Heat Flow and Thermal Maturity Modelling in the Northern

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Marine and Petroleum Geology 19 (2002) 1073–1088

Heat ﬂow and thermal maturity modelling in the Northern Carnarvon Basin, North West Shelf, Australia

Sheng Hea,b,*, Mike Middletonb

aDepartment of Petroleum Engineering, Faculty of Earth Resources, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China bDepartment of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Received 24 June 2002; received in revised form 17 December 2002; accepted 19 December 2002

Abstract The Northern Carnarvon Basin is located at the southern end of the North West Shelf of Australia. It was developed by rifting during the Jurassic–earliest Cretaceous. Heat flow and thermal maturity in seven wells, from three sub-basins, Rankin Platform and Exmouth Plateau of the Northern Carnarvon Basin, were modelled using BasinMod 1D program. Multiple thermal maturity parameters were used to constrain the influence of anomalously low vitrinite reflectance. Those Tmax data with reliable quality can be applied to correlate with vitrinite reflectance, establish thermal maturity and calibrate the modelled results. The modelled results indicated that the measured maturity data in some wells were consistent with the rift heat flow model (Jarvis & McKenzie) associated with the Jurassic rift and the earliest Cretaceous seafloor spreading events. The maximum values of heat flow were in the range from 67 mW/m2 (Jurabi-1) to 105 mW/m2 (Bowers-1) in the Exmouth and Barrow Sub-basins. On the Exmouth Plateau, the maximum values of heat flow were modelled to be 72 mW/m2 in the Jupiter-1 well and 78 mW/m2 in the Investigator-1 well. These maximum values were modelled to occur during syn-rift phase, which were 29–88% and 33–37% greater than their current heat flow values in the sub-basins and on the Exmouth Plateau, respectively. This study suggests that maturity indicators are less diagnostic of rifting thermal histories if the maximum thermal effect is associated with Cretaceous and Cainozoic burial in this basin. q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Heat ﬂow; Thermal maturity; Thermal modelling; Rock-Eval Tmax; Northern Carnarvon Basin

1. Introduction Vitrinite reflectance (Ro) is the most widely used indicator of thermal maturity (Allen & Allen, 1990; Hunt, The heat flow history of a basin is proposed by 1996; Tissot & Welte, 1984; Waples, 1985). Wilkins, establishing an agreement between a calculated (or Wilmshurst, Hladky, Ellacott, and Buckingham (1992a) modelled) maturity parameter and the equivalent observed pointed out that two major causes of the anomalously low maturity parameter (such as vitrinite reflectance, or Rock- vitrinite reflectance for some North West Shelf wells are: (1) Eval T ). The calculated, or modelled, thermal maturity suppression of vitrinite reflectance through marine influ- max ence; (2) difficulty of identifying vitrinite population in parameters are usually derived from models that use (1) dispersed organic matter. Marine influence resulting in empirically-based temperature and time integrals (Middle- lower R values was proposed by Hunt (1996), Gurba and ton, 1982; Waples, 1980), (2) the Arrhenius-reaction o Ward (1998), Kaiko and Tingate (1996) and Wilkins et al. approach (Lerche, Yarzab, & Kendall, 1984; Wood, (1992a). Samuelsson and Middleton (1998) stressed that the 1988), or (3) multiple Arrhenius-reaction models, which vitrinite reflectance suppression in this basin has led to an attempt to simulate the chemical reactions that produce underestimation of the true level of thermal maturity. As a maturation (Larter, 1988; Sweeney & Burnham, 1990). major thermal maturity indicator, these problems with vitrinite reflectance make it difficult to estimate the thermal * Corresponding author. Address: Department of Petroleum Engineering, history in many wells of this basin. Faculty of Earth Resources, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China. Tel./fax: þ86-27-8743-6106. The Northern Carnarvon Basin is the richest hydrocarbon E-mail address: [email protected] (S. He). province in Australia (Kopsen, 1994). A number of gas/

