Page 79 - Petrophysics

PROPERTIES OF SUBSURFACE FLUIDS 53

of oil is possible, however, if the sand is flushed with large quantities of
hot water. This process may help account for the oil-free sand found
below many hydrocarbon-saturated reservoirs, given the enormous
amount of geologic time accompanied by changes of temperature and
diastrophism.
Secondary migration of petroleum ends in the accumulation in a
structural or stratigraphic trap, and sometimes in a trap that is a complex
combination of the two. Levorsen observed that oil has been found
in traps that were not developed until the Pleistocene Epoch, which
implies that the minimum time for migration and accumulation is about
one million years [18]. The hydrocarbons accumulate at the highest
point of the trap and the fluids are stratified in accordance with their
densities, which shows that individual hydrocarbon molecules are free
to move within the reservoir. Inasmuch as the sedimentary rocks may
have formed during the Cretaceous Period or earlier, it is entirely possible
that the oil accumulation may have been disturbed by diastrophism, and
many changes of temperature and pressure. The petroleum accumulation
may (1) become exposed by an outcrop and develop an oil seep, or
(2) become uplifted and eroded to form a tar pit. In addition, petroleum
may be transported to another sedimentary sequence as a result of rapid
erosion and clastic transport. Levorsen identifies this type of secondary
accumulation as recycled oil, which should be low in paraffins because
of attack by aerobic bacteria [MI. Thus, the geologic history of an oil
reservoir may have been quite varied, and knowledge of the sedimentary
history, origin, migration, and accumulation is valuable for the overall
understanding of oil recovery processes and formation damage that may
develop during production of the oil.
The caprock, or oil trap seal, may not be absolutely impermeable
to light hydrocarbons. The capillary pressure relationship of the rocks
overlying the oil trap may form an effective vertical seal for liquid
petroleum constituents (C5+ hydrocarbons), but the seal may not be
completely effective in retaining lighter hydrocarbons.

PROPERTIES OF SUBSURFACE FLUIDS

A basic knowledge of the physics and chemistry of subsurface waters
and petroleum is essential for petroleum engineers because many
problems associated with exploration, formation damage or production
problems, enhanced oil recovery, wettability, and others are directly
associated with the physical and chemical behavior of subsurface waters
and petroleum as a whole, or as groups of constituents such as paraffins,
asphaltenes, etc.