Page 52 - Geology and Geochemistry of Oil and Gas

CAPROCKS 29
belongs to the fractures formed as a result of weathering and leaching (in particular,
hydrothermal leaching). High-quality reservoirs in such sequences, however, are
sporadic. One may encounter very diverse oil flow rates in the adjacent wells, from
almost nil in one well to a few thousand tons a day in the next one.
Shaly sequences are very common. The shales serve as a confining medium or
local seal, whereas the reservoir rocks within the shales consist of lenses and inter-
beds of sands, sandstones, and carbonates. At the beginning of the 20th century,
oil and gas flows were obtained from fractured schists in California, in some other
areas of the world, and from the bituminous shales of Bazhenov Formation in
western Siberia. As a rule, shales serving as reservoir rocks were significantly altered
during catagenesis and are in effect in-between shales and schists. Klubova (1984)
stated that these shales/schists ‘‘are mostly composed of illite, include significant
amounts of dispersed organic matter, and are silicified. Commercial capacity of
these rocks is due to the presence of a rigid silica frame and of organic matter sorbed
by clay minerals. Organic matter makes the surface of clay mineral monocrystals
hydrophobic, thereby making the contacts between these crystals and with
other rock microcomponents easy to separate, with subsequent release of the con-
tained oil’’.
With respect to the Bazhenov shales in particular, Klubova concludes that mi-
crotextural non-uniformity of the Bazhenov shales results in the presence of weak-
ness zones and resulting porosity. Zones of weakness are clearly observable in thin
sections and are sometimes misinterpreted as fractures. The separation of the rocks
along these zones of weakness causes rock fragmentation, i.e., leads to the same
results as fracturing. Despite a rather rigid silica frame in Bazhenov shales, their
plasticity is quite high, resulting in a low degree of fracturing (o10 fractures/m)
(Klubova, 1984).
Zones of weakness in the Bazhenov Formation have been finalized at the dia-
genetic stage, whereas the separation of these zones and the appearance of poro-
sity (fractures) occurred in tectonically active areas during the catagenetic stage
(Klubova, 1984).
2
Of course, the catagenesis (or epigenesis ) affects the reservoir properties not only
of shales but also of many other reservoir rocks. A systems treatment involves an
analysis (effects of individual factors) first and, then, synthesis. Before doing this,
however, one must review one more component of petroleum sequences, namely,
seals (caprocks).

2.3. CAPROCKS

For a long time, the only force causing the movement of oil and gas in the sub-
surface was believed to be buoyancy. If so, then to form oil and gas accumulation,
their migration paths must have been stopped by a roof, i.e., caprock (seal). Clays,
shales, carbonates, evaporites, and their combinations can form caprocks. Dobrynin
2
Epigenesis includes all processes that affect already formed sedimentary rocks after diagenesis.