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Petroleum Potential

The Malita Graben has in the past been viewed to be gas prone given the results of Jacaranda-1 (?tight gas zones in the Upper Vulcan/Flamingo), Heron-1 (gas shows in the Flamingo Formation) and Evans Shoal-1 (Plover Formation gas). The Echuca Shoals Formation contains potential oil prone source rocks with oil expulsion probably occurring initially in the Late Cretaceous but with only gas being expelled from the Early Tertiary onwards. The Elang and Plover formations probably expelled oil in the Malita Graben in the mid Cretaceous while gas was expelled from the Early Tertiary onwards. The main play types in this region are Cretaceous drape closures, Oxfordian drape and fault bound closures, Oxfordian-Callovian subcrop plays and Jurassic/Triassic tilted fault blocks/horsts (Figure 6). Hanging wall fault plays are also viable on the downthrown faulted margins of the Malita Graben. The relatively shallow Maastrichtian sequence offers a secondary target.

In NT03-4 the Darwin Shelf is a structurally positive feature variably eroded during episodes of peneplanation. It is blanketed by a thin Mesozoic section. A comprehensive review of the area is presented in Colwell and Kennard (1996). The main plays in this area relate to drape of Flamingo Formation/Darwin Formation over the rugose basement topography. The main risk is closure against the strong regional dip. Flamingo Formation onlap plays occur where these sands infill and onlap the eroded basement topography.

To the south in NT03-5, which extends to the flanks of the Petrel Sub-basin, the dominant plays are combination structural and stratigraphic traps within the Late Jurassic and Early Cretaceous sequences. In NT03-6 drape and fault closures at Permian levels provide the main play type. Carboniferous pinchout plays and Jurassic closures are secondary targets.

In this general area of the northern Petrel Sub-basin there are only a few wells which provide limited evidence of existing petroleum systems. Gas discovered in the Hyland Bay Formation reservoirs of the Petrel and Tern accumulations is likely to be have been generated from the Keyling or Hyland Bay Formations. The Mount Goodwin Formation provides regional seal for Hyland Bay Formation reservoirs, which are the most important in the Petrel Sub-basin.

Salt flow towards the Gull and Curlew diapirs has created four way closures around their fringes. These closures are expected to comprise sandstones of the Hyland Bay Formation sealed by shales of the Mount Goodwin Formation. Potential traps have formed as a result of salt withdrawal towards the nearby diapirs. The Petrel and Tern gas accumulations are examples of structures created by salt movement.

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