Area NT07-2

Money Shoal Basin

Basin Geology

Regional tectonic setting

This summary of the regional petroleum geology draws heavily on a recently completed Geoscience Australia study of the Arafura and Money Shoal basins (Earl, 2006; Struckmeyer, 2006a, b; Totterdell, 2006). The greater part of Area NT07-2 is underlain by the Mesozoic to Cenozoic Money Shoal Basin (Figure 2) which directly overlies the offshore continuation of the Pine Creek Inlier, a Palaeoproterozoic orogenic province containing a range of sedimentary, metamorphic and igneous rocks (Carson et al, 1999). The Money Shoal Basin is a mainly offshore basin and is up to 4.5 km thick. In the west, the basin is bounded by the Lynedoch Fault System, which separates the Money Shoal Basin from the Calder and Malita graben of the Bonaparte Basin. In the east, a Mesozoic hinge separates the Money Shoal Basin from the Carpentaria Basin. The southern basin boundary is defined by the depositional edge of Mesozoic to Recent sediments. The northern part of the basin extends beyond the Australian–Indonesian boundary. The basin thins rapidly eastwards but, in the release area, the thickness of the Money Shoal Basin typically ranges between 2.5 and 4.5 km.

In the southeastern part of the release area, the Money Shoal Basin is underlain by the Goulburn Graben of the western Arafura Basin (Figures 2 and 3), a Neoproterozoic to Permian intra-cratonic basin. The Goulburn Graben is a highly deformed, obliquely inverted rift that formed during a Late Carboniferous–Permian extensional event, and underwent contractional deformation during the Triassic. It contains up to 10 km of Proterozoic–Permian sedimentary section.

Basin evolution

The subsidence history of the Arafura Basin has been episodic, with periods of basin-wide subsidence in the Neoproterozoic, Middle Cambrian–Early Ordovician, Late Devonian and Late Carboniferous–Early Permian, separated by long, relatively quiescent periods of non-deposition and erosion (Figure 4). Deposition in the Arafura Basin commenced in the Neoproterozoic during a period of upper crustal extension. Northwest–southeast oriented extension resulted in the formation of northeast–southwest-trending half graben across much of the basin (Totterdell, 2006). Subsequent periods of subsidence in the Cambro–Ordovician and Late Devonian probably were the result of regional-scale stresses, generated by plate-margin events or thermal processes. Subsidence in the Late Carboniferous–Early Permian was driven by northeast–southwest directed extension, which was localised in the Goulburn Graben. Seismic data suggest that this extensional deformation was focused along a northwest–southeast oriented highly deformed zone within the Pine Creek Province. Prior to the Triassic, the basin underwent little deformation, and the entire Neoproterozoic to Permian succession appears to be structurally conformable.

During the Triassic, the Goulburn Graben underwent contractional, probably transpressional, deformation characterised by inversion on pre-existing faults, folding, uplift and the formation of thin-skinned thrust faults. This event is considered to be equivalent to the Middle–Late Triassic Fitzroy Movement (Forman and Wales, 1981), which affected the Canning Basin and adjacent regions, including the Bonaparte Basin (Colwell et al, 1996). Deformation was largely focused on the Goulburn Graben, but the rest of the basin was affected to a lesser extent. Erosion following the Triassic deformation eventually resulted in the development of a peneplain across the basin. During this period of erosion, the basin was affected by a minor extensional episode resulting in relatively small displacement planar normal faults in the upper part of the pre-Triassic section.

The Money Shoal Basin contains a sedimentary succession (Figure 4) equivalent to that of the Bonaparte Basin to the west (eg Mory, 1988, 1991; McLennan et al, 1990; Miyazaki and McNeil, 1998). However, the Money Shoal succession is thinner and less complete than that of the Bonaparte Basin because it consists of the proximal onlap edge of the Mesozoic to Cenozoic succession. In the release area, the thickness of the Money Shoal Basin ranges between about 2.5 and 4.5 km (1900–2800 ms two-way time, Figure 5). The basal sediments are Early Jurassic in age and onlap the regional angular unconformity of probable Triassic age. Although the Triassic event resulted in the formation of a peneplain across the region, it is likely that some topography remained, facilitating initial deposition of the Troughton Group equivalent. In the Bonaparte Basin, the Late Jurassic was characterised by Oxfordian to Tithonian rifting events that led to the formation of the Malita Graben, Calder Graben and Vulcan Sub-basin (eg Pattillo and Nicholls, 1990; Longley et al, 2002). In the region of the release area, this is reflected in relatively small-scale normal faulting along the boundaries of the Goulburn Graben, particularly along the southern boundary (Figure 6). These faults are likely to be reactivated Late Carboniferous faults, which controlled sedimentation during the Late Jurassic to Early Cretaceous. The Jurassic faults underwent further, compressional reactivation in the Neogene resulting in both small- and large-scale anticlinal features (Figure 6). Figure 7 shows the distribution and thickness (in milliseconds two-way time) of the Jurassic to Early Cretaceous succession and illustrates that the depocentres align along the western Goulburn Graben and continue northwest towards the Calder Graben.


