University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones?
and will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
I. Seals - the simple view
Seals are a) ductile (so that they don’t fracture) and
b) impermeable (so that fluids can’t pass though) strata
The most common seals are shales;
the most effective seals are evaporites.
Sandstones, on the other hand, are reservoirs
and pathways of migration.
But what about siltstones and silty shales?
But does the nature of the petroleum affect the effectiveness
of a potential seal?
Will all petroleum stop moving at the same permeability barriers?
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
II. Buoyancy and upward migration, or . . .
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
II. Buoyancy and upward migration, or
The interplay of buoyancy and pore size in determining what is a seal
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
II. Buoyancy and upward migration
Buoyancy is what drives upward migration of petroleum, and the limit
of upward migration is what defines the boundary between reservoir
and seal.
Upward migration of petroleum through water-filled pores of sedimentary
rocks is driven by the “Buoyancy Force”:
w = density of water (~1.01-1.10)
p = density of petroleum
(~0.7-0.8 for oil)
Buoyancy Force = h • g • (w-p)
h
h = vertical extent (height)
of the petroleum column
h = H = zo = Y in other presentations
~ 0.3 for oil (see above)
g = gravitational acceleration
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Seals and Reservoirs
II. Buoyancy and upward migration
Buoyancy is what drives upward migration of petroleum, and the limit
of upward migration is what defines the boundary between reservoir
and seal.
Upward migration of petroleum through water-filled pores of sedimentary
rocks is driven by the “Buoyancy Force”:
w = density of water (~1.01-1.10)
p = density of petroleum
(~0.7-0.8 for oil)
Buoyancy Force = h • g • (w-p)
h
h = vertical extent (height)
of the petroleum column
h = H = zo = Y in other presentations
~ 0.3 for oil (see above)
g = gravitational acceleration
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Reservoirs and Seals
II. Buoyancy and upward migration
Buoyancy is what drives upward migration of petroleum, and the limit
of upward migration is what defines the boundary between reservoir
and seal.
Passage of an immiscible fluid through pore throats is limited by the
“capillary resistance force” or “capillary injection pressure” or “displacement pressure” inherent in the interaction of fluid and pore throat:
 = wettability
 = interfacial tension
Resistance =
2•  • cos 
rt
rt = radius of pore throat
See next page for
more explanation.
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Passage of an immiscible fluid through pore throats is limited by the
“capillary resistance force” or “capillary pressure” or “displacement
pressure” inherent in the interaction of fluid and pore throat:
 = interfacial tension, a measure of the
immiscibilty of two liquids because
of the cohesion of like molecules in each.
If hydrocarbons were soluble in water,
this term would go to zero, and
resistance would go to zero. Relative to
water, gas > light oil > heavy oil. 
decreases with increasing
temperature.
 = wettability or wetting angle,
a rock-dependent term for the extent
to which water (or hydrocarbon in
some cases) is the fluid on the rock
surface.  is commonly so small, and
thus cos  so nearly
1.0, that this term
Water
is neglected.
2•  • cos 
Resistance =
rt

Oil
Rock
rt = radius of pore throat
(the smaller the pore throat, the greater the resistance).
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Berg (1975)
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Berg (1975)
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Downey (1984, AAPG Bulletin)
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
Conventional wisdom says that sandstones are reservoirs and shales are
seals, but the points above suggest that
• even the lousiest potential seal can be the seal of a short column of
hydrocarbons (ii & iv), and
• even the best siliciclastic seal (the tightest shale) will fail to seal a tall
column of hydrocarbons (i and iii), especially light hydrocarbons.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Implications of the comparison of buoyancy force
and displacement pressure:
i) Upward migration of petroleum will continue so long as the buoyancy
force of a hydrocarbon column exceeds the resistance of the pores in its
path.
ii) A horizon with pore throats small enough to generate resistance greater
than the buoyancy of the hydrocarbon column below it is a seal. The rock
below becomes a reservoir rather than a migration pathway.
iii) There is a limit to the vertical extent (h or zo) of the hydrocarbon column
that any given sealing rock (small-pore-throated rock) can hold.
iv) A rock with larger pore throats than those of shale (e.g., a siltstone)
can be the seal over a petroleum accumulation of lesser vertical extent.
v) Migration through rocks with large pores leaves behind less oil than
though rocks with smaller pores where some oil meets blind pathways.
vi) Migration of a hydrocarbon column can be limited by generation of
petroleum at its base (because separation of the base from its source
stops the increase of h and thus stops increase of the buoyancy force).
