PETROLEUM GEOLOGICAL SUMMARY RELEASE AREAS NT11-1 AND NT11-2, MONEY SHOAL BASIN, NORTHERN TERRITORY Bids Close – Thursday 12 April 2012 In the vicinity of giant gas accumulations in the Calder Graben, northeastern Bonaparte Basin. Close to Darwin operations base and the Wickham Point LNG plant supplied by the Bayu–Undan gas pipeline. Large under-explored Mesozoic basin. Shallow water depths (10–110 m). Numerous shallow Mesozoic stratigraphic and structural plays. Potential hydrocarbon charge from Mesozoic source rocks in the Malita and Calder graben, northeastern Bonaparte Basin. Potential hydrocarbon charge from Paleozoic source rocks in the Goulburn Graben, Arafura Basin. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 1 of 23 LOCATION Release Areas NT11-1 and NT11-2 are located about 150 km north of Darwin in the Arafura Sea. Water depths range from 10–110 m (Figure 1). The graticular block maps and graticular block listings for these Release Areas are shown in Figure 2. Release Area NT11-1 comprises 185 graticular blocks and covers an area of approximately 15,540 km2. Release Area NT11-2 consists of 136 full and part graticular blocks and covers an area of approximately 10,085 km2. Release Area NT11-1 is located in the Mesozoic to Cenozoic Money Shoal Basin (Figure 3). In the northeastern part of this Release Area, the Money Shoal Basin is underlain by the Goulburn Graben, which is part of the Neoproterozoic to Permian Arafura Basin (Link to Arafura Basin REGIONAL chapter). The eastern part of Release Area NT11-2 is located within the Money Shoal Basin whereas the western part lies on the Darwin Shelf which forms the northeastern margin of the Mesozoic to Cenozoic Bonaparte Basin (Figure 3). The Release Areas are located to the southeast of the Evans Shoal, Caldita and Barossa-Lynedoch gas accumulations in Australian waters and the Abadi gas accumulation in Indonesian waters. The Bayu–Undan to Darwin gas pipeline is located about 30 km south of Release Area NT11-2 (Figure 1). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 2 of 23 RELEASE AREA GEOLOGY Local Tectonic Setting This summary of the regional petroleum geology is compiled from a Geoscience Australia study of the Arafura and Money Shoal basins (Earl, 2006; Struckmeyer, 2006a, b; Totterdell, 2006). Release Area NT11-1 and the eastern portion of Release Area NT11-2 are underlain by the Mesozoic to Cenozoic Money Shoal Basin (Figure 3) which is a mainly offshore basin located off the Northern Territory coastline. The basin is a tilted, passive margin basin and contains sedimentary rock units that are generally monoclinal and undeformed. These sediments were deposited mostly in a marine environment with occasional incursions of deltaic and fluvial regimes. Figure 4 shows the distribution and thickness (in milliseconds two-way time) of the Jurassic to Lower Cretaceous succession and illustrates that the depocentre aligns with the western Goulburn Graben and continues northwest towards the Calder Graben. The total sedimentary section of the Money Shoal Basin is about 4.5 km thick. Figure 4 also demonstrates that the basin thins rapidly eastwards, with the base of the sedimentary section ranging in age from Middle Jurassic in the west to Late Cretaceous in the east. These sediments unconformably overlie the Arafura Basin and the offshore continuation of the Pine Creek Inlier, a Paleoproterozoic orogenic province containing a range of sedimentary, metamorphic and igneous rocks (Carson et al, 1999). The eastern Arafura Basin overlies the Paleoproterozoic– Mesoproterozoic (approximately 1815–1400 Ma) McArthur Basin (Struckmeyer, 2006b). Although the Money Shoal Basin is continuous with the Bonaparte Basin, the Lynedoch Fault System, which traverses Release Area NT11-2 (Figure 3), is used to separate the western margin of the basin from the Calder Graben and Darwin Shelf of the Bonaparte Basin (Link to Bonaparte Basin REGIONAL chapter). In the east, a Mesozoic hinge separates the Money Shoal Basin from the Carpentaria Basin. The southern boundary of the Money Shoal Basin is defined by the depositional edge of Mesozoic to Cenozoic sediments, while to the north, the basin extends into Indonesian waters. In the northeastern part of Release Area NT11-1, the Money Shoal Basin is underlain by the Goulburn Graben of the western Arafura Basin, a Neoproterozoic to Permian intracratonic basin. The Goulburn Graben is a highly deformed, obliquely inverted rift that formed during a Pennsylvanian (late Carboniferous) to Permian extensional event, and underwent contractional deformation during the Triassic (Struckmeyer, 2006b). It contains up to 10 km of Neoproterozoic to Permian sediments. The western part of Release Area NT11-2 overlies the Darwin Shelf, which is flanked by the Malita Graben of the northern Bonaparte Basin to the north and the Petrel Sub-basin to the southwest. The Bathurst Terrace is a zone of shallow basement between the Malita Graben and the Darwin Shelf that consists of a series of narrow fault blocks where Paleozoic and Mesozoic sediments are progressively truncated (Forman et al, 1974). The Darwin Shelf 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 3 of 23 is an offshore extension of the Proterozoic Sturt Block (Mory, 1991). It has been subjected to several episodes of peneplanation, and is covered by a thin veneer of Mesozoic and Cenozoic sediments which thicken northwards into the Malita Graben. Structural Evolution and Depositional History In terms of post-Triassic stratigraphy (Figure 5), the Money Shoal Basin contains a sedimentary succession equivalent