Page 181 - Geology and Geochemistry of Oil and Gas

150 FORMATION OF HYDROCARBON ACCUMULATIONS

Provenance areas of clastic sediments were relatively close to the shelf and
marginal sea basins, whereas the other sediment accumulation areas are hundreds
and even thousands of kilometers away from the source. This pattern may be broken
outside the shelf area only by turbid flows of major rivers. Rate of deposition and the
total thickness of the sediments significantly decrease from the marginal to the
central parts of the ocean. The bulk of organic matter is deposited over the marginal
zones. With increasing distance from the marginal zone, the amount of organic
matter drastically declines. It enters the sediments in a significantly altered form due
to long travel. Finally, common to all oceanic basins is a total absence of
aerodynamic weathering.
The conditions for the formation of clastic reservoir rocks are quite favorable in
the oceanic sedimentary basins. A high pore pressure due to the high water column
prevents compaction of deposits. Thus, good reservoir-rock properties can be
preserved before the secondary cementation processes. It is well known that the
‘‘marine’’ reservoir rocks are superior to the ‘‘continental’’ ones.
If marine carbonates are sufficiently compacted (near-shore environment; shelf;
some intraoceanic highs) and in the presence of water movement carbonate reservoir
rocks can often form due to erosion. Fractured reservoirs may occur only in rocks
that reached a certain degree of rigidity.
Compaction can occur in the sediments of the shelf and marginal seas; also in
some exceptional situations in other portions of the World Ocean. The latter can
involve the uplift resulting in an increased compaction.
The formation of the seals (caprocks) is a somewhat more involved issue.
Argillaceous caprocks form only if sediments are sufficiently compacted, which is
unlikely when the pore pressure is high. That is why numerous wells drilled in the
World Ocean encountered only uncompacted sediments. The compacted deposits
forming caprocks are only found over the shelves, within marginal seas and,
sometimes, over the continental slope. Under favorable depositional conditions in a
specific area, the formation of seals from evaporites also requires some compaction.
Seals may form in the upper sediment section also due to gas hydrates. Although gas
hydrates were discovered in many locations of the World Ocean, hydrocarbon
accumulations were found underneath these seals. Thus, the oceanic environment is
favorable for the formation of reservoir rocks, whereas the conditions for the
formations of seals are possible only within the ocean’s margins.
Organic matter introduced into the water of oceans was deposited mostly in the
oceanic margins. Any organic matter and hydrocarbons in the internal basins would
be formed at the expense of biologic resources. The formation of hydrocarbon
accumulations, however, would be problematic. The reason for this statement is that
prior to the sediment compaction, the avenues for the vertical migration in those
sediments will prevail over the lateral migration, and the dispersion processes in the
water medium will prevail over the accumulation processes.
In conclusion, the World Ocean should not be included as the area of vast
undiscovered oil and gas reserves. Relatively high potential may be expected only
within about 15% of the World Ocean area (marginal zones), whereas interoceanic
basins have low to zero potential.