Areas W07-1, W07-2 and W07-3
Londonderry High, Bonaparte Basin
Release Area Geology
The Londonderry High is a Permo-Triassic horst and graben complex. It is characterised by a highly faulted sequence of Palaeozoic and Triassic rocks that acted as a major source of sediment for adjacent depocentres during rifting, uplift and erosion in the Late Jurassic (Whibley and Jacobsen, 1990; de Ruig et al, 2000). The Palaeozoic and Triassic rocks are unconformably overlain by relatively unfaulted, Late Jurassic and younger sediments. Although most faulting terminates at the top of the Triassic succession, some faults show evidence of Miocene reactivation. On the higher parts of the Londonderry High, the Triassic section is deeply eroded. Uplift and erosion are less pronounced on the eastern and northern flanks where the unconformity is underlain by progressively younger sediments.
The major structural elements within and adjacent to the release areas are shown in Figure 2. Area W07-1 is located at the northern end of the Londonderry High on the Eider Horst, which formed during Late Jurassic faulting. To the west of this release area is the Dillon Ridge, which separates the Londonderry High from the Cartier Trough of the Vulcan Sub-basin. The Cartier Trough is a northeast–southwest-trending extensional feature that subsided rapidly as a result of collision of the Australian Plate with the southeast Asian microplates in the Neogene (Shuster et al, 1998; Whibley and Jacobson, 1990). To the north and northeast are the Nancar Trough, Sahul Syncline and Echo Syncline, respectively. The Nancar Trough is a major depocentre containing up to 8 km of Mesozoic–Cenozoic sediments. The Sahul Syncline is also a major Mesozoic depocentre, but it originally developed as the northerly extension of the Petrel Sub-basin. The Echo Syncline is a southwest-trending embayment of the Sahul Syncline, which sagged and became a depocentre during the Early Triassic and Late Jurassic.
Area W07-2 is located within the central portion of the Londonderry High. Area W07-3 is located on the western margin of the Londonderry High adjacent to the Vulcan Sub-basin, and includes the feature known as the Jacana Low.
Stratigraphy
The stratigraphy for the Londonderry High has been derived from the Petrel
and Vulcan sub-basins, with well control being provided on the north and
east of the high (Figure
3). The following synopsis is taken from Passmore et al (1997). The
oldest known sediments penetrated on the Londonderry High belong to the
Late Permian Hyland Bay Formation, although possibly older
sediments have been intersected at Osprey 1. The Hyland Bay Formation is
the primary reservoir target of the Londonderry High and comprises limestones,
sandstones and shales deposited in a marine shelf environment. Marine shales
of the Early Triassic Mount Goodwin Formation conformably
overlie the Hyland Bay Formation, and provide a regional seal. The conformable
Sahul Group is a thick clastic sequence of marginal marine
to marine sandstone, siltstone and dolomitic shale. Overmuch of the area,
the Sahul Group is unconformably overlain by the Late Jurassic–Early
Cretaceous Flamingo Group, which comprises predominantly
sandstones, with shale and siltstone interbeds. Hence, the Jurassic Plover
and Vulcan formations, which are the effective source rocks in the Vulcan
Sub-basin and northern Bonaparte Basin, are either thin and immature for
hydrocarbon generation, or are absent in the release areas on the Londonderry
High as a result of extensive erosion. The Plover Formation was intersected
at Tamar 1 in Area W07-2, and to the south of the release area in Plover
1. The Cretaceous Bathurst Island Group comprises a thick,
shale-dominated marine succession that is widespread across the Bonaparte
Basin, and acts as the regional seal.