316
7 THE SUBSURFACE ENVIRONMENT
Table 7.1
Classification of Subsurface Fluids a
Fluid
Origin
r~
CD
l
T
r~
C7'
Inert gases
Petroleum
Water
Helium }
Argon
Krypton
Radon
Nitrogen
Carbon dioxide
Hydrogen sulfide
Hydrogen
Methane
Condensate }
Crude oil
Mantle outgassing
Mixed
Organic
Meteoric
Surface runoff
Connate
Evolved from runoff or seawater
Juvenile
Magmatic
aCondensates are varieties of petroleum that are gaseous in the subsurface, but condense to liquids at normal temperatures and pressures. They include ethane, propane, and butane.
depths. The shallow reactions are normally caused by bacterial action, the deeper ones
by thermal effect. When these gases occur in volcanic emanations they may in part be
of recycled sedimentary origin as well as mantle derived. Nitrogen is essentially inert,
but carbon dioxide and hydrogen sulfide play a very important part in diagenesis. Carbon dioxide content affects the pH of the pore fluid, and hence the solubility of many
minerals, whereas hydrogen sulfide plays an important part in the formation of metallic sulfide ores (see Section 9.6.5).
7.3.2 Petroleum Fluids
Petroleum is the name given to fluid hydrocarbons. These occur in both gaseous and liquid states, partly controlled by their chemical composition, and partly according to the
pressure and temperature of their environment. Methane, often referred to as "dry
gas," is of diverse origin. Some methane forms biogenically near the surface of the earth
through the bacterial degradation of organic matter. This is often referred to as "marsh
gas." As discussed earlier this is a possible cause of "birds-eye" fabric in shallow water
carbonate muds (see Section 3.2.2.3.2). Methane also forms from the thermal alteration
of buried organic matter, and this is discussed in more detail later. Finally, some methane is derived from the mantle (Gold, 1979,1999; Gold and Soter, 1982). The diverse origin of methane can be established from its 12C-13C ratio, because this varies according
to its source. Particularly high amounts of mantle-derived methane have been encountered emerging from both continental and oceanic rift valleys (MacDonald, 1983).
"Condensate" is the term given to petroleum that is gaseous at high subsurface tem-
7 THE SUBSURFACE ENVIRONMENT
Table 7.1
Classification of Subsurface Fluids a
Fluid
Origin
r~
CD
l
T
r~
C7'
Inert gases
Petroleum
Water
Helium }
Argon
Krypton
Radon
Nitrogen
Carbon dioxide
Hydrogen sulfide
Hydrogen
Methane
Condensate }
Crude oil
Mantle outgassing
Mixed
Organic
Meteoric
Surface runoff
Connate
Evolved from runoff or seawater
Juvenile
Magmatic
aCondensates are varieties of petroleum that are gaseous in the subsurface, but condense to liquids at normal temperatures and pressures. They include ethane, propane, and butane.
depths. The shallow reactions are normally caused by bacterial action, the deeper ones
by thermal effect. When these gases occur in volcanic emanations they may in part be
of recycled sedimentary origin as well as mantle derived. Nitrogen is essentially inert,
but carbon dioxide and hydrogen sulfide play a very important part in diagenesis. Carbon dioxide content affects the pH of the pore fluid, and hence the solubility of many
minerals, whereas hydrogen sulfide plays an important part in the formation of metallic sulfide ores (see Section 9.6.5).
7.3.2 Petroleum Fluids
Petroleum is the name given to fluid hydrocarbons. These occur in both gaseous and liquid states, partly controlled by their chemical composition, and partly according to the
pressure and temperature of their environment. Methane, often referred to as "dry
gas," is of diverse origin. Some methane forms biogenically near the surface of the earth
through the bacterial degradation of organic matter. This is often referred to as "marsh
gas." As discussed earlier this is a possible cause of "birds-eye" fabric in shallow water
carbonate muds (see Section 3.2.2.3.2). Methane also forms from the thermal alteration
of buried organic matter, and this is discussed in more detail later. Finally, some methane is derived from the mantle (Gold, 1979,1999; Gold and Soter, 1982). The diverse origin of methane can be established from its 12C-13C ratio, because this varies according
to its source. Particularly high amounts of mantle-derived methane have been encountered emerging from both continental and oceanic rift valleys (MacDonald, 1983).
"Condensate" is the term given to petroleum that is gaseous at high subsurface tem-