Page 260 - Petroleum Geology

3. The persistent shale bodies which lie above and below the various sands of the Oficina
Formation should theoretically have served as effective barriers to vertical migration be-
tween sands.
4. The faults of the Greater Oficina area, while numerous, definitely have acted as bar-
riers rather than avenues of migration (West Guara cross section, Figure 15).
5. Sands of the basal Freites Formation immediately above the Oficina Formation in
the present fields invariably are barren of oil. These sands rest conformably on the Oficina
Formation and are separated by no greater shale barriers from the Oficina producing sands
than separate these latter from each other. These basal Freites sands occupy the same rela-
tion to structural traps, are of essentially the same degree of continuity, and are covered
by the same seal (Freites shale). It is hard to find any explanation of the absence of petro-
leum in the Freites Formation in the present fields other than that source material was
lacking.
6. The general tendency towards heavier oil with depth (although broken by numerous
exceptions) appears to accord with the general tendency toward more brackish and less
marine environment of deposition toward the base of the Oficina Formation.
7. Certain sands within the oil-producing section of the Oficina Formation are locally
barren of oil, although overlain and underlain by productive sands. The only reason for
these differences, provided there is no leak in the fault barrier forming the trap, seems to
be in the environment of deposition of the sands or of the adjacent shales.
Evidence for the geographically local origin of oil found at any particular horizon
(limited lateral migration) is as follows.
1. There are marked changes in gravity and character of oil in individual sands from
one area to another. For example, as mentioned in the previous section, the S sand pro-
duces 16OAPI oil in the segment just north of the Oficina fault, 41°API oil in the OG-116
area 7 kilometers distant, 3O"API oil in the OG-187 area, 7 kilometers farther west, 25'
API oil in YS-17, 13 kilometers still farther west, and 44OAPI wax-oil in the Agua Clara
field. Yet electrical-log correlation shows that essentially the same sand is represented in
all these cases.
2. Sands in the upper part of the Oficina Formation are barren of oil in the southern
and western parts of the Greater Oficina area in spite of favorable structural traps. These
same sands contain oil farther north and east where more marine conditions apparently
prevailed during the deposition of these beds.
3. Southward and westward from the Greater Oficina area where the whole Oficina
Formation becomes definitely less marine, heavy tar-oil replaces the lighter oils known in
the Greater Oficina fields.
4. Oil is commonly found in the Greater Oficina area in lenticular sands of small lateral
extent. It is difficult to see how it could have reached these sands by any extensive lateral
migration.
Considering the evidence outlined, the writers are inclined to believe that the oil pro-
duced from the Greater Oficina area had its origin largely in the shales of the Oficina For-
mation immediately above and below each of the productive sands and that it migrated
laterally only moderate distances in these sands within the Greater Oficina area. . .

They later note (Hedberg et al., 1947, p. 2138) that some lateral migra-
tion is required, and that the fact that almost all accumulations are on the
basinward side of faults and other barriers to migration suggested to them
that this had been up the regional dip, radially away from the centre of the
Eastern Venezuelan basin. Although some of the faults are growth faults
(Hedberg et al., 1974, pp. 2115, 2156, 2168 and fig. 18), the main faulting
that caused the accumulations occurred later, when there was a kilometre or