Regional Geology of the ARAFURA and MONEY SHOAL Basins

Regional geology of the ARAFURA AND MONEY SHOAL basinS

basin outline

The Arafura and Money Shoal basinsare located on the northern margin of Australia in the Arafura Sea and extend from onshore Northern Territory to beyond the Australian–Indonesian border (Figure1). These basins are located mostly in shallow water, with a maximum depth of about 400m. The Neoproterozoic to Paleozoic Arafura Basin unconformably overlies Archean to Mesoproterozoic basement terranes (the Pine Creek Inlier in the west and the northern McArthur Basin in the east) and is overlain by the Mesozoic to Cenozoic Money Shoal Basin. The Australian part of the Arafura Basin extends north from onshore Arnhem Land and covers an area of approximately 200,000km2.The Australian part of the Money Shoal Basin covers an area of about 230,000km2 and is bounded in the west by the Lynedoch Fault System, which separates it from the Calder Graben and Darwin Shelf 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 Cenozoic sediments.

The Arafura and Money Shoal basinsare under-explored with no commercial discoveries. Of the nine petroleum exploration wells drilled in theregion, all penetrated the MoneyShoalBasin succession and seven penetrated the underlying ArafuraBasin successionin the Goulburn Graben. The main Paleozoic depocentre to the north (Figure2) has not been tested. Likewise, the main Mesozoic depocentre to the northwest, which thickens into the Calder Graben of the Bonaparte Basin (Figure3), is under-explored, with the two most basinward wells being Tuatara1 and Cobra1A.

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tectonic development

The northern margin of Australia is structurally complex and contains three partially overlapping basins; the McArthurBasin (Paleoproterozoic–Mesoproterozoic), the Arafura Basin (Neoproterozoic–Paleozoic) and the MoneyShoalBasin (Mesozoic–Cenozoic) (Figure1 and Figure4). The Arafura Basin contains up to 15,000m of Neoproterozoic (Cryogenian–Ediacaran) to Paleozoic (Cisuralian) sediments, overlain by up to 4,000m of Mesozoic (Lower Jurassic) to Cenozoic sediments of the Money Shoal Basin (Figure5).

The following regional petroleum geology is compiled from a Geoscience Australia study of the Arafura and Money Shoal basins (Earl, 2006; Struckmeyer, 2006a, 2006b; Totterdell, 2006) and earlier work by Petroconsultants (1989), Bradshaw et al (1990), McLennan et al (1990), Labutis et al (1992) and Miyazaki and McNeil (1998).

Arafura Basin

The Arafura Basin formed in the Neoproterozoic in response to northwest–southeast extension that resulted in the formation of a series of northeast–southwest-trending half graben. Structurally, the ArafuraBasin consists of a southern and northern part, divided by the Goulburn Graben (Figure1).

Goulburn Graben

The Goulburn Graben is a northwest-trending asymmetric feature, over 400km long and up to 125km wide. Seismic data show that Neoproterozoic half graben extend beyond the Goulburn Graben on both sides. The Goulburn Graben probably formed in the Pennsylvanian (late Carboniferous) to Cisuralian (early Permian) in response to oblique extension associated with the break-up of Gondwana, and was deformed during a compressional episode in the Triassic. The combination of the thick sedimentary succession and large inversion structures focused hydrocarbon exploration on this region.

Northern and Southern Platforms

The areas of the ArafuraBasin to the north and south of the Goulburn Graben were either not affected by the aforementioned Phanerozoic extension and contractional events or restructuring was minor. Hence, previously, these areas were termed the northern and southern platforms (e.g. Bradshaw et al, 1990). In the northern area, the sedimentary section is up to 15km thick, whereas in the southern area, it is up to 3km thick (Figure2). However, seismic coverage is poor in the southern region and areas with a thicker succession may be present. Due to the low degree of deformation in the northern area, it is possible that any early formed traps and associated hydrocarbon accumulations have remained intact, thus up-grading the prospectivity compared with the Goulburn Graben. The southern inshore region probably has little hydrocarbon potential with inferred thin Paleozoic sediments over a Precambrian basement (Miyazaki and McNeil, 1998).

