Gas discoveries dominate in the central portions of the Petrel sub-basin. It conformably overlies the Treachery Shale and is in diachronous contact with the overlying Fossil Head Formation. At Petrel and Tern fields, the Hyland Bay Formation is 400-500 m thick and is characterized by sandstones and mudstones with minor coals and limestones.
Good permeability is indicated by high flow rates.
Trap Types Anticlines are the most common type of trap for accumulations of both oil and gas in the petroleum system. Draping structures are also important.