More and more sophisticated methods are therefore
used in modern exploration. We are on a global basis
not finding enough new oil fields oil to replace the
produced oil. Global reserves have however not
changed very much because of higher estimates of
recovery from existing fields and because unconventional oil like tar sand is now included in the reserves.
There is a major challenge for geoscientists to
develop ever better exploration methods and to optimise production.
Even if the global production of conventional oil
may be reduced there will be significant production for
many decades. This will mostly come from the tail
production of giant fields and from small reservoirs,
but it is rather labour intensive. This is also the case
with unconventional oil (tar sand and oil shale) and
also tight gas reserves and shale gas.
Until enough alternative sources of energy are
developed it is necessary to extract fossils fuels from
these resources. This should be done with as little
environmental damage as possible and this requires a
new generation of highly skilled geoscientists.
1.17 Conventional and Unconventional
Oil and Gas
Reservoir rocks have traditionally been defined as
rocks with sufficient permeability and porosity for
petroleum to be produced economically by drilling
wells. They have most commonly been sandstones
and limestones but fractured basement rocks may
also serve as reservoirs. Sandstone reservoirs normally
have porosities >10–12% and permeability >0.01
Darcy. Gas may be produced from tighter rocks. Petroleum generated in organic-rich source rocks must have
migrated to a reservoir rock to be produced as conventional oil and gas.
Exploration is then limited to structural traps or
pinch-outs trapping the hydrocarbons.
In many mature sedimentary basins such traps have
already been explored and produced.
In the last 5–10 years it has however been possible
to produce oil and gas directly from shales which are
source rocks even if their porosity and permeability
Carbon in
sedimentary rocks
Kerogen in sediments
Deposition of
organic material
Ocean floor
Oxidation of
organic matter
CO 2
C
Deposition
of carbonate
Oxidation and
biological processing
of organic material
Organic
plant matter
in delta
sediment
Delta
Oxidation of
organic matter
Transport of
organic matter
by rivers
Dissolution of
carbonate rocks
Geochemistry of carbonate minerals
Photosynthesis
Addition of organic matter
+ nutrients to the ocean
Photosynthesis
Breakdown
of biogenic
carbonate
CO 2
CO 2
CO
CO 2
CO 2
CO
2 exchange
CO 2
Fig. 1.14 Illustration of the carbon cycle. Carbon from organic
matter and carbonate rocks are the major sinks for carbon (CO 2 ).
The rate of precipitation of carbonate in the ocean by organisms
is limited by the supply of Ca
++ and Mg
++ from weathering of
silicate rocks brought in by rivers
1 Introduction to Petroleum Geology
25
used in modern exploration. We are on a global basis
not finding enough new oil fields oil to replace the
produced oil. Global reserves have however not
changed very much because of higher estimates of
recovery from existing fields and because unconventional oil like tar sand is now included in the reserves.
There is a major challenge for geoscientists to
develop ever better exploration methods and to optimise production.
Even if the global production of conventional oil
may be reduced there will be significant production for
many decades. This will mostly come from the tail
production of giant fields and from small reservoirs,
but it is rather labour intensive. This is also the case
with unconventional oil (tar sand and oil shale) and
also tight gas reserves and shale gas.
Until enough alternative sources of energy are
developed it is necessary to extract fossils fuels from
these resources. This should be done with as little
environmental damage as possible and this requires a
new generation of highly skilled geoscientists.
1.17 Conventional and Unconventional
Oil and Gas
Reservoir rocks have traditionally been defined as
rocks with sufficient permeability and porosity for
petroleum to be produced economically by drilling
wells. They have most commonly been sandstones
and limestones but fractured basement rocks may
also serve as reservoirs. Sandstone reservoirs normally
have porosities >10–12% and permeability >0.01
Darcy. Gas may be produced from tighter rocks. Petroleum generated in organic-rich source rocks must have
migrated to a reservoir rock to be produced as conventional oil and gas.
Exploration is then limited to structural traps or
pinch-outs trapping the hydrocarbons.
In many mature sedimentary basins such traps have
already been explored and produced.
In the last 5–10 years it has however been possible
to produce oil and gas directly from shales which are
source rocks even if their porosity and permeability
Carbon in
sedimentary rocks
Kerogen in sediments
Deposition of
organic material
Ocean floor
Oxidation of
organic matter
CO 2
C
Deposition
of carbonate
Oxidation and
biological processing
of organic material
Organic
plant matter
in delta
sediment
Delta
Oxidation of
organic matter
Transport of
organic matter
by rivers
Dissolution of
carbonate rocks
Geochemistry of carbonate minerals
Photosynthesis
Addition of organic matter
+ nutrients to the ocean
Photosynthesis
Breakdown
of biogenic
carbonate
CO 2
CO 2
CO
CO 2
CO 2
CO
2 exchange
CO 2
Fig. 1.14 Illustration of the carbon cycle. Carbon from organic
matter and carbonate rocks are the major sinks for carbon (CO 2 ).
The rate of precipitation of carbonate in the ocean by organisms
is limited by the supply of Ca
++ and Mg
++ from weathering of
silicate rocks brought in by rivers
1 Introduction to Petroleum Geology
25