Area NT07-1
Troubadour Terrace, Bonaparte Basin
Release Area Geology
Regional tectonic setting
This summary of the regional geology and hydrocarbon potential draws largely on studies by West and Passmore (1994), Shuster et al (1998), Longley et al (2002) and various papers from the Timor Sea Symposium volume (Ellis et al, 2004).
One of the main tectonic elements in the region is the Sahul Platform, which is a region of elevated Palaeozoic and Mesozoic sediments that overlie relatively shallow basement (Figure 2). Late Miocene to Pliocene convergence of the Australian and Eurasian plates resulted in flexural down-warp of the Timor Trough to the north, and the generation of the Kelp and Troubadour highs that are located respectively on the southwestern and northeastern parts of the Sahul Platform. The Troubadour Terrace is an area of relatively shallow basement that is arbitrarily separated from the Sahul Platform. The southern boundary of the Sahul Platform is marked by northeast-trending Mesozoic normal faults showing displacement down into the Malita and Calder graben, which are Mesozoic–Cenozoic depocentres that trend northeast–southwest and north-northeast–south-southwest, respectively (Figures 2 and 3). The Malita Graben is a symmetrical depocentre bound on both sides by large displacement normal faults that show some evidence of wrenching (Figure 4). The Calder Graben terminates at the Lynedoch Bank Fault System (Figure 2), which is a major depositional hinge connecting with the Halls Creek Mobile Zone to the south (McLennan et al, 1990), marking the eastern limit of the Bonaparte Basin. Regional structuring in the Timor Sea region resulted from episodic tectonic activity since the Precambrian.
Basin evolution
The relevant stratigraphic section is outlined in Figure 5. The first rifting in the northern Bonaparte Basin resulted from north-northwest–south-southeast orientated extension during the Late Carboniferous–Early Permian, which produced the incipient Malita Graben and its respective northeast–southwest-trending bounding faults (Shuster et al, 1998). The area underwent northwest–southeast extension during the Middle–Late Jurassic (Callovian–Oxfordian), producing the dominant northeast–southwest structural grain. Although the Late Jurassic sediments are important source rocks within the Bonaparte Basin, such source rocks have not been penetrated in the axis of the Malita Graben. Sediments of this age are represented by thin condensed sections in surrounding wells. A relatively thick syn-rift section is interpreted from seismic data to occur in the axis of this depocentre (Figure 6).
In the northern Bonaparte Basin, the Callovian tectonic event was relatively mild. Later Tithonian tectonism was a more important component in the tectonostratigraphic evolution of the region, and major block faulting and tilt reversal occurred at this time. Extension in the Late Jurassic–Berriasian (Flamingo Formation) formed thick sequences of syn-rift shales and coarse-grained clastics, the latter providing important reservoir targets.
The Early Cretaceous Valanginian unconformity relates to sea-floor spreading, and maintenance of a westerly tilt controlled deposition of the Bathurst Island Group, which comprises a series of high stand parasequences formed under prograding shelf and slope conditions. This section thickens locally into the Malita and Calder graben and thins markedly onto the Sahul Platform and Money Shoal Platform; the latter being controlled by the Lynedoch Bank Fault System. Eustatic falls in sea level during the Campanian and Maastrichtian resulted in the deposition of a marine siliciclastic sequence deepening to the northwest. Sandstones are widespread across the Malita Graben and Darwin Shelf (eg 158 m in Heron 1, 409 m in Evans Shoal 1).
Seismic mapping suggests the Cretaceous structural history varies markedly from the Malita Graben to the Calder Graben, as reflected by major variation of the lower and upper Bathurst Island Group isopachs (Figures 6 and 7), suggesting an important structural boundary exists between the two graben.
A westerly tilt, established during the Cretaceous, was maintained throughout the Cenozoic, resulting in thickening of the Paleocene to Miocene sequence along the axis of the Malita Graben, High subsidence rates in the Malita Graben persisted until the Holocene. Total sediment thickness in the Malita and Calder depocentres is unknown, as only the terraces and platform areas have been drilled. However, these depocentres are estimated to contain more than 10 km of post-Palaeozoic sediment fill (West and Passmore, 1994).
The last phase of tectonism in the area resulted from the Neogene collision of the Australia-India and Eurasia plates in the Banda Arc region. The resultant structural trends are explained by a combination of strike-slip faulting, and flexure induced by oblique convergence and partial subduction of the Australia-India Plate under the Eurasia Plate. Fundamental to this model is that the basement highs (eg the Money Shoals Platform and Sahul Platform) acted as rigid blocks and that the proposed shear (left lateral wrenching) preferentially reactivated the existing structural grain (Shuster et al, 1998). In the Calder Graben and Malita Graben areas this reflects high obliquity of the Banda Arc collision front with the Australia-India Plate motion. The resultant shear stress resulted in major Neogene strike-slip deformation on the Lynedoch Bank Fault System, and also along Mesozoic and older fault zones bounding the Malita Graben (Shuster et al, 1998).
