Chapter 6a Petroleum Origin, Chemistry & Generation

Question 1

What are the different theories of the origin of Petroleum?

Answer 1

• Inorganic origin
• Organic origin

Question 2

Explain the Inorganic Origin of petroleum.

Answer 2

• H and C came together under great P and T and formed O&G where chemical reactions occured
• The O&G then seeped through porous rocks to deposit in various natural underground traps
• It has also excluded the hypothesis that petroleum is a finite substance
• There are some theories that describe the inorganic origin of petroleum:
  
  • Metal Carbide Theory
  • Volcanic Theory
  • Earthquake Theory
  • Serpentinization Theory
  • Overwhelming evidences : - geographical location & stability with depth

Question 3

Explain what is the Organic Origin of Petroleum

Answer 3

1. The O&G are formed from remains of plants and animals.
   Remains of plants=coal
   Remains of animals=O&G
2. These remains will settled into the sea and accumulated and buried at the ocean floor.
3. The accumulation of the remains will be compressed for million of years.
4. The increase in P & T with the absence of Oxygen will change the sediments/mud/sand/silt into rock and organic matter into kerogen
5. Kerogen will be transformed into petroleum and natural gas via cracking
6. Evidences for Organic Origin of petroleum:
   • presence of brine with petroleum
   • petroleum is found in association with sedimentary rock
   • molecules of HC similar to organic matter

Question 4

What are the 4 Hydrocarbon series? (Refer to notebook for better memorization)

Answer 4

1. Paraffin
   - occur as chain-like structures
   - general formula: CnH2n+2
   - the carbon number ‘n’ ranges from methane (n=1) to over 40
   - C1-C5 = gaseous, C5 above = normally liquid
   - high molecular weight paraffins become viscous, waxy solids
   - Dry gas = natural gas composed of nearly pure methane
   - Wet gas = natural gas that contains other heavier paraffin gasses along with methane
2. Naphthenes
   - Form a closed ring structure
   - General formula: CnH2n
   - Compound of naphthenes series have chemical & physical properties similar to equivalent paraffins with the same carbon number
   - form the major components of most crude oils together with paraffins
3. Aromatics
   - General formula: CnH2n-6
   - Have a structure based on a hexagonal ring of carbons with alternate single and double bonds
   - Basic unit name is Benzene ring
   - Responsible for the strong odor and fluorescence in oils and extract
4. Resins & Asphaltenes
   - composed of fused benzene ring networks but they contain other atoms and are not true HCs
   - these impurities include the high molecular weight NSO compounds
   - the heaviest components of crude oil and the major components in many natural tars and asphalts

Question 5

Saturated HC vs Unsaturated HC

Answer 5

1. Saturated HC
   -contain only c-c single bonds
   -ethane
2. Unsaturated HC
   -contain two or more covalent bonds
   -ethene

Question 6

What is the method of classifying crude oil that was proposed by Tissot and Welte (1978) that represents 6 crude oil classes?

Answer 6

• Draw triangle by plotting paraffin, napthenes and combination of aromatic HC & NSO compounds

Question 7

What are the 3 main crude oil classes?

Answer 7

Most normal crude oils fall within 3 main classes:

1. Be rich in paraffins (paraffinic oils)
2. Have nearly equal amounts of paraffin and naphthenes - which together make up more than 50% of the crude (parafinitic-naphthenic oil)
3. Have subequal amounts of paraffins and naphthenes - which total less than 50% where the composition is dominated by aromatics & the resins and asphaltenes

Question 8

What is Kerogen?

Answer 8

• composed of a variety or organic materials, including algae, pollen, wood, vitrinite and structureless material

-the types of kerogen present in a rock largely control the type of HCs generated in that rock

• the H content of kerogen is the controlling factor for oil vs gas yields

Question 9

What are the 4 fundamentally different classes of kerogens?

Answer 9

1. Type I - Algal
   - derived mostly from the remains of algae
   - when it matures, it yields mainly crude oil
   - capable of generating the most petroleum of all the kerogen types
2. Type II - Mixed
   - consists mostly of amorphous material, derived from the bacterial & mechanical breakdown of a mixture of marine, one celled plants & animals
   - this kerogen is oil-prone but yields more natural gas than Type I
   - Type I and Type II kerogens are referred to as Sapropelic Kerogens
3. Type III - Coaly
   - derived from the higher land plants, is sometimes known as coaly kerogen
   -The humic material in Type III kerogen has a low capacity to form oil and yields mostly natural gas
   - Type III kerogen has a low capacity to form oil and yields mostly natural gas
4. Type IV - Inert
   - consists mostly of inert particles that have been highly oxidized before burial, like charcoal.
   - It is the rarest kerogen type and has practically no ability to generate either oil or gas.