Page 259 - Petroleum Geology

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it is waxy. The oil gravities vary also, both within and between reservoirs.
The range within reservoirs is probably due to gas de-asphalting (Renz et al.,
1958, p. 596; Evans et al., 1971, pp. 154-159). The totalranges of APIgravi-
ty are listed in Table 11-1: these are for the Oficina Formation in West Guara
field, from top to bottom over a depth range from about 1500 to 2500m.
The temperature of the M and N reservoirs, some 240 m above the deepest
horizon listed in Table 11-1, is about 88°C (Hedberg et al., 1947, p. 2132,
table 1x1.

TABLE 11-1

Total ranges of API gravity in successive reservoirs in the Oficina Formation, West Guara
field, E. Venezuela. (Data from Renz et al., 1958, p. 587, fig. 21)
OF.l 20:-21° OF.9 33"-51: D 14544, J 14O--35" P lO:-l5:
OF.5 100-371 OF.ll 241-56, E 130-431 L 101-3100 R 10o-l&o
OF.6 32,-48" OF.13 44"-47" F 13 -46 M 10-26 S 10-16
OF.7 36,-47, AB 210-46" H 14"-44" N 11,-22: T 11"-141
OF.8 22 -50 C 12 -47 I 12"-47" 0 11"-22 U 11'-14

The salinity of the formation waters also varies, and there is a tendency
for the heavier crude oils to be associated with the less saline waters. The in-
crease in crude oil gravity, decrease in API gravity, towards the base of the
Oficina Formation coincides with a general decrease in salinity and a trend
towards less-marine environments of deposition -that is, locally transgressive.
Hedberg et al. (1947, pp. 2136-2137) are well worth quoting in full:

The writers believe that the multiple-sand character and intricate system of segments
and reservoirs in the fields of this area present an exceptional opportunity for the study
of matters concerning place of origin, migration, and accumulation of petroleum, not al-
ways afforded by simpler single-reservoir fields. Evidence for the origin of the Oficina oil
at stratigraphic horizons very close to those in which it is at present found may be sum-
marized as follows.
1. In spite of essentially conformable deposition, essentially identical age, and a com-
mon geological history, there is a marked variation in the character of the oils found in
different sands within the same trap segment. This variation includes a range in gravities
from less than 10" to 57"API, differences in wax content from a negligible amount to
more than 15 per cent, differences in dissolved gas values, variations from undersaturated
oil to all gas, and differences in color, sulphur content, and other qualities. Moreover, these
variations show no more systematic relation to depth than is called for by the general
changes in environment of deposition from bottom to top of the Oficina Formation, and
marked differences in these qualities are commonly found to occur from one sand to the
next in the stratigraphic succession. A striking example is the presence of the highly waxy
D and E sand production of the Oficina field in the middle part of the productive section,
overlain and underlain by sands producing non-waxy oil.
2. Differences in the salinity of water associated with different sands in any one struc-
tural trap are not compatible with extensive migration from one bed to another or mixing
of fluids across stratigraphic boundaries.