Money Shoal Basin

The Money Shoal Basin extends across the Arafura Basin and has equivalent stratigraphy to the Mesozoic section of the Bonaparte Basin to the west (Mory, 1988, 1991; McLennan et al, 1990; Miyazaki and McNeil, 1998). The basin sediments form a stratigraphic wedge that thickens towards the west to up to 3sTWT (Figure3). The Money Shoal Basin succession is thinner and less complete than that inthe Bonaparte Basin because it comprises the proximal onlap edge of the Mesozoic to Cenozoic succession. The basal sediments are Early Jurassic in age and onlap the regional angular unconformity of Triassic age. Although the Triassic event resulted in the formation of a peneplain across the region, it is likely that some topographic relief remained in the area of the Goulburn Graben, facilitating initial deposition of the Troughton Group (Figure5).

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basin evolution

The subsidence history of the Arafura Basin has been episodic, with periods of basin-wide subsidence in the Neoproterozoic, Cambrian (Series 3)–Early Ordovician, Late Devonian and Pennsylvanian–Cisuralian (late Carboniferous–early Permian), separated by long, relatively quiescent periods of non-deposition and erosion (Figure4and Figure5).

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 a series of northeast–southwest-trending half graben across much of the basin (Totterdell, 2006). Subsequent periods of subsidence in the Cambrian–Ordovician and Late Devonian probably were the result of regional-scale stresses, generated by plate-margin events or thermal processes. Subsidence in the Pennsylvanian–Cisuralian (late Carboniferous–early Permian) was driven by northeast–southwest directed extension, which was localised in the Goulburn Graben. 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 and Bonaparte basins (Colwell et al, 1996). Deformation was largely focused on the Goulburn Graben with the rest of the basin being 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.

Arafura Basin and Goulburn Graben

The oldest succession in the ArafuraBasin is the Neoproterozoic (Cryogenian–Ediacaran) Wessel Group (Figure5), 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. Onshore, the group consists mainly of shallow marine sandstones and mudstones, with lesser amounts of conglomerates and carbonates (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 10,000m 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 Cambrian (Series 3)–Lower Ordovician Goulburn Group (Bradshaw et al, 1990; Nicoll et al, 1996; Zhen et al, 2011; Figure 5). The Goulburn Group has a sag- to sheet-like geometry overall and reaches a maximum thickness of about 2,500m. It represents prolonged deposition on a shallow marine shelf. The basal unit is the middle Cambrian (Series3) Jigaimara Formation (Nicoll et al, 1996), a shallow marine limestone, shale and dolomite succession. It is overlain by the largely dolomitic ?upperCambrian (Furongian)–lowest Ordovician Naningbura Formation (Nicoll et al, 1996; Zhen et al, 2011). The Lower Ordovician marine shelf mixed carbonate and clastic rocks of the Milingimbi and Mooroongga formations form the uppermost units of the Goulburn Group.

The UpperDevonian Arafura Group (Petroconsultants, 1989; Bradshaw et al, 1990; McLennan et al, 1990) overlies the Goulburn Group (Figure5). It has a sheet-like geometry, reaches a maximum thickness of approximately 1,500m, and consists of shallow marine to non-marine interbedded mudstone, siltstone, sandstone and minor carbonate. The oldest unit is the Djabura Formation, a dominantly shallow marine succession of interbedded clastics and minor limestone. Conodont biostratigraphy indicates an early Famennian age for the Djabura Formation (Nicoll, 2006), but palynological dating suggests that it is older (Frasnian; Purcell, 2006). It is overlain unconformably by the clastics of the ?Frasnian–Famennian Yabooma Formation, 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 Carboniferous to early Permian (Cisuralian) succession. Palynological studies by Helby (2006) have indicated that most of this succession is Cisuralian in age (G.confluens to C.alutas [Lower Stage 2 equivalent] spore-pollen zones) and that these clastic sediments are approximately equivalent in age to the Kulshill Group of the BonaparteBasin (Figure5). In Tasman1 and Kulka1, the basal part of theunit contains palynomorphs that are indicative of the D.birkheadensis to S.ybertii biozones (Esso Australia Ltd, 1983; Helby, 2006), which places it within the Carboniferous (Mississippian–Pennsylvanian).

Well intersections of the Kulshill Group equivalent 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 5km thick. The upper part of the succession represents post-rift deposition.