to that of the Bonaparte Basin (Mory, 1988, 1991; McLennan et al, 1990; Miyazaki and McNeil, 1998). However, the Money Shoal Basin 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. The basal sediments are Early Jurassic in age and onlap the regional angular unconformity of Triassic age (Figure 6 and Figure 7). Although the Triassic event resulted in the formation of a peneplain across the region, it is likely that some topographic relief remained, facilitating initial deposition of the Troughton Group. In the Bonaparte Basin, the Oxfordian to Tithonian (Late Jurassic) was characterised by rifting events that led to the formation of the Malita and Calder graben and further west, the Vulcan Sub-basin (Pattillo and Nicholls, 1990; Longley et al, 2002). This is reflected by relatively small-scale normal faulting along the boundaries of the Goulburn Graben, particularly along the southern boundary. These faults are likely to be reactivated Pennsylvanian (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 the development of both small- and large-scale anticlinal features. 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 4, Figure 5, Figure 6 and Figure 7). The Troughton Group is the oldest unit to have been penetrated and is represented only by its youngest component, the Plover Formation (Figure 5), which directly overlies the Triassic regional unconformity. The oldest sediments, intersected at Tuatara 1, are of Early Jurassic age (C. torosa to C. turbatus spore/pollen zone). The upper boundary of the unit is defined by the regional Callovian unconformity. Deposition of the Troughton Group occurred mostly in the western Goulburn Graben region where it is in excess of 564 m thick at Tuatara 1 (Struckmeyer, 2006b). It thins rapidly to the north and east and is absent from wells (Arafura 1, Goulburn 1 and Torres 1) in 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 serrated gamma log character, the absence of marine microfossils and the presence of coal indicate an overall fluvial depositional environment. Barber et 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 4 of 23 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 rivers. Lowe-Young et al (2004) postulated increasing marine influence in the upper Plover Formation in the Evans Shoal area of the northern Bonaparte Basin. Seismically, the Plover Formation is characterised by moderate to high amplitude, moderately continuous, parallel reflections but the sequence geometry often is below seismic resolution. The Flamingo Group includes all of the sedimentary succession above the Callovian breakup unconformity and below the Valanginian unconformity (Mory, 1988, 1991). The Callovian unconformity marks the commencement of an extensional event throughout the northwest of Australia (Labutis et al, 1998; Pattillo and Nicholls, 1990; McLennan et al, 1990). However, in the northern Bonaparte Basin the Callovian tectonic event was relatively mild and its effects are even less pronounced in the Money Shoal Basin. Nevertheless, the angular unconformity truncates the Plover Formation within tilted fault blocks where it is preserved beneath the transgressive sandstones of the Elang Formation. The Elang Formation is an important reservoir in the Malita and Calder graben (Figure 5). In the Money Shoal Basin, an increase in the thickness of the lower Flamingo Group is apparent across the reactivated boundary fault of the Goulburn Graben. In the northern Bonaparte Basin, the upper Flamingo Group is characterised by a condensed section of open marine deepwater sediments consisting mostly of mudstones (Frigate Shale). The laterally equivalent coarse to finegrained clastics of the Sandpiper Sandstone provide additional reservoir targets in the region. The ‘Tuatara Formation’ intersected in Beluga 1 comprises Berriasian-aged sandstones (BHP Petroleum, 1992) and hence is now placed within the upper Sandpiper Sandstone. In the western Money Shoal Basin at Tuatara 1, the Flamingo Group is mudstone-rich with a strong marine influence, but it becomes increasingly sand-prone towards the east, reflecting deposition in mostly fluvio-deltaic environments. Like 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 (Figure 4), and is absent from wells (Arafura 1 and Goulburn 1) in the eastern Goulburn Graben. A number of erosional surfaces and flooding surfaces have been identified within this group (Struckmeyer, 2006b). A major feature of the Flamingo Group is a fluvial channel system along the northern edge of the Goulburn Graben 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 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 intra-Valanginian unconformity separates the Flamingo Group from the overlying Cretaceous Bathurst Island Group that comprises the Echuca 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 5 of 23 Shoals Formation, Darwin Radiolarite, Wangarlu Formation and Puffin Formation. The Bathurst Island Group consists of a series of stacked prograding units that thicken into the Calder Graben near Lynedoch 1. Overall, the unit consists mostly of fine-grained rocks including claystone, marl and siltstone, with locally thick interbeds predominantly of fine-grained sandstones 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 extensive marine transgression occurred during the late Aptian to Cenomanian. The increasing presence of planktonic foraminifera at Lynedoch 1 suggests a westwardly deepening marine environment. Deeper water environments are also indicated by the presence of upwardly 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 shingled fan systems, consisting of slope fans and basin floor sands are present, particularly north and south of Tuatara 1. The Cenozoic carbonate-dominated Woodbine Group thickens rapidly towards the Calder Graben, reaching about 800 m at Tuatara 1 and 1,300 m at Lynedoch 1. The initial depositional setting of shallow marine to deltaic environments was followed by more widespread open marine conditions. