Page 74 - Petrophysics

48 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

the sediments are buried deeper, where the anaerobic environment
prevails and where the organic matter continues to transform to more
insoluble high-molecular-weight polymers due principally to the increase
of pore fluid pressure and temperature.
Anaerobic bacteria reduce sulfates to hydrogen sulfide and may
remove oxygen from some low-molecular-weight organic compounds,
but otherwise they add to the total biomass rather than depleting it, which
occurs in the aerobic regions. Some organically produced compounds
of calcium and silica dissolve in the water and later are precipitated
with the mixture of clay minerals and organics as they reach saturation
in the aqueous layer. The organic matter is gradually transformed into
new polymeric organic compounds that eventually become kerogen.
Considerable methane is formed and released-mixed with hydrogen
sulfide-as marsh gas. Low-molecular-weight water-soluble compounds
formed during diagenesis probably are lost to the interstitial water
percolating upward, leaving behind a solid organic mass compacted into
fine kerogen particles.

TRANSFORMATION KEROGEN INTO OIL AND GAS
OF

Consecutive deposition of sediments in the basin leads to deeper
burial reaching several thousand feet deep, which imposes an increase
of temperature and pressure on the kerogen mixed with the fine-grained
sediments. The increase of temperature with burial places the materials
once more out of thermodynamic equilibrium, which induces further
reactions and transformations (catagenetic stage). During catagenesis,
the reactions are catalyzed to some extent by the inorganic matrix. While
the organic material is undergoing major transformations, the sediments
are being compacted with expulsion of water and decrease of porosity
and permeability. The kerogen evolves through a liquid bitumen to
liquid petroleum. If the petroleum remains in the compacted source
rock undergoing deeper burial with continued heating, the kerogen is
ultimately reduced to graphite and methane.
The thermodynamic stability of the organic matter is never reached
because of the gradual increase in temperature as burial proceeds.
Chilingarian and Yen describe the approximate depths for the various
diagenetic and catagenetic changes: 10-20 feet is the zone of change
to humic materials; 20-1,500 feet is where the diagenetic changes
take place; 1,500-6,000 feet is the zone of catagenetic changes and
formation of oil from kerogen; and below 6,000 feet there is a zone
of metagenesis where petroleum changes to graphite and methane
(Figure 2.8) [15].