1074 S. He, M. Middleton / Marine and Petroleum Geology 19 (2002) 1073–1088 condensate and oil fields, associated with postulated a maturity indicator, also need to be further studied in this Triassic and Jurassic source rocks, have been discovered area. The aims of this study are (1) to investigate the (Baillie & Jacobson, 1997; Lawry & Carter, 1992; le applicability of the classical McKenzie-model (Jarvis & Poidevin & Lowden, 1994; Vincent & Tilbury, 1988). McKenzie, 1980; McKenzie, 1978) for rifting thermal Researchers have used alternative thermal parameters and history to the study area, (2) to examine and use Rock-Eval Ro correction for attempting to overcome the problems Tmax data for evaluation of thermal maturity compared with associated with the anomalously low Ro in studies of the other maturity parameters of Ro and Eq VR, and (3) to basin’s thermal history and the thermal evolution of source present detailed thermal modelling of individual wells based rocks. Alexander, Marzi, and Kagi (1990) applied molecular on the correlation between Tmax and Ro. markers as thermal indicators for modelling the palaeoheatflow of the Jupiter-1 well in the basin. Wilkins, Russell, and Ellacott (1994) evaluated thermal maturity of five Northern 2. Regional geological setting Carnarvon Basin wells using equivalent vitrinite reflectance (Eq VR) data, which is analysed by the technique of the The Northern Carnarvon Basin (Fig. 1) is located at the fluorescence alteration of multiple macerals (FAMM). southern end of the North West Shelf of Australian (AGSO Kaiko and Tingate (1996) used the spore coloration method (Australian Geological Survey Organisation) North West combined with Ro and FAMM data for assessment of Shelf Study Group, 1994; Hocking, Moors, & van De thermal maturity in the Barrow and Dampier Sub-basins. Graaff, 1987). The main subdivisions of the Northern Samuelsson and Middleton (1998) corrected vitrinite Carnarvon Basin include the Exmouth, Barrow, Dampier, reflectance values based on the method of Lo (1993) for and Beagle Sub-basins, the Rankin Platform and the reconstruction of heat flow histories in seven wells of the Exmouth Plateau (Fig. 1), which commonly refer to the basin. Mesozoic–Cainozoic basin overlying the pre-Triassic base- Studies of palaeoheatflow in rift-related basins indicate ment. Sediments usually extend to the continental-oceanic that high palaeoheatflow is commonly associated with crust boundary. The sub-basins contain thick sedimentary rifting and seafloor spreading (Mello & Karner, 1996; Zhou rocks of Mesozoic–Cainozoic (Fig. 2). Middle–Upper & Littke, 1999), and the reconstruction of thermal history Jurassic syn-rift sedimentary rocks are thinner on the from vitrinite reflectance is a function of the tectonic history Rankin Platform and the Exmouth Plateau than elsewhere of a sedimentary basin (Allen & Allen, 1990; Middleton, in the basin. 1982). Based on theoretical and empirical studies (Barber, The geological evolution of the Northern Carnarvon 1982; Driscoll & Karner, 1998; Hellinger & Sclater, 1983; Basin, as a part of the North West Shelf, commenced in the McKenzie, 1978; Middleton & Hunt, 1989; Polomka, Late Palaeozoic. During this time, the North West Shelf was Bruins, Spanninga, & Mennie, 1999; Tindale, Newell, situated on the eastern part of Gondwana and formed part of Keall, & Smith, 1998), the evolution of the Northern the southern Tethyan continental margin (Exon & Colwell, Carnarvon Basin from the Jurassic is commonly accepted to 1994). A previous extension of the North West Shelf basins be comprised of two major thermo-tectonic phases: (1) rapid terminated in the late Permian, and is represented by an subsidence and faulting, and (2) thermal subsidence. angular unconformity at the Permian–Triassic boundary Nielsen (1996) proposed that organic maturity indi- (Westphal & Aigner, 1997). The post-Permian basin cators tend to record only the maximum temperature evolution can be divided into (1) a Triassic pre-rift phase, encountered, and when organic matter in the present era (2) a syn-rift phase and continental breakup, and (3) a post- experiences the maximum geotemperature, maturity indi- rift (thermal sag) phase. cators are less sensitive to thermal reconstruction. Simi- larly, Kaiko, and Tingate (1996) noted that direct evidence 2.1. Pre-rift phase for estimating past heat flow from palaeotemperatures (derived from vitrinite reflectance, Rock-Eval Tmax,or The Triassic section shows little, or no evidence, of syn- similar parameters) in most Barrow and Dampier Sub- depositional extensional faulting along the entire North basin wells may have disappeared, since the palaeotem- West Shelf (Etheridge & O’Brien, 1994; Westphal & peratures experienced during the rifting phase may have Aigner, 1997). The Triassic sedimentary rocks are inter- been equalled, or exceeded, by those associated during preted to be present nearly everywhere on the continental burial in later sag-phase sedimentation. margin; these rocks comprise the marine Locker Shale and Palaeoheatflow models for the study region have been fluvio-deltaic Mungaroo Formation (AGSO North West proposed by Alexander et al. (1990), Samuelsson and Shelf Study Group, 1994; Barber, 1982). Middleton (1998) and Swift, Stagg, and Falvey (1988). However, their models are not directly coupled to the 2.2. Syn-rift phase and continental breakup algorithm for rifting heat flow history of Jarvis and McKenzie (1980). The evidence of rift heat flow needs to The early rift is interpreted to have commenced in the be further studied in this basin. Rock-Eval Tmax data, as latest Triassic to the earliest Jurassic, based on K/Ar ages 中国科技论文在线 http://www.paper.edu.cn

