Inpetroleum geology,source rock is a sedimentary rock which has generatedhydrocarbons or which has the potential to generate hydrocarbons.[1] Source rocks are one of the necessary elements of a workingpetroleum system. They are organic-rich sediments that may have been deposited in a variety ofenvironments including deep watermarine,lacustrine anddeltaic.Oil shale can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled.[2] Subsurface source rock mapping methodologies make it possible to identify likely zones of petroleum occurrence in sedimentary basins as well asshale gas plays.[citation needed]
Source rocks are classified from the types ofkerogen that they contain, which in turn governs the type of hydrocarbons that will be generated:[1]
With increasing depth of burial by sediments or dead organic matters and increase in temperature, the kerogen within the rock begins to break down. Thisthermal degradation orcracking releases shorter chain hydrocarbons from the original large and complexmolecules occurring in the kerogen.[1]
The hydrocarbons generated from thermally mature source rock are first expelled, along with other pore fluids, due to the effects of internal source rock over-pressuring caused by hydrocarbon generation as well as bycompaction. Once released into porous and permeablecarrier beds or intofaults planes, oil and gas then move upwards towards the surface in an overall buoyancy-driven process known assecondary migration.
Areas underlain by thermally mature generative source rocks in asedimentary basin are calledgenerative basins or depressions or elsehydrocarbon kitchens. Mapping those regional oil and gas generative "hydrocarbon kitchens" is feasible by integrating the existing source rock data into seismic depth maps that structurally follow the source horizon(s). It has been statistically observed at a world scale[3] that zones of high success ratios in finding oil and gas generally correlate in most basin types (such as intracratonic orrift basins) with the mapped "generative depressions". Cases of long distance oil migration into shallow traps away from the "generative depressions" are usually found inforeland basins.
Besides pointing to zones of high petroleum potential within a sedimentary basin, subsurface mapping of a source rock's degree ofthermal maturity is also the basic tool to identify and broadly delineateshale gas plays.
Certain source rocks are referred to as "world class", meaning that they are not only of very high quality but are also thick and of wide geographical distribution. Examples include:
