Petroleum geology Fate of organic matter

For the conversion of organic matter to petroleum you need atleast 2 things :- evolution of organic matter i.e abundance of organic matter and  preservation( need a platform area for deposition)

Platform:- water depth ≤ 200 m . this is generally the region where fine grain particles are deposited and they can preserver the organic matter at its best.

Preservation in coarse grains is not possible as water can move through its pore spaces and so does the oxygen which oxidizes the organic matter thus ceasing the chances of preservation.

vuggy porosity [ note: if there is a mistake here, plz post comment to get it corrected] :- vuggy porosity arises due to dissolution of carbonates like limestone.

Petroleum geology shales ,kerogens, bitumin overview

Oil shale:- shales containing oil are called oil shales. Oil was extracted from these shales by pyrolysis. pyrolysis is basically heating in absence of oxygen at 550° C .

Bitumin:- It is soluble in organic solvents , it is very well structured. Its of 3 types:-

a) labine -> very easily liberates hydrocarbon

b) inert -> high temp required for getting hydrocarbons

c) refractant -> can be used as an indicator of maturity in hydrocarbon source rocks, is a key method for identifying the temperature history of sediments in sedimentary basins.
(wiki pedia)

kerogens:- not structured and are insoluble in organic solvents, these the ones that have a good petroleum formation potential.

kerogen type-1   H:C ratio > 1.5

Vitrinite reflectance :- can be used as an indicator of maturity in hydrocarbon source rocks,  is a key method for identifying the temperature history of sediments in sedimentary basins.
(wiki pedia)

Catagenetic regime -> it comes after diagenisis stage and before metagenisis.  So VRo –> 0.5 – 2 %

Oil window -> VRo –> 0.7 – 1.3 %

Wish to know more,go here:-  petrocafe.us.to

Petroleum geology - Kerogens

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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Petroleum geology Duplex theory of origin

Duplex theory of origin of petroleum
This theory says that initially the hydrocarbon may be formed abiologically due Fischer - tropsch reaction. Then it must be acted upon by the living organisms like bacteria thus giving rise to the organic hydrocarbon.
The Fischer tropsch reaction is in general the reaction of carbon monoxide with hydrogen giving rise to the hydrocarbon and water.
It can be given as follow:- C+ Fe+H+OH------>CH4+C2H6+C3H8+C4H10+FeO
CO2+HOH+Fe---->CH4+FeO
C+HOH+Fe-->CH4+CO2+H2
Temp needed is 500°C
Robinson in 1963 and Szatmari in 1986 explained the same theory in different forms. Szatmari used plate tectonics to explain it.

Petroleum geology organic origin theory

Supports for organic origin theory:-
Vast amount of organic matter is found in sedimentary rocks. Hydrogen and carbon are predominant in remains of plants and animals.
Most of the crudes have porphyrin in them that is the most essential element of life form.

If  M in porphyrin is replaced by Fe then we get hemoglobin.
If M is replaced by Mg   then we get Chlorophyll
If M is replaced with Ni or V we get a cyclic porphyrin .
Porphyrin is not stable above 200 ° C . So inorganic theory is proved faulty, as the mantel is about 33 km below the surface and average temp gradient is about 25° C/km . So the temp till we reach mantel would be about 800 ° C where porphyrin cant exist, so it shouldnt be present in the crude if it was formed inorganically.
Most of the crudes contain Nitrogen which is the basics element of amino acids that form the basis of life form.
All crudes possess the property of rotating the plane of polarised light. such a property is possessed by cholesterol which is essential component of animals and plants. The inorganic substances that possess this property are cinnaber(HgS) and quartz(SiO2)

Petroleum geology- theories of origin

Petroleum geology

Theories of origin of petroleum:-

organic origin theory is widely accepted, as 97% of petroleum reservoirs are found in sedimentary rocks. Rest 3% are found in metamorphic as well as igneous rocks, that too with some conditions.

Inorganic theories:-

1) cosmic theory:- Bertheiot(1865) ,mendaleef(1877) suggested the formation of petroleum in earths interior by action of water on metallic carbides such as those of Ca and Fe.Another proposed mechanism involved in the interaction of alkali metals with co2 and water.

Metallic carbide + H2O vapour = CH4 + metallic hydroxide

Alkali metals + Co2 + H2O vapour = CH4 + alkali hydroxide

2) mantle theory/Gold’s earthquake out gassing theory:-

Enriched Delta(13C) of methane from hotspots of red sea, lake kivu(east africa) and pacific rise depicts an abiogenice origin(Mac donald).

Delta(13C) content in crude oil is -30%o(i.e per millenium)

some other key learning:-

oldest rock is found in greenland which is 4.2 billion years old.

2nd oldest rock is found in chota nagpur(singhbhum)

Origin of most petroleum in the world dates back to mesozoic to paleozoic

The origin of petroleum found in india lies in the cenozoic age.

NELP:- new exploration licensing policy [ it is now about 15 years old  ]

Please post your comments for corrections and suggestions

petroleum geology

[ there might be some errors in the text below]

porosity is of 2 types:-
bulk: The porosity calculated considering the total pore spaces.
effective: the porosity calculated considering the pores that can hold hydrocarbon.

The component that will occupy the pore depends upon the pore size

Sedimentary texture:-structure formed due to sedimentation. It is of two types:-
1)that forms on surface:- Depends on flow and deposional enviornment(also known as bed forms)[example:ripple marks]

2)that forms in beds:-current bedding. enourmous importance in studying the depositional history.

Sedimentary structures are used to know the paleocurrent flow.

most indian oils are generated from type-3 kerogens. type-3 kerogens are derived from terrestial organic matter.

Transgression—> flooding –effect–>ground water table also rises —>controls eroison–>sedimentation is less

Regression:- base level of eroison decreases, area available for deposition decreases.

During the last glacial period the sea level decreases about 138 metres. lot of shelf was exposed, the base level of erosion went down
so no deposition in vast area Because slope of shelf is very less. Such an kind of unconformity formed is known
as 1st order unconformity.

Primary migration:- low permiability rock, Fine grain rock.

secondary migration:- high permeability rock, coarse grain rock.

For any suggestions and questions please post comments.

Petroleum geology

Some key learning points:-

bubble point can be defined on basis of  Temp and also pressure(which is the conventional way) individually . Also it can be defined on basis of both Temp and Pressure simultaneously.

what is a petroleum system??

ans: the system consisting of a source rock, a migration mechanism( primary or secondary) , a reservoir rock and a cap rock is known as a petroleum system. There are in all 70 petroleum systems in india, out of which around 35 are identified to have a potential.

Around 3.14 million sq.Km is covered with sedimentary rocks in india.

In a petroleum system there are potential areas where petroleum is found which are called oil pools, ONGC calls oil pools as assets.