This kind of gas is often referred to as “shallow biogenic gas”.
14.7 Biodegradation
Hydrocarbons can be broken down by microorganisms (bacteria, yeast and fungi). This is a form
of biological oxidation whereby hydrocarbons are
oxidised to alcohols, ketones and various acids. The
biological breakdown of hydrocarbons proceeds far
more rapidly with smaller molecules (carbon number
<20). When it comes to molecules with the same
carbon number, n-paraffins will break down first,
and then isoparaffins, naphthenes and aromatics.
Isoprenoids, steranes and triterpanes show the greatest
resistance to biodegradation. Relatively rapid biodegradation depends on a supply of oxygen, usually
dissolved in water or in air. If sulphate is present
hydrocarbons may also be broken down by sulphatereducing bacteria which use the oxygen in sulphates
(e.g. gypsum) to oxidise some of the oil. Minerals
containing trivalent iron (Fe
3+ ), such as haematite,
can also contribute to the biodegradation and oxidation of oil deeper in the basin when ferric iron is
reduced to ferrous (Fe
2+ ). The content of haematite
and other minerals with trivalent iron in sediments can
thus also be an oxidant.
Anaerobic biodegradation may occur under reducing conditions deeper in a sedimentary basin and the
supply of nutrients such as phosphorus in the
sediments may then be critical.
The temperature must however be lower than about
80
C. The bacteria eat the lighter compounds so that
the larger, more asphaltic, compounds are enriched.
Biodegraded oils (tar) have therefore high viscosity.
Biodegradation occurs as soon as the oil flows out
at the surface as oil seepage and the lighter compounds
will evaporate. Bacteria can then use oxygen from the
air and the biodegradation is relatively fast.
Biodegradation may occur in relatively shallow
reservoirs (<1,500–2,000 m, <70–80
C). Very close
to the surface and in contact with groundwater flow,
the biodegradation may be oxic but for the most part is
not possible to supply oxygen from the surface and we
must assume that the biodegradation is anoxic. As a
result the oil is less valuable and difficult to produce.
Injection of steam to heat the oil and reduce its viscosity is the most effective method of increasing
production from reservoirs containing heavy,
biodegraded oil. Increasing oil prices have however
made production of heavy oil and tar sand more profitable. Heating the oil may however require up to 30%
of the energy recoved thus increasing the CO 2 emission (see tar sand and oil shales, Chap. 21). Excavation
and separation of oil from sand also require much
more energy than production of conventional oil.
Reservoirs buried to greater depth (>80–100
C)
are sterilised by the heat and may remain without
biodegradation also after uplift to lower temperatures.
It is difficult to introduce new bacteria. Biodegration
and the formation of heavy oil and tar sand occurs
most readily when oil migrates into reservoir rocks
which have not been sterilised at temperatures exceeding about 80
C. When oil forms seeps near the surface,
aerobic biodegradation occurs rather rapidly.
In recent years it has become possible and economical to produce oil and gas directly from shales that are
source rocks. This is the petroleum which has not been
expelled from the source rocks. This is shale oil
and shale gas which has been generated at greater
depth to become mature but is produced by drilling
at shallow depth in uplifted areas, mostly on land (see
Chaps. 1, 23).
Further Reading
Geoff, J.C. 1985. Hydrocarbon generation and migration from
Jurassic source rocks in the East Shetland Basin and Viking
Graben at the North Sea. Journal of the Geological Society
140, 445–474.
Hannah, J.L. and Stein, H.J. 2012. Re-Os geochemistry. In:
Melezhik, V.A., Kump, L.R., Fallick, A.E., Strauss, H.,
Hanski, E.J., Prave, A.R. and Lepland, A., (eds), Reading
the Archive of Earth’s Oxygenation, Volume 3: Global
Events and the Fennoscandian Arctic Russia – Drilling
Early Earth Project: Springer-Verlag, Berlin, Heidelberg,
pp. 1506–1514. DOI 10.1007/978-3-642-29670-3_10.
Hunt, J.M. 1996. Petroleum Geochemistry and Geology. Freeman and Co., New York, 743 pp.
Karlsen, D.A., Nedkvitne, T., Larter, S.R. and Bjørlykke, K.
1993. Hydrocarbon composition of authigenic inclusions –
Application to elucidation of petroleum reservoir filling history. Geochemica et Cosmochemica Acta 57, 3641–3659.
Løseth, H., Wensaas, L., Gading, M., Duffaut, K. and Springer,
M. 2011. Can hydrocarbon source rocks be identified on
seismic data? Geology 39, 1167–1170.
Makhous, M. and Galushkin, Y. 2005. Basin Analysis and
Modeling of the Burial, Thermal and Maturation Histories
in Sedimentary Basins. Editions Technip, Paris, 379 pp.
