The Subsurface Environment(s) of Petroleum University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth Typical depths: 1, ,000 feet / meters Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor in 4132 ft / 1259 m of water in Lower Tertiary strata Drilled by the Deepwater Horizon rig destroyed in April Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma (Anadarko Basin) (1974) 31,441 feet / 9583 m, P&A in molten sulfur Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit) Temperature Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth Typical depths: 1, ,000 feet / meters Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor in 4132 ft / 1259 m of water in Lower Tertiary strata Drilled by the Deepwater Horizon rig destroyed in April Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma (Anadarko Basin) (1974) 31,441 feet / 9583 m, P&A in molten sulfur Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit) Temperature Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

BP’s Tiber discovery well

The Subsurface Environment(s) of Petroleum I. Depth Typical depths: 1, ,000 feet / meters Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor in 4132 ft / 1259 m of water in Lower Tertiary strata Drilled by the Deepwater Horizon rig destroyed in April Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma (Anadarko Basin) (1974) 31,441 feet / 9583 m, P&A in molten sulfur Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit) Temperature Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth Typical depths: 1, ,000 feet / meters Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor in 4132 ft / 1259 m of water in Lower Tertiary strata Drilled by the Deepwater Horizon rig destroyed in April Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma (Anadarko Basin) (1974) 31,441 feet / 9583 m, P&A in molten sulfur. Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit) Temperature Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth Typical depths: 1, ,000 feet / meters Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor in 4132 ft / 1259 m of water in Lower Tertiary strata Drilled by the Deepwater Horizon rig destroyed in April Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma (Anadarko Basin) (1974) 31,441 feet / 9583 m, P&A in molten sulfur. Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit) Temperature Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth Deepest petroleum well to date: BP’s 2009 Tiber discovery well in Gulf of Mexico 35,055 ft / 10,685 m sub-seafloor Deepest (?) onshore petroleum well GHK #1-27 Bertha Rogers in Washita County, Oklahoma 31,441 feet / 9583 m, P&A in molten sulfur. Deepest drillhole to date: Kola Superdeep Borehole in Kola Peninsula, Russia (1989) 40,230 ft / 12,262 m (drilled non-rotary with a mud-motor bit: With a lot of rounding: Deepest onshore petroleum well: 30 thousand feet Deepest offshore petroleum well: 35 thousand feet Deepest well/borehole of any sort: 40 thousand feet University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Significance: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Significance: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology (Lowest T of oil generation to ~metamorphism)

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Significance: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

Geothermal gradients: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology From smu.edu/geothermal/heatflow/heatflow.htm

Geothermal gradients: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology Both are from smu.edu/geothermal/heatflow/heatflow.htm Heatflow (at right) = conductivity x gradient (at left)

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

Geothermal gradients: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology Alsharhan & Nairn 1997 Persian/Arabian Gulf

Geothermal gradients: University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology Alsharhan & Nairn 1997

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance of temperature: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance of temperature: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

North 1985

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance of temperature: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Diagenetic reactions that destroy porosity Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology *Diagenesis: the physical and chemical modification of sediments that turns them into sedimentary rocks, including but not limited to compaction (lessening of bulk volume) and cementation (infiling of pores with minerals).

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology North 1985

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance of temperature: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Diagenetic reactions that destroy porosity Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature Relevant range: °C Geothermal gradients: °C/km Typically ~25 °C/km Bottom-hole Temperatures (BHTs) Measured during logging, well after circulation has stopped. Significance of temperature: (past) Thermal maturation of kerogen to yield petroleum Oil window: ~65-160°C Diagenetic reactions that destroy porosity Decreased resistivity of formation waters Degradation / melting of drill bit Pressure Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry Increasing total dissolved solids / salinity with depth Thus increasing density with depth Cl - typically the dominant anion Na + and Ca 2+ the dominant cations University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

North 1985 sw

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry Increasing total dissolved solids / salinity with depth Thus increasing density with depth Cl - typically the dominant anion Na + and Ca 2+ the dominant cations University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry Increasing total dissolved solids / salinity with depth Thus increasing density with depth Cl - typically the dominant anion Na + and Ca 2+ the dominant cations University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry Increasing total dissolved solids / salinity with depth Thus increasing density with depth Cl - typically the dominant anion Na + and Ca 2+ the dominant cations University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

North 1985

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

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The dashed curve labeled “Natural water” is the pressure trajectory of a water column with porewaters increasing from G = 1.00 in the uppermost 1000 feet to 1.08 at 20,000 feet depth. The dashed curve labeled “Natural strata” is the pressure trajectory of a stratigraphic section with a mineral G of 2.65 with porosity decreasing from 25% in the uppermost 1000 feet to 4% at 20,000 feet.

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology These two are not nearly synonymous in the “isotropic” sense sometimes used in structural geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology These two are not nearly synonymous in the “isotropic” sense sometimes used in structural geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology These two are not nearly synonymous in the “isotropic” sense sometimes used in structural geology

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology The dashed curve labeled “Natural water” is the pressure trajectory of a water column with porewaters increasing from G = 1.00 in the uppermost 1000 feet to 1.08 at 20,000 feet depth. The dashed curve labeled “Natural strata” is the pressure trajectory of a stratigraphic section with a mineral G of 2.65 with porosity decreasing from 25% in the uppermost 1000 feet to 4% at 20,000 feet.

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

North 1985 Note dearth of data in (e).

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

Dallmus, in Weeks (1958) Note the inverted vertical scale.

University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology Dallmus, in Weeks (1958)

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The dashed curve labeled “Natural water” is the pressure trajectory of a water column with porewaters increasing from G = 1.00 in the uppermost 1000 feet to 1.08 at 20,000 feet depth. The dashed curve labeled “Natural strata” is the pressure trajectory of a stratigraphic section with a mineral G of 2.65 with porosity decreasing from 25% in the uppermost 1000 feet to 4% at 20,000 feet. Overpressure

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Less extensive compaction ii)Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Less extensive compaction ii) Fracturing of rock ii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

The Subsurface Environment(s) of Petroleum I. Depth II. Temperature III. Water Chemistry IV. Pressure Force/area Weight/area (psi) Lithostatic: Weight of overlying rock Hydrostatic: Weight of overlying column of fluid (in which density typically increases downward) Results Greater pressure at depth Compaction of sediments/rocks Overpressure: subsurface liquid/gas pressure greater than hydrostatic pressure Pore fluids sealed below an impermeable stratum are pressurized a) because of compaction (decrease of pore volume) or b) because of diagenetic chemical reactions that release liquid or gas (increase of fluid volume) Potential results of overpressure: i) Less extensive compaction ii) Fracturing of rock iii) Blowout of well Water Chemistry University of Georgia Department of Geology GEOL 4320/6320 Petroleum Geology

200 foot flames at a 1998 natural gas well blowout near Bakersfield, CA. Image from Sandia National Laboratories via a Wilderness Society webpage..

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