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Arafura Basin (Goulburn Graben) stratigraphy

The oldest succession in the Arafura Basin is the Neoproterozoic Wessel Group (Figure 4), which outcrops onshore (Plumb and Roberts, 1992; Rawlings et al, 1997), and is present throughout the offshore extent of the basin. Offshore, the fill of the basal half graben and the overlying post-rift succession are interpreted as belonging to the Wessel Group (Figure 3). Onshore, the group consists mainly of shallow marine sandstone and mudstone, with lesser conglomerate and carbonate rocks (Plumb and Roberts, 1992; Rawlings et al, 1997). The age of the Wessel Group is poorly constrained, but limited radiometric data and stratigraphic constraints suggest that it is Neoproterozoic (Rawlings et al, 1997). The group reaches a maximum thickness of approximately 10 km in the central part of the basin, northeast of the Goulburn Graben, but is likely to be thinner in the graben itself.

The Wessel Group is overlain disconformably by the early Middle Cambrian–Early Ordovician Goulburn Group (Bradshaw et al, 1990; Nicoll et al, 1996; Figure 4). The Goulburn Group has a sag- to sheet-like geometry overall and reaches a maximum thickness of about 2500 m. The Goulburn Group represents prolonged deposition on a shallow marine shelf. The basal unit is the early Middle Cambrian Jigaimara Formation (Nicoll et al, 1996), a shallow marine limestone, shale and dolomite succession. It is overlain by the largely dolomitic ?Middle Cambrian–earliest Ordovician Naningbura Formation (Nicoll et al, 1996). The Early Ordovician marine shelf mixed carbonate and clastic rocks of the Milingimbi and Mooroongga formations form the uppermost units of the Goulburn Group.

The Late Devonian Arafura Group (Petroconsultants, 1989; Bradshaw et al, 1990; McLennan et al, 1990) overlies the Goulburn Group (Figure 4). The Arafura Group has a sheet-like geometry overall and reaches a maximum thickness of approximately 1500 m. The Arafura Group consists of shallow marine to non-marine interbedded mudstone, siltstone, sandstone and minor carbonate. The basal unit is the Frasnian Djabura Formation, a dominantly shallow marine succession of interbedded clastics and minor limestone. It is overlain unconformably by the interbedded clastics of the ?Frasnian–Famennian Yabooma Formation (Bradshaw et al, 1990), which is also interpreted to represent dominantly shallow marine deposition. The overlying Famennian Darbilla Formation is a mudstone and siltstone dominated succession interpreted to have been deposited in a largely non-marine environment (Petroconsultants, 1989; Bradshaw et al, 1990).

The Arafura Group is overlain unconformably by a Late Carboniferous–Early Permian succession that is approximately equivalent in age to the Kulshill Group of the Bonaparte Basin (Figure 4). Well intersections of the Kulshill Group consist of non-marine to marginal marine interbedded sandstone, siltstone and claystone, with minor coal and dolomitic rocks. In the Goulburn Graben, where the lower part of the section comprises an extensional growth wedge, the Kulshill Group is up to 5 km thick. The upper part of the succession represents post-rift deposition.


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Money Shoal Basin stratigraphy

The Money Shoal Basin unconformably overlies the Arafura Basin succession and comprises Jurassic to Cretaceous siliciclastic sediments and Cenozoic carbonates that thin rapidly towards the east (Figure 5). The oldest unit, the Troughton Group, is represented only by its youngest component, the Plover Formation, which directly overlies the Triassic regional unconformity. The oldest sediments, intersected at Tuatara 1 (Figure 8), are of late Early Jurassic age (C. torosa to C. turbatus spore/pollen zone). The upper boundary of the unit is defined by the regional Callovian unconformity (cal). Deposition of the Troughton Group occurred mostly in the western Goulburn Graben region where it is up to 564 m thick at Tuatara 1. It thins rapidly to the north and east and is absent in wells of the eastern Goulburn Graben.

The Plover Formation comprises fine- to coarse-grained sandstones with interbedded siltstones and claystones, and minor coal. A generally blocky to serrate gamma log character, the absence of marine microfossils and the presence of coal all indicate an overall fluvial depositional environment. Barber et al (2004) suggested that a series of braided river systems fed into a wide, northeast–southwest-trending marine shelf, with the Goulburn Graben the focus of one of these ‘trunk rivers’. Lowe-Young et al (2004) postulated increasing marine influence in the upper Plover Formation in the Evans Shoal area to the west of the study area. Seismically, the Plover Formation is characterised by moderate to high amplitude, moderately continuous, parallel reflections (Figure 6), but the sequence geometry often is below seismic resolution.

