-The oil and gas must somehow migrate out of the oil shale and accumulate in a reservoir rock where it is more easily accessible. -To be a reservoir rock, a rock must have high porosity and permeability. Oil traps are impermeable and allow oil to accumulate in a restricted area. -There must be a seal to prevent the fluids from flowing out. The seal and reservoir rock bodies must have a proper geometry so hydrocarbons can collect in a restricted area. -A seal is a rock with low permeability -Folding during mountain building forms one type of trap: anticline trap- Salt-dome traps are formed when deeply buried salt flows like plastic. -Stratigraphic traps are created when depositional features create changes in permeability and porosity Tar sands: viscous, tar-like “heavy oil” or bitumen trapped in sand or sandstone -Deposits of residual petroleum in sand reservoirs that have lost lighter hydrocarbons by bacterial digestion. -The residual heavy oil, or bitumen, is all that is left of a former oil field. The remaining hydrocarbon is too viscous to be pumped, so tar sands must be mined and processed. -It takes 2 tons of tar sands to produce one barrel of oil; 1 barrel of oil and 6 barrels of water to produce 2 barrels of oil from tar sands Oil shale: organic-rich shale that have not been subjected to temperatures of the oil window, or did not stay within the oil window long enough to complete the transformation of oil, still contain a high proportion of kerogen.
-Oil shales must be heated to 500 degrees to convert the kerogen into liquid hydrocarbons. Natural gas: consists of volatile short-chain hydrocarbons, including methane, ethane, propane,and butane. Gas floats on top in an oil reservoir, but below the oil window, gas occurs without oil. -Gas burns more cleanly than oil and produces more energy per unit carbon but requires expensive high-pressure pipelines and ships to transport it. Gas hydrates: methane molecule surrounded by a cage-like arrangement of water molecules. Gas hydrates form when anaerobic bacteria eat organic matter, such as dead plankton, that hasbeen incorporated into marine sediment. When bacteria degrade the organic matter, they produce methane as a by-product. -Gas hydrates occur as layers interbedded with sediment and/or as a cement holding together the sediment. Coal: black, brittle, carbon-rich sedimentary rock made up of the altered remains of fossil vegetation with minor amounts of quartz and clay -Important global energy source and a dominant CO2emitter. The formation of coal (coalification):under anaerobic conditions bacterial degradation is slowand plant matter is converted to peat as a result of compaction and partial decay. -Where coal accumulated were ancient swamps, because oxygen is used u rapidly by the decomposition of organic matter and is not replenished rapidly.
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- Fall '09
- Plate Tectonics, Magnetic Field, rocks