Petroleum – the environment and stages of formation
An analysis of hydrocarbon formation stages should involve a plethora of environmental factors, including:
- type and quality of the supplied organic matter,
- hydrodynamics of the environment,
- type of plants and animals living in the water reservoir,
- nutrient supply source and rates,
- type of bottom sediments and their mineral compositions.
All of these parameters vary from one environment to another and are subject to constant dynamic changes within a single sedimentary basin. Therefore, petroleum formation processes are slightly different in various petroleum systems. Nonetheless, there are some generic stages of hydrocarbon formation, namely: diagenesis, catagenesis and metamorphism.
Diagenesis
Diagenesis of organic matter contained in the sediment falls into two stages.
The first stage, which takes place immediately upon organic matter deposition, occurs in the presence of oxygen with involvement of aerobic organisms at the water/sediment interface. Oxygen contained in the water column helps to boost primary production while enhancing biodegradation and oxygenation of organic matter.
The processes of organic matter decomposition by the organisms feeding at the bottom (biodegradation), including aerobic bacteria, deplete organic matter of more labile components. Mostly labile organic matter is decomposed by these processes. The balance – semi-labile and refractory organic matter – is buried in the sediment to undergo subsequent decomposition with an involvement of anaerobic bacteria at the sulfate reduction level. This marks the beginning of the second (anaerobic) stage of diagenesis.
Sediment depth at which redox fronts (nitrate and sulfate reduction levels, respectively) occur are contingent primarily on the volume of organic matter supply and the degree of oxygenation of water at the bottom and in upper layers of the sediment. The depth ranges from zero to a few centimetres in eutrophic environments and up to several tens of centimetres in oligotrophic environments.
In addition to the aforementioned factors, the quantity of organic matter buried in the sediment for subsequent diagenesis will depend on the hydrodynamics of the environment.
Organic matter preservation in the sediment is primarily controlled by:
- the degree of water mixing in the column,
- abrasive action of the wave base and bottom currents,
- the share of bottom current in lateral transport and sorting of organic matter particles.
Environments with the following patterns:
- a high primary production,
- low hydrodynamics (stagnant water),
- stratified water column,
- poor benthic fauna,
- dysoxic (anoxic) conditions prevailing at the bottom,
are the best prospects for the development of oil source rocks.
The phase of aerobic alteration is reduced in these environments so that much more organic matter less depleted in labile particles is buried.
A lower hydrodynamics of the environment is reflected by finer size of grains (silt and clay fraction) that are deposited at the bottom of the basin; an important factor considering both sealing and catalytic properties (i.e. increasing the rate of chemical reactions that lead to the formation of the hydrocarbons).
In the anoxic phase, buried organic matter is further altered by anaerobic bacteria. Biogenic methane, hydrogen sulfide and carbon dioxide are released as part of their metabolic processes. Upon reaching a burial depth at which temperatures are in the order of 50oC, the organic matter first altered by anaerobic processes undergoes polycondensation so that insoluble kerogen and small amounts of bituminen are formed.
A deeper burial and related higher temperatures trigger thermocatalytic reactions and transition to the stage of catagenesis.
Further reading:
- McCarthy, K., Rojas, K., Niemann, M., Palmowski, D., Peters, K., Stankiewicz, A., 2011. Basic Petroleum Geochemistry for Source Rock Evaluation. Oilfield Review Summer 2011, 23, no. 2.
author: Katarzyna Dybkowska