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Fig. 1. The Exmouth, Barrow, Dampier and Beagle Sub-basins, Rankin Platform and Exmouth Plateau in the Northern Carnarvon Basin with adjacent three abyssal plains (modiﬁed from AGSO North West Shelf Study Group, 1994; Polomka et al., 1999). Also showing the cross-section of the Exmouth and Barrow Sub-basins, Alpha Arch and Exmouth Plateau (simpliﬁed from Tindale et al., 1998). See Fig. 3 for well locations. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 2. Generalised stratigraphic column of the sub-basins in the Northern Carnarvon Basin (after Blevin et al., 1994; Labutis, 1994; Polomka et al., 1999).

from three volcanic samples on the Wombat Plateau dated until the continental breakup of Gondwana in the Late from 213 to 192 Ma (von Stackelberg et al., 1980)(Exon & Jurassic and the Valanginian (Barber, 1982; Polomka et al., Colwell, 1994; Westphal & Aigner, 1997). Extensional 1999; Tindale et al., 1998; Westphal & Aigner, 1997). The tectonism in the sub-basins of the Northern Carnarvon Basin syn-rift ﬁll is characterised by rapid subsidence and began in the Early Jurassic (about 190 Ma), and continued deposition of a thick sequence of dominantly ﬁne-grained 中国科技论文在线 http://www.paper.edu.cn

S. He, M. Middleton / Marine and Petroleum Geology 19 (2002) 1073–1088 1077 sediments in grabens, whereas uplift and erosion occurs on 3. Data used in the study the shoulders of rift grabens. The extension and differential subsidence developed (1) a series of four NE–SW and 3.1. Data sources NNE–SSW-trending grabens and half-garbens, (2) numer- ous parallel normal faults, and (3) variable erosion of Seven wells (Fig. 3) from various subdivisions of the several hundred metres to 2500 m along the Rankin Northern Carnarvon Basin were selected on the basis of Platform, and the Peedamullah and Lambert Shelves, suitability for thermal modelling using BasinMod 1D (Barber, 1982; Crostella, Iasky, Blundell, Yasin, & Ghori, (version 7.06, Platte River Associates, Inc.). The input 2000; Hocking, 1988; Kopsen & McGann, 1985; Westphal data used for this modelling were collected from open-file & Aigner, 1997). Based on regional deep seismic data sources, such as well-completion reports from the Depart- interpreted by AGSO on the Exmouth Plateau, a number of ment of Minerals and Energy of Western Australia. The small-scale half-grabens developed during rifting. These initial porosity, matrix density, matrix thermal conductivity faults controlled deposition; up to one thousand metres of and matrix heat capacity were adopted from the default Jurassic syn-rift sediments were deposited on the down- values in BasinMod 1D. thrown sides of these faults, while syn-rift deposits are thin, or absent, on the up-thrown side of the faults. Erosion of 3.2. Thermal maturity several hundred metres has been interpreted on the structural highs of the Exmouth Plateau as a result of rifting Observed thermal maturity data for this study include (Barber, 1982). Ro, Eq VR and Rock-Eval Tmax. The vitrinite reflectance The earliest continental breakup in this region appears to values in dispersed organic matter are mean random have occurred at about 155 Ma (Ludden, 1992) to the north vitrinite reflectance in oil (Ro%). The Eq VR data by the of the basin where the Argo Abyssal Plain formed the FAMM technique are believed to be equivalent to earliest part of the Indian Ocean. Seafloor spreading (further unsuppressed vitrinite reflectance, and calibrated in units extension) is interpreted to have begun in the Valanginian, of ‘mean random vitrinite reflectance’. They range from ca. 136 Ma (Mu¨ller, Mihut, & Baldwin, 1998), associated about 0.45 to 1.20% (Wilkins et al., 1992a; Wilkins et al., with separation of Australia and Greater India, and the 1992b; Wilkins, Wilmshurst, Hladky, Ellacott, & Buck- formation of the Gascoyne and Cuvier Abyssal Plains to the ingham, 1995). west and south of the basin. The actual continental breakup Rock-Eval Tmax (8C) is the temperature at the maximum is commonly dated as being concomitant with an unconfor- S2 (pyrolysable hydrocarbons, mg HC/g rock) as a function mity, called the ‘main unconformity’ in the northern of the thermal maturity (Espitalie´, Marquis, & Barsony, Carnarvon Basin. As a result of the breakup, the main 1984; Peters, 1986; Tissot, Pelet, & Ungerer, 1987). The unconformity in the Beagle and Dampier Sub-basins is seen value of Tmax increases with thermal maturation when as a contact between the Callovian and Oxfordian sequences reliable values are obtained (Espitalie´ et al., 1984; Peters, (Blevin, Stephenson, & West, 1994) and between the Lower 1986; Tissot & Welte, 1984). Hunt (1996), Peters (1986) Oxfordian and Middle–Upper Oxfordian rocks (Barber, and Tissot and Welte (1984) have discussed the factors of 1994; Vincent & Tilbury, 1988) respectively. The main influence Tmax, such as type of organic matter, contami- unconformity in the Barrow and Exmouth Sub-basins is the nation and the mineral matrix. Tmax is a good indicator for boundary between the Barrow Group and the Winning maturity in type III kerogen derived from terrestrial plants Group (Valanginian to early Late Cretaceous) (Polomka (Tissot et al., 1987) and it can be correlated to vitrinite et al., 1999; Tindale et al., 1998; Westphal & Aigner, 1997). reflectance for humic coal and type III kerogen (Espitalie´ et al., 1984; Teichmu¨ller & Durand, 1983; Tissot & Welte, 1984; Waples, 1985). However, some contaminants, such as 2.3. Post-rift phase drilling-mud additives and natural bitumen, can raise, or lower, the Tmax, depending on their composition (Hunt, Following breakup, the basin developed as a passive 1996; Peters, 1986). Recycled organic matter can alter Tmax continental margin and underwent a thermal-sag subsidence up to about 10 8C(Peters, 1986). Tmax might be suppressed phase. The Cretaceous sedimentary rocks deposited about 1 8C for each 50 HI (hydrogen index ¼ S1/S1 þ S2) immediately after breakup are dominated by marine shales unit, when HI is above 100–150 for typical type III organic with local sandstones (Hocking, 1988). From the Santonian, matter (Snowdon, 1995). calcareous sediments are the prime lithologies (Barber, In this study, we also found that the following 1982). During the Cainozoic, short-episodes of com- attributes affected the reliability of Tmax data in assess- pression, due to the convergence between Australian and ment of thermal maturity in the studied area (1) Asian plates, led to some localised structural deformation contamination by drilling mud additives, (He, Middleton, (Westphal & Aigner, 1997). The Cainozoic strata are Kaiko, Jiang, & Li, 2002), (2) suppression due to carbonate-dominated sequences in the Northern Carnarvon HI . 150, (3) natural bitumen, (4) recycled organic Basin (Hocking, 1988). matter, and (5) cavings. Abnormal Tmax data from 中国科技论文在线 http://www.paper.edu.cn

