Storage rock
Storage rocks are porous or fissured rocks , mostly sedimentary rocks , in which natural gas and oil have accumulated to so-called conventional deposits over the course of the earth's history . The most common storage rocks are sandstones and certain types of limestone , which are now stored at a depth of around 0.5 to 5 km.
KW migration and storage rocks
The hydrocarbons (HC) of many oil and gas fields have not been formed in the rocks from which they are promoted today, they are the place of their education - the bedrock by - Migration immigrated to the current deposit. The pressure increase due to the weight of the overlying rock layers as a result of the filling of the corresponding sediment basin causes an increasing compaction of the sediments, which leads to the expulsion of the hydrocarbons from the bedrock ( primary migration ). They rise in the pore space of permeable rocks - following the pressure gradient and under the influence of groundwater currents - in the direction of the earth's surface ( secondary migration ) and collect under impermeable layers in suitable geological structures in the subsurface (so-called oil and natural gas traps ), for example in the core of Anticline structures . The permeable rock in which the KW accumulate is called reservoir rock, the KW-saturated reservoir rock in the trap structure is called conventional storage , provided that the amount of KW allows profitable extraction.
properties
The suitability of a rock for the migration and enrichment of hydrocarbons depends primarily on its void volume ( porosity ) and permeability ( permeability ). The higher the value of these two quantities, the better the suitability. Therefore z. For example, relatively coarse-grained, well-sorted (i.e. characterized by a relatively uniform grain size), weakly cemented sandstones are excellent storage rocks, because the larger the grains, the better the sorting and the lower the degree of cementation, the larger the interstitial space and thus the pore space . With these rock properties, it is also guaranteed that all intergranular spaces are connected to one another (pore interconnectivity) - an important prerequisite for high permeability. If the pore space is large, but the pore interconnectivity is low, the permeability is also low. Bioclastic limestones and especially reef limestone often have high porosity and permeability , which also offer the advantage that fossil reef structures in the subsurface form sedimentary KW traps (cores of "pseudo-anticlines"). Among other things, there are numerous conventional oil deposits in the Permian Basin (Texas) and the Gulf region in reef limestone.
However, its suitability as storage rock does not necessarily have to depend on sedimentary features. Extensive fissures as a result of tectonic processes can also turn an originally relatively dense, slightly porous and impermeable rock into a suitable storage rock. Thus it is possible that oil and natural gas can also accumulate in igneous or metamorphic rocks .
Unconventional storage facilities
Since around the year 2000, accompanied by technical and methodological innovations in hydraulic fracturing (fracking) and rising oil and natural gas prices, the energy companies increasingly focus on KW-bearing, weakly fissured, relatively dense, impermeable rocks. These unconventional deposits include: a. around classic KW parent rocks (see, inter alia, shale gas ) or former migration and storage rocks that have subsequently lost their permeability through diagenetic processes. In this context, the English expression “reservoir rock”, which traditionally only refers to the storage rocks of conventional deposits, is increasingly being used for the classic bedrocks.
literature
- Dieter Richter: General Geology . 4th edition. Walter de Gruyter, 1992, ISBN 3-11-012242-1 ( p. 134 in the Google book search).
- Bernhard P. Tissot, Dietrich H. Welte: Petroleum Formation and Occurrence . Springer-Verlag, Berlin Heidelberg 1978, ISBN 978-3-642-96448-0 , p. 315-323 .
- Oil and gas in Austria . In: Friedrich Brix, Ortwin Schultz (ed.): Publications from the Natural History Museum in Vienna . 2nd Edition. Natural History Museum, 1993, ISBN 3-85028-236-8 .