There is some evidence of magmatic activity in the basin during this extensional phase. Sills and dykes can be seen on seismic and one, a dolerite of Carboniferous–Permian age (Diamond Shamrock Oil Company (Australia) Pty Ltd, 1985), was intersected in Kulka1. In addition, a large magmatic body within the Goulburn Graben, in the vicinity of Kulka1 and Money Shoal1, is interpreted on the basis of seismic and magnetic data (Struckmeyer, 2006b).

Money Shoal Basin

Unconformably overlying the Arafura Basin is the Mesozoic to Cenozoic succession of the Money Shoal Basinthat thins rapidly towards the east. The detailed stratigraphy of itsJurassic to Cretaceous siliciclastic sediments and the Paleogene to Holocene carbonates can be found in the Money Shoal Release Area Geology.

regional hydrocarbon potential

No commercial discoveries have been made in either the MoneyShoalBasin or ArafuraBasin. However, there are numerous hydrocarbon indications in wells drilled in the Goulburn Graben. Some of the most significant oil shows that occur throughout Paleozoic reservoirs were intersected within the Arafura Groupin Arafura1 and Goulburn1, and within the Goulburn Group in Arafura1. Kulka1 and Tasman1 discovered oil shows in the Kulshill Group, while Cobra1A, Money Shoal1 and Tuatara1 all contain oil indications in Mesozoic reservoirs, and Chameleon1 contains oil indications in both Paleozoic and Mesozoic reservoirs. A review of available geological data (Earl, 2006; Struckmeyer, 2006a, 2006b), together with the results from a survey investigating potential hydrocarbon seepage in the Arafura Basin (Logan et al, 2006), show that the region contains not only all the required essential petroleum system elements to generate, expel and trap hydrocarbons, but also evidence that this generation and expulsion has occurred.

The following section documents the petroleum system elements for the Arafura Basin. The petroleum system elements for the Mesozoic to CenozoicMoneyShoalBasin can be found in the Money Shoal Release Area Geology.

Regional Petroleum Systems

A recent oil-source correlation study in the Georgina Basin (Boreham and Ambrose, 2005) identified three Cambrian petroleum systems related to source rocks of algal/bacterial origin. One of these, the Thorntonia(!) Petroleum System, has similar geochemical and isotopic characteristics to oil stains in early Paleozoic rocks at Arafura1 and Goulburn1 (Boreham and Ambrose, 2005; Boreham, 2006). This suggests that the effective source rock in the ArafuraBasin is likely to lie within the Jigaimara Formation, which is an approximate age equivalent of the Thorntonia Limestone in the GeorginaBasin. The presence of abundant interstitial bitumen in association with oil stains in early Paleozoic samples in Arafura1 is indicative of a multi-charge history from a prolific source nearby (Sherwood et al, 2006).

Source Rocks

In the Arafura Basin, potential source rocks occur within the Goulburn Group (Cambrian–Ordovician), the Arafura Group (Devonian) and the Kulshill Group equivalent (Carboniferous–Permian). Potential source rocks may be present within the Wessel Group, but no data are available for this section.

Samples from the Cambrian–Ordovician Goulburn Group have total organic carbon (TOC) contents up to 8.6%. The higher values represent migrated oil and solid bitumen (Keiraville Konsultants, 1984; Sherwood et al, 2006) rather than dispersed organic matter, as reported in previous publications (Bradshaw et al, 1990; Edwards et al, 1997).

Modelling by Struckmeyer et al (2006b) indicates that the major phase of hydrocarbon (light oil and gas) expulsion from the Cambrian source rock within the Goulburn Graben occurred in response to Devonian and Permo-Carboniferous subsidence (Figure6). However, this expulsion pre-dates the Triassic Fitzroy Movement and potential trap formation, which probably resulted in the loss and/or degradation of the majority of these hydrocarbons. The mapped expulsion and preservation limit of hydrocarbons from the Cambrian source rock indicates that oil may be preserved within Release Area NT12-1 and possibly within the southeasternmost corner of Release Area NT12-2 (Figure7).

