petroleum geology Petroleum geology is the study of the origins, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons ( oil exploration). ...

, source rock is a sedimentary rock which has generated

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...

or which has the potential to generate hydrocarbons. Source rocks are one of the necessary elements of a working

petroleum system A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...

. They are organic-rich sediments that may have been deposited in a variety of

environments Environment most often refers to: __NOTOC__ * Natural environment, referring respectively to all living and non-living things occurring naturally and the physical and biological factors along with their chemical interactions that affect an organism ...

including deep water marine,

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

and deltaic.

Oil shale Oil shale is an organic-rich Granularity, fine-grained sedimentary rock containing kerogen (a solid mixture of Organic compound, organic chemical compounds) from which liquid hydrocarbons can be produced. In addition to kerogen, general compos ...

can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled. Subsurface source rock mapping methodologies make it possible to identify likely zones of petroleum occurrence in sedimentary basins as well as

shale gas Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s, a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and ...

plays.

Types of source rocks

Source rocks are classified from the types of

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kero ...

that they contain, which in turn governs the type of hydrocarbons that will be generated: * Type I source rocks are formed from algal remains deposited under

anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...

conditions in deep lakes: they tend to generate waxy crude oils when submitted to

thermal stress In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, m ...

during deep burial. * Type II source rocks are formed from marine

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic and bacterial remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial. * Type III source rocks are formed from terrestrial plant material that has been decomposed by

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

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 depth of burial by sediments or dead organic matters and increase in temperature, the kerogen within the rock begins to break down. This

thermal degradation Thermal decomposition, or thermolysis, is a chemical decomposition of a substance caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic ...

cracking Cracking may refer to: * Cracking, the formation of a fracture or partial fracture in a solid material studied as fracture mechanics ** Performing a sternotomy * Fluid catalytic cracking, a catalytic process widely used in oil refineries for crac ...

releases shorter chain hydrocarbons from the original large and complex

molecules A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry ...

occurring in the kerogen. 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 by compaction. Once released into porous and permeable carrier beds or into faults planes, oil and gas then move upwards towards the surface in an overall buoyancy-driven process known as ''secondary migration''.

Mapping source rocks in sedimentary basins

Areas underlain by thermally mature generative source rocks in a

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock They form when long-term subsidence ...

are called ''generative basins or depressions'' or else ''hydrocarbon 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 that zones of high success ratios in finding oil and gas generally correlate in most basin types (such as intracratonic or

rift basins In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben w ...

) with the mapped "generative depressions". Cases of long distance oil migration into shallow traps away from the "generative depressions" are usually found in

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

s. Besides pointing to zones of high petroleum potential within a sedimentary basin, subsurface mapping of a source rock's degree of

thermal maturity In petroleum geology, the maturity of a rock is a measure of its state in terms of hydrocarbon generation. Maturity is established using a combination of geochemical and basin modelling techniques. Rocks with high total organic carbon, (termed sou ...

is also the basic tool to identify and broadly delineate

plays.

World class source rocks

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: *

Middle Devonian In the geological timescale, the Middle Devonian epoch (from 393.3 ± 1.2 million years ago to 382.7 ± 1.6 million years ago) occurred during the Devonian period, after the end of the Emsian age. The Middle Devonian epoch is subdivided into two ...

to lower Mississippian widespread marine

oil and gas source beds in the Mid-Continent and

Appalachia Appalachia ( ) is a geographic region located in the Appalachian Mountains#Regions, central and southern sections of the Appalachian Mountains in the east of North America. In the north, its boundaries stretch from the western Catskill Mountai ...

areas of North America: (e.g. the Bakken Formation of the

Williston Basin The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North Dakota, South Dakota, southern Saskatchewan, and south-western Manitoba that is known for its rich deposits of petroleum and potash. The basin is a ...

, the Antrim Shale of the

Michigan Basin The Michigan Basin is a geologic basin centered on the Lower Peninsula of the U.S. state of Michigan. The feature is represented by a nearly circular pattern of geologic sedimentary strata in the area with a nearly uniform structural dip towar ...

, the

Marcellus Shale Marcellus may refer to: People * Marcellus (name) * Marcellus of Ancyra, fourth-century Christian bishop and theologian * Marcus Claudius Marcellus, Roman commander * Marcellus of Tangier * Marcellus Empiricus * Marcellus (nephew of Augustu ...

of the

Appalachian Basin The geology of the Appalachians dates back more than 1.2 billion years to the Mesoproterozoic era when two continental cratons collided to form the supercontinent Rodinia, 500 million years prior to the development of the range during the form ...

). *

Kimmeridge Clay The Kimmeridge Clay is a sedimentary rock, sedimentary deposit of fossiliferous marine clay which is of Late Jurassic to lowermost Cretaceous age and occurs in southern and eastern England and in the North Sea. This rock formation (geology), form ...

– This

upper Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

marine

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

or its stratigraphic equivalents generated most of the oil found in the

North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...

and the

Norwegian Sea The Norwegian Sea (; ; ) is a marginal sea, grouped with either the Atlantic Ocean or the Arctic Ocean, northwest of Norway between the North Sea and the Greenland Sea, adjoining the Barents Sea to the northeast. In the southwest, it is separate ...

. *

La Luna Formation LA most frequently refers to Los Angeles, the second most populous city in the United States of America. La, LA, or L.A. may also refer to: Arts and entertainment Music * La (musical note), or A, the sixth note *"L.A.", a song by Elliott Smi ...

– This

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

(mostly

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

) formation generated most of the oil in northwestern

Venezuela Venezuela, officially the Bolivarian Republic of Venezuela, is a country on the northern coast of South America, consisting of a continental landmass and many Federal Dependencies of Venezuela, islands and islets in the Caribbean Sea. It com ...

. *

Late Carboniferous Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * Late (The 77s album), ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudo ...

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

s – Coals of this age generated most of the gas in the southern North Sea, the Netherlands Basin and the northwest German Basin. * Hanifa Formation – This upper Jurassic laminated carbonate-rich unit has sourced the oil in the giant

Ghawar Ghawar (Arabic: الغوار) is an oil field located in Al-Ahsa Governorate, Eastern Province, Saudi Arabia. Measuring (some ), it is by far the largest conventional oil field in the world, and accounts for roughly a third of the cumulative ...

field in

Saudi Arabia Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in West Asia. Located in the centre of the Middle East, it covers the bulk of the Arabian Peninsula and has a land area of about , making it the List of Asian countries ...

.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
/ref>

References

{{reflist

External links

Australian Source Rock and Fluid Atlas

Source rock publications
Petroleum geology