Page 226 - Geology and Geochemistry of Oil and Gas

ACCUMULATIONS BASED ON THE TYPES OF TRAPS 195
central areas of tectonic plates, the traps are gentle and sometimes very large.
Over the plate margins, transition zones and, especially, collision zones the
folds are higher, steeper and with a clearly expressed trend. The accumulations
may be classified using some other parameters (mentioned earlier), too. In
particular, oil–water contours in such accumulations are closed and, in plan
view, have oval or more intricate shapes, and form rings.
II. Traps formed within various buildups. Accumulations formed within various
buildups are usually associated with the massive-type reservoirs. Most common
are accumulations in biogenic buildups (reefs and bioherms). Sometimes, biost-
romes are mistakenly attributed to the same class. Included here are large
accumulations with huge flow rates due to the presence of fractures and vugs in
carbonates. Some investigators also include in this group the erosional projec-
tions of the metamorphic and volcanic rocks (fault-bounded or bounded by
erosional surfaces), which may contain accumulations, e.g., White Tiger Field
in Vietnam.
III. Traps that are limited by the depositionally imposed facies changes. Lithologic
and stratigraphic traps of Group III include facies pinch-outs, stratigraphic
unconformities, and contact of the reservoir with the impermeable rock upsec-
tion. Such traps may be associated with the bedded reservoirs on the monoc-
lines or on the flanks of anticlines. These traps may contain rather large
accumulations. They may be associated with bedded reservoirs confined on
every side. In such a case, they form large accumulations. Water saturation
contours impinge on the trap (impermeable barrier). This type of accumulations
is very common: about 50% of all known accumulations.
Accumulations of Types I, II, and III are formed in accordance with the grav-
itational (‘‘anticlinal’’) theory. By far, not all known accumulations, however, belong
in the described three types or combinations thereof. Also, not all of them formed in
accordance with the gravitational theory. These unconventional accumulations are
discussed below (Types IV through VII).
IV. Dominance of capillary forces over the gravity force. Oil or gas found in hy-
drophilic rocks occupy coarser-grained reservoir rocks, which are sealed by
water-saturated fine-grained reservoir rocks. Examples of such accumulations
associated with the relatively coarse-grained sandstone lenses (e.g., 100-ft sand-
stone in Appalachian oil and gas province, USA) were presented by Brod
(1957). The authors of this book reviewed large number of commercial accu-
mulations all over the world and have not been able to find another such clear-
cut example. Although the appearance of capillary forces is frequently ob-
served, the formation and preservation of the accumulations cannot be attrib-
uted to these forces. It should be kept in mind that water and gas lenses exist
within oil accumulations; water is sometimes encountered updip in pinched-out
reservoirs (e.g., Productive Series of the Absheron Peninsula in Azerbaijan, and
Maykopian sandstones in the Northwest Caucasus).
V. Dominance of hydraulic forces. The hydraulic forces (Fig. 10.12) can cause a tilt
in the oil–water interface. To determine that tilt, a trigonometric function (see