oil and gas

• Oil and gas are hydrocarbons. 
• Oil and gas are formed from plankton in the ocean. When the plankton die, it falls down to the ocean floor to make an ooze. 
• Where there is no oxygen there are few scavengers so the ooze accumulates. Anaerobic bacteria decompose some of the ooze. Over time black organic rich shale/mudstone form due to accumulation and lithification.  
• As black shale is buried it is heated, first turned into kerogen - a solid form of hydrocarbon. 
• At 75 dc most kerogen is liquified into oil. 
• At 100 dc kerogen is mostly changed into gas. 
• Above 200 dc hydrocarbons are destroyed or denatured where there is no hydrogen left. 
• The oil window is between 50-120 dc and the gas window is between 100-120 dc. 
• When oil and gas is denatured it breaks down into smaller hydrocarbon chain - finally becoming graphite after oil and gas. 
• 150 million years ago conditions were suited to build up lots of black shale. Oil formed in warm shallow seas due to the lack of oxygen. 
• Kimmeridge clay is a black shale with 50% organic matter and is the main source of rock for the north sea oil field. 
• Some oil basins are formed from hydrogen rich coals and some gas comes from carboniferous coals. 
• Factors determining migration:

  • Permeability 
  • Pressure - high to low 
  • Density differences - oil and gas is less dense than water in pore spaces 
  • Viscosity of oil - high temperatures mean low viscosity 
• Good reservoirs:

  • Highly porous 
  • Highly permeable 
  • Rounded grains 
  • Poor cementations 
  • Desert sandstone 
• Cap rocks are impermeable rocks above reservoirs that prevent further movement of petroleum to the surface - mudstones, evaporites. 
• Traps are geological situations that concentrate hydrocarbons in one place. 
• Structural traps:

  • Anticline 
  • Fault 
  • Salt domes
• Lithological traps:

  • Unconformity 
  • Stratigraphic traps 
• Anticline traps are the most common traps, open folds contain more than a tight fold. Gas, oil and water lie horizontally on boundaries. 
• Stratigraphic traps are often old limestone reefs, where the limestone is surrounded by impermeable rocks and covered with another fine rock. 
• Oil and gas can be denatured due to burial, volcanic activity and metamorphism where temperature increases. 
• Oil and gas can be lost due to erosion and removal of cap rock, faulting and escaping upwards along unsealed fault planes.