance trendsof the Cooper Basin without invoking extreme Late Triassic-Early Jurassic exhumationas required by previous geothermal histories, modeling was undertaken using the thirdgeothermal history. In this third attempt to model the vitrinite reflectances, geothermalgradients during the deposition of the Eromanga Basin were the same as in the sec-ond geothermal history (i.e., increasing over the last 90 Ma), but the gradients duringthe deposition of the Cooper Basin were taken to be 5C/km higher than the presentgeothermal gradients (Figure 2). The effect of increasing the geothermal gradients duringthe deposition of the Cooper Basin is to reduce the amount of exhumation required atthe time of Late Triassic-Early Jurassic unconformity (i.e., between the deposition of theCooper and Eromanga Basins). The precise geothermal histories used for each well ineach of the three scenarios are summarized in Table 1.

One of the objectives of this study was to compare the exhumation values of this studywith the exhumation estimates based on the compaction analysis of the Eromanga Basin(Mavromatidis and Hillis, 2005). In compaction analysis, the terms apparent exhumationand total exhumation have been estimated and the same terms have been adopted andused herein. In general, total exhumation at the time the rocks were being elevated (ET )

Exhumation in the Cooper-Eromanga Basins, Australia 635

Table 1Present and palaeogeothermal gradients that best fit vitrinite reflectance data

assuming different geothermal histories

First geothermal history Second geothermal history Third geothermal history

WellAge,Ma

Geothermal gradient,C/km

Age,Ma

Geothermal gradient,C/km

Age,Ma

Geothermal gradient,C/km

Alkina-1 0 45 0 45 0 4591 45 91 30 91 30

198 45 198 30 198 30286 45 286 30 286 50

Baryulah-1 0 45 0 45 0 4591 45 91 30 91 30

208 45 208 30 208 30286 45 286 30 286 50

Battunga-1 0 40 0 40 0 4010 40 10 30 10 3091 40 91 30 91 30

183 40 183 30 183 30286 40 286 30 286 45

Beanbush-1 0 37 0 37 0 3791 37 91 27 91 27

225 37 225 27 225 27286 37 286 27 286 42

Bungee-1 0 44 0 44 0 4488 44 88 34 88 34

193 44 193 34 193 34286 44 286 34 286 49

Burley-2 0 55 0 55 0 5591 55 91 40 91 40

193 55 193 40 193 40286 55 286 40 286 60

Copai-1 0 45 0 45 0 4591 45 91 30 91 30

193 45 193 30 193 30Curalle-1 0 48 0 48 0 48

91 48 91 33 91 33198 48 198 33 198 33250 48 250 33 250 53

Innamincka-4 0 47 0 47 0 4791 47 91 32 91 32

193 47 193 32 193 32258 47 258 32 258 52

Jackson-1 0 44 0 44 0 4410 44 10 34 10 3491 44 91 34 91 34

193 44 193 34 193 34286 44 286 34 286 49

Lycium-1 0 38 0 38 0 3810 38 10 28 10 2891 38 91 28 91 28

193 38 193 28 193 28268 38 268 28 268 43

(continued)

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Table 1(Continued)