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Fig. 3. A map showing the locations of the studied wells in the Northern Carnarvon Basin (after Scott, 1992; Woodside Offshore Petroleum, 1988). the maturity trend in a well profile should be ignored 4. Absolute ages. The geological time scale is referenced in when significantly affected by these problems. The Labutis (1994) and Polomka et al. (1999). REESA (Rock-Eval Expert System Analysis) rules in 5. Thermal parameters including formation temperatures BasinMod 1D were used for filtering Rock-Eval data. (from drill stem tests (DSTs), the extrapolated bottom The Upper Triassic organic matter in this basin is type III hole temperatures (BHTs) using the Horner plot correc- kerogen and the Jurassic organic matter in the studied wells tion method and BHTs were corrected by adding 10% of is mainly type III kerogen (Fig. 4). The relationship between the raw BHT value to derive an ‘approximate equili- Ro versus Tmax by He et al. (2002), as an average conversion brium temperature’ (Horstman, 1988)), seabed tempera- trend of two maturity data sets (Table 1), has been used to tures (the current seabed temperatures were based on the convert Tmax data into Ro values in BasinMod 1D software studies by Horstman (1988) and Pickard and Emery mainly for the Triassic Mungaroo Formation and the (1982)), default thermal conductivity and heat capacity. Jurassic sequence in the selected wells. Some present-day thermal parameters of the selected wells as shown in (Table 2). 6. Erosional thickness base on seismic and well-log data. 3.3. Input data for the modelling 7. Palaeobathymetry and sea-level variations based on the studies of Bradshaw et al. (1988), Haq, Hardenbol 1. Measured thermal maturity including Ro, Eq VR and and Vail (1987) and Kaiko and Tait (2001). Rock Eval Tmax. 2. Stratigraphic thickness based on well completion reports. In addition, the porosity–depth relationship for decom- 3. Percentages of five lithologies (sandstone, siltstone, paction correction of Falvey and Middleton (1981), used in shale/claystone, limestone and coal). BasinMod 1D, was employed to model burial histories. Table 1

