In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working hydrocarbon system. They are organic rich sediments that may have been deposited in a variety of environments including deep water marine, lacustrine and deltaic. Oil shale can be regarded as an immature source rock from which little or no oil has been expelled.
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Types of source rock
Source rocks are classified from the types of kerogen that they contain, which in turn governs the type of hydrocarbons that will be generated.
- Type 1 source rocks are formed from algal remains deposited under anoxic conditions in deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep burial
- Type 2 source rocks are formed from marine planktonic remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial.
- Type 3 source rocks are formed from terrestrial plant material that has been decomposed by bacteria and fungi under oxic or sub-oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial. Most coals and coaly shales are generally Type III source rocks.
Maturation and expulsion
With increasing burial by later sediments and increase in temperature, the kerogen within the rock begins to break down. This thermal degradation or cracking releases shorter chain hydrocarbons from the original large and complex molecules found in the kerogen.
The hydrocarbons generated from the source rock are expelled, along with other pore fluids, due to the continuing effects of compaction and start moving upwards towards the surface, a process known as migration.
World class source rocks
Certain source rocks are referred to as 'world class', meaning that they are not only of very high quality but of wide geographical distribution. Examples include
- Kimmeridge clay - This upper Jurassic marine mudstone or its stratigraphic equivalents generated most of the oil found in the North Sea and the Norwegian Sea [1]
- La Luna shale - This Turonian formation generated most of the oil in Venezuela [2]
- Late Carboniferous coal - Coals of this age generated most of the gas in the southern North Sea, the Netherlands Basin and the northwest German Basin[3]
- Hanifa & Tuwaiq Mountain formations - These middle and upper Jurassic shales source the oil in the huge Ghawar field in Saudi Arabia[4]
References
- ^ Kimmeridgian Shales Total Petroleum System of the North Sea Graben Province - USGS Bulletin
- ^ James,K.H. 2000. The Venezuelan hydrocarbon habitat, Part 2: hydrocarbon occurrences and generated-accumulated volumes. Journal of Petroleum Geology, 23, 133-164
- ^ Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary - USGS Bulletin
- ^ Total Petroleum Systems of the Paleozoic and Jurassic, Greater Ghawar Uplift and Adjoining Provinces of Central Saudi Arabia and Northern Arabian-Persian Gulf - USGS Bulletin
See also
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