Conventional wisdom says that sandstones are reservoirs and shales are
seals, but the points above suggest that
• even the lousiest potential seal can be the seal of a short column of
hydrocarbons (ii & iv), and
• even the best siliciclastic seal (the tightest shale) will fail to seal a tall
column of hydrocarbons (i and iii), especially light hydrocarbons.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Scan Selley p. 175 or Berg original
Selley 1998
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Gluyas & Swarbrick 2004
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Scan G&S p. 145
Gluyas & Swarbrick 2004
A better caption:
Maximum vertical extent of a gas or oil
column (horizontal axis) as a function
of depth (vertical axis) for a given
mudstone or shale seal. Greater
columns are possible for oil than gas
because of greater buoyancy of gas,
so that gas columns overcome
resistance of small pore throats.
Shapes of curves depend on interplay
of (1) decrease of interfacial tension
with increasing temperature at depth,
(2) decrease of oil density with
increasing temperature at depth, and
(3) decreasing size of pore throats in
mudstone with increasing depth.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
• Migration through finingupwards sandstones leaves
oil scattered in pore throats;
migration through
coarsening-upwards sandsandstones leave behind
less oil.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
• Reservoir pore water,
with zero interfacial
tension against seal pore
water, can escape
upwards out of a
petroleum as the trap
fills.
• Gas, with its greater
buoyancy than, can
escape upwards out of
a trap that holds oil.
Cant, 1986, AAPG Bulletin 70: 155-160.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
• Reservoir pore water,
with zero interfacial
tension against seal pore
water, can escape
upwards out of a
petroleum as the trap
fills.
• Gas, with its greater
buoyancy than, can
escape upwards out of
a trap that holds oil
(hence “gas chimneys”).
Cant, 1986, AAPG Bulletin 70: 155-160.
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
III.
Faults - seals or pathways of migration?
A. Faults as zones rather than planes
B. Factors favoring faults as seals
1) Abundance of clay/shale along fault
Clay smear” or “Shale smear”
2) Faulting early in burial history
“Clay smear” vs. “Shale smear” vs. “Shale Gouge”
3. Greater time between faulting and arrival of petroleum
Infilling/cementing minerals block open volumes
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Shepherd 2009
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
III.
Faults - seals or pathways of migration?
A. Faults as zones rather than planes
B. Factors favoring faults as seals
1) Abundance of clay/shale along fault
“Clay smear” or “Shale smear”
2) Faulting early in burial history
“Clay smear” vs. “Shale smear” vs. “Shale Gouge”
3. Greater time between faulting and arrival of petroleum
Infilling/cementing minerals block open volumes
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Aydin & Eyal 2002 AAPG Bulletin
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
van der Zee et al. www.ged.rwth-aachen.de/Ww/projects/faults/Clay%20injection/Clay%20injection.html
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Several statistical algorithms are being used in practice
for the purpose of estimating the distribution and
relative amount (percentage) of shale along fault zones
in the subsurface and the associated sealing (Yielding
et al., 1997; Skerlec, 1999). . . . The general conclusions
from these statistically based methods are that
1) thicker shale beds produce thicker shale smears
2) the percentage of shale decreases with distance
from the [shale] source layer, and
3) the relative percentage of shale [in the fault
gouge/smear] increases with the number of [shale]
source beds passing a point on the fault plane.
Koledoye et al. 2003,
AAPG Bulletin
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Shepherd 2009
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Shepherd 2009
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
III.
Faults - seals or pathways of migration?
A. Faults as zones rather than planes
B. Factors favoring faults as seals
1) Abundance of clay/shale along fault
“Clay smear” or “Shale smear”
2) Faulting early in burial history
“Clay smear” vs. “Shale smear” vs. “Shale Gouge”
3. Greater time between faulting and arrival of petroleum
Infilling/cementing minerals block open volumes
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
III.
Faults - seals or pathways of migration?
A. Faults as zones rather than planes
B. Factors favoring faults as seals
1) Abundance of clay/shale along fault
“Clay smear” or “Shale smear”
2) Faulting early in burial history
“Clay smear” vs. “Shale smear” vs. “Shale Gouge”
3. Greater time between faulting and arrival of petroleum
Infilling/cementing minerals block open volumes
University of Georgia Department of Geology GEOL 4320 Petroleum Geology
Selley 1998
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology
Sources
White sans-serif Helvetica text
Light gray Times New Roman text
Asquith and Krygowski 2004
Assaad 2008
AAPG Basic Well Log Analysis course notes
Baker-Hughes Atlas of Log Responses
Small white text
Bjørlykke 2010
Conaway 1999
Crain’s Petrophysical Handbook
Title
Glover’s Petrophysique
Gluyas & Swarbrick 2004
North 1980
Jonathan B. Martin UF class notes
Rigzone
Schlumberger Log Interpretation P&I
Schlumberger Oilfielld Glossary
Shell Petroleum Handbook (1983)
Shepherd 2009
Notes
Selley 1998
Tissot & Welte (1984)
Wikipedia
Selley 1978, Porosity gradients in North Sea oil-bearing sandstones: Jo. Geol. Soc. London v. 135, 119-132.