Relationship between Rock-Eval Tmax values and vitrinite reﬂectance data related to type III kerogen in the Northern Carnarvon Basin

R0 (%) 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.35 1.5 1.8 2.0 Tmax (8C) 420 428 434 436 438 439 443 447 453 463 471 484 508 中国科技论文在线 http://www.paper.edu.cn

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Fig. 4. Plots of atomic H/C versus atomic O/C and hydrogen index versus Tmax showing type III kerogen. (A) showing atomic H/C versus atomic O/C in Jurabi- 1; (B), (C) and (D) showing hydrogen index versus Tmax in Bowers-1, Anchor-1 and Madeleine-1; (E) showing hydrogen index versus Tmax for the Triassic (Tr.) organic matter in North Gorgon-1, Investigator-1 and Jupiter-1. 中国科技论文在线 http://www.paper.edu.cn

1080 S. He, M. Middleton / Marine and Petroleum Geology 19 (2002) 1073–1088

Table 2 Some present-day thermal parameters of the selected seven wells in the Northern Carnarvon Basin

Well name Geothermal gradient Sea ﬂoor temperature Heat ﬂow Water depth Subdivision (8C/100 m) (8C) (mW/m2) (m)

Jurabi-1 3.50 23 52.1 60 Exmouth Sub-basin Bowers-1 3.50 21 55.3 133 Barrow Sub-basin Anchor-1 3.30 24 53.6 18 Barrow Sub-basin Madeleine-1 2.90 23 48.2 69 Dampier Sub-basin North Gorgon-1 3.60 20 57.1 215 Rankin Platform Investigator-1 3.40 5.5 56.8 841 Exmouth Plateau Jupiter-1 3.20 5 54.3 960 Exmouth Plateau

4. Methods for calculations of heat flow, geotemperature driven flow and anomalous heat intrusion are less and thermal maturity influential. In our study, 1D models are used where the heat flow is transferred by vertical conduction, and lateral The thermal history in a sedimentary basin is governed influences are neglected. Both rift heat flow and constant by heat flow from the mantle, radiogenic heat produced in heat flow models can be evaluated by comparing observed the crust, and regional water flow (Allen & Allen, 1990; and produced maturity data. Lachenbruch, 1970). The increase in heat flow during In BasinMod 1D software, two basic assumptions for rifting is related to the lithospheric thinning, which heat flow histories can be adopted: (1) steady state, a influences heat entering the basin from the asthenosphere. constant heat flow over time, and (2) non-steady state, a The principal mode of heat transfer in sedimentary rocks variable heat flow over time such as a rift heat flow is by thermal conduction, determined by sediment model. BasinMod 1D provides a modified Jarvis and lithology, porosity and nature of pore fluids. Convection McKenzie (1980) algorithm to calculate rift heat flow. thermal effects related to compaction-driven and gravity- The rift heat flow model, which incorporates a higher heat

Fig. 5. Jurabi-1 thermal maturity modelling showing the measured maturity data and calculated maturity curves (A). Also showing the rift heat flow profile (B). Curve 1: modelled maturity curve obtained from the rift heat flow history; Curve 2: calculated maturity curve using the constant heat flow history (52.1 mW/m2 and a seabed temperature of 23 8C). Fm: formation. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 6. Bowers-1 thermal maturity modelling showing the observed maturity data and modelled maturity curves (A). Also showing the rift heat flow model (B). Curve 1: modelled maturity curve obtained from the rift heat flow history; Curve 2: calculated maturity curve using the current heat flow of 56.0 mW/m2 and seabed temperature of 20 8C. Ro%(1) and Ro%(2) were measured by Robertson Research Australia Pty. Ltd and Keiraville Konsultants, respectively. Fm: formation.