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 6 of 23 EXPLORATION HISTORY Petroleum exploration in the Arafura region began in the 1920s when several boreholes were drilled on Elcho Island in response to reported bitumen strandings. In the 1960s and early 1970s, stratigraphic drilling occurred on Bathurst and Melville islands (McLennan et al, 1990). During this time Shell Development (Australia) was awarded exploration permits covering the western region of the Arafura Sea and drilled the first well, Money Shoal 1 (1971) in the offshore Money Shoal Basin which also penetrated sediments of underlying Arafura Basin. This well was drilled primarily to test the Mesozoic Money Shoal Basin sequence. At the same time, Aquitaine was operating in the central southern region of the Arafura Sea. The two operators carried out extensive mapping based on seismic data and defined the Goulburn Graben as an important structural feature. The next phase of exploration occurred in the early 1980s with several wells (Arafura 1, Goulburn 1, Kulka 1, Tasman 1 and Torres 1) being drilled primarily to test the Paleozoic sequence within the Goulburn Graben of the Arafura Basin. Petroleum exploration continued in the late 1980s and early 1990s, targeting mostly Mesozoic plays in the Goulburn Graben with three exploration wells (Chameleon 1, Cobra 1A and Tuatara 1) being drilled. Since this time, seismic acquisition has continued, but no further wells have been drilled. Well Control Nine petroleum exploration wells have been drilled in the offshore Money Shoal Basin, all in the Goulburn Graben area. No wells have been drilled in Release Areas NT11-1 and NT11-2 (Figure 1). However, four wells (Cobra 1A, Kulka 1, Money Shoal 1 and Tuatara 1) lie to the north and northeast of Release Area NT11-1 and provide stratigraphic control for the Money Shoal Basin. Beluga 1, and Caldita 1 and 2 have been drilled to the north of Release Area NT11-2 in the Calder Graben of the Bonaparte Basin, and Newby 1, drilled on the margin of the Darwin Shelf and northern Petrel Sub-basin, lies to the southwest of this Release Area. Three wells, Bathurst Island 1, 2 and Tinganoo Bay 1, were drilled on Bathurst Island and Melville Island, respectively, to the south of Release Area NT11-2. Newby 1 (1969) Newby 1 was drilled by Australian Aquitaine Petroleum Pty Ltd (1970) to test a stratigraphic pinch-out and erosional wedge along the northwestern margin of the Petrel Sub-basin, and targeted Upper Paleozoic and Mesozoic reservoirs. The well reached 1148.5 mKB, penetrating Cretaceous to Jurassic sediments before terminating in Proterozoic basement. No significant hydrocarbon indications were encountered. This well is not regarded as a valid test because of the absence of suitable trapping geometry. Money Shoal 1 (1971) Money Shoal 1 was drilled by Burmah Oil Company of Australia Limited in 69 m of water and was the first well drilled in the Arafura Sea (Shell Development (Australia) Pty Ltd, 1971). It targeted a faulted anticline within 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 7 of 23 the Goulburn Graben and was an important well for establishing the stratigraphy of the region. The primary objective was Lower Cretaceous sandstones (upper Flamingo Group) sealed by overlying Cretaceous shales. The secondary objectives were Upper Cretaceous sandstones (Bathurst Island Group) and Jurassic sandstones (lower Flamingo and Troughton groups). The well was initially believed to have penetrated a ‘Tertiary’ to pre-Cambrian succession with a TD of 2590 mKB. However, re-interpretation of the well (Earl, 2006) demonstrates that it intersected sediments of Cenozoic (Woodbine Group), Cretaceous to Jurassic (Bathurst Island, Flamingo and Troughton groups) and Permian (Kulshill Group) age. The Jurassic sandstones (Troughton Group) exhibit good porosity; however, only minor oil indications were encountered. Post-drill analysis indicates that the Jurassic target probably does not constitute a valid structural closure. Despite the Cretaceous sandstones probably being within closure, the failure to find significant hydrocarbons in either the Jurassic or Cretaceous sandstones is due to either lack of hydrocarbon charge or fault/seal breach, with late-formed faults cutting the crest of the structure and a sandy Cretaceous (Bathurst Island Group) seal. Kulka 1 (1984) Kulka 1 was drilled in 77 m of water by Diamond Shamrock Oil Company (Australia) Pty Ltd (1985) to test a Mesozoic stratigraphic trap and a Paleozoic down-thrown faulted anticline beneath the Jurassic angular unconformity. The primary reservoirs targeted were Carboniferous and Devonian clastics and carbonates, with secondary reservoirs being within the Mesozoic section. The well provides stratigraphic control for the upper Paleozoic and Mesozoic sections. It reached a total depth of 3998 mKB in a Pennsylvanian (upper Carboniferous) siltstone unit (Kulshill Group). The Cretaceous (Bathurst Island Group) sediments are dominantly composed of claystones with minor, interbedded sandstones and mudstones that overlie a sandstone package (Darwin Radiolarite and Flamingo Group). The Permo-Carboniferous section (Kulshill Group) is thicker than anticipated with the fine grained