Geology of Southern California.pdf - Grossmont College

26 Geology of Southern California
� Figure SC.42 The light brown exposures of 17-million-year-old
lava and ash flows in the western Transverse Ranges.
organic-rich marine sediments were subjected to elevated
temperatures and pressures. The original organic residues
were converted to hydrocarbon compounds that comprise
oil and natural gas. Once formed, the fluid and gaseous
hydrocarbons can migrate through rock bodies in response
to subsurface pressures.
The formation of a hydrocarbon reservoir in the subsurface
requires some mechanism to interrupt the subsurface
flow oil and gas and the presence of porous or
fractured rock that can absorbed the fluids and gases.
Porous Cenozoic sandstone is an important reservoir
rock in many southern California oil fields. In addition,
the many faults and folds in the region create structural
traps (Figure SC.43) that block the flow of hydrocarbons
and allow them to saturate the underground reservoir rock.
The combination of organic-rich source rock, porous
reservoir rock, and structural traps occurs in many areas in
the Transverse Ranges and adjacent interior basins.
Examples include the Los Angeles basin, the Long Beach
area, and the offshore oil fields in the Santa Barbara area
(Figure SC.44). In addition to being historically important
producers of petroleum and natural gas, the Cenozoic
rocks of southern California still contain substantial
reserves of petroleum in the subsurface reservoirs and offshore
fields.
� Figure SC.43 Faults and folds can block the migration of oil and
gas in the subsurface, trapping it in layers of porous rock.
Frank DeCourten
� Figure SC.44 Offshore oil platform west of the southern
California coast.
Some of the subsurface oil in southern California naturally
rises to the surface through fractured rock, within confined
layers of porous rock reaching the surface, or along
faults. Natural oil seeps occur in many places, both on land
and on the seafloor (Figure SC.45). Oil seeping to the
surface tends to lose the volatile and more fluid components
to the atmosphere, leaving behind thick and viscous
tar deposits. People commonly notice tar globs on their
shoes or feet after strolling on some of the southern
California beaches. This tar generally originates from oil
seeps on the neighboring seafloor, and arrives on the beach
as a sticky residue after the more fluid components have
escaped. In some places, oil flows to the surface in large
enough volume for a pool of viscous hydrocarbons to form.
The LaBrea tar pits, famous for the remains of ice age animals
trapped in the sticky tar, is the best-known example of
such a natural oil pool (Figure SC.46).
The Channel Islands: Offshore from southern California
are several prominent islands including Santa Catalina and
� Figure SC.45 Natural oil seep near Ojai in the western
Transverse Ranges.
Dick Hilton
Dick Hilton