Petroleum Geological Summary s1

Petroleum geological summary

Release areas NT12-1 and NT12-2, MONEY SHOAL BASIN AND ARAFURA Basin, NORTHERN TERRITORY

HIGHLIGHTS

· Under-explored large frontier Paleozoic Arafura Basin and Mesozoic Money Shoal Basin

· Shallow water depths 50–390m

· Proximal to large gas accumulations in the adjacent northern Bonaparte Basin

· Large structural plays and numerous stratigraphic plays

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Location

Release Areas NT12-1 and NT12-2 are located about 240km northeast of Darwin in the Arafura Sea. Water depths range from 50m to 390m (Figure1). The graticular block maps and graticular block listings for these Release Areas are shown in Figure2. Release Area NT12-1 comprises 300 graticular blocks and covers an area of approximately 24,610km2. Release Area NT12-2 consists of 207 graticular blocks and covers an area of approximately 16,385km2. Release Areas NT12-1 and NT12-2 both cover part of the Mesozoic to Cenozoic Money Shoal Basin and the underlying Neoproterozoic to Permian Arafura Basin (Figure3). The western part of Release Area NT12-2 covers part of the Calder Graben of the Mesozoic to Cenozoic Bonaparte Basin (Figure3). The Release Areas are located to the east of the Evans Shoal, Caldita and Barossa-Lynedoch gas accumulations in Australian waters and the Abadi gas accumulation in Indonesian waters (Figure1). Wells in the permit areas have oil shows and oil and gas indications.

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Release Area Geology

Local tectonic setting

Within Release Areas NT12-1 and NT12-2, Mesozoic to Cenozoic rocks of the Money Shoal Basin overlie a Neoproterozoic to Permian Arafura Basin succession (Figure3). In the southern part of Release Area NT12-1, the Money Shoal Basin is underlain by the Goulburn Graben of the Arafura Basin. The northern part of the Release Area overlies the western edge of the northern Arafura Basin. Release Area NT12-2 extends into the Calder Graben (Mesozoic to Cenozoic northern Bonaparte Basin) to the west and the Goulburn Graben in the east (Figure1 and Figure3). Both Release Areas are underlain by an extension of the Archaean–Paleoproterozoic Pine Creek Inlier Basement Terrane, which outcrops onshore.

Structural evolution and depositional history of the area

The Arafura Basin formed in the Neoproterozoic as a result of NW-SE upper crustal extension producing a series of NE-SW trending half graben across much of the basin. During this time the dominantly clastic sediments of the Wessel Group were deposited. Subsequent periods of subsidence in the Cambro-Ordovician, Late Devonian and late Carboniferous to early Permian were separated by long, tectonically quiescent periods of non-deposition and erosion. The predominantly marine carbonates of the Cambro-Ordovician Goulburn Group are overlain by shallow marine to non-marine clastics and carbonates of the Devonian Arafura Group and fluvio-deltaic clastics of the late Carboniferous to early Permian Kulshill Group equivalent.

The Goulburn Graben formed in the Late Carboniferous to early Permian in response to oblique extension associated with the break-up of Gondwana. Subsequent contraction in the Triassic resulted in oblique inversion of the Goulburn Graben, uplift and erosion of up to 3.5km of sediment. The deformational event affected areas to the north and south of the Goulburn Graben to a lesser extent. Subsequent erosion resulted in formation of a peneplain across the basin upon which the sediments of the Money Shoal Basin were deposited (Totterdell, 2006).