sandstones containing volcanic rock fragments and the presence of a diorite sill being recorded at 2,555 mKB, which has been dated as 293 3 Ma. Devonian sediments were not intersected. A number of oil indications (fluorescence) were recorded within Upper Jurassic sandstones and an oil show was interpreted to occur within Pennsylvanian (upper Carboniferous) shaly sandstones. However, silica and carbonate cementation have reduced the porosity and permeability of the Carboniferous fine grained sandstones to such an extent that they have no reservoir potential. Tuatara 1 (1990) Tuatara 1 was drilled by BHP Petroleum Pty Ltd (1991) in 115 m of water on the northwestern edge of the Goulburn Graben and tested a fault block within a broad faulted anticline. The primary objectives were sandstones within the Lower–Middle Jurassic Plover Formation, sealed by Upper Jurassic–Lower Cretaceous claystones of the Flamingo Group. Secondary objectives were 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 8 of 23 Upper Jurassic lower Flamingo Group clastics and Paleozoic clastic and carbonate reservoirs below the base Jurassic unconformity. The structure formed in response to normal faulting in the Early–Middle Jurassic, with fault location being controlled by the reactivation of Paleozoic faults. Pre-drill interpretation implied that the more distal Mesozoic sediments at Tuatara 1 would be clay-rich in comparison to Kulka 1, hence minimising source and seal risks. The well penetrated the base of the Money Shoal Basin and was terminated at 3874.2 mKB in a sandstone unit of presumed Devonian age. However, there is no biostratigraphic evidence for a Devonian age and re-interpretation of seismic data suggests that the well bottomed in Jurassic rocks that overlie probable Cambrian dolomites (Earl, 2006). The well encountered numerous oil indications (fluorescence) throughout the Mesozoic section and oil and gas indications within Lower Cretaceous sandstones. The porosities of the target reservoirs were lower than expected because all seismic picks were 9–10% deeper than originally interpreted (BHP Petroleum, 1991). The Upper Jurassic lower Flaming Group does not contain an adequate seal to the Plover Formation reservoir. The well failed due to insufficient seals and poor quality reservoirs. Beluga 1 (1991) Beluga 1 was drilled on the Bathurst Terrace that flanks the southern Malita Graben (BHP Petroleum Pty Ltd, 1992). It was drilled to evaluate a structural/stratigraphic play comprising an Upper Jurassic sand-dominated fan (referred to as the Beluga Sequence), sealed by the transgressive marine claystones of the Flamingo Group. The closure is modified to the southeast by down-to-the-basin normal faulting. The results from this well are discussed in detail by Anderson et al (1993), but they do not use the same informal stratigraphic nomenclature (‘Jacaranda’, ‘Beluga’ and ‘Tuatara’ formations) presented in the well completion report. The well reached a TD of 3100 mKB in the Plover Formation and encountered gas indications in the Bathurst Island Group, and gas shows within the Flamingo Group and Plover Formation. The most significant gas shows and hydrocarbon fluorescence were encountered in tight Flamingo Group argillaceous sandstones within the Beluga Sequence [re-interpreted as the Sandpiper Sandstone]. The Beluga Sequence comprises distal shelf and slope fan sands. Log analysis indicates that potential reservoir sands were encountered in the upper Flamingo Group over the depth range 2520– 2543 mKB (11–16% porosity; informal ‘Tuatara Formation’). These sandstones are interpreted to have been deposited as channel fill in a shallow marine setting. The lower Flamingo Group sequence (informal ‘Beluga Formation’), the primary reservoir objective in the Beluga closure, was deposited in a distal shelf or upper slope environment, away from the clastic source. Abundant detrital clay, silica and minor carbonate cement, and physical compaction account for the low porosity (average 10%) and very low permeability (<1 mD) of this sequence. Dip analysis suggests that the dominant direction of sediment transport was to the south, such that the majority of the Flamingo Group sediments were derived from the Sahul 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 9 of 23 Platform to the north and transported across a broad continental shelf. This sequence does not represent a viable reservoir objective in this distal setting due to the high amount of detrital argillaceous material, but it may be satisfactory in more proximal facies to the north. Furthermore, quartz sandstones of the fluvial to shallow marine Plover Formation were also found to be tight due to silica cement and quartz overgrowth (average porosity 6%). Tithonian outer shelf to upper continental slope fan sediments derived from the north (‘Beluga Formation’) were deposited on uplifted and eroded shallow marine deltaic sediments of late Callovian–Oxfordian age (‘Jacaranda Formation’). Transgressive marine claystones subsequently onlapped the Tithonian sequence to create the potential stratigraphic closure. During the late Tithonian to Berriasian a series of progradational/aggradational cycles progressively filled a broad shallow basin. Towards the end of the Berriasian the onset of basin inversion is indicated by a change in sediment transport direction (derived from the south/southeast) and normal faults parallel to the graben margin which show increasing displacement stepping to the north. The combination of depositional setting and structural inversion rotated the Beluga feature down to the northwest