Stratigraphy
The oldest sedimentary unit penetrated on the Sahul Platform is the Permian Hyland Bay Formation, in which gas was reservoired in Kelp Deep 1. The top Permian disconformity, which marks the base of the Triassic section, is an important seismic event that can be mapped over much of the northern Bonaparte Basin. In Troubadour 1, a thin section of Late Permian carbonates were found to overlie granitic basement.
Triassic sediments were deposited during a period of tectonic quiescence related to the foundering of the passive margin associated with the Permo-Carboniferous cycle of rifting (Whittam et al, 1996). The nearest relevant section to Area NT07-1 occurs to the west in Troubadour 1, where Late Permian to Early Triassic marine siltstones and shales of the Mount Goodwin Formation are overlain by mixed clastics/carbonates of the Sahul Group. Regression in the Late Triassic resulted in the deposition of fluvio-deltaic redbeds (Nome and Malita formations). Uplift and erosion at the end of the Triassic terminated sedimentation.
The Early–Middle Jurassic, prerift Plover Formation is ubiquitous in the northern Bonaparte Basin, being deposited in a northeast–southwest-trending ‘sag’ basin in fluvio-deltaic to marginal marine depositional environments. To the southwest of the release area in the Vulcan Sub-basin/Cartier Trough, Ambrose (2004a and b) established a four fold subdivision of the Plover Formation that is valid as far north as Thornton 1; there is a transition from this well to more marine facies to the northwest across the Sahul Platform. The only complete Plover section (300 m thick) relevant to Area NT07-1 occurs in Troubadour 1. Here a basal alluvial–fluvial floodplain section, about 150 m thick, is largely defined by upward fining (GR log) channel sandstone–overbank shale cycles, overlain by braided channel facies. These in turn are succeeded by marine shales–siltstones capped by shoreface sandstones. The latter are probably of Bathonian age, and may equate with well developed shoreface sandstones in the upper Plover Formation in the Evans Shoal field. Here the best quality Plover Formation reservoirs occur in an upper zone of shoreface sandstones (~125 m thick), which comprise fine to coarse-grained pyritic sandstone deposited in several upward coarsening (GR log) cycles.
Over wide areas of the northern Bonaparte Basin, the Plover Formation is para-conformably overlain by thin (<50 m thick) marine sandstone and shale of late Callovian to early Oxfordian age (equivalent to the Elang/Laminaria Formation). The basal sand-rich, sheet-like section of the formation (W. digitata, R. aemula dinoflagellate zones) is ubiquitous. However, with the possible exception of the Malita Graben, synrift shales of the Oxfordian/Kimmeridgian (Frigate Formation) are relatively thin in this area (20–30 m thick); being less than 30 m thick in Evans Shoal 1.
Bounding the synrift Laminaria/Frigate formations is a major unconformity of Tithonian age that is overlain by shales and subordinate sandstones of the Flamingo Group (Tithonian–Berriasian in age). The section is relatively thin on the structural highs, but thickens into the adjacent graben. For example, in Heron 1, which tested an inversion structure in the Malita Graben, Flamingo Formation shales are over 850 m thick, but thin via onlap onto the Troubadour High.
Renewed sea-floor spreading and subsidence from the mid-Valanginian resulted in widespread deposition of a condensed section of glauconitic, marine claystone (Valanginian to Early Aptian, Echuca Shoals Formation), which is about 30 m thick in Evans Shoal 1. The peak of this transgression is represented by a condensed sequence of radiolarian chert, claystone and calcilutite (Darwin Formation, Whittam et al, 1996). The top of the radiolarite (Aptian) is a prominent seismic marker in the region, and is often used as a ‘phantom’ horizon to determine approximate structure at the ‘near top’ Plover level.
South of Area NT07-01 in Evans Shoal 1, an Aptian to Maastrichtian progradational sequence (~2200 m thick) comprises claystone, calcilutite, marl and sandstone (Bathurst Island Group), which resulted from foundering of the Australian margin following continental break up. The basal Wangarlu Formation (~1800 m thick) comprises mainly claystone with subordinate siltstone and minor sandstone. The overlying Maastrichtian sequence (~400 m thick) comprises mainly marine sandstones with subordinate claystone, and is probably equivalent to the Puffin Formation documented in the Vulcan Sub-basin.
The Neogene collision of the Australia-India and Eurasia plates occurred with high obliquity on the Band Arc collision front. The resultant shear stress facilitated Neogene strike-slip along fault lines bounding the Malita Graben. Neogene subsidence in the Malita Graben, which is interpreted as a pull-apart rhomb-graben related to sinistral shear, resulted in deposition of a relatively thick Tertiary carbonate sequence.