First geothermal history Second geothermal history Third geothermal history

WellAge,Ma

Geothermal gradient,C/km

Age,Ma

Geothermal gradient,C/km

Age,Ma

Geothermal gradient,C/km

Macadama-1 0 44 0 44 0 4491 44 91 34 91 34

183 44 183 34 183 34286 44 286 34 286 49

Mackillop-1 0 45 0 45 0 4591 45 91 30 91 30

193 45 193 30 193 30258 45 258 30 258 50

Nulla-1 0 37 0 37 0 3790 37 90 27 97 27

193 37 193 27 193 27286 37 286 27 286 42

Okotoko-1 0 41 0 41 0 4191 41 91 31 91 31

193 41 193 31 193 31286 41 286 31 286 46

Putamurdie-1 0 40 0 40 0 4090 40 90 30 90 30

193 40 193 30 193 30Russel-1 0 42 0 42 0 42

10 42 10 32 10 3291 42 91 32 91 32

258 42 258 32 258 47Tirrawarra 0 35 0 35 0 35North-1 91 35 91 30 91 30

193 35 193 30 193 30286 35 286 30 286 40

Ullenbury-1 0 42 0 42 0 4291 42 91 32 91 32

204 42 204 32 204 32245 42 245 32 245 47

Wareena-1 0 43 0 43 0 4391 43 91 33 91 33

198 43 198 33 198 33253 43 253 33 253 48

Warnie East-1 0 52 0 52 0 5210 52 10 37 10 3791 52 91 37 91 37

193 52 193 37 193 37286 52 286 37 286 57

is the sum of apparent exhumation (EA) and post-exhumational burial (BE):

ET = EA + BE. (1)

Maximum burial-depth (BT ) is constrained by the apparent exhumation (EA) and not theamount of exhumation at the time the rocks were being exhumed (ET ). It is given by

Exhumation in the Cooper-Eromanga Basins, Australia 637

the sum of the present burial-depth (BP ) and apparent exhumation (EA):

BT = EA + BP . (2)

In this study, the total exhumation was firstly determined and then for comparison pur-poses the apparent exhumation was estimated and used for comparisons with compactionanalysis study.

Results and DiscussionAssuming the third geothermal history, there are no wells where vitrinite reflectanceis best modeled with no exhumation associated with both the Late Cretaceous-Tertiaryand Late Triassic-Early Jurassic unconformities (Figure 3). The best fit to the observedvitrinite reflectance data, implies no Late Cretaceous-Tertiary exhumation at: Beanbush-1,Bungee-1, Innamincka-4, and Tirrawarra North-1. In the rest of the wells, Late Cretaceous-Tertiary exhumation is inferred, but Late Triassic-Early Jurassic exhumation is higher.Figure 4 shows apparent exhumation inferred from Cooper Basin vitrinite reflectancecrossplotted against apparent exhumation inferred from Eromanga Basin vitrinite re-flectance.

The results suggest that Late Cretaceous-Tertiary exhumation from maximum burial-depth increases from approximately 200 m in the south and west of the NappamerriTrough to 800 m in the northeastern/Queensland part of the Nappamerri Trough (Fig-ure 5). This supports the evidence of Shaw (1991) based on stratigraphic estimates thatexhumation increases eastwards. Late Cretaceous-Tertiary exhumation reaches maximumvalues of approximately 600 m in the Wareena Anticline (north of the Jackson-Naccowlaharea) and of approximately 700 m in the Naryilco Anticline (south of the Jackson-Naccowlah area). The other area of maximum exhumation, which reaches approximately800 m, is near the northeastern boundary of South Australia, in the vicinity of theCuralle-1 well.

The change of gradient in the vitrinite reflectance/depth profiles between the Cooperand Eromanga Basins is characteristic of the majority of the wells studied. This changeoccurs at the unconformity between the Cooper and the Eromanga Basins. Observedvitrinite reflectances were difficult to model consistently. The data points from the Ero-manga Basin were lower than expected at shallow levels and those from the CooperBasin higher than expected at deeper levels. A key feature of the observed vitrinite re-flectance data from the Cooper-Eromanga Basins is that measured vitrinite reflectancesare lower than would be expected from present temperatures, particularly in the Ero-manga Basin. This demonstrates that the present high geothermal gradients in the area,typically 4050C/km, are a relatively recent phenomenon. The cause of this increase ingeothermal gradient is unclear. The results of the compaction analysis indicate signif-icant exhumation over the area (Mavromatidis and Hillis, 2005). Hence, it is temptingto suggest that advective transfer of heat to the surface associated with exhumation mayhave been responsible for the increase in geothermal gradient. However, if this was so,vitrinite reflectances should be consistent with present temperatures. Alternatively, assuggested by Stuart et al. (1993), there may have been significant heat transport throughgroundwater movement in aquifers. The Eromanga Basin is, of course, part of the GreatArtesian Basin, and thus subject to groundwater movement through its aquifers. Further-more, there is significant contemporary hot spring activity in the Flinders Ranges (e.g.,Paralana Hot Springs) to the south of the Cooper-Eromanga Basins (Foster et al., 1994),

638 A. Mavromatidis

Exhumation in the Cooper-Eromanga Basins, Australia 639

Figure 3. Plots of observed (black dots) and modeled (broken line) vitrinite reflectance Ro (%) vs.depth (in km). Modeling assumes that burial/exhumational events took place in Late Triassic-EarlyJurassic times, after the deposition of the Cooper Basin, and in Late Cretaceous-Tertiary times,after the deposition of the Eromanga Basin. Apparent exhumation (EA) and total exhumation (ET )values (in meters) used in the modeling are also shown (IM = interval missing).