and produced the potential for a closed feature. Base seal, which was considered a significant pre-drill risk, was provided by the base of the coarsening-upward ‘Beluga Formation’. Source potential of the Upper Jurassic to Lower Cretaceous claystones was found to be relatively poor, showing an affinity for the generation of gas to light oil. The Middle Jurassic Plover Formation is considered to be early mature, based on a vitrinite reflectance measurement of 0.6%. The apparent sediment source direction for the Upper Jurassic sequences and present structural geometry indicates that significant basin inversion occurred during the latest Jurassic to Early Cretaceous. The majority of the Flamingo Group sediments have been derived from the north, probably from the Sahul Platform. Poor reservoir quality of the lower Flamingo Group sandstones reduces the prospectivity for similar play types in this area, but improved reservoir potential is predicted to the north along the northern margin of the Malita Graben. Cobra 1, 1A (1993) Cobra 1A was drilled in 78 m water depth by BHP Petroleum Pty Ltd (1993) on the northwestern margin of the Goulburn Graben and is the most recent well drilled in the Arafura Sea. The exploration target was a stratigraphic trap comprising sandstones within a faulted, erosional dome formed as a remnant topographic high between two Upper Jurassic channels. The primary objective was a Tithonian sandstone unit sealed by Berriasian claystones within the Flamingo Group. Secondary objectives were Lower Cretaceous (upper Flamingo Group) and Middle Jurassic (Plover Formation) sandstones. The well reached a TD of 2542 mRT in Plover Formation sandstones. The well intersected Money Shoal Basin sediments of Cenozoic (Woodbine Group), Cretaceous (Bathurst Island Group and upper Flamingo Group) and Jurassic (lower Flamingo Group and Plover Formation) age. The Bathurst Island Group is dominated by marine mudstones with minor interbeds of sandstone and traces of coal and is 1,192 m thick (Earl, 2006). The Flamingo 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 10 of 23 Group consists of blocky sandstones with interbedded mudstones deposited in a variety of deltaic environments. Although the Cretaceous and Jurassic sandstones recorded excellent porosity, only minor oil indications (fluorescence) were encountered. The well relied on the Tithonian channel fill to act as a seal over the crest of the closure. However, the ‘channel’ comprised interbedded sandstones and claystones in an older channel that was cross cut by a younger channel. The nature of the channel fill was unexpected and interpreted to be a thief zone, which is the primary reason for well failure (Earl, 2006). Caldita 1 (2005) Caldita 1 was drilled in 2005 by ConocoPhillips Exploration Australia Pty Ltd. The well targeted Elang and Plover sandstones within a large faulted anticline (ConocoPhillips Australia Exploration Pty Ltd, 2006). Caldita 1 is reported as a gas discovery, having a gas column that on test flowed at a rate of up to 33 MMscfd (DST 2, 3765.5–3775.4 mRT). The accumulation has P50 reserves of 2.9 Tcf of gas (http://www.nt.gov.au/d/Minerals_Energy/Content/File/html/Petroleum_Reserv es_Resources_Summary.htm). The well reached a TD of 4037 mRT within the Plover Formation, with a calculated extrapolated static bottom hole temperature of 166.39°C, with a temperature gradient of 3.8°C per 100 m (seabed temperature was 12.1°C, as measured by a remotely operated vehicle (ROV) (ConocoPhillips Australia Exploration Pty Ltd, 2006). The interpretation report for this well is currently confidential and no further information is available. Caldita 2 (2007) Caldita 2 was drilled by ConocoPhillips Australia Exploration Pty Ltd (2007) in Permit NT/P61 as an appraisal well of the Caldita accumulation after the acquisition of a 3D seismic survey. Caldita 2 is located 6 km west-northwest of Caldita 1 and the primary objectives were the Elang and Plover formation reservoirs as intersected in the discovery well. The well was drilled in a water depth of 136.5 m and reached a TD of 3972 mRT (ConocoPhillips Australia Exploration Pty Ltd, 2007). No testing was carried out in this well. The interpretation report for this well is currently confidential and no further information is available. For further details regarding wells and available data follow this link: http://www.ret.gov.au/Documents/par/data/documents/Data%20list/data%20lis t moneyshoal_AR11.xls 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 11 of 23 Data Coverage During the early 1990s Geoscience Australia (then the Bureau of Mineral Resources) acquired a total of 5,342 km of regional deep seismic data across the Arafura Basin. In the past 10 years, exploration activities have contributed to the available dataset and have high-graded the prospectivity of the region. These include, for example, non-exclusive regional 2D seismic data sets by TGS Nopec in 1998 and Veritas DGC in 2002, and Synthetic Aperture Radar acquisition and interpretation across the region by Infoterra (2003). Recently, the PGS GeoStreamer Northern Margin Australia–Arafura Multiclient 2D (NMWT) seismic survey has been completed (http://www.pgs.com/Data_Library/Asia-Pacific/Australia/Arafura-NMAA-092D/), which is applicable to Release Areas NT11-1 and NT11-2. To view image of seismic coverage follow this link: http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL No commercial discoveries have been made in either the Money Shoal Basin or Arafura Basin. However, there are numerous hydrocarbon shows and indications in wells drilled in the Goulburn Graben (Figure 3) (Figure 5). Some of the most significant oil shows were intersected in Arafura 1, and pervasive oil indications occur in Goulburn 1. Tasman 1 encountered an oil show in an unnamed Carboniferous carbonate, and Kulka 1 discovered an oil show in the Kulshill Group. 