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Geology _ Geophysics in Oil Exploration

Sinceitislateinthegeologictimescaleonlyaminorpartofith

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Unformatted text preview: ied from the types of kerogen that they contain • source rocks are formed from algal remains deposited under anoxic conditions in deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep burial • Source rocks are formed from marine planktonic remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial. • Source rocks are formed from terrestrial plant material that has been decomposed by bacteria and fungi under oxic or sub‐oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial. Most coals and coaly shales are generally Type 3 source rocks. Maturation and expulsion When temperatures of the organic‐rich sedimentary rocks exceed 120o C (250o F) the organic remains within the rocks begin to be "cooked" and oil and natural gas are formed from the organic remains and expelled from the source rock. It takes millions of years for these source rocks to be buried deeply enough to attain these maturation temperatures and additional millions of years to cook (or generate) sufficient volumes of oil and natural gas to form commercial accumulations as the oil and gas are expelled from the source rock into adjacent reservoir rocks. If the organic materials within the source rock are mostly wood fragments, then the primary hydrocarbons generated upon maturation are natural gas. If the organic materials are mostly algae or the soft parts of land plants, then both oil and natural gas are formed. Gas can be generated in two ways in the natural systems; it can be generated directly from woody organic matter in the source rocks or it can be derived by thermal breakdown of previously generated oils at high temperatures. Oil window: oil maturation begins at 120`F (50`C) peaks at 190`F (90`C) & ends at 350`F (175`C). Above and below Oil Window, natural gas is generated. At higher temperatures above 500°F (260°C), the organic material is carbonized & destroyed as a source material. So, if source beds become too deepl...
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