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Source rock

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Title: Source rock  
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Subject: Petroleum geology, Orphan basin, Economy of Seychelles, Marcellus Formation, Enhanced coal bed methane recovery
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Source rock

In shale gas plays.

Contents

  • Types of source rocks 1
  • Maturation and expulsion 2
  • Mapping source rocks in sedimentary basins 3
  • World class source rock 4
  • See also 5
  • References 6
  • Source rock publications 7

Types of source rocks

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 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 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 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. 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 [1] 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 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:

See also

Basin modelling

References

  1. ^ 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
  2. ^ Kimmeridgian Shales Total Petroleum System of the North Sea Graben Province – USGS Bulletin
  3. ^ James,K.H. 2000. The Venezuelan hydrocarbon habitat, Part 2: hydrocarbon occurrences and generated-accumulated volumes. Journal of Petroleum Geology, 23, 133–164
  4. ^ Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary – USGS Bulletin
  5. ^ 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

Source rock publications

  • Source rock publications
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