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 Review Guide 1

Last Update Feb 11, 2003

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Readings are in red
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Table of contents

A. Origin of Petroleum
1. Introduction
2. The petroleum system
3. What is Petroleum?
4. The subsurface environment
Temperature
Pressure
Fluids

 

5. Review of Sedimentary Basins and Sedimentary rocks
6. The source: How oil forms
Source rock characteristics
Maturation of Kerogen.

A. Origin of Petroleum

1. Introduction
Historical Review of Petroleum Exploration.

Petroleum= Rock Oil
Petroleum is known since antiquity but only in the late 1800’s 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

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.






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:

  1. Mature source rock.
  2. Permeable reservoir rock.
  3. Trap composed of structure, seal and reservoir.
  4. Migration path from source to trap.
  5. Right timing of all these elements.
  6. Preservation

Reading: Ch. 5 pp. 181-191, Selley


3 . What is Petroleum?

Definitions:

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:

Physical Properties:
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=-k dT/dz
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.

Pressure

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
 

  • Lithostatic Stress= due to the load of the overlying column of rock
  • typical lithostatic gradient = 1 psi/ ft
  • Hydrostatic Pressure = due to the load of the interconnected column of water in the pore spaces within the rock

  • hydrostatic gradient depends on the salinity of the water (and Temp)
  • fresh water gradient= 0.43 psi/ft
  • moderate salinity water = 0.465 psi/ft (88,000 ppm solids)
  •  
  • Overpressure


    Explain how overpressure can exist. Why is it important?

    Subsurface waters

    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-


    Salinity:

     

    Why is the salinity of water important in petroleum geology, what does it tell you about the history of the reservoir?

     

    Fluid thermodynamics

    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.

    Sedimentary environments

    Continental: Fluvial, lacustrine, deltaic

    Marine: Barrier Island, Lagoon, open shelf, reef, basinal turbidites

    Important concepts:

  • Characteristic rock sequence for each environment.
  • Role of the different rock types in the petroleum system
  • Patterns of grain size variations,
  • High energy vs. low energy environments,
  • Unconformity
  • The vertical stacking of rock facies corresponds to the lateral changes through the sedimentary environment
  • Progradation
  • Subsidence
  • Reading: Read the chapter on "Sedimentary Environments" from Earth Systems History textbook. Copies on reserve in Colson Library.


    6. The source: How oil forms

    Black shales are the main petroleum sources. They can contain 3-12% organic carbon
     
    Organic Productivity
    Best source rocks contain remnants of marine algae, phytoplancton.
    High organic productivity needed for a good source rock. This is found in areas where there is:
    • -sunlight (Upper 100 m, clear water)
    • -Abundant nutrients (Phophates, nitrates)
     
     Upwelling

     This happens in zones of upwelling of deep water.

    Explain how up welling works, where it is found, and why it causes algal blooms.

     Preservation of Organic Matter.
     
     
     Anoxic bottom waters are required for organic rich sediments to be preserved. Otherwise the organic matter just feeds other critters, or bacteria.
     

     Anoxic water is found where the water column does not mix:

      • -stratified lakes in the tropics
      • -some restricted marine basins
      • -on the open shelves in areas of high productivity with a oxygen minimum layer

    Explain how these settings develop, sketch them. Why is water stratified?

    Formation of Kerogen.
    Kerogen= solid organic matter disseminated in sediments which is insoluble in organic solvents. It is the precursor to petroleum.
    Explains the steps in the process that leads from deposition of dead algae to petroleum.
     
    Generation of hydorcarbons
    What are the different processes that lead to the generation of hydrocarbons?
     
    Maturation of Kerogen.
    What is meant by "maturation"?
    What happens to the source rock with increasing temperature?
     
    Types of organic matter
    What are the three types of kerogen? What hydrocarbons are generated by each?
    Reading: Ch. 5, pp. 191-206, Selley