GIPPSLAND'S OLD AND NEW OIL

The Gippsland Basin, initiated in the late Cretaceous, accumulated as much as 4,500 m. (15,000 feet) of sediment before the first major structural movement in the early Eocene, when faulted anticlinal structures and pronounced regional westerly dip were developed in the Latrobe Group.Over the next 13 m.y. of the Eocene, sediment supply was reduced and much of it trapped in the western portion of the basin. On the eastern marine edge of the basin the Tuna-Flounder Channel was cut and filled over a period of 4 m.y. Subsequent erosion, sometimes severe, particularly in the Marlin area, created the significant unconformity on top of the Latrobe Group reservoir sediments. Much of that surface was covered with fine-grained marine sediment of early Oligocene age, leaving only a few high-standing areas unsealed for a further period of 25 m.y. until the mid-Miocene.Later structural movements, in the mid-Miocene (10 m.y.B.P.), were largely vertical with some anticlinal warping. New potential traps were created then and some older structures rejuvenated. Following the latter period of anticlinal growth, a major marine channel system was formed by erosion 9 m.y.B.P. and subsequently engulfed by rapid deposition of prograded wedges of sediment on the continental margin.Oil and gas have been formed from land-derived organic matter deposited in the Latrobe Group during late Cretaceous to Eocene times (100.37 m.y.). Subsequently the oil and gas accumulations have developed their distinctive geographical distribution with the major oil fields buried deeper than the major gas fields. It appears that oil has migrated and been trapped at intervals over the last 60 m.y. under varying overburdens from about 100 m. to about 2,000 m. as indicated by the saturation pressures of the crude oils. Migration of oil into the Kingfish and Halibut fields apparently took place no later than 10 to 24 m.y.B.P. Gas migration into Marlin and associated gas fields took place later. There is evidence that oil and gas is forming at present, leading to the conclusion that both old and new oil exist in the basin.