The overlying Flamingo Group ranges in age from Callovian (upper W. digitata dinoflagellate zone) to Hauterivian (M. testudinaria dinoflagellate zone). The base of the Flamingo Group is defined by the Callovian unconformity (cal), which also marks the commencement of a minor extensional event. This is reflected in the clear increase in thickness of the lower Flamingo Group across a reactivated boundary fault of the Goulburn Graben (Figure 6). A clastic unit of Hauterivian age, equivalent in age to the Darwin Formation, is here included in the Flamingo Group rather than the Bathurst Island Group as defined by Hughes (1978) and Mory (1991), because a major Barremian to late Aptian hiatus observed in all wells of the Goulburn Graben is interpreted to represent the boundary between two major depositional cycles. In the Bonaparte Basin, the age equivalent of this unit is generally known as the Echuca Shoals Formation (eg Pattillo and Nicholls, 1990; Whittam et al, 1996).

In the Bonaparte Basin, the Flamingo Group is characterised by a condensed section of open marine strata consisting mostly of mudstones (eg Longley et al, 2002). At Tuatara 1, the unit is mudstone-rich with a strong marine influence, but it becomes increasingly sand-prone towards the east, reflecting deposition in mostly fluvio-deltaic environments. Similar to the Troughton Group, the Flamingo Group is thickest in the western Goulburn Graben region (up to 1230 m at Money Shoal 1), thins rapidly to the north and east, and is absent in wells of the eastern Goulburn Graben. A number of erosional surfaces and flooding surfaces can be identified within this unit (Figure 6). A major feature of the Flamingo Group is a fluvial channel system along the northern edge of the Goulburn Graben (Figure 6) that straddles the hanging wall of the reactivated graben-bounding fault system (Enclosure 4 of Miyazaki and McNeil, 1998). It is present along the entire length of the Goulburn Graben, but is most pronounced near Kulka 1. A distinct flooding surface (mtithflood: Figure 4) defines the upper limit of the channel fill and separates the fluvio-deltaic sediments of the lower Flamingo Group from the prograding marine deltaic deposits of the upper Flamingo Group.

The Bathurst Island Group of the eastern Money Shoal Basin is late Aptian (D. davidii dinoflagellate zone) to Maastrichtian in age. The base of the group is defined by a major unconformity of Aptian age (apt), which forms the base of a series of thick prograding packages. At the toe of the progrades, in the more distal parts of the basin, this unconformity merges with the overlying downlap surface. Overall, the unit consists of mostly fine-grained rocks including claystone, marl and siltstone, with locally thick interbeds predominantly comprising fine-grained sandstones. In the release area, it reaches a thickness of up to 2000 m (1400 ms two-way time; Figure 9), but seismic data show that it thickens further into the Calder Graben near Lynedoch 1.

The Bathurst Island Group consists of a series of stacked prograding units deposited in deltaic to open marine environments. The distribution of the group in the offshore Money Shoal Basin and the age of sediments mapped onshore (Hughes, 1978; Carson et al, 1999) suggest that the most far-reaching marine transgression occurred during the Aptian to Cenomanian. The increasing presence of planktonic foraminifera at Lynedoch 1 to the west of the release area suggests a westwards deepening marine environment. Deeper water environments are also indicated by the presence of upward fining units suggestive of turbidites, particularly in the upper part of the Bathurst Island Group. Submarine canyons and coeval deep-water deposits, such as basinward building shingled fan systems, consisting of slope fans and basin floor sands (Figure 10) are present, particularly north and south of Tuatara 1.

Sediments of the Woodbine Group are typically Late Miocene and younger in age and are generally less than 400 m thick. West of Kulka 1, where the unit includes Middle Miocene sediments, it thickens rapidly towards the Calder Graben, reaching about 800 m at Tuatara 1, and 1300 m at Lynedoch 1 to the west of the release area. The Woodbine Group was sampled in a limited number of wells only. For example, at Cobra 1A, a lower unit of probable Lower to Middle Miocene, coarse, quartzitic sandstones with claystone interbeds and minor coal and dolomite is overlain by an upper unit of Late Miocene and ?younger calcareous claystone and marl with calcarenite interbeds (BHP Petroleum, 1993). This suggests initially localised, shallow marine to deltaic sedimentation followed by more widespread open marine environments in the Late Miocene. In the release area, the Woodbine Group is generally unstructured.


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