Table of contents
A. Origin of Petroleum
Historical Review of Petroleum Exploration.Petroleum= Rock Oil Petroleum is known since antiquity but only in the late 1800s it became an important resource. Early uses: waterproofing, lubricant, medicinal 19th Century- kerosene for lamps 20th Century fuel for internal combustion engines and then a huge number of other applications in all industries
What triggered the sudden growth of the oil industry in the second half of the 19th century?
Early explortion was based on the locations of oil seeps: places where oil flowed at the surface.
The future of the Petroleum Industry: When will we run out of oil?
Petroleum is a large but finite resource.
Peak of oil discoveries was in 1962. Since then discovery of new reserves has been declining in spite of technological improvements. We are currently near the peak of production . This scenario implies that due to lack of new discoveries production will begin to decline by about 2010.
Discuss this scenario.
The price of oil has historically been closely tied to political events, particularly regarding the Middle East where half of the world's oil reserves are located.
Why has the price of oil tripled in the last year? Why has it not affected the economy to the same degree that it did in 1974 oil crisis?
Readings: Ch. 1, pp. 1-11, Selley., Ivanhoe, 1997, Get Ready for Another Oil Shock, in: http//dieoff.org/page90.htm
2. The petroleum system
Organic vs. inorganic origin of petroleum
Most scientific evidence shows that petroleum originates from the decay of organic matter buried in sediments.
What is this evidence?
Small amounts of hydrocarbons are also found in settings far from organic influence (meteorites, CH4 in Jupiter and Saturn, some metamorphic and igneous complexes, CH4 in volcanoes) so there must also be a minor inorganic source of petroleum.
The Carbon Cycle
Organic Carbon Cycle
Through photosynthesis plants and algae take CO2 from the air or water and turn it into organic molecules for their cells. These organisms support the food chain. Once a living thing dies it either is decomposed back to CO2 and H2O or is buried. If buried it may turn into hydrocarbons if all conditions are right. When we burn hydrocarbons CO2 returns to the atmosphere.
Make a diagram showing the organic carbon cycle.
In light of the carbon cycle, what are the environmental consequences of burning hydrocarbons?
Inorganic Carbon cycle
Most of carbon on earth is stored as carbonate ( CO3)- in rocks such as limestones and dolomites. Carbonate is in equilibrium with seawater, marine organisms take it from the water to make their skeletons.
Overview of the formation of a petroleum deposit from source to trap
These are necessary conditions for a petroleum accumulation:
Make a sketch of an active petroleum system with all its elements.
Reading: Ch. 5 pp. 181-191, Selley
3 . What is Petroleum?
Petroleum? Hydrocarbon? Natural gas? Oil? Crude? Condensate? API gravity? Light oil? Heavy oil? Gasoline? Kerosine?
Chemical Properties of Petroleum:
Basic components: C, H, O, N, S
Carbon forms covalent bonds with itself allowing for long chains and networks of rings. An immense variety of molecular structures is possible.
Basic molecular types: Parafins, Naphtenes, Aromatics, Asphaltics.
What is the basic structure of each? What are the uses of each?
Composition of an typical crude:
Longer hydrocarbons chains are increasingly denser, more viscous, and have higher boiling temperatures
API gravity is a density scale calibrated so that density of water is 10 degrees API. Increasing API gravity = decreasing density
The changes in boiling temperature of the different hydrocarbons are used to separate crude using a distillation process.
Explain how a distillation tower works.
Readings: Ch. 2, pp. 13-16, 26-33 (skip sections 2.1.2 and 2.2), Selley
4. The subsurface environment
Temperature within the earth
The generation of petroleum from a source rock is controlled by temperature. Oil window 60°C- 120°C. Gas window 120°C-220°C.
Temperature increases with depth due to heat produced by radioactive elements in rocks, mainly U, Th, K. These are concentrated in the crust.
Heat flow equation:
q= heat flow (mW/m2); k= thermal conductivity; dT/dz= geothermal gradient
Average surface heat flow is about 50 mW/m2
Average geothermal gradient = 25°C/km (ranges from 18 to 55°C/km)
Given a thermal gradient, what is the depth of the oil window?
Thermal conductivity of rocks:
Because of heat flow equation low conductivity leads to high thermal gradient.
Given a rock column, sketch the thermal gradient.
Flow of water through aquifers can transport heat producing thermal anomalies.
What would be the effect of an active aquifer on temperature measured in a well
Temperature measurements: BHT (bottom hole temperature). Temperatures measured in a well right after drilling are not accurate because circulation of the drilling mud has cooled the well. It is necessary to wait until equilibrium temperature is reached, or make a correction. One needs to know the time since circulation ended in the well.
Why do we care about pressure?
P= pressure= force/area (units Pascals, or psi)
Not to be confused with stress which has same units, but is not a scalar quantity
P= rgz = density x accel. of gravity x depth
Why can two oilfields located
near each other have different pressure gradients?
Explain how overpressure can exist. Why is it important?
Most open space in the subsurface is filled with water
Free water is able to flow
Irreducible water is attached to the pore walls by surface tension or to minerals as OH-
Why is the salinity of water important in petroleum geology, what does it tell you about the history of the reservoir?
Physical state of hydrocarbons depends on the temperature and pressure conditions. For example condensate (C5) will be a gas in the reservoir, but turn into a liquid as it is brough to the surface and cools. The gas/oil ratio of a crude changes with P and T.
On a pressure-temperature graph sketch the path followed by condensate as it is produced by a well
Reading: Ch. 4, pp. 147-177, Selley
5. Review of Sedimentary Basins and Sedimentary rocks
Why are there sedimentary rocks on earth?
Explain the sedimentary rock cycle.
Shale, siltstone, sandstone, conglomerate
Continental: Fluvial, lacustrine, deltaic
Marine: Barrier Island, Lagoon, open shelf, reef, basinal turbidites
Reading: Read the chapter on "Sedimentary Environments" from Earth Systems History textbook. Copies on reserve in Colson Library.
6. The source: How oil forms
This happens in zones of upwelling of deep water.
Explain how up welling works, where it is found, and why it causes algal blooms.
|Anoxic bottom waters are required for organic rich sediments to be preserved. Otherwise the organic matter just feeds other critters, or bacteria.|
Explain how these settings develop, sketch them. Why is water stratified?