Is oil shale considered a biochemical sedimentary rock?

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Discussion Overview

The discussion revolves around the classification of oil shale as a biochemical sedimentary rock. Participants explore the composition of oil shale, its relationship to other sedimentary rocks like limestone, and the processes involved in the formation of organic-rich sedimentary rocks.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that oil shale is a mixture of kerogen and mud sediments, questioning whether it can be classified as a biochemical sedimentary rock since the organic material does not form the rock itself.
  • Another participant clarifies that in limestone, the rock is primarily formed from hard calcium shells and not from soft organic matter, which has typically decomposed.
  • A later reply points out that oil shale is classified as an organic-rich sedimentary rock according to Wikipedia.
  • One participant elaborates that oil shale is a sedimentary rock containing a mixture of organic matter and mineral matrix that is not thermally mature, discussing the conditions necessary for the preservation of significant organic matter in sediments.
  • The same participant notes that the classification of organic-rich sedimentary rocks varies based on the depositional environment and the types of organic matter present.

Areas of Agreement / Disagreement

Participants express differing views on the classification of oil shale and its comparison to limestone. There is no consensus on whether oil shale should be classified as a biochemical sedimentary rock, and the discussion remains unresolved.

Contextual Notes

The discussion highlights the complexity of sedimentary rock classification and the varying definitions of organic-rich sediments. There are unresolved assumptions regarding the definitions and characteristics of the materials involved.

Asmaa Mohammad
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Could we classify oil shale as a biochemical sedimentary rock ? Oil shale is a muddy rock contains Kerogen. So I think in oil shale to be a mixture of Kerogen and mud sediments, and that's not the case, for instance, with the biochemical limestone where the solid remains of the living organisms formed the rock. So in limestone the organic material formed the rock, but in oil shale the organic material didn't, it is just, in my thoughts, preserved in the rock, or I am wrong ? enlighten me, please !
 
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Asmaa Mohammad said:
So in limestone the organic material formed the rock,

generally no it didn't ... it's just the hard calcium shells ( seashells, corals etc) or bone that formed the rock
along with all the deposition of other sedimentary material
The soft organic matter has long gone. Usually rotted away whilst still surrounded by the water sea/or fresh
 
davenn said:
generally no it didn't ... it's just the hard calcium shells ( seashells, corals etc) or bone that formed the rock
along with all the deposition of other sedimentary material
The soft organic matter has long gone. Usually rotted away whilst still surrounded by the water sea/or fresh
Oh, ok, and what about the oil shale? How would we classify it?
 
Asmaa Mohammad said:
and what about the oil shale? How would we classify it?

Wiki says it's classified as an organic-rich sedimentary rock
 
Oil shale in the strictest sense is a sedimentary rock with a mixture of organic matter and mineral matrix that is not yet thermally mature.

Organic-rich sedimentary rock covers a lot of ground. Most sediments don't have significant organic matter because it was not present in the first place, was oxidized, or was consumed by burrowing infauna. Those sediments that preserve significant organic matter (more than a % or so) range from oil shale or carbonaceous shale (from aquatic single celled or terrestrial plants, respectively, with dominant mineral matrix) to kukersites / torbanites or coal (lacking mineral matrix). Organic matter deposition requires an environment with a combination of high organic productivity and low inorganic deposition, like swamps or anoxic (oxygen deficient) lakes and seafloor far from sediment sources like rivers or turbidites. The mineral contribution can be organic calcite (such as coccoliths) and silica (diatoms), or inorganic chemical (calciie, chert, evaporites) and detrital (silt and clay). All typically very fine grained, but composition varying based again on the depositional environment. Once deposited, the organic-rich sedimentary rock undergoes further changes through diagenesis - compaction and dewatering; changes in mineral composition and texture; thermal conversion of the organic matter to solid (coally), liquid (oil) and gaseous (natural gas) phases which may migrate away.
 
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