Page 61 - Geology and Geochemistry of Oil and Gas

38 OIL- AND GAS-BEARING ROCKS
the sheet-type (bedded) rather than the massive-type reservoirs). Inasmuch as the
bioherms are very similar to reef buildups, they should be considered as massive
reservoirs.
To form a trapping mechanism, beside the caprock, the massive reservoirs require
isolating steep lateral limitations. As an example, numerous present-day coral reefs
in the Indian Ocean do not form subsurface reservoirs not only because of the
absence of caprock, but also due to the absence of lateral barriers (lateral isolation).
Fresh water accumulating within such bodies floats on the surface of heavier sea-
water.
The hydrodynamic system of the massive reservoirs is poorly studied. It is possible
that they communicate at depth with the bedded (sheet-type) reservoirs and are, in
effect, just a veriety of a sheet-type reservoir.
Reservoirs lithologically limited from all directions include all types of reservoirs
where the liquid or gaseous hydrocarbons present from the time of formation of the
reservoir are surrounded from all directions with practically impermeable rocks.
Fluid movement within such reservoir is limited by its size. There is some superficial
similarity between the massive reservoirs and the differentiated sheet-type reservoirs.
The similarity is in the limitation (lithologic isolation). The difference is in the timing
of the emergence of the latter. The massive reservoir is a result of depositional
processes, whereas the differentiated sheet-type reservoir is a result of stresses during
the basin subsidence. The former type is originally small (certainly, any bed is a large
lens, but this approach is not used here). The latter type is a separate portion of a
previous, possibly large hydrodynamic system. Prevalent elastic energy is typical for
both, but the latter type of reservoirs has a greater stress level. The capacity of any
type of reservoir is defined by its size and reservoir-rock properties. Energy of a
reservoir is associated with its capacity, and the energy is what is important for the
extraction of oil and gas (and associated water).
The identification of the above four reservoir types is tentative, because such well-
rooted concepts as reservoir rock and caprock (fluid barrier) are also tentative. Even
in the same state, the same rock may be a fluid barrier to one fluid and a reservoir
rock to the other, depending on the physicochemical properties of fluids and rocks
(especially, wettability), and on the subsurface temperature and pressure. For in-
stance, a prominent projection of a massive reservoir may be just a complication of a
regional sheet-type reservoir and be a part of the same hydrodynamic system. This
phenomenon is especially common in carbonate sequences. It is possible to imagine a
conversion of a sheet-type differentiated (block) reservoir into a massive reservoir if
the caprock loses its sealing properties over a fault or flexure. The hydrocarbon
accumulation may then occur underneath another, shallower caprock.