640 A. Mavromatidis

Figure 4. Crossplot of apparent exhumation (in meters) from Eromanga Basin units against ap-parent exhumation (in meters) from Cooper Basin units. The line illustrating the 1:1 relationshipbetween apparent exhumation values from each pair of units analyzed is shown.

where the presumed basement of the area outcrops. The role played by the basementgranites in producing the high geothermal gradients is discussed later in this section.

The most striking feature of the observed vitrinite reflectance data from the Cooper-Eromanga Basins is the increase in vitrinite reflectance/depth gradient from the EromangaBasin sequence to the Cooper Basin sequence. This increase in gradient reflects exhuma-tion during Late Triassic-Early Jurassic times, and/or higher palaeogeothermal gradientsduring the deposition of the Cooper Basin sequence, prior to Late Triassic-Early Juras-sic exhumation. The balance of exhumation and/or palaeogeothermal gradient used tomodel the observed vitrinite reflectance trends is inherently a non-unique one. Indeed,it is clear from this study that there almost as many different estimates of exhumationand palaeogeothermal gradients as there are vitrinite reflectance modeling studies in thearea. High geothermal gradients during the deposition of the Cooper Basin sequence arelikely to be the consequence of Carboniferous tectonic activity and associated igneousintrusion. Granites have been widely postulated to play a role in both high present andhigh palaeogeothermal gradients in the area (e.g., Pitt, 1986; Gallagher, 1988). Kantsleret al. (1983) suggested that the high geothermal gradients could have existed for 300 Myrwith even higher gradients in the Permo-Triassic, on the basis that the early high gradi-ents were associated with granite intrusion. Much of the Nappamerri Trough is underlainby granites (Gatehouse, 1986). The granites encountered in wells in the area are datedradiometrically at about 305360 Ma (Gatehouse, 1986), which implies that the elevatedgeothermal gradients may have existed for up to 50 Myr before the onset of sedimentdeposition in the Late Carboniferous-Early Permian. The distribution of granites must beconsidered, laterally, more extensive underneath the basement lithologies than the rep-resented by intersection wells (Gallagher, 1988). Present-day high geothermal gradientsin the Nappamerri Trough are considered to reflect the fact that the area is underlain byhot granitic basement (Russell and Bone, 1989). In contrast, lower geothermal gradients

Exhumation in the Cooper-Eromanga Basins, Australia 641

Figure 5. Apparent exhumation (in meters) based on vitrinite reflectance in Eromanga Basinstratigraphic units. Well control points and tectonic elements are also shown (GMI = Gidgealpa-Merrimelia-Innamincka, NM = Nappacoongee-Murteree, Patch = Patchawarra, PNJ = Pepita-Naccowlah-Jackson South, RW = Roseneath-Wolgolla).

in the Patchawarra Trough may reflect the fact that this area is underlain by older, midPalaeozoic (meta-) sediments of the Warburton Basin (Kantsler et al., 1983; Gallagher,1988). The GMI trend appears to represent a thermal hinge between the Patchawarraand Nappamerri Troughs. The period of lower geothermal gradients during the deposi-tion of Eromanga Basin sediments may have been anomalous if the basement granitesof the area are responsible for the long term high geothermal gradients. Lower geother-mal gradients at this time may have been associated with thermal blanketing due to rapidJurassic-Early Cretaceous sedimentation. Alternatively, the high heat flow associated withgranites may have decayed during Jurassic-Early Cretaceous times and the recent increase

642 A. Mavromatidis

may have a different origin. It is significant that in areas such as Strzelecki and Jackson,where there is no evidence of granites, that there is a present thermal high (Pitt, 1986),suggesting that the more recent increase in geothermal gradients is not everywhere as-sociated with the granites and that it may be related to groundwater movement (Stuartet al., 1993). Clearly granite intrusion prior to the onset of Cooper Basin sedimentationcannot alone account for high geothermal gradients during the deposition of the CooperBasin and the recent increase in geothermal gradients. Exhumation estimates in LateTriassic-Early Jurassic times are not considered to be representative due to this complexpalaeogeothermal environment and hence only indications can be deduced.