The Money Shoal Basin is bounded to the west by the Lynedoch Fault System, which separates the 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 Holocene sediments. The northern part of the basin extends beyond the Australian-Indonesian boundary. The basin thins rapidly eastwards to less than 500ms two-way time (< 600m) of ?Upper Cretaceous and upper Cenozoic sediments. The basin sediments form a stratigraphic wedge that thickens towards the west, ranging from 1–4sTWT depth across both Release Areas (Figure5 and Figure6; Struckmeyer, 2006b). The Money Shoal Basin has equivalent stratigraphy to the Mesozoic section of the Bonaparte Basin (Figure4; 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 (Figure3 and Figure6). 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). In the Money Shoal Basin, this is reflected by relatively small-scale normal faulting along the boundaries of the Goulburn Graben, particularly 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 (Struckmeyer, 2006b).

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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 (McKirdy and Horvath, 1976; Summons et al, 1993). 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 Shoal1 (1971), in the offshore Money Shoal Basin which also penetrated sediments of the 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 (Arafura1, Goulburn1, Kulka1, Tasman1 and Torres1) 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 (Chameleon1, Cobra1A and Tuatara1) being drilled. Since this time, seismic acquisition has continued, but no further wells have been drilled. In 2007, exploration acreage in the northern Arafura Basin, largely to the east of the current Release Areas, was granted to Samson International (Australia) who acquired over 3000km of 2D seismic data.

Well control

Nine petroleum exploration wells have been drilled in the offshore Money Shoal Basin, all in the Goulburn Graben area. Release Area NT12-1 contains 6 wells (Chameleon1, Cobra1A, Kulka1, Money Shoal1, Tasman1 and Torres1). Release Area NT12-2 contains one well, Tuatara1 (Figure1). Arafura1 and Goulburn1 also provide stratigraphic control in the Goulburn Graben to the east of the Release Areas.

Barossa1ST1, Caldita1 and2 and Lynedoch1ST1 and 1ST2 have been drilled to the west of Release Area NT12-2 in the Calder Graben of the Bonaparte Basin.

Money Shoal1 (1971)

Money Shoal1 was drilled by Burmah Oil Company of Australia Limited in 69m of water and was the first well drilled in the Arafura Sea (Shell Development (Australia) Pty Ltd, 1971). It targeted a faulted anticline within the Goulburn Graben and was an important well in establishing the stratigraphy of the region. The primary objective were 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 Precambrian succession with a TD of 2,590mKB. However, reinterpretation 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 equivalent) age. The Jurassic sandstones (Troughton Group) exhibit good porosity; however, only minor oil indications were encountered. Post-drill analysis indicated 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 either to a 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.

Lynedoch1ST1 (1973)

Lynedoch1 was drilled in 247m of water by Shell Development (Australia) Pty Ltd (1973) about 350km north-northwest of Darwin. It was the first well drilled in the Calder Graben, and the second well to be drilled in the northern Bonaparte Basin. The well targeted a seismically defined, low-relief anticlinal structure mapped at the near base-Albian seismic horizon (labelled ‘P’ in the well completion report, or base of the limestone unit within the Darwin Formation, and interpreted as the Aptian Disconformity by Shell Development (Australia) Limited (1999)). There was no closure at either the Upper Cretaceous or Cenozoic levels.

Lynedoch1 was sidetracked when the drill pipe became stuck in tight Cretaceous shales and a fish was lost in the hole. The sidetracked well was logged. Lynedoch1ST1 penetrated a thick Cenozoic to Jurassic succession of sedimentary rocks and reached a TD of 3,967mRT within Valanginian (Oxfordian in WCR) sediments (Flamingo Group).

A thin (9.7m) gas-bearing zone was encountered within a limestone unit (3,674–3,715mRT) of the Darwin Formation, but it was not tested. Methane to butane was reported in the gas kick at 3,698mRT. Porosities range from 8% to18% over a depth of 3,689–3,715mRT, with low gas saturation occurring in the matrix and higher gas saturation occurring in the fractured zones. Log analysis indicates a further zone of possible gas saturation in water-bearing sandstones of the uppermost Jurassic section (upper Flamingo Group). Due to the thinness of the Bathurst Island Group limestone reservoir and the presence of tight Jurassic sandstones in this well, the Mesozoic structures in this area were interpreted to be uneconomic for hydrocarbons.

