FLOUNDER — A COMPLEX INTRA-LATROBE OIL AND GAS FIELD

Abstract

The Flounder Field is the deepest producing field in the Gippsland Basin. Since discovery in 1968 by Flounder 1, five delineation wells and 15 development wells have been drilled on the structure. The main T-1.1 oil and gas reservoir is trapped at the crest of a highly faulted anticline within the Latrobe Group. The Flounder structure has a complex deformational history with the Latrobe Group sequence undergoing two main phases of deformation. Late Cretaceous to Late Paleocene north-west trending normal faults are overprinted by a Late Eocene to mid Miocene north-east trending anticline. Generally within the Latrobe sequence in the Gippsland Basin, faulting destroys the integrity of the anticlinal features by breaking the lateral continuity of potential intra-formational seals. However, at the Flounder Field, the estuarine sands of the T-1.1 reservoir are overlain by a marine shale of adequate thickness to provide an effective seal across the faults.The T-1.1 oil and gas reservoir has excellent reservoir parameters. Separate gas caps are trapped in the multiple faulted crests of the structure and have had a major influence on the development of the field due to the resultant variation in gross oil column thickness.In addition, several small oil accumulations have been structurally and stratigraphically trapped in sediments filling the Tuna-Flounder Channel and the deeper Latrobe Group sequence.The Flounder Field commenced production in December 1984. Current estimated reserves for the T-1.1 reservoir are 155 billion cubic feet (BCF) (wet) gas and 115 million barrels (MMB) of oil, a dramatic increase over the 1978 pre-development estimated reserves of 86 BCF (wet) gas and 57 MMB oil.