Page 60 - Geology and Geochemistry of Oil and Gas

OIL AND GAS RESERVOIRS 37
constant over significant areas. Still, a substantial thickness change can occur locally
or at the edge of the reservoir development, which may result in a pinch-out of the
reservoir rock. The reservoir rocks in bedded reservoirs are usually lithologically
continuous, but may have a more complex nature. A bedded reservoir may have a
single hydrodynamic system.
Reservoir energy in bedded reservoirs is distributed in accordance with the hy-
drostatic or hydrodynamic environment of the artesian basins. However, reservoirs
with that kind of energy distribution are typical only for the uppermost portion of
the sediment cover. As a result of subsidence and sediment compaction and various
secondary geochemical processes, reservoirs may be separated into diverse portions
as a consequence of previously described phenomena.
Leaving aside changes in the sediment composition, drastic changes occur in the
major reservoir-rock properties (porosity and permeability). Even if prior to sub-
sidence the reservoir rocks were reasonably uniform in terms of porosity and per-
meability, subsequent to subsidence non-uniformities appear between various
portions of reservoir so that they may turn out to be totally separated from one
another. An indication of such a change may be a change in a hydrodynamic drive
from artesian to ‘‘elision’’ type and the appearance of abnormally high pressure. The
beginning of the process involves (1) lateral fluid migration, and (2) gradual change
in the reservoir energy. Potential energy of the accumulations relative to the total
energy of the reservoir is small. As the sheet-type reservoir differentiates, lateral
migration becomes increasingly more obstructed, with formation of numerous hy-
draulic fractures. Fluid migration from the reservoir to other favorable zones (if they
are available) may become prevalent. An increase in the elastic potential energy is
observed (Abnormally-High Formation Pressure, AHFP). Energy distribution be-
comes discrete. The difference in potential energy between the accumulations and the
reservoir as a whole becomes smaller, and within some zones (blocks) they become
identical.
Thus, it is reasonable to recognize a separate type, i.e., differentiated sheet-type
reservoir, which under certain circumstances becomes a bedded reservoir.
Massive reservoir is a thick permeable sequence overlain at the top and restricted
from the sides by low-permeable rocks. Its bottom may be at a depth that has not yet
been penetrated by wells (e.g., Tengiz Field, Kazakhstan). Reservoir rocks com-
prising massive reservoirs may be homogeneous or heterogeneous. Homogeneous
massive reservoir rocks may be carbonates and metamorphic or volcanic rocks.
Their porosity and permeability is due to the presence of vugs and fractures. Porous
and permeable zones in massive reservoir rocks are not stratigraphically related.
Isolated high-porosity and high-permeability zones cutting through stratigraphic
surfaces within a body of a massif are common.
Buried reefs are often assigned to this reservoir type. Among the best examples are
the Ishimbay group of fields in Bashkortostan, Russia, and Rainbow Oilfield in
Alberta, Canada. Usually, the thickness (height) of massive reservoirs is greater than
the width. The length of possible vertical fluid migration is similar or greater than the
lateral migration within the beds. The flanks of the reservoir and its contacts with the
contemporaneous sediments are steep (thus, the biostromes should be classified as