Regional Geology of the Arafura Basin

Regional Geology of the ARAFURA Basin

Basin Outline

Introduction

The Arafura Basin is located on the northern margin of Australia in the Arafura Sea and extends from the onshore Northern Territory to the Australian–Indonesian border (Figure1). It is located in mostly shallow water, with a maximum depth of 230m. The region is under-explored with no commercial discoveries; nine wells have been drilled within the Goulburn Graben but the main depocentre to the north (Figure2) has not been tested.

Basin Summary

The northern margin of Australia is structurally complex and contains three partially overlapping basins; the McArthur Basin (Paleoproterozoic–Mesoproterozoic), the Arafura Basin (Neoproterozoic–Paleozoic) and the Money Shoal Basin (Mesozoic–Cenozoic) (Figure1 and Figure3). The Australian part of the Arafura Basin extends north from the onshore McArthur Basin and covers an area of approximately 200,000km2. The basin contains up to 15km of late Neoproterozoic (Cryogenian–Ediacaran) to Paleozoic (Cisuralian) sediments, overlain by up to 4km of Mesozoic (Early Jurassic) to Cenozoic sediments of the Money Shoal Basin.

The following regional petroleum geology is compiled from a Geoscience Australia study of the Arafura and Money Shoal basins (Earl, 2006; Struckmeyer, 2006a, b; 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).

Basin Evolution and Tectonic Development

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 Arafura Basin consists of a southern and northern part, divided by the Goulburn Graben (Figure1).

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, with subsequent contraction in the Triassic. The combination of the thick sedimentary succession and large inversion structures focused hydrocarbon exploration on this region.

The areas of the Arafura Basin to the north and south of the Goulburn Graben were either not affected by the aforementioned 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 thicker sediments may be present. 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 pre-Cambrian basement (Miyazaki and McNeil, 1998).

The subsidence history of the Arafura Basin has been episodic, with periods of basin-wide subsidence in the Neoproterozoic, Cambrian (Series 2)–Early Ordovician, Late Devonian and Pennsylvanian–Cisuralian (late Carboniferous–early Permian), separated by long, relatively quiescent periods of non-deposition and erosion (Figure4).

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 Cambro-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. 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 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 (Goulburn Graben) Stratigraphy

The oldest succession in the Arafura Basin is the Neoproterozoic (Cryogenian–Ediacaran) Wessel Group (Figure4), 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 10km 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 2)–Early Ordovician Goulburn Group (Bradshaw et al, 1990; Nicoll et al, 1996). The Goulburn Group has a sag- to sheet-like geometry overall and reaches a maximum thickness of about 2,500m. The Goulburn Group represents prolonged deposition on a shallow marine shelf. The basal unit is the middle Cambrian (Series2) Jigaimara Formation (Nicoll et al, 1996), a shallow marine limestone, shale and dolomite succession. It is overlain by the largely dolomitic ?middle Cambrian (Series 3)–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 (Figure4). It has a sheet-like geometry and reaches a maximum thickness of approximately 1,500m. The Arafura Group 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 sediments are approximately equivalent in age to the Kulshill Group of the Bonaparte Basin (Figure4). However, the basal ~100m of this succession intersected in Tasman1 contains palynomorphs that are indicative of the D. birkheadensis to S. ybertii biozones (Esso Australia Ltd, 1983), which places these carbonates within the late Carboniferous (Mississippian–Pennsylvanian). This carbonate unit is referred to as an ‘unnamed carbonate’ on Figure4, and is age equivalent to the Aquitaine Formation in the Petrel Sub-basin.

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 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 the Kulka1 and Money Shoal1, is interpreted on the basis of seismic and magnetic data (Struckmeyer, 2006b).

Regional Hydrocarbon Potential

No commercial discoveries have been made in either the Money Shoal Basin or Arafura Basin. However, numerous hydrocarbon indications in wells drilled in the Goulburn Graben. Some of the most significant oil shows that occur throughout Paleozoic reservoirs were intersected in Arafura1, and pervasive oil indications occur in Goulburn1. Tasman1 encountered an oil show in an unnamed Carboniferous carbonate, and Kulka1 discovered an oil show in the Kulshill Group. Chameleon1, Cobra1A, Money Shoal1 and Tuatara1 all contain oil indications in Mesozoic and Paleozoic reservoirs. A review of available geological data (Earl, 2006; Struckmeyer, 2006a, b) 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.

Source Rocks

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

Samples from the Cambro-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). 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). This suggests that the effective source rock in the Arafura Basin is likely to occur in the Jigaimara Formation, which is an age equivalent of the Thorntonia Limestone in the Georgina Basin. 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).

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. However, expulsion pre-dates the Triassic Fitzroy Movement and potential trap formation resulting 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 in the northeasternmost corner of Release Area NT11-1 (Figure5).

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 subsidence of the Money Shoal Basin (Figure6).

Good to very good potential source rocks are present in the Permo-Carboniferous Kulshill Group. 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 at 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.

Reservoir Rocks

Potential reservoir rocks in the Arafura Basin 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 Group. The Goulburn Group dolomite hosts an oil show and gas indication in Arafura1 and oil indications in Goulburn1 (Figure4). The unit has a maximum porosity of 7.7% that relies on the development of secondary porosity through 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 and indications in Arafura1 and Goulburn1, with the better quality reservoir occurring at 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 encountered an oil show in an unnamed Carboniferous carbonate, and Kulka1 recorded an oil show 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).

Seals

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 Cambro-Ordovician Goulburn Group may also provide a seal for adjacent carbonate reservoirs, and Permo-Carboniferous dolerite sills such as that intersected in Kulka1 may provide localised seals.