flow episode during the rift phase and an exponential 5. Heat flow and thermal maturity modelling reduction during the post-rift phase (McKenzie, 1978), has been introduced as a fundamental consideration for the 5.1. Jurabi-1 thermal modelling relationship between thermal history and tectonic evolution. Allen and Allen (1990) indicate that the heat flow The Jurabi-1 well is located on the southwestern flank of in an active rift (syn-rift) is about 63–110 mW/m2 with an the Muiron Anticline in the eastern Exmouth Sub-basin average heat flow value of 80 mW/m2, and in a thermally (Fig. 3). The boundary between the Upper Jurassic strata and subsiding rift (post-rift) is about 37–66 mW/m2 with an Muderong Shale is the main unconformity. Whilst the average value of 50 mW/m2. Lysak (1992), who investi- Barrow Group is absent at the well location, we interpret that gated heat flow variations versus structural positions in several hundred metres were eroded during the breakup in the active continental rift systems, found that the highest heat Valanginian, based on seismic and sonic log data. Never- flow is distributed along subsiding grabens, active fault theless, there are more than 1 km Cretaceous–Cainozoic zones and around volcanic centres, while heat flow on rift sedimentary rocks in this well. Three formation temperatures shoulders is generally lower than the axial graben system. were determined from BHTs using Horner plots. The current BasinMod 1D provides the chemical kinetic model of heat flow of 52.1 mW/m2 is calculated from the formation Sweeney and Burnham (1990) and the default values of temperatures with a seafloor temperature of 23 8C. kinetic parameters. The calculated vitrinite reflectance by The maturity profiles in this well with Ro (measured by the model ranges from 0.3 to 4.5%. A good agreement A.C. Cook, 1982) from side-wall cores and Tmax data between observed and calculated Ro values and tempera- (measured by PGA Consultants, 1992 and Geotech, 1995) tures imply that the 1D model thermal history may are shown in Fig. 5(A). It can be seen that three of the four represent that similarly experienced by the sediments in Ro values are in the Upper Triassic sequence, one is an geological past. anomalously low value in the Lower–Middle Jurassic For our modelling, temperature was calculated using the sequence, and all the Tmax data are in the Middle and Upper transient heat flow model. Time and depth steps of 1 Ma and Jurassic sequence. Note that some Tmax values, between 100 m, respectively, were used. The method to calculate 1500 and 2400 m with HI ranging from 168 to 314, are temperature takes into account the thermal conductivities likely to be suppressed about 1–3 8C, according to the study and heat capacities of the lithologies. of Snowdon (1995). The apparently high Tmax data from 中国科技论文在线 http://www.paper.edu.cn

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Fig. 7. Investigator-1 thermal maturity modelling showing the observed maturity data and modelled maturity curves (A). Also showing the rift heat flow model (B). Curve 1: calculated maturity curve obtained from the rift heat flow history; Curve 2: calculated maturity curve using the current heat flow of 57.1 mW/m2 and seabed temperature of 5.5 8C. Fm: formation.

1305 to 1470 m are possibly related to recycled organic sequence, but there is no palynological evidence that matter. Thus, Tmax is not as reliable as non-anomalous indicates the existence of the Middle–Upper Jurassic strata. vitrinite reflectance, as a maturity indicator, in this well. In this case, two formation temperatures were derived from The thermal modelling, using the current heat flow Horner-plot corrections and a seafloor temperature is 21 8C. (projected back into the past), does not give a match The calculated current heat flow is 55.3 mW/m2.The between the observed maturity profile and calculated vitrinite reflectance values (measured by Robertson maturity curve (Fig. 5(A)). A good fit to the maturity data, Research Australia Pty. Ltd and Keiraville Konsultants, however, was achieved from a rift heat flow model, with 1989), Tmax data (measured by Robertson Research Australia 600 m erosion at the main unconformity and exponentially Pty. Ltd and Analab, 1982) and Eq VR values (supplied by decreasing heat flow to the present (Fig. 5(B)). The CSIRO Petroleum) are plotted in Fig. 6(A). It can be seen that modelled maturity curve preferably links the Tmax data in the Ro data from Triassic rocks, obtained from different the Middle–Upper Jurassic with the Ro data in the lower laboratories, both support high thermal maturity, which is Jurassic and Upper Triassic (Fig. 5(A)). The proposed syn- also consistent with the Tmax data. Most Ro data (measured by rift phase in this sub-basin started at 190 Ma and terminated Keiraville Konsultants, 1989) in the Jurassic to Cretaceous 2 at 136 Ma. The maximum heat flow of about 67 mW/m is formations are too low to be modelled even with a constant 29% greater than the current heat flow. present-day heat flow. The best match, between the measured valid maturity data (Tmax and Ro) and the calculated maturity 5.2. Bowers-1 thermal modelling curve, was again obtained from a rift heat flow model. The model assumes that the syn-rift phase began at 190 Ma and The Bowers-1 well was drilled on a faulted anticlinal ended at 136 Ma, which is when the maximum rift heat flow structure on the eastern flank of the Alpha Arch in the Barrow of 105 mW/m2 occurred (Fig. 6(B)). The modelled maxi- Sub-basin (Fig. 3). The well contains a thin Jurassic mum heat flow is 88% higher than the present-day heat flow. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 8. Jupiter-1 thermal maturity modelling showing the observed maturity data and modelled maturity curves (A). Also showing the rift heat flow model (B). Curve 1: calculated maturity curve obtained from the rift heat flow history; Curve 2: calculated maturity curve using the constant heat flow history (54.3 mW/m2 and a seabed temperature of 5 8C). Fm: formation.

