Final answer:
Source rocks refer to sedimentary rocks where oil and gas are generated from organic material. These rocks accumulate sediment, including mineral grains and fragments of other rocks, which cement together or precipitate from solutions forming reservoirs for hydrocarbons.
Step-by-step explanation:
Source rocks are always sedimentary. The term source rock in a geological context often refers to the type of rock where oil and natural gas are generated from organic material. More specifically, in the rock cycle, source rocks are primarily understood as sedimentary rocks because they are derived from the accumulation of sediment, which can include mineral grains, organic matter, and fragments of other rocks. The sediment is deposited in various environments, such as lakes, rivers, and oceans.
Sedimentary rocks are categorized based on their mode of formation. They can form through the cementation of rock fragments derived from weathering processes (clastic sedimentary rocks like sandstone and shale) or through the precipitation of minerals from solutions (chemical sedimentary rocks like limestone and salt).
These processes result in the trapping of organic material, which under certain conditions of temperature and pressure, can generate hydrocarbons like oil and natural gas, hence their designation as source rocks.
While igneous rocks, which form from cooled magma or lava, and metamorphic rocks, which form when existing rocks are altered by heat and pressure, also make up the Earth's crust, they are not termed source rocks in this context. Sedimentary rocks are unique in this respect because they are the ones that typically contain sufficient organic material to generate hydrocarbons and sustain the exploration of fossil fuels.