Source rock
Overview
 
In petroleum geology
Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...

, 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 petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 including deep water marine, lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 and deltaic
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

. Oil shale
Oil shale
Oil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which liquid hydrocarbons called shale oil can be produced...

 can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled.
Source rocks are classified from the types of kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

 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
    Hypoxia (environmental)
    Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

     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 and bacterial 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
    Bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

     and fungi under oxic or sub-oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial.
Quotations

It may be necessary to use methods other than constitutional ones.

"ZAPU deposes Mr. Nkomo as Leader", The Times, 9 July 1962, p. 9.

Our votes must go together with our guns. After all, any vote we shall have, shall have been the product of the gun. The gun which produces the vote should remain its security officer – its guarantor. The people's votes and the people's guns are always inseparable twins.

Martin Meredith, "Our Votes, Our Guns: Robert Mugabe and the Tragedy of Zimbabwe".

We are still exchanging blows with the British government. They are using gay gangsters. Each time I pass through London, the gangster regime of Blair

expresses its dismay'.

What we hate is not the color of their skins but the evil that emanates from them.

Speech in Harlem, New York (September 2000), quoted in Michael Radu, "State of Disaster", National Review, 27 May 2002

Our party must continue to strike fear in the heart of the white man, our real enemy!

"Whites are real enemy, warns Mugabe", Irish Times, 15 December 2000, p. 11.

The white man is not indigenous to Africa. Africa is for Africans. Zimbabwe is for Zimbabweans.

ibid.

We have fought for our land, we have fought for our sovereignty, small as we are we have won our independence and we are prepared to shed our blood…. So, Blair keep your England, and let me keep my Zimbabwe.

Speech at the Earth Summit in Johannesburg (2 September 2002), quoted in John Battersby and Andrew Grice, "Anti-West anger at summit as Mugabe rounds on Blair", The Independent, 3 September 2002, p. 1.

Let Blair and the British government take note and listen. Zimbabwe is for Zimbabweans. Our people are overjoyed, the land is ours. We are now the rulers and owners of Zimbabwe.

Michael White, Andrew Meldrum, "Commonwealth leaders delay decision on defiant Mugabe", The Guardian, 6 December 2003, p. 2.

Encyclopedia
In petroleum geology
Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...

, 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 petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments
Sedimentary depositional environment
In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 including deep water marine, lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 and deltaic
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

. Oil shale
Oil shale
Oil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which liquid hydrocarbons called shale oil can be produced...

 can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled.

Types of source rock

Source rocks are classified from the types of kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

 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
    Hypoxia (environmental)
    Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

     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 and bacterial 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
    Bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

     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 3 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
Cracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...

 releases shorter chain hydrocarbons from the original large and complex molecules found 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
Compaction (geology)
Compaction refers to the process by which a sediment progressively loses its porosity due to the effects of loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually...

. Once released into porous and permeable carrier beds or into faults planes, oil and gas then move upwards towards the surface, 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 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 (Demaison,1984) that zones of high success ratios in finding oil and gas generally correlate in most basin types (such as intracratonic or rift basins) with the mapped "generative depressions". Cases of long distance oil migration into shallow traps away from the "generative depressions" are usually found in foreland basins.

Besides pointing to zones of high petroleum potential within a sedimentary basin, subsurface mapping of a source rock's degree of thermal maturity is also the basic tool to identify and broadly delineate shale gas
Shale gas
Shale gas is natural gas produced from shale. Shale gas has become an increasingly important source of natural gas in the United States over the past decade, and interest has spread to potential gas shales in the rest of the world...

 plays.

World class source rock

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 to lower Mississipian widespread marine anoxic oil and gas source beds in the Mid-Continent and Appalachia
    Appalachia
    Appalachia is a term used to describe a cultural region in the eastern United States that stretches from the Southern Tier of New York state to northern Alabama, Mississippi, and Georgia. While the Appalachian Mountains stretch from Belle Isle in Canada to Cheaha Mountain in the U.S...

     : (e.g. the Bakken shale
    Bakken Formation
    The Bakken formation, initially described by geologist J.W. Nordquist in 1953,is a rock unit from the Late Devonian to Early Mississippian age occupying about of the subsurface of the Williston Basin, underlying parts of Montana, North Dakota, and Saskatchewan...

     of the Williston Basin
    Williston Basin
    The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North and South Dakota, and southern Saskatchewan known for its rich deposits of petroleum and potash. The basin is a geologic structural basin but not a topographic depression; it is transected by the...

    , the Antrim Shale
    Antrim Shale
    The Antrim Shale is a formation of Upper Devonian age in the Michigan Basin, in the US state of Michigan, and extending into Ohio and Indiana. It is a major source of natural gas in the northern part of the basin....

     of the Michigan Basin
    Michigan Basin
    The Michigan Basin is a geologic basin centered on the Lower Peninsula of the US 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 toward the center of the peninsula.The basin is centered in...

    , the Marcellus Shale of the Appalachian Basin ).
  • Kimmeridge clay
    Kimmeridge Clay
    The Kimmeridge Clay Formation is a sedimentary deposit of fossiliferous marine clay which is of Jurassic age. It occurs in Europe.Kimmeridge Clay is arguably the most economically important unit of rocks in the whole of Europe, being the major source rock for oil fields in the North Sea hydrocarbon...

     - This upper Jurassic marine mudstone
    Mudstone
    Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

     or its stratigraphic equivalents generated most of the oil found in the North Sea
    North Sea
    In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

     and the Norwegian Sea
    Norwegian Sea
    The Norwegian Sea is a marginal sea in the North Atlantic Ocean, northwest of Norway. It is located between the North Sea and the Greenland Sea and adjoins the North Atlantic Ocean to the west and the Barents Sea to the northeast. In the southwest, it is separated from the Atlantic Ocean by a...

  • La Luna shale - This Turonian formation generated most of the oil in Venezuela
    Venezuela
    Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...

  • Late Carboniferous coals - 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 field in Saudi Arabia
    Saudi Arabia
    The Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World...


Further reading

  • Gerard Demaison: "The Generative Basin Concept" in: American Association of Petroleum Geologists (AAPG) Memoir #35 : "Petroleum Geochemistry and Basin Evaluation", 1984 , Edited by Gerard Demaison and Roelof J. Murris, ISBN:0-89181-312-8
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