Source potential for the Devonian fluvio-deltaic Arafura Group sediments is typically poor, with the exception of one lamalginite-rich sample from Arafura1 that has TOC contents of 0.85% (Sherwood et al, 2006). Modelling by Struckmeyer et al (2006b) implies that the Djabura and Darbilla formations are mature in the Goulburn Graben and northern Arafura Basin, but that expulsion only occurred where these units were buried to about 4km depth: in the case of Cobra1A, this resulted from the Money Shoal Basin subsidence (Figure8).

Good to very good potential source rocks are present in the Permo-Carboniferous Kulshill Groupequivalent. The typical TOC content ranges from <0.4% to 3% with a maximum hydrogen index (HI) of 321mgHC/gTOC. Several samples in the central Goulburn Graben have TOC contents up to 9% and comprise land plant-derived organic matter such as vitrinite, sporinite and liptodetrinite (Sherwood et al, 2006). Based on vitrinite reflectance data from Kulka1 (0.9–2.4%Ro), the Kulshill Group in the western Goulburn Graben is mature to overmature for oil generation and mature for gas generation due to loading by the Money Shoal Basin. Elsewhere in the Arafura Basin, the Kulshill Group is immature for hydrocarbon generation.

Reservoirs and Seals

Potential reservoir rocks in the ArafuraBasin include shallow marine limestones and dolomites of the Cambro-Ordovician Goulburn Group, and terrestrial to fluvio-deltaic interbedded sandstones and mudstones of the Devonian Arafura Group and Permo-Carboniferous Kulshill Groupequivalent. The Goulburn Group dolomite hosts an oil show and gas indication in Arafura1 and oil indications in Goulburn1 (Figure5). The unit has a maximum porosity of 7.7% that relies on the development of secondary features such as vugs and fractures (Earl, 2006). A risk associated with this unit is cementation reducing secondary porosity. The cementation is probably at least partly related to Triassic contraction and uplift. Siltstones and sandstones of the Arafura Group host the oil shows in Arafura1 and Goulburn1, with the better quality reservoir occurring in Goulburn1 (maximum porosity of 19% and maximum permeability of 7.83mD). A significant proportion of the primary porosity has been destroyed by diagenetic effects, including silica overgrowths and carbonate cementation. Tasman1 and Kulka1 recorded oil shows in the Kulshill Group. Although no hydrocarbons have been found within the uppermost part of the Kulshill Group, these sediments display good reservoir characteristics, averaging 5.5% porosity, with a maximum porosity of 17.7% being recorded at Tasman1. Carbonate cements are sporadic throughout the group but there is evidence of multiple fracture sets (such as at Chameleon1), which could enhance the overall permeability and porosity (Earl, 2006).

There is little information about potential Paleozoic seals; however, oil shows and indications below thick Devonian fine-grained sediments in Arafura1 and Goulburn1 attest to the sealing capacity of this unit (Petroconsultants, 1989). Oil indications above this seal in Arafura1 are the result of fault migration (Labutis et al, 1992; Earl, 2006). Mudstones at the top and base of the Cambrian–Ordovician Goulburn Group may also provide a seal for adjacent carbonate reservoirs, and Carboniferous–Permian dolerite sills, such as that intersected in Kulka1, may provide localised seals.

Timing of Generation

Peak oil generation and migration from potential Paleozoic source rocks in the Goulburn Graben, where all exploration wells are located, pre-dates the Triassic structural event and thus potential trap formation (Moore et al, 1996; Struckmeyer, 2006a, 2006b). Despite this, modelling by Struckmeyer (2006b) demonstrated that some areas in the western Goulburn Graben could have experienced a late phase of generation and expulsion from potential Paleozoic source rocks (Figure6, Figure7 and Figure8). For example, this includes the possibility of a minor phase of late expulsion of light oil from a Type I/II Cambrian source rock at Tuatara1, where the lack of success is considered to be due to an absent or inadequate seal (Earl, 2006). Struckmeyer (2006b) found that the modelling of hydrocarbons expelled from source rocks within the Devonian Arafura Group and the Carboniferous–Permian Kulshill Group was highly sensitive to the amount of Triassic erosion interpreted for any location. Bearing this in mind, expulsion of hydrocarbons appears to have occurred in the late Cenozoic from the western Goulburn Graben, including areascovered by the Release Areas NT12-1 and NT12-2 (Figure8).