Page 329 - Petroleum Geology

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-1700 TO 2586.3 1 TO 3373 6

VERTICAL EXAGGERATION 10 :1 c 4 KILOMETRES I
Fig. 13-13. Cross-section through Marlin field, Gippsland basin. (Courtesy of Esso Australia
Ltd.)

sealed at their subcrops by the Lakes Entrance Formation. This new field,
Fortescue, has its oil/water contact 24 m deeper than that in Halibut (Fig.
13-14).
The reservoirs of all these fields are fluvial-deltaic sands with good poro-
sity and very high permeability in a predominantly non-marine sequence that
includes coals. The crude oils vary from field to field, but are typically paraf-
fin-base of about 45"API. Halibut crude has a very high wax content of 26.8% ;
Kingfish has 13% wax, and so is also classified as a high-wax crude. The small
oil accumulation in Marlin is a 5O"API crude with 2.7% wax. The gas compo-
sitions are typically 85% methane, 6% ethane and 3% propane. Pressures are
normal hydrostatic.
The concordant water contacts within fields indicates that either the La-
trobe unconformity surface (as it is called) is an imperfect seal in itself, but
seals by virtue of its configuration, or that the reservoir sands themselves are
interconnected. The evidence of the Halibut and Fortescue fields, with their
separate oil/water contacts within a single culmination of the unconformity
surface, suggests that the reservoir sands are interconnected within a field
because here the unconformity surface has effectively sealed the two accu-
mulations. This conclusion is supported by the chemistry of the Halibut and
Fortescue crudes. These are similar, and were probably generated from similar
source rocks; but they are considered to have some differences that would
not exist if they were a single accumulation (Thornton et al., 1980).