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A-17 A -14
FORTESCUE-1 FORTESCUL-3 WEST HALIBUT-1 A - 12 A-19
I I I I I II

DEPTH FORTESCUE HALIBUT

Fig. 13-14. Cross-section through FortescuP and Halibut fields, Gippsland basin. (Courtesy
of Esso Australia Ltd.)

The general similarities of the accumulations - their stratigraphy and trap
configurations - suggests that these fields have similar source-reservoir rela-
tionships. The choice, therefore, is between non-marine pre-unconformity
source rocks and marine post-unconformity source rocks. Although the post-
unconformity Lakes Entrance Formation contains a small quantity of heavy,
15.7"API, asphaltic crude oil near the coast at Lakes Entrance itself (from
which the formation name came), which was exploited from 1925 to 1956
for a total of only 1300 m3, there is little evidence that it was the source of
the offshore oil and gas. This small heavy oil accumulation occurs where the
Lakes Entrance Formation lies nonconformably on Devonian granite, and
has been severely degraded during secondary migration. It could have been
sourced from contiguous formations down-dip, offshore. Thomas (1982) re-
ported that geochemical studies of the offshore Latrobe Group had revealed
abundant potential source rocks, rich in oil- and gas-generating macerals (re-
sinite, cutinite, sporinite). Given the right conditions, these could have gen-
erated oil in some places, gas in others. These studies found the post-uncon-
formity mudstones organically barren.
There seems little doubt, therefore, that the Gippsland basin petroleum
was generated from pre-unconformity, non-marine source rocks. These are
likely to be close stratigraphically to the reservoirs. A certain amount of
migration across stratigraphic boundaries is likely from the evidence of the
fields themselves, but the occurrences of small oil accumulations in the gas