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 petroleum systems elements to generate, expel and trap hydrocarbons, but also evidence that generation and expulsion has occurred. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 12 of 23 Table 1: Petroleum Systems Elements Summary for Money Shoal Basin Bonaparte and Money Shoal basins Lower–Middle Jurassic gas-prone fluvio-deltaic source rocks in the Plover Formation. Sources Potential oil- and gas-prone source rocks in the Upper Jurassic to Lower Cretaceous Frigate Shale and Echuca Shoals Formation. Goulburn Graben, Arafura Basin Middle Cambrian marine source rocks in the Jigaimara Formation. Potential oil and gas-prone fluvio-deltaic source rocks in the Upper Devonian Arafura Group and Permo-Carboniferous Kulshill Group. Bonaparte and Money Shoal basins Primary clastic reservoirs are Lower–Middle Jurassic Plover Formation and Middle–Upper Jurassic Elang Formation. Secondary clastic reservoirs are Upper Jurassic–Lower Cretaceous Sandpiper Sandstone and Upper Cretaceous Wangarlu and Puffin formations. Reservoirs A secondary carbonate reservoir is fracture porosity in the Lower Cretaceous Darwin Radiolarite. Goulburn Graben, Arafura Basin Cambro-Ordovician shallow marine limestones and dolomites of the Goulburn Group. Upper Devonian siltstones and sandstones of the Arafura Group. Bonaparte and Money Shoal basins Lower Cretaceous Echuca Shoals Formation, together with the Upper Jurassic Frigate Shale, forms the regional seal. Seals Intra-formational seals occur within the Lower–Middle Jurassic Plover Formation and Upper Cretaceous Wangarlu Formation. Goulburn Graben, Arafura Basin Cambro-Ordovician mudstones at the top and base of the Goulburn Group. Upper Devonian mudstones at the top of the Arafura Group. Permo-Carboniferous dolerite sills. Play Types Bonaparte and Money Shoal basins 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 13 of 23 The main structural plays are broad faulted anticlinal structures over tilted fault blocks at the base of the regional seal. Tilted horst blocks are attractive targets on faulted terraces adjacent to the Calder Graben. Combination structural/stratigraphic traps are possible within the turbidite sandstones of the Middle Jurassic–Lower Cretaceous Flamingo Group and Upper Cretaceous Puffin Formation. Onlap, drape closure, channel and low stand wedge plays in the Money Shoal Basin. Goulburn Graben, Arafura Basin Large faulted anticlines and fault blocks, sub-unconformity plays, diagenetic and other stratigraphic traps. Source Rocks Goulburn Graben, Arafura Basin The Arafura Basin contains several units with potential source rocks; the Neoproterozoic Wessel Group, Cambrian–Ordovician Goulburn Group, Devonian Arafura Group, and the Permo–Carboniferous Kulshill Group (Boreham, 2006; Struckmeyer, 2006a, b). However, the only effective source rock within the Arafura Basin is a marine shale of Cambrian age, probably within the Jigaimara Formation, which has sourced the majority of oil shows and indications within the wells drilled in the Goulburn Graben (Moore et al, 1996; Boreham and Ambrose, 2005; Sherwood et al, 2006; Boreham, 2006). Migration and preservation of Cambrian-derived hydrocarbons have the potential to be preserved in the northern Arafura Basin, and along the northeastern and southwestern margin of the Goulburn Graben (Struckmeyer 2006b), including the northeasternmost corner of Release Area NT11-1 (see Link to Arafura Basin REGIONAL Figure 5; Arafura Basin Regional Geology chapter). Potential source rocks within the Arafura Group and the Kulshill Group have been modelled by Struckmeyer (2006b) to demonstrate the possible generation and expulsion of hydrocarbons within the Goulburn Graben (Figure 8). Figure 8 also shows that hydrocarbons generated from these Paleozoic source rocks may have migrated into the northeasternmost corner of Release Area NT11-1. The lack of a Kulshill Group-derived accumulation at the nearby well Cobra 1A is attributed to lack of an effective seal rather than source and reservoir issues (Earl, 2006). Money Shoal and Northern Bonaparte Basins The Lower–Middle Jurassic Plover Formation (Troughton Group) contains effective source rocks within the Bonaparte and Money Shoal basins (Figure 5), as well as being a proven reservoir target for hydrocarbon discoveries in the northern Bonaparte Basin (Longley et al, 2002; Ambrose, 2004 a, b; Barrett et al, 2004; Edwards et al, 2004). The Plover Formation is a fluvio-deltaic unit that contains fair to good oil- and gas-prone source rocks; 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 14 of 23 however, these source rocks appear to be dry gas-prone within the Calder Graben. In the Money Shoal Basin, the Plover Formation is composed of blocky sands with minor interbedded mudstones and coals. Other effective source rocks for oil and gas within the Bonaparte Basin include the organic-rich shales of the Upper Jurassic–Lower Cretaceous Frigate Shale (Flamingo Group) and the basal marine shales of the Lower Cretaceous Echuca Shoals Formation (Bathurst Island Group) (Preston and Edwards, 2000; Longley et al, 2002). Where intersected in the northeastern Bonaparte Basin, these source rocks appear to become gas-prone due to the dilution and inertinitic nature of the organic matter (West and Passmore, 1994). In the Money Shoal Basin, the Mesozoic sediments show a range in thermal maturity as they thicken to the west; the lower Flamingo Group is marginally mature to mature for oil generation (VR values of 0.6–0.79% are recorded at Tuatara 1) and the Echuca Shoals Formation is immature to marginally mature for hydrocarbon generation. Reservoir Rocks Goulburn Graben, Arafura Basin 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 Arafura 1 and oil indications in Goulburn 1. 