Quantification of Exhumation Using the Compaction AnalysisMavromatidis and Hillis (2005) have been estimated the exhumation in the EromangaBasin using the compaction method. Their study showed that exhumation increases east-wards from the Patchawarra Trough, through the GMI Trend and Nappamerri Trough intoQueensland, with values of approximately 600 m in the Jackson-Naccowlah area. Theother area of maximum exhumation, which also reaches approximately 1 km, lies nearthe extreme northeastern boundary of South Australia, in the vicinity of the Curalle-1well. The study was based on the use of sonic log in 210 wells from 7 units in the Ero-manga Basin. The similarity of exhumation values from different units and the extensivedataset make these estimates to be a robust reference for comparison with this study andfor other exhumation studies.

Comparison of Exhumation Estimates from the Two Different MethodsThe exhumation values derived from vitrinite reflectance modeling for the EromangaBasin sequence are comparable with, or less than, those derived from compaction analysis(Table 2). The differences between exhumation values for the two techniques are lessthan 350 m except at Innamincka-4, Jackson-1, and Warnie East-1, where the differencesare 650, 370, and 480 m, respectively. Both methods suggest lower Late Cretaceous-Tertiary exhumation values in the Patchawarra Trough and the Nappamerri Trough andan increase in exhumation in the Jackson-Naccowlah area (to approximately 600 m) andthe Curalle anticline (to approximately 1000 m) (Figure 5 in this study and Figure 7 inMavromatidis and Hillis (2005) study). However, the tendency for exhumation values,from compaction analysis, to increase from the Patchawarra to Nappamerri Trough, isnot born out by the vitrinite reflectance modeling. This may simply be because lesswells were analyzed in the vitrinite reflectance modeling than in the compaction analysis(210 wells). The other area of maximum Late Cretaceous-Tertiary exhumation, inferredfrom vitrinite reflectance modeling, which reaches approximately 800 m, is near thenortheastern boundary of South Australia, in the vicinity of the Curalle-1 well. Theseresults are consistent with those based on compaction analysis, and with the fact that theTertiary is absent or very thin over these areas.

Influence of Exhumation on Source Rock MaturityThe source rock thermal history required to model observed maturity is generally de-termined from burial history and palaeogeothermal gradients or palaeo-heatflow/thermal

Exhumation in the Cooper-Eromanga Basins, Australia 643

Table 2Apparent exhumation estimatesa in Eromanga Basin

(Late Cretaceous-Tertiary times)

Well

Apparent exhumation(in meters) based onvitrinite reflectance

Apparent exhumation(in meters) based oncompaction analysis(after Mavromatidis

and Hillis, 2005)Alkina-1 200 529Baryulah-1 600 366Battunga-1 250 302Beanbush-1 0 67Bogala-1 578Bungee-1 0 278Burley-2 200 385Copai-1 300 531Challum-1 389Curalle-1 800 996Innamincka-4 0 666Jackson-1 400 775Lycium-1 150 250Macadama-1 300 336Mackillop-1 350 446Morney-1 824Nulla-1 300 230Okotoko-1 350 517Pepita-2 431Putamurdie-1 230 542Russel-1 350 337Tirrawarra North-1 0 185Ullenbury-1 520 453Wareena-1 800 871Warnie East-1 60 542Watson-1 613

aWells without apparent exhumation values means that no data have been collected(i.e., in vitrinite reflectance).

conductivity (e.g., Falvey and Deighton, 1982; Bray et al., 1992). To assess the influenceof Late Triassic-Early Jurassic and Late Cretaceous-Tertiary exhumation on source rockmaturity, vitrinite reflectance levels have been modeled in Jackson-1. The palaeogeother-mal gradients used in modeling were those of in geothermal history according to thirdscenario (Table 1). Source rock maturity has been modeled (in terms of vitrinite re-flectance) for the following three scenarios:

without considering exhumation;

644 A. Mavromatidis

considering exhumation only in Late Cretaceous-Tertiary times; considering exhumation in Late Triassic-Early Jurassic; and Late Cretaceous-Tertiary times (Figure 6).

