Lecture23

Lecture 23

Source rock
1. rocks with high TOC (total organic carbon), algae/plankton rich, spores, pollen, land plant remains
2. formed in low energy, stagnant (no circulation), anoxic (low oxygen) environment
3. often fine-grained
4. rate of decomposition - preserve organic materials
  1. too slow- bacteria /oxidation on surface
  2. too rapid- diluted by sediments
5. production of organic matter - accumulation/concentration in sediments - preserve from bacteria/oxidation
6. Decomposition environment
  1. lake, basin, swamps, marshes, river, delta, anoxic zones
Generation
1. source kitchen
  1. area where source rocks have matured
2. burial and chemical alternation of organic material - organic matters broken into kerogen which then broken into petroleum
3. Kerogen
  1. Lipid rich kerogen & sapropelic kerogen
    1. marine organisms
      1. oil
  2. Humic kerogen
    1. land organisms
      1. gas
4. Maturation process
  1. oil generation requires temp: 50-90 to be mature, cease at 140-170degree (oil windown)
  2. controlled by temp & time
  3. gas generation increases above 150degree, cease at 200-230degree (gas window)
  4. degree determine by kerogen colour: darker-high temp-more mature
  5. source potential: not reach oil window (immature), hotter than gas window (overmature), max. temp reached
5. Temperature
  1. lower limit
    1. Present day temperature in wells
  2. upper limit- max.temp reached
    1. Vitrinite reflectance
    2. spore/pollen colouration index
    3. kerogen colouration index
  3. rock can experience different temp from present temp in the past due to temp gradient decrease with time/ uplifted
  4. below 80: biodegradation: bacteria destroy oil
6. Burial history (geohistory of basin)
  1. model, evaluate if/when source mature before
Migration
1. Primary
  1. from rock to carrier bed (solution in water, gas, droplets, globules, microfacturing)
2. Secondary
  1. from carrier bed (permeable layers) - trap (buoyancy)
  2. affect by poresize, rock type
3. Tertiary
  1. through seal (tilting, faulting, capillary failure)
4. buoyancy force vs capillary pressure
5. water washing
  1. removal of gas+light HC by solution in moving water
6. Fault/seal breach
7. Movement of petroleum from source -trap
Preservation
1. Decrease with time as processes (heating, lift, fold, erosion)
  1. Age distribution of oil fields often young - require: little/no tectonic activity

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