5.3. Investigator-1 thermal modelling Fig. 7(B), and the modelled result is shown in Fig. 7(A). The rift thermal model matches the measured Ro and Tmax The Investigator-1 well was drilled in the southern profiles in the Triassic and Jurassic, but observed values are Exmouth Plateau. The depositional and tectonic evolution lower than those produced for the Barrow Group interval. of the Exmouth Plateau have been discussed by many This suggests that the measured Ro values in the Barrow authors (Barber, 1982, 1988; Exon & Colwell, 1994; Group should be inferred to be suppressed or incorrect. The Tindale et al., 1998). Based on the palynological analyses syn-rift phase is assumed to commence in the earliest in the well-completion report, there is a condensed Jurassic Jurassic (208 Ma) and continue to breakup (136 Ma). The interval (188 m) and a thick succession of the Barrow Group modelled maximum heat flow, which subsequently decayed (1619 m). The Lower–Middle Jurassic interval is probably exponentially to the present-day value, was determined to 2 not completely represented, and is only 44 m thick. The be 78 mW/m , and was 37% higher than the present heat 2 formation temperatures are based on one Horner-plot- flow of 56.8 mW/m . corrected temperature of 97.2 8C at 3592 m, and two corrected BHTs (raw BHT þ 10%). The maturity profiles 5.4. Jupiter-1 thermal modelling with vitrinite reflectance (measured by A.C. Cook, 1979) and the Tmax data (measured by BHP, 1993) are shown in The Jupiter-1 well was drilled on an upthrown block of a Fig. 7(A). There is an obvious break in the Ro profile at fault half-graben on the central Exmouth Plateau. The well about 3000 m, and there are no Tmax data in the Barrow has a condensed Upper Jurassic interval of 15 m from the Group. As a result of the breakup in the Valanginian, the Callovian to the Tithonian. The Lower–Middle Jurassic unconformity is comprised of a contact between the Barrow sequence and Barrow Group are absent. The three suites of Group and the Muderong Shale (Tindale et al., 1998). The the measured maturity data include vitrinite reflectance two segments of the Ro profile are unlikely to be caused by a (measured by A.C. Cook, 1979 (also from Cook, Smyth, & discontinuous thermal effect between the Jurassic sequence Vos, 1985) and Amdel, 1996), Eq VR (CSIRO Petroleum) and the Barrow Group resulting from erosional period and Tmax data (measured by Geotech, 1996) within the during the Middle Jurassic. The model, based on the current Mungaroo Formation of the Middle–Upper Triassic heat flow of 56.8 mW/m2 (seafloor temperature ¼ 5.5 8C) (Fig. 8(A)). The formation temperatures are all Horner- back in time, is inconsistent with the measured Ro values plot-corrected BHTs. It is difficult to get a fit between the (Fig. 7(A)). A rift palaeoheatflow scenario is shown in observed maturity data and modelled maturity curve, based 中国科技论文在线 http://www.paper.edu.cn

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Fig. 9. Anchor-1 thermal maturity modelling showing the measured maturity data and the calculated maturity curve using the constant heat ﬂow history and seabed temperature. Fm: formation.

on the present heat flow of 54.3 mW/m2 (seabed maximal thermal regime is the strongest influence on temperature ¼ 5 8C) in this well (Fig. 8(A)). A good fit organic matter maturity in the well location (Fig. 9). between the observed and the calculated maturity for this well is obtained using the rift heat flow model (Fig. 8(A) and (B)). The peak thermal influence during rifting on this 5.6. Madeleine-1 thermal modelling well location is considered to be from 208 to 155 Ma. The modelled maximum value of the rift palaeoheatflow was The Madeleine-1 well was drilled in the Dampier Sub- 2 about 72 mW/m , which exponentially decreased to the basin (Fig. 3). In this case, six formation temperatures are present value during the post-rift phase. The peak heat flow corrected from the measured well temperatures value is about 33% increase from the present heat flow. (BHT þ 10%). A current heat flow of 48.2 mW/m2 (seafloor temperature ¼ 23 8C) is estimated. Most values 5.5. Anchor-1 thermal modelling of the two sets of vitrinite reflectance data (measured by Robertson Research Australia Pty. Ltd and Woodside The Anchor-1 well was drilled in the southern margin Petroleum) from core and cutting samples fall significantly of the Barrow Sub-basin. In this case, six formation lower than the calculated maturity curve for the constant temperatures are corrected from the BHTs (raw current heat flow model. 2 BHTs þ 10%). A current heat flow of 53.6 mW/m is Tmax data (measured by Robertson Research Australia estimated, with a seabed temperature of 24 8C. The Pty. Ltd and Woodside Petroleum) from both core and vitrinite reflectance values (measured by Robertson cutting samples can be used to evaluate the thermal Research Australia Pty. Ltd) and the Tmax data (measured maturity trend in this well (Fig. 10). The Tmax data with by Robertson Research Australia Pty. Ltd, BHP and S1 # 1 used for the thermal modelling was examined AGIP) are shown in Fig. 9. Both Ro and Tmax data can be using the REESA rules. The constant current heat flow matched with the present heat flow and seabed tempera- model provides a good match to the relatively reliable ture. This modelled result suggests that both Ro and Tmax Tmax data (Fig. 10). The fact that the present-day heat values can be used to evaluate the thermal maturity trend flow model adequately matches the Tmax data suggests in this well and also suggests that the present day that the thermal effect due to present-day burial has 中国科技论文在线 http://www.paper.edu.cn

