Page 258 - Petroleum Geology

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Fig. 11-1. Probable source-accumulation relationship in fossil reefs.

It has long been observed that in fields with multiple sandstone reservoirs
heavier oil tends to be at shallower depth and associated with fresher forma-
tion waters; lighter oils tend to be at greater depth, associated with more
saline formation waters. Bunju field, on the island of that name in northern
Kalimantan, Indonesia, is a particularly interesting example of that associa-
tion.
The shallowest reservoirs, in common with many fields of this type, con-
tain asphaltic crude oil, and the transition to paraffinic crudes occurs at about
800 m. The paraffinic crudes become lighter with depth, and at about 950 m
they are 29"API (s.g., 0.84). Below this, they become heavier again until at
1900 m they are 2O"API (s.g., 0.88) with an increase in waxy residue from
20 to 45%. Concomitant with this decrease in API gravity, the salinity of the
formation waters decreases from 10 to 1.8 g/l, with an increase in carbonate
content (Weeda, 1958, pp. 1345-1346). Weeda does not give the tempera-
tures of these reservoirs. The data of Kenyon and Beddoes (1977) suggests
that the geothermal gradient is unlikely to be much less than 30"C/km, so
we may suppose that the temperature at 1900 m is at least 80°C.
While we cannot eliminate bacteria as a possible cause of the changes of
gravity, the facts as presented permit no deep source for the oil. The associa-
tion of crude oil gravities and formation water salinities strongly suggests an
interbedded source, closely related stratigraphically to the reservoirs, with
short primary migration paths. The variation in crude oil gravity appears to
be due to variation in source material due to variation of facies and the envi-
ronment in which the source rocks accumulated.
In eastern Venezuela, oil has accumulated in numerous Tertiary sandstone
reservoirs, in stratigraphic and fault traps, in several fields. Hedberg et al.
(1947), in a classic paper that should still be read, described the area and
examined the geological evidence for the position of the source of the oil.
Most of the crude oil comes from the Oficina Formation, but not all the
sands contain oil or gas. The oil varies greatly in composition, and some of