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Why is petroleum usually associated with marine sedimentary rocks?

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Multiple large offshore petroleum oil rigs with cranes and equipment stand in the ocean, connected by walkways above the water. — 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.

Petroleum is primarily associated with marine sedimentary rocks because it originates from the remains of microscopic aquatic organisms, such as planktonic plants and animals, that accumulated in subaqueous environments.12

The process begins with the rapid burial of these organic remains within fine-grained sediments, such as clays or shales, in settings where a lack of free oxygen prevents decay. This preservation is essential for the organic matter to undergo diagenesis, a series of biological, chemical, and physical changes that convert the material into [kerogen].12

As these sediments are buried deeper over geologic time, increasing temperatures and pressures within the oil window trigger the thermal degradation of kerogen, transforming it into the various liquid and gaseous hydrocarbons that constitute petroleum. Because these source rocks are typically fine-grained and compact, the generated hydrocarbons must migrate into more permeable reservoir rocks to form the commercial accumulations we discover today.345