ORIGIN AND OCCURRENCE OF OIL 39
Petroleum accumulations are generally found in relatively
coarse-grained porous and permeable rocks that contain little or no
insoluble organic matter. It is highly unlikely that petroleum originated from organic matter of which no trace remains now. Rather,
it appears that petroleum constituents are generated through geochemical action on organic detritus that is usually found in finegrained sedimentary rocks. In addition, it may be anticipated that
some of the organic material would also change sufficiently from
the original organic material to remain in the sediment through and
beyond the oil-generating stage. Thus, it is generally assumed that
the place of origin of oil and gas is not identical to the locations
where it is found. The oil has had to migrate to present reservoirs
from the place of origin.
When sediments such as mud and clay are deposited, they contain water. As the layers of sediment accumulate, the increasing
load of material causes compaction of the sediment and part of the
water is expelled from the pores. Migration of oil probably begins
at this stage and may involve a substantial horizontal component.
Initially, the oil hydrocarbons may be present in the water, either
suspended as tiny globules or dissolved, with hydrocarbons being
slightly soluble in water. The loss of hydrocarbons from a trap is
referred to as dismigration.
However, the processes by which petroleum migrates into pools
are not well defined and are speculative (Table 2.2), but it is known
that crude oil and gas occur in pools or fields in which the oil or gas
occupies pore space in the rock. Some pools are large, extending laterally over many square kilometers, with their vertical extent ranging from several meters to as much as several hundred meters. The
crude oil or gas occupying the pore space in the pool has displaced
the water that was initially present in the pores. The presence of a
pool of oil or gas therefore implies that the oil or gas has migrated
into the pool. Within the pool, there may be a single phase (oil and
gas dissolved in water) or a multiphase (separate water and hydrocarbon phases) fluid system.
2.2.3 Distribution of Fluids in the Reservoir
The specific gravity of gas and petroleum, the latter generally
between specific gravity (at 60°F, 15.6°C) that varies from about 0.75
to \ .00 ($7 to Λ0° ÄP\>, "with the specific gravity oí most crude oils falling in the range 0.80 to 0.95 (45 to 17°API), are considerably lower
than those of saline pore waters (specific gravity: 1.0 to 1.2).
Petroleum accumulations are generally found in relatively
coarse-grained porous and permeable rocks that contain little or no
insoluble organic matter. It is highly unlikely that petroleum originated from organic matter of which no trace remains now. Rather,
it appears that petroleum constituents are generated through geochemical action on organic detritus that is usually found in finegrained sedimentary rocks. In addition, it may be anticipated that
some of the organic material would also change sufficiently from
the original organic material to remain in the sediment through and
beyond the oil-generating stage. Thus, it is generally assumed that
the place of origin of oil and gas is not identical to the locations
where it is found. The oil has had to migrate to present reservoirs
from the place of origin.
When sediments such as mud and clay are deposited, they contain water. As the layers of sediment accumulate, the increasing
load of material causes compaction of the sediment and part of the
water is expelled from the pores. Migration of oil probably begins
at this stage and may involve a substantial horizontal component.
Initially, the oil hydrocarbons may be present in the water, either
suspended as tiny globules or dissolved, with hydrocarbons being
slightly soluble in water. The loss of hydrocarbons from a trap is
referred to as dismigration.
However, the processes by which petroleum migrates into pools
are not well defined and are speculative (Table 2.2), but it is known
that crude oil and gas occur in pools or fields in which the oil or gas
occupies pore space in the rock. Some pools are large, extending laterally over many square kilometers, with their vertical extent ranging from several meters to as much as several hundred meters. The
crude oil or gas occupying the pore space in the pool has displaced
the water that was initially present in the pores. The presence of a
pool of oil or gas therefore implies that the oil or gas has migrated
into the pool. Within the pool, there may be a single phase (oil and
gas dissolved in water) or a multiphase (separate water and hydrocarbon phases) fluid system.
2.2.3 Distribution of Fluids in the Reservoir
The specific gravity of gas and petroleum, the latter generally
between specific gravity (at 60°F, 15.6°C) that varies from about 0.75
to \ .00 ($7 to Λ0° ÄP\>, "with the specific gravity oí most crude oils falling in the range 0.80 to 0.95 (45 to 17°API), are considerably lower
than those of saline pore waters (specific gravity: 1.0 to 1.2).