1.3 APPLIED SEDIMENTOLOGY
9
with traces of elements such as vanadium and nickel. Hydrocarbons occur in solid, liquid, and gaseous states (see Section 7.3.2). Solid hydrocarbons are variously known as
asphalt, tar, pitch, and gilsonite. Liquid hydrocarbons are termed crude oil or simply
"crude." Gaseous hydrocarbons are loosely referred to as natural gas, ignoring inorganic natural gases such as those of volcanic origin.
It is a matter of observation that hydrocarbons occur in sedimentary basins, not in
areas of vulcanism or of regional metamorphism. Most geologists conclude, therefore,
that hydrocarbons are both generated and retained within sedimentary rocks rather
than in those of igneous or metamorphic origin. (See Hunt, 1996, for expositions of the
Western orthodox view, but see the references cited earlier for an exposition of the abiogenic theory.) The processes of hydrocarbon generation and migration are complex and
controversial. Detailed analyses are found in the references previously cited. Almost all
sedimentary rocks contain some traces of hydrocarbons. The source of hydrocarbons is
generally thought to be due to large accumulations of organic matter, vegetable or animal, in anaerobic subaqueous environments. During burial and compaction this source
sediment becomes heated. Hydrocarbons are formed and migrate out of the source rock
into permeable carrier beds. The hydrocarbons will then migrate upward, being lighter
than the pore water. Ultimately the hydrocarbons will be dissipated at the earth's surface through natural seepages. In some fortunate instances, however, they are trapped
by an impervious rock formation. They form a reservoir in the porous beds beneath
and await discovery by the oil industry. This brief summary of a complex sequence of
events shows that a hydrocarbon accumulation requires a source rock, a reservoir rock,
a trap, and a cap rock. Any rock with permeability is a potential reservoir. Most of the
world's reservoirs are in sandstones, dolomites, and limestones. Fields also occur in igneous and metamorphic rocks, commonly where porosity has been induced by weathering beneath unconformities, and where fracture porosity has been induced tectonically.
The impermeable cap rocks that seal hydrocarbons in reservoirs are generally shales or
evaporites. Less commonly "tight" (nonpermeable) limestones and sandstones may be
cap rocks. Sedimentology assists in the search for oil in all types of traps, and in the subsequent effective exploitation of a reservoir.
The exploitation of hydrocarbons from an area falls into a regular time sequence. Initially, broad regional stratigraphic studies are carried out to define the limits and architecture of sedimentary basins. With modern offshore exploration this is largely based
on geophysical data. Preliminary magnetic and gravity surveys are followed by more
detailed seismic shooting. This information is used to map marker horizons. Though
the age and lithology of these strata are unknown at this stage, the basin can be broadly
defined and prospective structures located within it. The first well locations will test
structural traps such as anticlines, but even at this stage seismic data can detect such sedimentary features as delta fronts, reefs, growth faults, and salt diapirs. Today it is possible to directly locate petroleum accumulations because seismic reflections of petroleum" water contacts can sometimes actually be observed. The first wells in a new basin
provide a wealth of information. Regardless of whether these tests yield productive hydrocarbons, they give the age and lithology of the formations previously mapped seismically. Geochemical analysis tells whether source rocks are present and whether the
basin has matured to the right temperature for oil or gas generation. As more wells are
drilled in a productive basin, so are more sedimentological data available for analysis.
9
with traces of elements such as vanadium and nickel. Hydrocarbons occur in solid, liquid, and gaseous states (see Section 7.3.2). Solid hydrocarbons are variously known as
asphalt, tar, pitch, and gilsonite. Liquid hydrocarbons are termed crude oil or simply
"crude." Gaseous hydrocarbons are loosely referred to as natural gas, ignoring inorganic natural gases such as those of volcanic origin.
It is a matter of observation that hydrocarbons occur in sedimentary basins, not in
areas of vulcanism or of regional metamorphism. Most geologists conclude, therefore,
that hydrocarbons are both generated and retained within sedimentary rocks rather
than in those of igneous or metamorphic origin. (See Hunt, 1996, for expositions of the
Western orthodox view, but see the references cited earlier for an exposition of the abiogenic theory.) The processes of hydrocarbon generation and migration are complex and
controversial. Detailed analyses are found in the references previously cited. Almost all
sedimentary rocks contain some traces of hydrocarbons. The source of hydrocarbons is
generally thought to be due to large accumulations of organic matter, vegetable or animal, in anaerobic subaqueous environments. During burial and compaction this source
sediment becomes heated. Hydrocarbons are formed and migrate out of the source rock
into permeable carrier beds. The hydrocarbons will then migrate upward, being lighter
than the pore water. Ultimately the hydrocarbons will be dissipated at the earth's surface through natural seepages. In some fortunate instances, however, they are trapped
by an impervious rock formation. They form a reservoir in the porous beds beneath
and await discovery by the oil industry. This brief summary of a complex sequence of
events shows that a hydrocarbon accumulation requires a source rock, a reservoir rock,
a trap, and a cap rock. Any rock with permeability is a potential reservoir. Most of the
world's reservoirs are in sandstones, dolomites, and limestones. Fields also occur in igneous and metamorphic rocks, commonly where porosity has been induced by weathering beneath unconformities, and where fracture porosity has been induced tectonically.
The impermeable cap rocks that seal hydrocarbons in reservoirs are generally shales or
evaporites. Less commonly "tight" (nonpermeable) limestones and sandstones may be
cap rocks. Sedimentology assists in the search for oil in all types of traps, and in the subsequent effective exploitation of a reservoir.
The exploitation of hydrocarbons from an area falls into a regular time sequence. Initially, broad regional stratigraphic studies are carried out to define the limits and architecture of sedimentary basins. With modern offshore exploration this is largely based
on geophysical data. Preliminary magnetic and gravity surveys are followed by more
detailed seismic shooting. This information is used to map marker horizons. Though
the age and lithology of these strata are unknown at this stage, the basin can be broadly
defined and prospective structures located within it. The first well locations will test
structural traps such as anticlines, but even at this stage seismic data can detect such sedimentary features as delta fronts, reefs, growth faults, and salt diapirs. Today it is possible to directly locate petroleum accumulations because seismic reflections of petroleum" water contacts can sometimes actually be observed. The first wells in a new basin
provide a wealth of information. Regardless of whether these tests yield productive hydrocarbons, they give the age and lithology of the formations previously mapped seismically. Geochemical analysis tells whether source rocks are present and whether the
basin has matured to the right temperature for oil or gas generation. As more wells are
drilled in a productive basin, so are more sedimentological data available for analysis.