Bituminous rocks

Organic-rich sedimentary rocks are a specific type of sedimentary rock that contains significant amounts (>3%) of organic carbon. The most common types include coal, lignite, oil shale, or black shale. The organic material may be disseminated throughout the rock giving it a uniform dark color, and/or may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material. Organic-rich sedimentary rocks may act as source rocks which generate hydrocarbons that accumulate in other sedimentary "reservoir" rocks (see oil sands and petroleum geology). Potential source rocks are any type of sedimentary rock that the ability to dispel available carbon from within it (limestone is a classic example of a source rock). Good reservoir rocks are any sedimentary rock that has high pore-space availability. This allows the hydrocarbons to accumulate within the rock and be stored for long periods of time (a sandstone commonly makes a good source rock). Highly permeable reservoir rocks are also of interest to industry professionals, as they allow for the easy extraction of the hydrocarbons within. The hydrocarbon reservoir system is not complete however without a cap rock. Cap rocks are rock units which have very low porosity and permeability, which trap the hydrocarbons within the units below as they try to migrate upwards.

Sandstone with fossil shells

Organic carbon is derived from ancient biological deposition of matter (kerogen is the name given to this by geologists), and this organic matter is buried with mineral and rock fragments into sedimentary rocks. The temperature and pressure of the burial conditions will affect the material's diagenetic processes, and determine whether or not the material will be transformed into petroleum. Fossiliferous organic carbon can also be reworked through tectonic processes and returned to the continental surface in the form of an outcrop. Erosion and chemical weathering can cause the organic carbon materials to be transported throughout the modern environment, in rivers, soils, and eventually the oceans. This process occurs over a very large time scale, and acts as one of the major mechanisms for fossiliferous organic carbon to be released back into the environment.

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