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). Siltstones and sandstones of the Arafura Group are important reservoirs in the region, hosting oil shows and indications in Arafura 1 and Goulburn 1, with the better quality reservoir occurring at Goulburn 1. Tasman 1 encountered an oil show in an unnamed Carboniferous carbonate. The upper part of the Kulshill Group contains some good quality reservoir units; however, only a minor oil show deeper in the section has been recorded at Kulka 1. Money Shoal and Northern Bonaparte Basins In the northern Bonaparte Basin, Mesozoic reservoirs, notably the Jurassic Plover Formation and basal Flamingo Group (Elang Formation), host numerous hydrocarbon accumulations including the Caldita, Evans Shoal, Lynedoch-Barossa, Sunrise and Abadi gas accumulations (Longley et al, 2002; Barrett et al, 2004; Cadman and Temple, 2004; Lowe-Young et al, 2004) (Figure 3 and Figure 5). The Sandpiper Sandstone (upper Flamingo Group) and the upper Wangarlu and Puffin formations (Bathurst Island Group) are also excellent quality reservoirs within the Bonaparte Basin (Barber et al, 2004; De Boer, 2004). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 15 of 23 Similarly, the Money Shoal Basin also contains reservoir quality clastics in the Plover Formation, Flamingo Group and the Bathurst Island Group. These Mesozoic reservoirs are well positioned to receive any late hydrocarbon charge from underlying potential Paleozoic source rocks within the Goulburn Graben, as well as the long range migration of Mesozoic-derived hydrocarbons from the Calder Graben. Mesozoic hosted hydrocarbon indications occur at Chameleon 1, Cobra 1A, Kulka 1, Money Shoal 1 and Tuatara 1 (Struckmeyer, 2006b), with most occurring within the Plover Formation and Flamingo Group. However, there is currently no geochemical evidence that determines the origin of these Mesozoic reservoired hydrocarbons. In the Money Shoal Basin, fluvio-deltaic sandstones of the Plover Formation have average porosities of 8.5% with a maximum of 27% at Tasman 1. Blocky fluvio-deltaic sands of the Flamingo Group have an average porosity of 18.5%, with a maximum of 32% at Tasman 1 and a range of 5–17% at Tuatara 1 (Earl, 2006). Within the upper Flamingo Group, there are sandstones associated with an upper Tithonian incised channel complex running the length of the western Goulburn Graben and a postulated turbidite fan system in the Calder Graben (Barber et al, 2004). The Bathurst Island Group contains potential reservoir units; where it is sandstone-rich, the Darwin Radiolarite (eg at Kulka 1) has excellent porosity (average of 25 %). In the upper Bathurst Island Group at Tuatara 1 porosities range from 13–33%, although no permeability data are available (Earl, 2006). The reservoir potential of basin floor sands (Puffin Formation) within this group is untested. Seals Goulburn Graben, Arafura Basin The Paleozoic section contains numerous mudstone units which could provide intraformational seals. Argillaceous sediments within the Goulburn and Arafura groups in Arafura 1 and Goulburn 1 provide intraformational seals for the most significant oil shows and indications. The generally poor reservoir quality of the Paleozoic sediments in the Goulburn Graben means that fluid movement is restricted within the section, with lateral decreases in reservoir quality possibly providing sealing and trapping structures. Money Shoal and Northern Bonaparte Basins In the Bonaparte Basin, the regional seals for the Plover Formation and Flamingo Group reservoirs are the thick claystones of the Frigate Shale (Flamingo Group) and Echuca Shoals Formation (lower Bathurst Island Group) (Figure 5). In the Money Shoal Basin, the claystones of the Wangarlu Formation (upper Bathurst Island Group) are laterally and vertically extensive; therefore, fault breach is unlikely due to the thickness of this unit. Other seals in the region are less homogenous. The Upper Jurassic to Lower Cretaceous Flamingo Group tends to be sand-dominated, but contains extensive mudstones. These include maximum flooding surfaces and abandoned channel fill and overbank deposits that could provide good intraformational seals. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 16 of 23 Timing of Generation Goulburn Graben, Arafura Basin 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, b). 