Key learning of last two lectures:-
In a lagoon or silled condition we get more of sulphur while in an open water body (the water mass that is connected to the sea/ocean) we find less of sulphur in the sediments, this is because of presence of bacterias
In an open condition where there is agitation of air with water due to sea waves which dissolves the oxygen in water the bacteria Thiobacillus is found. This bacteria is responsible for converting sulphur in SO4. Thus decreases the content of sulphur in the sediments. The bacteria named desulfovibrio is responsible for the conversion of SO4 into S. In a Lagoon the oxygen content is depleted, so thiobacillus cannot survive as it is an aerobic bacteria and thus only desulfovibrio is present which converts the SO4 into S thus increasing the content of sulphur into the sediments.
Diagenesis:- Temp and pressure conditions for its initiation are:- 50 degree Celsius and pressure is 500-600 bar. It is the process which is responsible for converting organic matter into kerogens.
Pristane – phytane :-
The presence of pristane and phytane in a crude is a supportive point for the organic theory of origin.
The chlorophyll when undergoes a oxydizing environment it finally yields the pristane and if goes through an reducing one the yields phytane. Thus depending upon the ratio of pristane to phytane we can make out what kind of environment was present during that period. These two are known as geochemical fossils.
The content of particulate organic matter is much less than dissolves organic matter in the ocean.
The process of formation of kerogen:-
Protein and polysaccharides(highly organic bipolymers) à enzymatic microbiological degradationà Amino acids and sugar (mononers) à random polymerisation and condensation à Fumic acid, humic acid, humin ( geo polymers) à increasing condensation and insolubalisation à Kerogens
Lipids(highly structured) à preservation with minor alteration yields kerogens
Organic matter obtained from algae is always saprophylic , which is rich in hydrogen and has less of NSO( nitrogen, sulphur and oxygen)
Type of kerogens:-
Type- 1 , type-2 , type-3
TYPE- 1 :-
Has high H:C ratio 1.5 or more
Has low O:C ratio less than 0.1 , small amount of oxygen that too in ester 4.6%
Much lipid material , particularly aliphatic.
Produces large yield of volatile and extractable compounds , upto 80% by wt prolific oil shales.
Sources:- selective ammulation of algae and severe degradation of OM. Abundance:- low
TYPE-2 :-
High H:C ratio
Low O:C ratio
Abundance ofModerate length chains of aliphatics and naphthenic rings
Rather abundant amount of ester group oxygen = 10.3 %
Can provide commercial oil shales but less than that of type-1
Source: Marine sediments of phytoplanktons , zooplanktons, bacteria deposited in reducing environment
Abundance:- abundant
TYPE-3 :-
Low H:C ratio less then 0.1
High O:C ratio 0.2 to 0.3
No ester group, oxygen content is still in other forms = 27.5 %
Less efficient for oil but a good gas source
Source:- continental plants
Abundance:- high
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