Chapter 10
Subsurface Water and Fluid Flow in Sedimentary Basins
Knut Bjørlykke
The pore spaces in sedimentary basins are mostly
filled with water. Oil and gas are the exceptions and
most of the information we have about fluid flow in
sedimentary basins is derived from the composition of
water and the pressure gradients in the water phase. It
is therefore important to characterise and understand
the variations in the composition of these waters. All
the porewater may be referred to as subsurface water
but the water that is analysed from exploration wells or
is produced during oil production is usually called
formation water or oil field brines.
The composition of subsurface water can provide
vital information for several different practical
purposes:
(1) The water composition may give information
about the origin of the water and the pattern of
fluid flow in the basin.
(2) Differences in water composition with respect to
salinity or isotopic content may provide important
information about communication (permeability
and fluid flow) across barriers like shale layers or
faults.
(3) The composition of water produced together with
oil may give information about the rock units from
which the water is being drained. When water
injection is applied to a reservoir, the composition
of the injected seawater is different from that of
the formation water and this can be used to
determine if there is a breakthrough of the injected
water to the production well.
(4) The water composition determines the density of
the water column which is required to interpret
and calibrate the fluid pressure data from wells.
(5) Water composition and its resistivity are critical
for the calibration of well logs. The resistivity is
crucial for calculating the formation factor F,
which is the resistivity of the formation water
relative to the resistivity of the whole fluidsaturated rock.
Water buried with the sediments has long been
referred to as connate water and thought to represent
the original seawater. This is an unfortunate term
because the origin of such water is very complex and
meteoric water has been found to have a much stronger influence in sedimentary basins than earlier
assumed. In addition, seawater changes its composition significantly as soon as it is buried; the first
major change is the removal of sulphate ions in the
sulphate-reducing zone (Figs. 10.1 and 10.2). Then the
porewater will gradually approach equilibrium with
the minerals present in the sediments as the temperature increases.
Subsurface waters are mainly derived from:
(1) Seawater buried with the sediments.
(2) Meteoric water, which is groundwater (originally
rainwater) that can flow from land to far offshore
along permeable beds, mostly sandstones and
limestones.
(3) Water released by dehydration of minerals i.e.
from gypsum, or clay minerals like smectite and
kaolinite.
(4) Hydrothermal water introduced by igneous activity. This is also referred to as juvenile water.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_10, # Springer-Verlag Berlin Heidelberg 2015
279
Subsurface Water and Fluid Flow in Sedimentary Basins
Knut Bjørlykke
The pore spaces in sedimentary basins are mostly
filled with water. Oil and gas are the exceptions and
most of the information we have about fluid flow in
sedimentary basins is derived from the composition of
water and the pressure gradients in the water phase. It
is therefore important to characterise and understand
the variations in the composition of these waters. All
the porewater may be referred to as subsurface water
but the water that is analysed from exploration wells or
is produced during oil production is usually called
formation water or oil field brines.
The composition of subsurface water can provide
vital information for several different practical
purposes:
(1) The water composition may give information
about the origin of the water and the pattern of
fluid flow in the basin.
(2) Differences in water composition with respect to
salinity or isotopic content may provide important
information about communication (permeability
and fluid flow) across barriers like shale layers or
faults.
(3) The composition of water produced together with
oil may give information about the rock units from
which the water is being drained. When water
injection is applied to a reservoir, the composition
of the injected seawater is different from that of
the formation water and this can be used to
determine if there is a breakthrough of the injected
water to the production well.
(4) The water composition determines the density of
the water column which is required to interpret
and calibrate the fluid pressure data from wells.
(5) Water composition and its resistivity are critical
for the calibration of well logs. The resistivity is
crucial for calculating the formation factor F,
which is the resistivity of the formation water
relative to the resistivity of the whole fluidsaturated rock.
Water buried with the sediments has long been
referred to as connate water and thought to represent
the original seawater. This is an unfortunate term
because the origin of such water is very complex and
meteoric water has been found to have a much stronger influence in sedimentary basins than earlier
assumed. In addition, seawater changes its composition significantly as soon as it is buried; the first
major change is the removal of sulphate ions in the
sulphate-reducing zone (Figs. 10.1 and 10.2). Then the
porewater will gradually approach equilibrium with
the minerals present in the sediments as the temperature increases.
Subsurface waters are mainly derived from:
(1) Seawater buried with the sediments.
(2) Meteoric water, which is groundwater (originally
rainwater) that can flow from land to far offshore
along permeable beds, mostly sandstones and
limestones.
(3) Water released by dehydration of minerals i.e.
from gypsum, or clay minerals like smectite and
kaolinite.
(4) Hydrothermal water introduced by igneous activity. This is also referred to as juvenile water.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_10, # Springer-Verlag Berlin Heidelberg 2015
279