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 (Figure 8). Money Shoal and Northern Bonaparte Basins Potential source rocks in the Plover Formation are typically immature in all wells in the Goulburn Graben, apart from Tuatara 1. Here the unit reaches oil maturity and has probably generated some oil, but expulsion is unlikely to have occurred. However, modelling by Struckmeyer (2006b) at pseudo-well site G, to the west of Tuatara 1 (Figure 8) and north of Release Area NT11-1, where a thicker Money Shoal overburden (about 4 km) is present, has demonstrated that oil expulsion occurred from within the Plover Formation and lower Flamingo Group during the Late Cretaceous to Cenozoic. Play Types Goulburn Graben, Arafura Basin The structural setting of the Goulburn Graben is conducive to a variety of structural, stratigraphic and combination traps. The structural styles vary significantly between the Money Shoal and Arafura basins, with some wells testing different trap styles in both stratigraphic sections (for example Chameleon 1 and Kulka 1). The structure of the Paleozoic section of the Arafura Basin is dominated by features formed during the Triassic Fitzroy Movement. This includes fault blocks, fault roll-overs, anticlines and subunconformity traps. Money Shoal and Northern Bonaparte Basins In the northern Bonaparte and Money Shoals basins, a variety of structural, stratigraphic and combined structural/stratigraphic plays are present for hydrocarbons expelled from Mesozoic source kitchens. In the case of the Money Shoal Basin where it overlies the northern Goulburn Graben, hydrocarbons may also be sourced from Paleozoic source rocks. Tilted fault blocks, faulted anticlines and broad, low relief anticlinal drape over tilted fault blocks provide the main structural plays. Tilted horst blocks are attractive targets on faulted terraces adjacent to the Calder Graben. The possibility of hanging wall fault traps on the down-thrown side of the bounding faults provides a secondary play type that is relevant in Release Area NT11-2. Potential stratigraphic traps in both basins include pinch-outs and onlap plays associated with the Triassic unconformity, channel fills and slope- and basinfloor fans. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 17 of 23 A Tithonian channel system runs along the major bounding faults of the Goulburn Graben, from east to west (Barber et al, 2004). Although the channel system has been unsuccessfully tested at several locations (such as Chameleon 1, Cobra 1A and Kulka 1) the feature provides numerous untested stratigraphic/structural traps within the channel fill and associated erosion features and may have relevance to the northwesternmost part of Release Area NT11-1. The Upper Cretaceous prograding shelf and contiguous slope and basin deposits of the upper Bathurst Island Group provide untested plays within the northern Bonaparte and Money Shoal basins. Significantly, seismic data show bright amplitudes within interpreted basin floor fans possibly indicating the presence of hydrocarbons (Struckmeyer, 2006b). Critical Risks A recent audit of exploration wells in the Goulburn Graben (Earl, 2006) identified timing of hydrocarbon charge, breach of structure and reservoir quality as the major reasons for the failure of the wells. Thus, reservoir quality of Paleozoic rocks and the timing of generation, expulsion and trap formation in relation to the major structuring event in the Triassic are regarded as the key risks. Risks for the younger, mostly stratigraphic plays include the presence of suitable seals. The lack of large structures, adequate seals, and the absence of Plover Formation reservoirs on the Darwin Shelf are issues, as is the quality of the Flamingo Group reservoirs, which are tight at Beluga 1. The long range migration of hydrocarbons from the Malita and Calder graben source kitchens onto the Darwin Shelf and into the western Money Shoal Basin is largely unproven, with only gas shows being recorded within the Plover Formation and Flamingo Group at Beluga 1. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 18 of 23 FIGURES Figure 1: Location map of Release Areas NT11-1 and NT11-2 in the Money Shoal Basin and northeastern Bonaparte Basin. Figure 2: Graticular block map and graticular block listings for Release Areas NT11-1 and NT11-2 in the Money Shoal Basin and northeastern Bonaparte Basin. Figure 3: Regional structural elements of the northeastern Bonaparte Basin, Money Shoal Basin and Arafura Basin. The locations of the seismic sections in Figure 6 and Figure 7 are shown. Figure 4: Thickness (milliseconds two-way time) of the Jurassic to Lower Cretaceous sediments (Troughton and Flamingo groups) in the Money Shoal Basin (Struckmeyer, 2006b). Figure 5: Stratigraphic correlations between the Malita and Calder graben, northern Bonaparte Basin and the Money Shoal Basin, based on the Bonaparte Basin Biozonation and Stratigraphy Chart (Nicoll et al, 2009). Geologic Time Scale after Gradstein et al (2004) and Ogg et al (2008). Hydrocarbon shows are displayed. Seismic horizons after Kennard and Colwell (2001). Figure 6: Seismic line AGSO 118-14 through Release Area NT11-1 in the Money Shoal Basin (location of line segment shown in Figure 3). Figure 7: Seismic line AGSO 118-05 through Release Area NT11-2 on the Darwin Shelf, northeastern Bonaparte Basin (location of line segment shown in Figure 3). Figure 8: Interpreted hydrocarbon expulsion map from potential Devonian (Arafura Group) and Permo-Carboniferous (Kulshill Group) source rocks in the Goulburn Graben, Arafura Basin and expulsion from Jurassic (Plover Formation, Troughton Group) source rocks in the Money Shoal Basin and northeastern Bonaparte Basin (Struckmeyer, 2006b). Inset: hydrocarbon generation and expulsion rates of oil from the Plover Formation at pseudo-well site G. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Areas NT11-1 and NT11-2, Money Shoal Basin, NT Release Area Geology Page 19 of 23 REFERENCES AMBROSE, G.J., 2004a—Jurassic pre-rift and syn-rift sedimentation in the Bonaparte Basin – new models for reservoir and source rock development. In: Ellis, G.K., Baillie, P.W. and Munson, T.J. (editors), Timor Sea Petroleum Geoscience: Proceedings of the Timor Sea Symposium, Darwin Northern Territory, 19-20 June 2003, 125–142. 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