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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

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slide2

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?

slide3

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?

slide4

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?

slide5

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?

slide6

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?

slide7

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?

slide8

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?

slide9

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

Seals and Reservoirs

II. Buoyancy and upward migration, or . . .

slide10

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

slide11

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”:

p= density of petroleum

(~0.7-0.8 for oil)

w= density of water (~1.01-1.10)

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

slide12

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”:

p= density of petroleum

(~0.7-0.8 for oil)

w= density of water (~1.01-1.10)

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

slide13

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 “displace-

ment pressure” inherent in the interaction of fluid and pore throat:

= wettability

  • = interfacial tension

See next page for

more explanation.

2•  • cos 

Resistance =

rt

rt = radius of pore throat

slide14

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

is neglected.

Water

Oil

Rock

2•  • cos 

Resistance =

rt

rt = radius of pore throat

(the smaller the pore throat, the greater the resistance).

slide17

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

Downey (1984, AAPG Bulletin)

slide19

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).

slide20

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).

slide21

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).

slide22

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).

slide23

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).

slide24

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).

slide25

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.

slide26

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.

slide27

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

Scan Selley p. 175 or Berg original

Selley 1998

slide28

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

Gluyas & Swarbrick 2004

slide29

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

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.

Scan G&S p. 145

Gluyas & Swarbrick 2004

slide30

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

• Migration through fining-

upwards sandstones leaves

oil scattered in pore throats;

migration through coarsening-upwards sand-

sandstones leave behind

less oil.

slide31

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.

slide32

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.

slide33

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

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

slide36

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

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

slide39

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

Aydin & Eyal 2002 AAPG Bulletin

slide40

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

slide42

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

slide45

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

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

slide47

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

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

slide52

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

Sources

White sans-serif Helvetica text

Asquith and Krygowski 2004

Light gray Times New Roman text

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)

Selley 1998

Shepherd 2009

Tissot & Welte (1984)

Wikipedia

Notes

Selley 1978, Porosity gradients in North Sea oil-bearing sandstones: Jo. Geol. Soc. London v. 135, 119-132.