14.3 Hydrocarbon Habitats
expulsion efficiency (50% and more), even if the
total organic carbon content is relatively low. Consequently, the yield of lighter hydrocarbons, condensate, and gas is reduced after further burial. In contrast, thick homogeneous shale source rocks require a
higher organic carbon content and an advanced level
of maturity to expel oil in a certain period of time.
Hence they tend to retain a large proportion of heavy
oil products until they reach higher maturity in the
course of further subsidence. Then they expel light
oils, condensate, and gas, but their expulsion efficiency is lower (5 to 25%) than that of the
interbedded source rocks.
Secondary Migration (in Liquid Phase)
After expulsion from their source rocks, the hydrocarbons enter the water-saturated wider pores of
more permeable carrier beds and finally reach reservoir rocks. This secondary oil migration commonly
occurs as multiphase flow. Oil globules, liquid gas
(high pressure!), or gas bubbles in por.e water tend to
move upward sole1y due to buoyancy or driven by
hydrodynamic conditions. The hydrocarbons tend to
move upward into reservoirs located in structural
highs (Fig. 14.7e). Oil and also gas are trapped if
they can no longer be pressed through fine rock
pores. The pore space of traps is, however, never
fully occupied by petroleum, but always contains
some residual water which cannot be replaced by
hydrocarbons (Fig. 14.7b).
Other Transport Mechanisms
It has been frequently discussed whether and to
which extent the oil generated in source rocks migrates as a liquid hydrocarbon phase separated from
water (two-phase flow) or in molecular solution in
pore water. Most workers now assurne that the firstmentioned process is most important. The oil moves
as a immiscible liquid phase through water-saturated
rocks (volume flow or Darcy flow), preferentially in
the coarser layers of carrier beds.
Transport of hydrocarbons by molecular diffusion
plays some role for small hydrocarbon molecules
(including natural gas) and over short distances (cf.
Sect. 13.3.4). Considering the long time periods involved and the fact that diffusion transport can overcome layers practically impermeable for water, this
mechanism may be relevant under certain circumstances, for example for the dissipation of gas.
Source rocks generating abundant gas and little oil
may carry this oil in gaseous solution, which may
bring about a certain fractionation of this oil.
Hydrocarbon transport by water in molecular solution cannot be very effective during primary migra713
tion because the solubility of hydrocarbons in water
is low and the volume of compaction flow is limited
(cf. Sects. 13.2.3 and 13.3.2), particularly at the
depths where the source rocks become thermally mature. The situation may be different for secondary
hydrocarbon migration. Large-volume, long-Iasting
meteoric water flow, passing the top of a source rock
expelling low quantitities of oil per time unit, may
take up most of this oil in solution and carry it along
the preferential flow paths of an aquifer system to a
reservoir. This "water-drive" seems to be a major
mechanism in transporting oil in those cases where
the tops of the petroleum transporting strata have
very low dips (generally less than 5°). For these reasons, some authors see large-scale regional
subsurface flow systems as an important medium for
secondary migration of oil and gas (mostly in solution) in permeable rocks.
The problems mentioned in this section have been addressed by many workers (e.g. Magara 1978; Roberts and
CordeIl 1980; Durand 1987; Davis 1987; Mann 1990;
Leythaeusser and Poelschau 1991; Demaison and Huizinga
1994; England 1994; Killops et al. 1996; Krooss and
Leythaeusser 1996; Matthews 1996, and further articels in
Schumacher and Abrams 1996; Baskin 1997; Surdam
1997). Long-distance hydrocarbon transport in solution has
been advocated, e.g., by Toth (1980), Bachu (1997),
Rostron et al. (1997). Further references are listed above
and below.
14.3 Examples of Hydrocarbon Habitats
The general concept of hydrocarbon generation in a
subsiding basin (Fig. 14.7e) is demonstrated here in a
few examples of well known oil and gas fields.
We mainly follow the concept proposed by Demaison
(1985) which was later discussed and supplemented by
Magoon and Dow (1994). Detailed descriptions of oil and
gas fields all over the world have been published in numerous articles and a number of special volumes (e.g. Roehl
and Choquette 1985; Shannon and Naylor 1989; Magoon
and Dow 1994; Tankard et al. 1995; Surdam 1997;
MacGregor et al. 1998).
14.3.1 Lacustrine Source Beds
Lacustrine black shales deposited in rift basins and
buried sufficiently deep below younger strata represent excellent source rocks for the generation of hydrocarbons (cf. Sect. 2.5.8; Sladen 1994) in some
regions. However, small lake basins have a limited
reservoir potential. The organic matter of lake deposits is either autochthonous (predominantly algae;
kerogen type I) andJor originates from land-derived
plants as often observed in wet tropical regions.
