ORIGIN AND OCCURRENCE OF OIL 41
as a filling of pores with unusually small throats connecting with
adjacent pores, or, to a much smaller extent, as wetting films on the
surface of the mineral grains when the rock is preferentially wet by
water. The water may occur as wetting films, or collars, around the
sand grains as well as in some completely filled pores.
2.2.4 Migration of Reservoir Fluids
The predominant theory assumes that as the sedimentary layers superimposed in the source bed became thicker, the pressure
increased and the compression of the source bed caused liquid
organic matter to migrate to sediments with a higher permeability,
which as a rule is sandstone (or porous limestone).
Primary migration is the movement of hydrocarbons from
mature, organic-rich source rocks to a point where the oil and gas
can collect as droplets or as a continuous phase of liquid hydrocarbon. Secondary migration is often attributed to various aspects of
buoyancy and hydrodynamics (Schowalter, 1979; Barker, 1980) and
is the movement of the hydrocarbons as a single, continuous fluid
phase through water-saturated rocks, fractures, or faults followed
by accumulation of the oil and gas in sediments (traps) from which
further migration is prevented.
It is believed that during migration petroleum does not move
through the bulk of the non-source rock and shale bodies but
through faults and fractures that may be in the form of a channel network that permits leakage of oil and gas from one zone to
another. During this migration, the composition of the oil may be
changed through physical causes, such as filtration and adsorption,
in a manner analogous to the Chromatographie separation of petroleum. Reactions with minerals such as elemental sulfur or even
with sulfur-containing minerals (e.g. sulfates) may also occur.
Source beds do not generally coincide with reservoir rocks and
the belief is that petroleum, before it comes to rest in a trap, migrates
large distances. Various examples are known of vertical migration,
and migration upward for some kilometers is considered possible.
Nevertheless the converse may also be true, and the in situ theory (juxtaposition of source bed and reservoir rock) advocates that
petroleum migrates very little, if at all, and that even the amount of
vertical migration is negligible.
Once the oil has accumulated in the reservoir rock, gravitational
forces are presumed to be dominant, thereby causing the oil, gas,
as a filling of pores with unusually small throats connecting with
adjacent pores, or, to a much smaller extent, as wetting films on the
surface of the mineral grains when the rock is preferentially wet by
water. The water may occur as wetting films, or collars, around the
sand grains as well as in some completely filled pores.
2.2.4 Migration of Reservoir Fluids
The predominant theory assumes that as the sedimentary layers superimposed in the source bed became thicker, the pressure
increased and the compression of the source bed caused liquid
organic matter to migrate to sediments with a higher permeability,
which as a rule is sandstone (or porous limestone).
Primary migration is the movement of hydrocarbons from
mature, organic-rich source rocks to a point where the oil and gas
can collect as droplets or as a continuous phase of liquid hydrocarbon. Secondary migration is often attributed to various aspects of
buoyancy and hydrodynamics (Schowalter, 1979; Barker, 1980) and
is the movement of the hydrocarbons as a single, continuous fluid
phase through water-saturated rocks, fractures, or faults followed
by accumulation of the oil and gas in sediments (traps) from which
further migration is prevented.
It is believed that during migration petroleum does not move
through the bulk of the non-source rock and shale bodies but
through faults and fractures that may be in the form of a channel network that permits leakage of oil and gas from one zone to
another. During this migration, the composition of the oil may be
changed through physical causes, such as filtration and adsorption,
in a manner analogous to the Chromatographie separation of petroleum. Reactions with minerals such as elemental sulfur or even
with sulfur-containing minerals (e.g. sulfates) may also occur.
Source beds do not generally coincide with reservoir rocks and
the belief is that petroleum, before it comes to rest in a trap, migrates
large distances. Various examples are known of vertical migration,
and migration upward for some kilometers is considered possible.
Nevertheless the converse may also be true, and the in situ theory (juxtaposition of source bed and reservoir rock) advocates that
petroleum migrates very little, if at all, and that even the amount of
vertical migration is negligible.
Once the oil has accumulated in the reservoir rock, gravitational
forces are presumed to be dominant, thereby causing the oil, gas,