3. Setting
• Oil and natural gas (hydrocarbons) produced from oil and gas
fields are in porous and permeable rock or reservoirs, in which
liquids have collected over geologic time
• Coincidence of four types of geologic features:
• Hydrocarbons source rocks
• Reservoir rocks
• Seals, and
• Traps
4. Setting
• Oil and natural gas (hydrocarbons) produced from oil and gas
fields are in porous and permeable rock or reservoirs, in which
liquids have collected over geologic time
• Coincidence of four types of geologic features:
• Hydrocarbons source rocks
• Reservoir rocks
• Seals, and
• Traps
5. Source
• Sedimentary rocks that were deposited usually in still swamps
on land, shallow quiet marine bays or in deep submarine
• Comprised of very small mineral fragments
• In between them were organic remains like algae, wood or
soft parts of plants
• Buried by the deposition of later, overlying sediments
• Increased heat and pressure with depth turned them into rock
• If further burial temperature > 120oC (250oF) then organic
remains begin to “cook” into hydrocarbon over time
6. Source
• Sedimentary rocks that were deposited usually in still swamps
on land, shallow quiet marine bays or in deep submarine
• Comprised of very small mineral fragments
• In between them were organic remains like algae, wood or
soft parts of plants
• Buried by the deposition of later, overlying sediments
• Increased heat and pressure with depth turned them into rock
• If further burial temperature > 120oC (250oF) then organic
remains begin to “cook” into hydrocarbon over time
7. Reservoir
• Must be porous and permeable to contain anything
• Contain interconnected passageways of microscopic pores or
holes in between the mineral grains of the rocks
• Hydrostatic pressure will naturally expel hydrocarbons from
source rocks
• They will migrate to adjacent reservoir rocks
• Mostly sandstone, limestone or dolomite
8. Reservoir
• Must be porous and permeable to contain anything
• Contain interconnected passageways of microscopic pores or
holes in between the mineral grains of the rocks
• Hydrostatic pressure will naturally expel hydrocarbons from
source rocks
• They will migrate to adjacent reservoir rocks
• Mostly sandstone, limestone or dolomite
9. Seals
• Hydrocarbons are relatively free to move once they are in the
reservoir rock
• Those are originally filled with saline water ~ 1.03 g/cm3
• Oil (0.82 - 0.93 g/cm3) and gas (0.12 g/cm3) will rise up
through pore space until they meet an impermeable barrier
• Seals are generally fine-grained rock with no over very small
pore space that prevents fluids from entering
• Oil and natural gas then accumulate in the reservoir against
those seals into what is called a trap
10. Seals
• Hydrocarbons are relatively free to move once they are in the
reservoir rock
• Those are originally filled with saline water ~ 1.03 g/cm3
• Oil (0.82 - 0.93 g/cm3) and gas (0.12 g/cm3) will rise up
through pore space until they meet an impermeable barrier
• Seals are generally fine-grained rock with no over very small
pore space that prevents fluids from entering
• Oil and natural gas then accumulate in the reservoir against
those seals into what is called a trap
11. Traps, structural
• Formed when reservoir rock and overlying seal have been
deformed by folding or faulting
• Usually millions or 100s of millions of years after the
deposition of the sediments turned into seals or reservoir rock
• Hydrocarbons migrate upward in pore spaces, through
buoyancy, to the highest part of the structure
• Likewise hydrocarbons layer above water, and gas layers above
oil
12. Traps, structural
• Formed when reservoir rock and overlying seal have been
deformed by folding or faulting
• Usually millions or 100s of millions of years after the
deposition of the sediments turned into seals or reservoir rock
• Hydrocarbons migrate upward in pore spaces, through
buoyancy, to the highest part of the structure
• Likewise hydrocarbons layer above water, and gas layers above
oil
13. Traps, stratigraphic
• Formed when reservoir rocks are deposited in a discontinuous
layer - abut against or turn into a seal sideways - in other
words seals are deposited beside or on top of reservoirs
• Common example is a coastal barrier island, which is an
elongate lens of sandstone sandwiched in between shale,
which in turn could be source rocks too
• It also helps if it gets all tilted up sideways by further uplift,
not strong enough to deform rocks, but helps abut the
hydrocarbons against the seal through natural buoyancy
14. Traps, stratigraphic
• Formed when reservoir rocks are deposited in a discontinuous
layer - abut against or turn into a seal sideways - in other
words seals are deposited beside or on top of reservoirs
• Common example is a coastal barrier island, which is an
elongate lens of sandstone sandwiched in between shale,
which in turn could be source rocks too
• It also helps if it gets all tilted up sideways by further uplift,
not strong enough to deform rocks, but helps abut the
