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Why do deeper petroleum deposits generally have higher pressure and more gaseous hydrocarbons?
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petroleum
Liquid petroleum (crude oil)—which is refined into gasoline, diesel, heating fuels, asphalt, plastics, and other useful chemical compounds—is often extracted from the ground using a series of drills and pumps.
Deeper petroleum deposits generally contain more gaseous hydrocarbons and exist under higher pressure due to the effects of burial, temperature, and chemical maturation.
As source rocks are buried deeper by continuing sedimentation, they are subjected to increasing temperatures and compaction from the weight of overlying layers. This pressure helps initiate the migration of fluids from source beds.12
Regarding the composition of hydrocarbons, the process is driven by thermal degradation:
- The Oil Window: At depths of approximately 760 to 4,880 metres, temperatures between 65 °C and 150 °C facilitate the transformation of kerogen into a full range of petroleum compounds.1
- Thermal Cracking: Below about 4,800 to 5,000 metres, temperatures become high enough that crude oil is no longer stable. In this postmature stage, existing liquid hydrocarbons undergo thermal cracking, where larger, heavier molecules are broken down into lighter ones, primarily resulting in methane gas.31
Consequently, while shallower deposits may contain a mix of oil and gas, deeper environments are increasingly dominated by dry thermal gas due to the destruction of liquid hydrocarbons.31