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?... 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?.... 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

Buoyancy Force = h g (  w -  p ) Seals and Reservoirs II. Buoyancy and upward migration, or The interplay of buoyancy and pore size in determining what is a seal 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”: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology g = gravitational acceleration h = vertical extent (height) of the petroleum column h = H = z o = Y in other presentations  w = density of water (~ )  p = density of petroleum (~ for oil) ~ 0.3 for oil (see above) h

Buoyancy Force = h g (  w -  p ) 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”: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology g = gravitational acceleration h = vertical extent (height) of the petroleum column h = H = z o = Y in other presentations  w = density of water (~1.01 to 1.10)  p = density of petroleum (~0.7 to 0.8 for oil) ~ 0.3 for oil (see above) h

≠ University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology tutorvista.com

2  cos  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: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology r t = radius of pore throat Resistance = rtrt  = interfacial tension  = wettability See next page for more explanation. Berg (1975)

2  cos  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: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology r t = radius of pore throat (the smaller the pore throat, the greater the resistance). Resistance = rtrt  = 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.  Oil Water Rock 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 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 z o ) 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 z o ) 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 z o ) 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). Hydrocarbon columns growing with migration from below Seals 1234 Changing position and effectiveness of seal with progressive accumulation of 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 z o ) 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 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 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. Gluyas & Swarbrick 2004 Oil column Gas column

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 z o ) 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 z o ) 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 Migration through fining- upwards sandstones leaves oil scattered in pore throats; migration through coarsening-upwards sand- sandstones leave behind less oil.

University of Georgia Department of Geology GEOL 4320 Petroleum Geology 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 z o ) 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 through increasingly small pores can be limited by cessation of generation of petroleum at the column’s 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 z o ) 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 z o ) 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 Cant, 1986, AAPG Bulletin 70: 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. Zero

University of Georgia Department of Geology GEOL 4320 Petroleum Geology Cant, 1986, AAPG Bulletin 70: 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. (There is no buoyancy force for water, so this concept assumes a compacting reservoir to generate pressure head) Zero

University of Georgia Department of Geology GEOL 4320 Petroleum Geology Cant, 1986, AAPG Bulletin 70: 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 oil, can escape upwards out of a trap that holds oil (hence “gas chimneys”).

University of Georgia Department of Geology GEOL 4320 Petroleum Geology Gas, with its greater buoy- ancy than, can escape upwards out of a trap that holds oil (hence “gas chimneys”).

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 Shepherd 2009

University of Georgia Department of Geology GEOL 4320 Petroleum Geology

New PGSG slides:

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

Aydin & Eyal 2002 AAPG Bulletin

University of Georgia Department of Geology GEOL 4320 Petroleum Geology van der Zee et al. l

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 ? ? A B C D Note new meaning of “source”.

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 Really, really, really – caly/shale smear can be a seal:

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

Light gray Times New Roman text White sans-serif Helvetica text University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology Title Notes Bjørlykke 2010 Gluyas & Swarbrick 2004 Selley 1998 Sources Schlumberger Oilfielld Glossary Conaway 1999 Assaad 2008 Shepherd 2009 Shell Petroleum Handbook (1983) Rigzone Crain’s Petrophysical Handbook North 1980 AAPG Basic Well Log Analysis course notes Schlumberger Log Interpretation P&I Glover’s Petrophysique Asquith and Krygowski 2004 Baker-Hughes Atlas of Log Responses Jonathan B. Martin UF class notes Wikipedia Tissot & Welte (1984) Small white text Selley 1978, Porosity gradients in North Sea oil-bearing sandstones: Jo. Geol. Soc. London v. 135,