Modeling was undertaken using the BasinModTM software in which vitrinite reflectance iscalculated using the kinetics of Sweeney and Burnham (1990). The major potential sourcerocks for liquid hydrocarbon generation are the Patchawarra and Toolachee Formations inthe Cooper Basin (Jenkins, 1989), and the Basal Jurassic (Hawkins et al., 1989), BirkheadFormation (Jenkins, 1989), and Murta Member (Michaelsen and McKirdy, 1989) in theEromanga Basin. In modeling, without allowance for burial/exhumation, the PatchawarraFormation reaches a vitrinite reflectance level of 0.5%Ro, equivalent to early maturityfor oil generation during Late Cretaceous times and the rest of the source rocks duringTertiary times. Without allowance for exhumation no source rocks reach mid-maturity(0.71.0%Ro) (Figure 6a). However, with the incorporation of Late Cretaceous-Tertiaryexhumation, all source rocks reach a vitrinite reflectance of 0.5%Ro, during Late Cre-taceous times and the Patchawarra Formation reaches a vitrinite reflectance of 0.7%Ro,equivalent to mid maturity for oil generation during Tertiary times (Figure 6b). When mat-uration modeling incorporates Late Triassic-Early Jurassic exhumation the Patchawarraand Toolachee Formations pass through early oil generation during mid-Triassic times andthe Patchawarra Formation reaches mid mature during Late Triassic times (Figure 6c).Considering Late Triassic-Early Jurassic exhumation, the Toolachee Formation reachesmid-mature oil generation at around the Late Cretaceous/Tertiary boundary. Robinson(1982) quoted observed reflectances in the Jackson-1 well, which include 0.56% at1.1 km in the Murta Member, 0.58% at 1.4 km in the Birkhead Formation, 0.74% at1.5 km in the Toolachee Formation and 0.87% at 1.6 km in the Patchawarra Formation.Hence, the only maturation modeling that is consistent with the observed reflectancesin the Jackson-1 well is that which incorporates Late Triassic-Early Jurassic and LateCretaceous-Tertiary exhumation. More broadly, incorporating Late Triassic-Early Jurassicand Late Cretaceous-Tertiary exhumation in maturation modeling is consistent with thesourcing of the oil fields of the Jackson-Naccowlah area, in southwestern Queensland,from the above source rocks (Vincent et al., 1985).

In summary, the combination of any given palaeogeothermal gradients with a burialhistory plot for a potential hydrocarbon source that allows for exhumation indicatesearlier and higher levels of organic maturity than the same palaeogeothermal gradientscombined with a burial history plot that does not allow for exhumation. This is morediscernible when Late Triassic-Early Jurassic exhumation is incorporated in maturationmodeling. Thus, estimates of exhumation, such as those presented, should be incorporatedin maturation modeling of wells not at their maximum burial-depth.

Figure 6. Burial/exhumation and maturity histories for the Jackson-1 well: (a) without allowancefor exhumation; (b) with allowance for Late Cretaceous-Tertiary exhumation; and (c) with allowancefor Late Cretaceous-Tertiary and Late Triassic-Early Jurassic exhumation. Modeling was undertakenusing the kinetics of Sweeney and Burnham (1990) BasinModTM software. All burial/exhumationhistories were decompacted using the methodology of Sclater and Christie (1980) with allowancefor the effect of exhumational event. Ages were taken from the operators composite logs andgeochronologically calibrated after the time scale of Harland et al. (1989). The apparent exhumationvalue (in meters) is shown (Pch = Patchawarra Formation, Tlc = Toolachee Formation, BJr =Basal Jurassic, Brk = Birkhead, Mrt = Murta Member, BGL = below ground level).

