Conversion of Organic Matter into Petroleum

v Conversion of Organic Matter into Petroleum:

(Production, Accumulation and Preservation of Organic Matter)

The production, accumulation and preservation of organic matter are the pre-requisites for existence of petroleum source rocks.

The term organic matter includes the organic molecules in monomeric or polymeric form that derived directly or indirectly from organic part of the organisms (excluding the mineral part).

The production, accumulation and preservation of organic matter varies in the earth history with conditions.

Ø Production of Organic Matter:

Photosynthesis leads to basis for the mass production of organic matter on earth.

Photosynthesis is “the transfer of hydrogen from water to carbon dioxide to produce organic matter in form of glucose and oxygen”

Photosynthesis is accomplished by availability of H2O+CO2+Sunlight and Chlorophyll.

Oxygen is freed from water and from glucose the synthesis of polysaccharides, such as cellulose and starch, took place with help of plants and autotrophic organisms.

 v How Mass Production Starts? Evolution of Biosphere:

Calvin (1969) proposed chemical or abiological evolution that starts about 3.3 billion years ago, when first primitive algae and bacteria emerged and starts producing organic molecule.

About 3 billion years ago, first heterotrophic organism evolved which utilized the organic molecule produced by algae and bacteria, which consumed organic molecule as produce by algae and bacteria as a source of energy.

Then, photosynthetic bacteria evolved, photosynthesis were used as secondary source of energy, organism used H2S in stead of H2O and evolved S2 instead of O2.

Then about 2 billion years ago certain blue-green algae evolved from photosynthetic bacteria and were the first to produce oxygen, well established photosynthesis prevailed and it is called zero reference point.

The autotrophic organism were superior than heterotrophic and soon dominated the biological realm and photosynthesis emerged as world wide phenomena.

A foundation were laid for food pyramid and the evolution of higher form of life were laid.

The evolution of biosphere is connected with the formation of

petroleum, because it is believed that some extinct benthonic,

zooplanktons and algae serves as source of organic material.

Evolution of biosphere is connected with three categories:

·         Phytoplankton

·         Bacteria

·         Higher Plants

v Phytoplankton:

Production starts in Precambrian before 2 billion year ago, it were the main producers increases through early Palaeozoic and decrease sharply at late Devonian.

They are considered the largest contributors to organic matter.

Divided into two periods on account of production

First Period: Pre-Cambrian to Early Palaeocene, high production governed by characteristically organic walled plankton such as blue-green algae, green algae and acritarchs.

Second Period: Late Jurassic to Cretaceous, production was dominated by nano-planktons which were calcareous e.g., cocolithophorids, dianoflagellates and diatoms: These appear in Late Cretaceous increase in Cenozoic. 

v Bacteria:

They are also considered the largest contributors to the organic matter along with phytoplankton, even today they are the main producers at 50-60% .

Fossils record is not sufficient to understand bacteria in geologic record, may be due to small size and lack hard parts but considered to be the same as today’s bacteria.

However, the fossilized bacteria may found in association with organic matter of plant tissue, animal and insects remain.

They may be heterotrophic or autotrophic or both.

They are pioneers, found every where in the earth even in the soil attached to the organic matter. 

v High Plants:

Third greater supply of organic matter.

Remains appear in Silurian age, evolution takes in plants step by step as:

Blue-green Algae

                            Greeen Algae

                                                 Finally Seeweed (Kelp)

These production increase in Devonian, most plants appears as primitive representative in Late Deveonian: Early Ferns, which dominates the land flora.

Late Devonian plants has small roots, leaves and secondary wood.

Early Carboniferous first seed (ferns) appeared, in Late Carboniferous large variety exists as shrubs, large trees eventually forming forest and thick wood, a factory of coal.

Towards the end of Palaeozoic, the gymnosperms appears, dominated between Late Permian to Early Cretaceous called “Era of Gymnosperm”.

Early Cretaceous; suddenly character of land plant changed due to appearance of angiosperm, which quickly dominated flora.

Summary/Conclusion:

Precambrian till Devonian: Sole primary producer of organic matter were marine phytoplankton.

Since Devonian, primary production has been contributed by higher terrestrial land plants.

 v The Primary Producers:

Most important contributors to organic matter in sediments are:

Phytoplankton/Zooplankton (planktonic foraminifera, radiolarian, and planktonic crustacean)

Higher Plants

Bacteria.

Presently, marine phytoplankton and higher terrestrial plants produce equal amount of organic carbon.

However, the productivity of marine organism would be preserved rather than terrestrial plants due to availability of excess O2, which destroyed the organic matter.

Higher organized animals, such fishes, contribute so little to organic matter that they are practically neglected.

According to law of food chain, zooplankton shows tendency to be high in areas of high phytoplankton productivity. Heterotrophic bacteria would be greater where organic matter is available as a source of food.

Hence, the marine environment suits more for the organic matter to produce and accumulate in the sediments.