Arafura1 (1983)

Arafura1 was drilled in 64.4m of water by Petrofina Exploration Australia S.A. (1983). Arafura1 is an important well in the region, providing information on Paleozoic reservoirs and source rocks and proving the occurrence of oil generation. The well confirmed that all mapped seismic horizons correlated to lithological changes within two main reservoir sections; Arafura Group siltstone and Goulburn Group dolomite, both of which hosted oil shows. The well reached a total depth of 3,635mKB and terminated in the Neoproterozoic Wessel Group.

Fluid movement in the reservoirs is restricted, with low porosity and permeability. There is evidence of secondary porosity with many vugs, fractures and burrows containing oil. Lateral improvement of the reservoir also remains a possibility. The presence of both biodegraded and non-biodegraded oil in the reservoirs indicates at least two hydrocarbon charges. The oil has been typed to a Cambrian source rock that is organic rich and mature for hydrocarbon generation. While it is not vertically extensive, the section still has good source potential due to its organic richness and lateral continuity.

The reservoirs are sealed by mudstones that appear to be effective, so fluid movement was probably facilitated by faults. This is reinforced by a Cambrian oil signature in the base of the Kulshill Group equivalent, where the main fault intersects the well section. Ultimately fault leakage breached the structure and is the primary reason for well failure.

Tasman1 (1983)

Tasman1 was drilled in 65.8m of water by ESSO Exploration and Production Australia Inc. (1983a). The Tasman1 structure was originally interpreted to be a salt-related anticline feature. Post-drill analysis has concluded that the feature is a raised fault block associated with carbonates, rather than salt. Tasman1 was drilled to test the youngest structured sediments beneath the base Jurassic unconformity. It was believed that the shallow nature of the Paleozoic sediments would optimise the chances of a good quality reservoir. The well reached 2,720mKB and terminated in the Cambrian to Ordovician Goulburn Group.

Well results indicate that while the porosity of the target horizon (Kulshill Group equivalent) is acceptable at 13% there is a lack of large sand-rich units and sealing units. Oil indications were found within the section and are believed to be locally sourced from early mature sediments and are evidence of migration. There is no evidence of fluid movement through the high quality reservoirs of the Mesozoic, which lack structural closure at this location. Source rock data indicates that there has not been significant hydrocarbon generation and expulsion. A high quality underlying Cambrian source rock, as intersected at Arafura1, is possibly the source of hydrocarbons in this well.

Torres1 (1983)

Torres1 was drilled in 67m of water by Esso Exploration and Production Australia Inc. (1983b) into the giant Torres anticline along the axis of the Goulburn Graben. The well targeted Devonian clastics of the Arafura Group, Cambrian dolomites of the Goulburn Group and a section underlying the Cambrian dolomites. The latter was not intersected. A secondary objective was to obtain stratigraphic information because little was known about the Paleozoic succession. The well reached a TD of 2,758mKB in the Cambrian to Ordovician Goulburn Group.

The well confirmed the presence of high quality Mesozoic reservoirs (which were not within structural closure) and poor quality Paleozoic reservoirs. The well was one of the biggest disappointments in the Arafura Basin due to the large size of the structure and the lack of significant shows within the reservoirs. Only minor gas traces were found below seals in the Cretaceous Upper Flamingo Group, Devonian Arafura Group siltstone and Cambro-Ordovician Goulburn Group dolomite.

As there is no evidence of a remnant hydrocarbon accumulation, such as dead oil and bitumen, and the structure is viable, it is likely that well failure was due to lack of hydrocarbon charge. This could be due to a lack of fluid movement in the poor quality Paleozoic reservoirs and/or no access to migration pathways (such as faults). There are no viable source rocks in the well section to locally source hydrocarbons. Gas traces are likely to have been generated from a postulated underlying Cambrian source rock (as seen at Arafura1).