Exhumation in the Cooper-Eromanga Basins, Australia 645

Figure 6. (Figure caption on page 644.)

646 A. Mavromatidis

DiscussionIt is worth noting that in Jackson-1 (Figure 6) the excess of Late Triassic-Early Jurassicexhumation over subsequent burial is relatively small, and greater maturities are attainedby Cooper Basin source rocks in Tertiary times than were attained in Late Triassic-EarlyJurassic times. Hence, hydrocarbons generated by Cooper Basin source rocks could havecharged reservoirs in these areas in Tertiary times. Thus, in areas where the excess ofLate Triassic-Early Jurassic exhumation over subsequent burial is relatively small, CooperBasin sourced oils could have directly charged Eromanga Basin reservoirs and/or CooperBasin reservoirs may have been charged with Cooper Basin sourced oils in Late Tertiarytimes, and such oils need not have been preserved in reservoirs since Late Triassic-EarlyJurassic times. However, where the excess of Late Triassic-Early Jurassic exhumationover subsequent burial is large (in excess of at least 400 m), it is considered unlikely thatCooper Basin sources could have filled Eromanga Basin reservoirs. Indeed geochemicalwork (Michaelsen and McKirdy, 1989) has suggested that Eromanga Basin sourced oilsform a significant component of Eromanga Basin reservoired oils. There are not yetsufficient geochemical data to compare geochemically-based determinations of sourcerock with the exhumation values determined herein.

Conclusion and Recommendations for Further ResearchThis study has shown that exhumation in Late Cretaceous-Tertiary times increases to-wards the Queesnland sector of the Cooper-Eromanga Basins. The exhumation estimatesare similar between this study and the compaction analysis study. However, the results forLate Triassic-Early Jurassic times are not reliable due to uncertainties in palaeogeother-mal history of the area. It is clear from this study that the Cooper-Eromanga Basin withits complex thermal history is not an ideal area in which to investigate whether exhuma-tion values yielded by modeling vitrinite reflectance are consistent with those from thecompaction methodology. Nevertheless, previous research has generally paid little atten-tion to exhumation in these basins and its consequences. In particular the significance ofLate Triassic-Early Jurassic times has not been widely recognized. It is not surprisingthat maturation studies which have not incorporated the effects of exhumation have failedto produce satisfactory models (Kantsler et al., 1983; Middleton, 1979; McKirdy, 1982;Pitt, 1986; Russell and Bone, 1989; Stuart et al., 1993). Quantification of exhumationin Late Triassic-Early Jurassic times can be estimated using the available porosity datafrom the Cooper Basin units and apply the compaction analysis in a similar way thatMavromatidis and Hillis (2005) have been quantified the exhumation in Late Cretaceous-Tertiary times. Vitrinie reflectance modeling exhumation results can then be tested withmore confidence and maybe determine better palaeogeothermal gradients for modelingpurposes.

Similar age unconformities to those in Cooper-Eromanga Basins, are observed inthe other basins of eastern Australia. Due to the availability of vitrinite reflectance andporosity data and the low cost of the techniques, it is suggested that vitrinite reflectancemodeling and compaction analysis could be used to quantify the amount of Late Triassic-Early Jurassic exhumation between the Galilee and Eromanga Basins, between the Bowenand Surat Basins, and between the Esk Trough and Moreton Basins. These techniquescould also be applied to determine Late Cretaceous-Tertiary exhumation in the Surat andMoreton Basins. Accurate knowledge of exhumation in the eastern part of the Australiancontinent will be a useful to petroleum exploration in these areas to applying a regional

Exhumation in the Cooper-Eromanga Basins, Australia 647

tectonic model for the formation and evolution of the eastern part of the continent andits sedimentary basins.

AcknowledgmentThe present work has been made possible thanks to Santos Ltd. for providing the dataand the well completion reports. I warmly thank Prof. R. Hillis and Dr. P. Tingate,University of Adelaide, Australia, for their critical reviews; P. Siffleet and G. Jacquierfor fruitful discussions; and D. Warner and M. Zwigulis of Santos Ltd. for suggestionsof improvement of this research.

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