14 Source Rocks and Petroleum Geochemistry
371
14.7 Biodegradation
Hydrocarbons can be broken down by microorganisms (bacteria, yeast and fungi). This is a form
of biological oxidation whereby hydrocarbons are
oxidised to alcohols, ketones and various acids. The
biological breakdown of hydrocarbons proceeds far
more rapidly with smaller molecules (carbon number
<20). When it comes to molecules with the same
carbon number, n-paraffins will break down first,
and then isoparaffins, naphthenes and aromatics.
Isoprenoids, steranes and triterpanes show the greatest
resistance to biodegradation. Relatively rapid biodegradation depends on a supply of oxygen, usually
dissolved in water or in air. If sulphate is present
hydrocarbons may also be broken down by sulphatereducing bacteria which use the oxygen in sulphates
(e.g. gypsum) to oxidise some of the oil. Minerals
containing trivalent iron (Fe
3+ ), such as haematite,
can also contribute to the biodegradation and oxidation of oil deeper in the basin when ferric iron is
reduced to ferrous (Fe
2+ ). The content of haematite
and other minerals with trivalent iron in sediments can
thus also be an oxidant.
Anaerobic biodegradation may occur under reducing conditions deeper in a sedimentary basin and the
supply of nutrients such as phosphorus in the
sediments may then be critical.
The temperature must however be lower than about
80
C. The bacteria eat the lighter compounds so that
the larger, more asphaltic, compounds are enriched.
Biodegraded oils (tar) have therefore high viscosity.
Biodegradation occurs as soon as the oil flows out
at the surface as oil seepage and the lighter compounds
will evaporate. Bacteria can then use oxygen from the
air and the biodegradation is relatively fast.
Biodegradation may occur in relatively shallow
reservoirs (<1,500–2,000 m, <70–80
C). Very close
to the surface and in contact with groundwater flow,
the biodegradation may be oxic but for the most part is
not possible to supply oxygen from the surface and we
must assume that the biodegradation is anoxic. As a
result the oil is less valuable and difficult to produce.
Injection of steam to heat the oil and reduce its viscosity is the most effective method of increasing
production from reservoirs containing heavy,
biodegraded oil. Increasing oil prices have however
made production of heavy oil and tar sand more profitable. Heating the oil may however require up to 30%
of the energy recoved thus increasing the CO 2 emission (see tar sand and oil shales, Chap. 21). Excavation
and separation of oil from sand also require much
more energy than production of conventional oil.
Reservoirs buried to greater depth (>80–100
C)
are sterilised by the heat and may remain without
biodegradation also after uplift to lower temperatures.
It is difficult to introduce new bacteria. Biodegration
and the formation of heavy oil and tar sand occurs
most readily when oil migrates into reservoir rocks
which have not been sterilised at temperatures exceeding about 80
C. When oil forms seeps near the surface,
aerobic biodegradation occurs rather rapidly.
In recent years it has become possible and economical to produce oil and gas directly from shales that are
source rocks. This is the petroleum which has not been
expelled from the source rocks. This is shale oil
and shale gas which has been generated at greater
depth to become mature but is produced by drilling
at shallow depth in uplifted areas, mostly on land (see
Chaps. 1, 23).
Further Reading
Geoff, J.C. 1985. Hydrocarbon generation and migration from
Jurassic source rocks in the East Shetland Basin and Viking
Graben at the North Sea. Journal of the Geological Society
140, 445–474.
Hannah, J.L. and Stein, H.J. 2012. Re-Os geochemistry. In:
Melezhik, V.A., Kump, L.R., Fallick, A.E., Strauss, H.,
Hanski, E.J., Prave, A.R. and Lepland, A., (eds), Reading
the Archive of Earth’s Oxygenation, Volume 3: Global
Events and the Fennoscandian Arctic Russia – Drilling
Early Earth Project: Springer-Verlag, Berlin, Heidelberg,
pp. 1506–1514. DOI 10.1007/978-3-642-29670-3_10.
Hunt, J.M. 1996. Petroleum Geochemistry and Geology. Freeman and Co., New York, 743 pp.
Karlsen, D.A., Nedkvitne, T., Larter, S.R. and Bjørlykke, K.
1993. Hydrocarbon composition of authigenic inclusions –
Application to elucidation of petroleum reservoir filling history. Geochemica et Cosmochemica Acta 57, 3641–3659.
Løseth, H., Wensaas, L., Gading, M., Duffaut, K. and Springer,
M. 2011. Can hydrocarbon source rocks be identified on
seismic data? Geology 39, 1167–1170.
Makhous, M. and Galushkin, Y. 2005. Basin Analysis and
Modeling of the Burial, Thermal and Maturation Histories
in Sedimentary Basins. Editions Technip, Paris, 379 pp.
14 Source Rocks and Petroleum Geochemistry
371