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Fig. 10. Madeleine-1 thermal maturity modelling showing the observed maturity data and modelled maturity curve obtained from the constant heat ﬂow history

(the present-day heat ﬂow and seabed temperature). Ro%(1): Ro data from core and cutting samples were measured by Robertson Research Australia Pty. Ltd; Ro%(2): Ro data from core samples were measured by Woodside Petroleum. Winning: Winning Group (Valanginian to early Late Cretaceous). Fm: formation.

Fig. 11. North Gorgon-1 thermal maturity modelling showing a fit to the measured maturity data indicating that the maturity data is correspondence with the current heat flow of 56.8 mW/m2 and seafloor temperature 20 8C irrespective of a missing section of the Upper Triassic and Lower Jurassic from 0 to 3200 m. Fm: formation. 中国科技论文在线 http://www.paper.edu.cn

1086 S. He, M. Middleton / Marine and Petroleum Geology 19 (2002) 1073–1088 exceeded any earlier or significant syn-rift thermal effects 72 mW/m2 in Jupiter-1, and 78 mW/m2 in Investigator-1. as noted by Kaiko and Tingate (1996). The study also indicates that syn-rift thermal perturbations associated with lithospheric thinning and extension for 5.7. North Gorgon-1 thermal modelling most of the wells in the sub-basins have been overprinted by higher temperature during the Later Cretaceous and The North Gorgon-1 well was drilled in the northern part Cainozoic burial. of the Gorgon structure of the southern part of the Rankin In general, the reliable Tmax data are consistent with the Platform (Fig. 3), a relatively shallow structure, which non-anomalous (non-suppressed) Ro data and Eq VR data. forms the western boundary of the Barrow–Dampier Sub- Our study indicates that the reliable Tmax data are important, basins. The Gorgon structure is a series of northeast– and useful, to evaluate thermal maturity. This study shows southwest trending, en-echelon horst blocks of the Rankin the correlation developed between Tmax and Ro in the Platform. The horst blocks consist of Triassic rocks. During detailed thermal modelling of individual wells and also the Early–Middle Jurassic the structure underwent uplift indicates that a good, reliable correlation between Tmax and and erosion associated with syn-rift faulting. Structural Ro may go some way towards overcoming the problems models by Stein (1994) show that some parts of the Rankin associated with anomalously low vitrinite reflectance. Platform have experienced more than 1.5 km of rift-related However, it should be noted that the relationship between uplift. Based on missing successions in the well data and Ro and Tmax, used in this study, may result in ^3–10% error interpreted seismic lines from AGSO, the Lower Jurassic for the calculated vitrinite reflectance. and the upper part of the Mungaroo Formation are missing and the erosional thickness is estimated to be about 1500– 2000 m. Formation temperatures here are based on DST Acknowledgements temperatures. The maturity profiles, with Ro data (measured by Keiraville Konsultants) and Tmax data (measured by We wish to extend our thanks to the Department of Amdel) in Fig. 11, indicate no obvious break at the Minerals and Energy of Western Australia for the data boundary between the Mungaroo Formation and Barrow supplied; to Platte River Associates, INC. for the academic Group. The measured maturity data can be matched by our loan of BasinMod software package to support this study; model, which assumes a constant current heat flow of and to AGSO, CSIRO Petroleum and BHP Petroleum for the 57.1 mW/m2 (seafloor temperature ¼ 20 8C), irrespective of supply of data. We would like to thank Drs Lindsay Collins, the missing section of the Upper Triassic and Lower Jurassic John Kennard and Alan Tait for their kind help. We would from 0 to 3200 m. This case also illustrates that the graben like to thank Drs Peter Tingate and Alex Kaiko and two shoulder has experienced higher heat flows, associated with anonymous reviewers for their reviews and discussions. We Cretaceous and Cainozoic burial, than in the syn-rift phase. wish to thank Professor David G. Roberts for his guidance.

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