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Regional Geology

Block NT03-3 includes a portion of the Malita Graben which is a symmetrical depocentre, initiated during the Triassic, and bordered by the Sahul Platform to the northwest and the Darwin Shelf to the southeast. The graben is bound by east-northeast trending faults with large displacements. This flexural depression probably developed on the inboard part of an upper plate rift margin (O'Brien, 1993). The central graben has not been drilled and Mory (1988) suggests a Mesozoic and Cainozoic sediment pile exceeding 10 km, underlain by Precambrian basement. The pre-rift Plover Formation is expected to thicken into the graben, possibly containing Early Jurassic source rocks . The sedimentary record of the earliest synrift (i.e. Callovian - Oxfordian) is uncertain, as is the presence of key source rocks of Oxfordian age present to the west in the JPDA. Recent interpretations suggest the key Oxfordian sequence, which includes organic-rich rocks elsewhere, thickens into the Malita Graben and could be up to 700 m thick. Thick shales of the Flamingo Group are expected and also Tithonian turbidite sands, both of which were intersected in Heron-1. The Cretaceous Bathurst Island Group exceeds 2,000 m in thickness in the Malita Graben. The basal unit, the Echuca Shoals Formation, is an important potential source rock, and may thicken significantly into the Malita Graben.

Areas NT03-4 to 7 cover the major part of the Darwin Shelf and the northeastern part of the Petrel Sub-Basin of the Bonaparte Basin. The Darwin Shelf has long been a structurally positive province and is flanked by the Petrel Sub-basin to the southwest and by the Malita Graben to the northwest (Figure 2). It consists of Proterozoic basement rocks although Lee and Gunn (1988) have suggested the northern Petrel Sub-basin is underlain by early Palaeozoic oceanic crust.

The structural and stratigraphic setting of the Petrel Sub-basin is most recently discussed by Kennard et al. (2002). This sub-basin contains mainly Palaeozoic and Mesozoic sediments underlain by Proterozoic crystalline basement and sediments of the Proterozoic Kimberley Basin (Colwell and Kennard, 1996). The Phanerozoic formations are 15km in thickness in the northern/central Petrel Sub-basin.

The Petrel Sub-basin architecture was initially governed by rifting during deposition of the Antrim Plateau Volcanics (Early Cambrian). The overlying section contains extensive evaporite deposits of Ordovician, Silurian or Devonian age; salt withdrawal was an integral part of forming the anticlines hosting the Petrel and Tern gas accumulations.

The Late Devonian-Carboniferous rift-sag system was orthogonally overprinted in the latest Carboniferous-Early Permian by northeast trending rift basins to form the proto-Malita Graben along the northwest margin of the Petrel Sub-basin. A succession of northwest-thickening shallow marine to fluvio-deltaic Permian and Triassic sediments was deposited across the Petrel Sub-basin.

Late Triassic compressional inversion produced numerous fault related inversion structures and anticlines within the sub-basin (O'Brien et al., 1996). This structuring was followed by widespread deposition of Lower-Middle Jurassic fluvio-deltaic clastics (Kennard et al., 2002).

Thick Late Jurassic, rift related sequences contain organic-rich source rocks but these lack maturity in the Petrel Sub-basin (Messent et al., 1994). Post-rift regional thermal subsidence resulted in the deposition of a thick Cretaceous-Tertiary passive margin succession deposited in a northwest-plunging synclinal sub-basin.

Late Miocene-Pliocene convergence of the Australian and Eurasian plates reactivated faults in the area of the Malita Graben but this event had little effect on the Petrel Sub-basin (Kennard et al., 2002). Regional tectonic elements are displayed in Figures 2 and 3, and the stratigraphy is shown in Figure 5.

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Petroleum Systems

At least two active petroleum systems have been identified and defined in the offshore portion of the Petrel Sub-basin (Kennard et al., 2002):

  1. An Early Carboniferous oil and gas system probably sourced from marine mudstones of the Milligans Formation, and,

  2. A Permian gas (and possible oil) system sourced form either pro-delta marine mudstones of the Hyland Bay Formation or shallow marine and coastal plain mudstones and coaly mudstones of the keyling Formation.

Maps and schematic chronostratigraphic sections of these systems are presented in Figures 20 to 24 in Kennard et al. (2002).

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