These basin settings are frequently associated with
expulsion efficiency (50% and more), even if the
total organic carbon content is relatively low. Consequently, the yield of lighter hydrocarbons, condensate, and gas is reduced after further burial. In contrast, thick homogeneous shale source rocks require a
higher organic carbon content and an advanced level
of maturity to expel oil in a certain period of time.
Hence they tend to retain a large proportion of heavy
oil products until they reach higher maturity in the
course of further subsidence. Then they expel light
oils, condensate, and gas, but their expulsion efficiency is lower (5 to 25%) than that of the
interbedded source rocks.
Secondary Migration (in Liquid Phase)
After expulsion from their source rocks, the hydrocarbons enter the water-saturated wider pores of
more permeable carrier beds and finally reach reservoir rocks. This secondary oil migration commonly
occurs as multiphase flow. Oil globules, liquid gas
(high pressure!), or gas bubbles in por.e water tend to
move upward sole1y due to buoyancy or driven by
hydrodynamic conditions. The hydrocarbons tend to
move upward into reservoirs located in structural
highs (Fig. 14.7e). Oil and also gas are trapped if
they can no longer be pressed through fine rock
pores. The pore space of traps is, however, never
fully occupied by petroleum, but always contains
some residual water which cannot be replaced by
hydrocarbons (Fig. 14.7b).
Other Transport Mechanisms
It has been frequently discussed whether and to
which extent the oil generated in source rocks migrates as a liquid hydrocarbon phase separated from
water (two-phase flow) or in molecular solution in
pore water. Most workers now assurne that the firstmentioned process is most important. The oil moves
as a immiscible liquid phase through water-saturated
rocks (volume flow or Darcy flow), preferentially in
the coarser layers of carrier beds.
Transport of hydrocarbons by molecular diffusion
plays some role for small hydrocarbon molecules
(including natural gas) and over short distances (cf.
Sect. 13.3.4). Considering the long time periods involved and the fact that diffusion transport can overcome layers practically impermeable for water, this
mechanism may be relevant under certain circumstances, for example for the dissipation of gas.
Source rocks generating abundant gas and little oil
may carry this oil in gaseous solution, which may
bring about a certain fractionation of this oil.
Hydrocarbon transport by water in molecular solution cannot be very effective during primary migra713
tion because the solubility of hydrocarbons in water
is low and the volume of compaction flow is limited
(cf. Sects. 13.2.3 and 13.3.2), particularly at the
depths where the source rocks become thermally mature. The situation may be different for secondary
hydrocarbon migration. Large-volume, long-Iasting
meteoric water flow, passing the top of a source rock
expelling low quantitities of oil per time unit, may
take up most of this oil in solution and carry it along
the preferential flow paths of an aquifer system to a
reservoir. This "water-drive" seems to be a major
mechanism in transporting oil in those cases where
the tops of the petroleum transporting strata have
very low dips (generally less than 5°). For these reasons, some authors see large-scale regional
subsurface flow systems as an important medium for
secondary migration of oil and gas (mostly in solution) in permeable rocks.
The problems mentioned in this section have been addressed by many workers (e.g. Magara 1978; Roberts and
CordeIl 1980; Durand 1987; Davis 1987; Mann 1990;
Leythaeusser and Poelschau 1991; Demaison and Huizinga
1994; England 1994; Killops et al. 1996; Krooss and
Leythaeusser 1996; Matthews 1996, and further articels in
Schumacher and Abrams 1996; Baskin 1997; Surdam
1997). Long-distance hydrocarbon transport in solution has
been advocated, e.g., by Toth (1980), Bachu (1997),
Rostron et al. (1997). Further references are listed above
and below.
14.3 Examples of Hydrocarbon Habitats
The general concept of hydrocarbon generation in a
subsiding basin (Fig. 14.7e) is demonstrated here in a
few examples of well known oil and gas fields.
We mainly follow the concept proposed by Demaison
(1985) which was later discussed and supplemented by
Magoon and Dow (1994). Detailed descriptions of oil and
gas fields all over the world have been published in numerous articles and a number of special volumes (e.g. Roehl
and Choquette 1985; Shannon and Naylor 1989; Magoon
and Dow 1994; Tankard et al. 1995; Surdam 1997;
MacGregor et al. 1998).
14.3.1 Lacustrine Source Beds
Lacustrine black shales deposited in rift basins and
buried sufficiently deep below younger strata represent excellent source rocks for the generation of hydrocarbons (cf. Sect. 2.5.8; Sladen 1994) in some
regions. However, small lake basins have a limited
reservoir potential. The organic matter of lake deposits is either autochthonous (predominantly algae;
kerogen type I) andJor originates from land-derived
plants as often observed in wet tropical regions.
These basin settings are frequently associated with