7.3 SUBSURFACE FLUIDS
317
peratures, but condenses to a liquid at normal temperatures and pressure at the earth's
surface. Chemically, condensate includes ethane, propane, and butane. Liquid petroleum is referred to as crude oil, and is a complex mix of paraffinic, aromatic, naphthenic,
and other compounds. The origin of petroleum is beyond the scope of this text. It must
be considered briefly, however, because it is a common pore fluid and obviously an important one. The stimulus for most sedimentological research lies in the quest for petroleum. From the days of Mendele'ev down to Gold (1979, 1999), chemists and astronomers have argued for an inorganic abiogenic origin for petroleum. They note that
complex hydrocarbons occur in space, and on earth in chondritic meteorites, and escaping from volcanoes. It has been argued that hydrocarbons may form in the mantle in
a reaction analogous to that which produces acetylene from calcium carbide and water:
CaC2 + 2H20 = C2H2 + Ca(OH)2.
The evidence for mantle outgassing of methane and radiogenic gases has already been
discussed. It is further argued, however, that the range of petroleum compounds encountered in sediments form from mantle-derived methane by polymerization. Space
does not permit this theory to be reviewed, but it is a fact that commercial quantities
of petroleum normally occur in sedimentary rocks. Where large amounts of petroleum
occur in igneous and metamorphic rocks there are sediments nearby. Detailed chemical analysis shows the original biogenic origin of petroleum and can identify the source
formation from which it was derived (Tissot and Welte, 1984; Hunt, 1996; Selley, 1998).
Petroleum geologists now generally believe that some organic matter is buried in various anoxic environments (see Sections 6.3.2.4.4 and 6.3.2.7.1). On burial, organic matter
evolves into kerogen, defined as hydrocarbon, insoluble in normal petroleum solvents,
that generates liquid petroleum when heated. There are different types of kerogen,
some (such as coal) generate only gas, some kerogen gives off both oil and gas, some
principally oil. As kerogen is heated it undergoes decarboxylation, generating carbon
dioxide in solution as carbonic acid. This may have an important role in generating secondary porosity in sandstones (see Section 8.5.3.4). After decarboxylation, oil is generated between about 60 and 120~ and gas between about 120 and 220~ Above these
temperatures the kerogen evolves into graphite and is essentially inert. Petroleum emigrates from the kerogen bearing shales that normally serve as source beds. The exact
mechanics of migration, whereby petroleum emigrates from an impermeable formation, is still a matter for debate (Baker, 1996). There is now, however, a growing opinion that the primary migration of petroleum from a source bed takes place during the
episodic expulsion of fluids from overpressured shales (e.g., Cartwright, 1994; Roberts
and Nunn, 1995). Once in permeable strata, however, petroleum is lighter than the ambient connate fluid. It will thus migrate upward in response to buoyancy. Petroleum
may ultimately reach the earth's surface and be dissipated as an oil or gas seep.
Occasionally, however, petroleum may be trapped in the subsurface. There are many
types of petroleum traps. Some are due to structure, such as folds or faults; some are in
domes that are overly salt or overpressured clay diapirs; and some are due to stratigraphy. Stratigraphic traps include depositional pinchouts and truncations, as well as channels, sandbars, and reefs (Fig. 7.8).
317
peratures, but condenses to a liquid at normal temperatures and pressure at the earth's
surface. Chemically, condensate includes ethane, propane, and butane. Liquid petroleum is referred to as crude oil, and is a complex mix of paraffinic, aromatic, naphthenic,
and other compounds. The origin of petroleum is beyond the scope of this text. It must
be considered briefly, however, because it is a common pore fluid and obviously an important one. The stimulus for most sedimentological research lies in the quest for petroleum. From the days of Mendele'ev down to Gold (1979, 1999), chemists and astronomers have argued for an inorganic abiogenic origin for petroleum. They note that
complex hydrocarbons occur in space, and on earth in chondritic meteorites, and escaping from volcanoes. It has been argued that hydrocarbons may form in the mantle in
a reaction analogous to that which produces acetylene from calcium carbide and water:
CaC2 + 2H20 = C2H2 + Ca(OH)2.
The evidence for mantle outgassing of methane and radiogenic gases has already been
discussed. It is further argued, however, that the range of petroleum compounds encountered in sediments form from mantle-derived methane by polymerization. Space
does not permit this theory to be reviewed, but it is a fact that commercial quantities
of petroleum normally occur in sedimentary rocks. Where large amounts of petroleum
occur in igneous and metamorphic rocks there are sediments nearby. Detailed chemical analysis shows the original biogenic origin of petroleum and can identify the source
formation from which it was derived (Tissot and Welte, 1984; Hunt, 1996; Selley, 1998).
Petroleum geologists now generally believe that some organic matter is buried in various anoxic environments (see Sections 6.3.2.4.4 and 6.3.2.7.1). On burial, organic matter
evolves into kerogen, defined as hydrocarbon, insoluble in normal petroleum solvents,
that generates liquid petroleum when heated. There are different types of kerogen,
some (such as coal) generate only gas, some kerogen gives off both oil and gas, some
principally oil. As kerogen is heated it undergoes decarboxylation, generating carbon
dioxide in solution as carbonic acid. This may have an important role in generating secondary porosity in sandstones (see Section 8.5.3.4). After decarboxylation, oil is generated between about 60 and 120~ and gas between about 120 and 220~ Above these
temperatures the kerogen evolves into graphite and is essentially inert. Petroleum emigrates from the kerogen bearing shales that normally serve as source beds. The exact
mechanics of migration, whereby petroleum emigrates from an impermeable formation, is still a matter for debate (Baker, 1996). There is now, however, a growing opinion that the primary migration of petroleum from a source bed takes place during the
episodic expulsion of fluids from overpressured shales (e.g., Cartwright, 1994; Roberts
and Nunn, 1995). Once in permeable strata, however, petroleum is lighter than the ambient connate fluid. It will thus migrate upward in response to buoyancy. Petroleum
may ultimately reach the earth's surface and be dissipated as an oil or gas seep.
Occasionally, however, petroleum may be trapped in the subsurface. There are many
types of petroleum traps. Some are due to structure, such as folds or faults; some are in
domes that are overly salt or overpressured clay diapirs; and some are due to stratigraphy. Stratigraphic traps include depositional pinchouts and truncations, as well as channels, sandbars, and reefs (Fig. 7.8).