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UNIT 3
GEOLOGICAL AND PHYSICAL CHARACTERISTICS OF
OIL AND GAS FIELDS

The main reason for the migration and the trapping of
petroleum hydrocarbon in reservoirs is the existing system of static
and dynamic pressures existing in the pores (such as overburden
pressure - pressure due to the weight of overlying sediments an the
average of 1 psi/ft-). The main result of overburden pressure is the
compaction of rock that will reduce the pore space and pushing the
liquid content out of the core. Liquid will try to find the way out to
the upper low pressure sections. During compaction water
covering the rock surface called “connate water”.
Compaction of sediments will remain a permanent process
over geologic time. The squeezed out water is kept moving for the
same time even longer and the path of its migration may cover
hundred of kilometers. Many of these migration paths may lead
into open and the oil droplets will be lost by migration. Others will
be ended by so called traps or reservoirs forming oil or gas fields.
It may take million of years to fill such a reservoir with a
considerable amount of hydrocarbons.

Oil and Gas Migration
After its formation, petroleum may migrate from the source
rock into porous and permeable beds where it accumulates and
continues its migration until finally trapped. Petroleum created
from the decomposed remains of animals and plants (which were
deposited and accumulated in deep sedimentary strata) finds its
way to natural storage basins by traveling through porous rocks or
layers of rocks. The tiny spaces or pores of sandstone or the ores
or cracks of limestone, dolomite or other sedimentary rocks, form
the “avenues” by which the petroleum products wind their way
upward, carried by their original salt water environment. The oil,
gas and water eventually stop when they reach a structure or trap,
having a cap rock seal, that forms a reservoir to hold them; that is
what the oil and gas industry took for in exploration operations.

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