hydrocarbons against the seal through natural buoyancy
22. Methods
• Hydrocarbons are usually buried deep underground
• They form a discontinuity in the rock formation
• Though porosity that is filled or empty of hydrocarbons
• Through changes from reservoir rocks to seal rocks
• Therefore ideally suited for geophysical prospecting
• The indirect measurement of rock properties underground
23. Methods
• Hydrocarbons are usually buried deep underground
• They form a discontinuity in the rock formation
• Though porosity that is filled or empty of hydrocarbons
• Through changes from reservoir rocks to seal rocks
• Therefore ideally suited for geophysical prospecting
• The indirect measurement of rock properties underground
24. Gravity
• Measure the spatial variation in the earth’s gravity filed
caused by the differences in density of the underlying rocks
• The measure of changes in gravitational acceleration is
expressed in gravity anomalies in milligals (10-5 m/s2)
• As a deviation from a reference value along a geoid
(surface along which gravitational accelerations is the same)
• Gravity is measured as variations is value between different
points on the earth’s surface
• It is a scalar value (intensity measured at each point)
25. Gravity
• Measure the spatial variation in the earth’s gravity filed
caused by the differences in density of the underlying rocks
• The measure of changes in gravitational acceleration is
expressed in gravity anomalies in milligals (10-5 m/s2)
• As a deviation from a reference value along a geoid
(surface along which gravitational accelerations is the same)
• Gravity is measured as variations is value between different
points on the earth’s surface
• It is a scalar value (intensity measured at each point)
27. Magnetics
• Same thing as gravity, except the anomalies measured are
magnetic in Tesla (V m/s2 , or very faint in nano Tesla = 10-9)
• Also magnetic susceptibility or remanance is a vector
(meaning it has a direction as well as a scalar value)
• It is left behind by magnetic elements in rocks and minerals
that vary with the type of rocks in the subsurface
• Anomaly = regional (background) – residual (local) fields
28. Magnetics
• Same thing as gravity, except the anomalies measured are
magnetic in Tesla (V m/s2 , or very faint in nano Tesla = 10-9)
• Also magnetic susceptibility or remanance is a vector
(meaning it has a direction as well as a scalar value)
• It is left behind by magnetic elements in rocks and minerals
that vary with the type of rocks in the subsurface
• Anomaly = regional (background) – residual (local) fields
29. Magnetics
• Same thing as gravity, except the anomalies measured are
magnetic in Tesla (V m/s2 , or very faint in nano Tesla = 10-9)
• Also magnetic susceptibility or remanance is a vector
(meaning it has a direction as well as a scalar value)
• It is left behind by magnetic elements in rocks and minerals
that vary with the type of rocks in the subsurface
• Anomaly = regional (background) – residual (local) fields
31. Electromagnetics
• Same as magnetic except that:
• instead of measuring the magnetism in the ground, measure:
• the change resulting from inducing a current in the ground
• Also that is a 2-dimensional vector
• Anomaly = primary field (natural) – secondary field (induced)
modified by what lies underground
• It is used for example in passive sea-bed logging
• Similarities in seismic and well logging to follow
32. Electromagnetics
• Same as magnetic except that:
• instead of measuring the magnetism in the ground, measure:
• the change resulting from inducing a current in the ground
• Also that is a 2-dimensional vector
• Anomaly = primary field (natural) – secondary field (induced)
modified by what lies underground
• It is used for example in passive sea-bed logging
• Similarities in seismic and well logging to follow
33. Electromagnetics
• Same as magnetic except that:
• instead of measuring the magnetism in the ground, measure:
• the change resulting from inducing a current in the ground
• Also that is a 2-dimensional vector
• Anomaly = primary field (natural) – secondary field (induced)
modified by what lies underground
• It is used for example in passive sea-bed logging
• Similarities in seismic and well logging to follow
34. Seismic
• Echo- or depth-sounding off rocks in the subsurface:
• from a pulse caused artificially on the earth’s surface
• the reflection of waves against variations in rock types
• and recording of travel + arrival times of various waves
37. Seismic
• Echo- or depth-sounding off rocks in the subsurface:
• from a pulse caused artificially on the earth’s surface
• the reflection of waves against variations in rock types
• and recording of travel + arrival times of various waves
38. Seismic
• Echo- or depth-sounding off rocks in the subsurface:
• from a pulse caused artificially on the earth’s surface
• the reflection of waves against variations in rock types
• and recording of travel + arrival times of various waves
