7.3 SUBSURFACE FLUIDS
315
Fig. 7.7. Depth versus density graph showing typical sediment burial curve and porosity relationships. Note
how porosity is preserved in overpressured intervals. There is often an overcompacted zone immediately
above. Evaporites do not compact significantly, and at about 800 m they are thus less dense than the overburden and, therefore, salt diapirs may be anticipated.
7.3 SUBSURFACE FLUIDS
Diagenesis is controlled both by temperature and pressure, the fundamental geological
aspects of which have just been outlined. Now it is approprite to consider the chemistry
of the fluids that infill the pores of sediments. These are many and varied, both gaseous
and liquid. They are listed and classified in Table 7.1 and are now described.
7.3.1 Nonhydrocarbon Gases
Sediments above the water table will have their pores infilled with atmospheric gases.
These normally generate an oxidizing environment at the present day. It was noted earlier, however, when discussing the origin of Pre-Cambrian uraniferous conglomerates
(see Section 6.3.2.2.4), that this was not always so. The earth's early atmosphere was enriched in carbon dioxide, and was probably reducing until the rise of the algae whose
photosynthetic reactions modified the atmosphere.
Traces of the inert gases helium, argon, krypton, and radon occur in the subsurface,
normally dissolved in connate fluids. They are believed to form from the decay of radioactive minerals in the mantle and to move up into the sedimentary cover along faults
and fractures (Sugisaki et al., 1983; Gold, 1979, 1999; Gold and Soter, 1982). Nitrogen,
carbon dioxide, hydrogen sulfide, and hydrogen also occur in the subsurface, again normally dissolved in connate waters. They appear to be of mixed origin. These gases have
been recorded emanating from volcanoes, and may thus originate from the mantle.
They may also form through the diagenesis of organic matter both at shallow and great
315
Fig. 7.7. Depth versus density graph showing typical sediment burial curve and porosity relationships. Note
how porosity is preserved in overpressured intervals. There is often an overcompacted zone immediately
above. Evaporites do not compact significantly, and at about 800 m they are thus less dense than the overburden and, therefore, salt diapirs may be anticipated.
7.3 SUBSURFACE FLUIDS
Diagenesis is controlled both by temperature and pressure, the fundamental geological
aspects of which have just been outlined. Now it is approprite to consider the chemistry
of the fluids that infill the pores of sediments. These are many and varied, both gaseous
and liquid. They are listed and classified in Table 7.1 and are now described.
7.3.1 Nonhydrocarbon Gases
Sediments above the water table will have their pores infilled with atmospheric gases.
These normally generate an oxidizing environment at the present day. It was noted earlier, however, when discussing the origin of Pre-Cambrian uraniferous conglomerates
(see Section 6.3.2.2.4), that this was not always so. The earth's early atmosphere was enriched in carbon dioxide, and was probably reducing until the rise of the algae whose
photosynthetic reactions modified the atmosphere.
Traces of the inert gases helium, argon, krypton, and radon occur in the subsurface,
normally dissolved in connate fluids. They are believed to form from the decay of radioactive minerals in the mantle and to move up into the sedimentary cover along faults
and fractures (Sugisaki et al., 1983; Gold, 1979, 1999; Gold and Soter, 1982). Nitrogen,
carbon dioxide, hydrogen sulfide, and hydrogen also occur in the subsurface, again normally dissolved in connate waters. They appear to be of mixed origin. These gases have
been recorded emanating from volcanoes, and may thus originate from the mantle.
They may also form through the diagenesis of organic matter both at shallow and great