OIL AND GAS PROSPECTS OF THE GELTWOOD BEACH ANTICLINE, MILLICENT, SOUTH AUSTRALIA

Abstract

The Geltwood Beach (buried) anticline is located near the ocean coast in south-east South Australia, directly west of the agricultural and industrial town of Millicent. The structure is developed in Mesozoic to Tertiary sediments forming the inner part of the continental sedimentary terrace which in this situation coincides also with the Nelson "half-graben".

The Geltwood Beach anticline is more than five miles long by two or more miles wide. It is part of a still larger regional development which pitches south-east into the deeper known portions of the Gambier-Otway Cretaceous to Tertiary Basin. There is no surface expression to the structure.

Structural "closure" on the base of the Tertiary may not exceed 100 feet, but an extensive area of structural flattening along the crest of the anticline (defined by structural drilling and geophysical techniques) overlies a zone of extensive sedimentary wedge-out within the predicted and prospective cretaceous sediments in depth. The wedging is predicted to be in the nature of progressive overlap onto structural "nosing" or alternatively, buried-ridge development in presumed Otway Group sediments in depth.

A thickness of 5,000 to 8,000 feet of unconsolidated Upper Cretaceous to Tertiary sediments, wedging to the north-west along the crest of the anticline in the deeper developments is expected to include the prospective Belfast Mudstone equivalents and related beds of the Port Campbell (Victoria) Association.

The Geltwood Beach structure lies approximately half way between the Mt. Salt No. 1 and Beachport No. 1 wells. In the distance of 45 miles between the latter wells, the dominantly Cretaceous (post-Otway) sedimentary section wedges spectacularly from 7,000 feet (possibly considerably more) to no more than 100 feet. The available geophysical evidence suggests that most of this wedging occurs within the zone of the Geltwood Beach anticline. For this reason, the anticline is believed to be well located for the development of structural and stratigraphic traps in a marginal continental shelf environment of proven thick sedimentation.

In the Mt. Salt No. 1 well, clays and shales encountered at at least five stratigraphic levels within the Lower Tertiary to Middle Cretaceous section provided adequate capping to underlying highly porous and permeable reservoir sands, the lowermost of which were brine-bearing.

Geltwood Beach is a locale of preferred coastal bitumen stranding. The weight of published evidence now points to nearby submarine seepage within the reach of erosive storm waves: recorded earthquakes in this vicinity are known to have greatly affected the activity of these seepages.

The conclusion is reached that the Geltwood Beach anticline is favourably situated up-dip on the inner continental shelf margin to accumulate hydrocarbons in potentially commercial quantities. The structure lies south of the Beachport-Kalangadoo "hinge-line" of the Nelson half-graben in a zone of submarine oil seepage. A proposed deep test well to be located near the culmination of shallower structure is expected to provide a satisfactory test in respect to both structural and (to a lesser extent) deeper stratigraphic entrapment of petroleum.