Petroleum is formed as a result of biochemical and thermal alteration of organic matter from the debris of plants and animals.
Accumulation of sufficient amounts of organic matter is the first stage in the formation of the future petroleum reservoirs. This happens if accumulation of sediments prevails over erosional processes in the conditions of uninterrupted ample supply of organic debris. These conditions are most often satisfied in marine and lacustrine environments.
Water reservoirs with a high level of primary production, eutrophication of bottom sediments and a low hydrodynamic environment and/or stratification of the water accumulate the thickest beds of organic matter.
An increase in primary production due to a high supply of nutrients to the photic zone is followed by massive blooms of the phytoplankton, mostly sea algae, which is the main source of marine labile (unstable) organic matter.
As a result of higher phytodetritus supply to the reservoir bottom, oxygen is depleted in upper portions of the sediments and in the bottom water, as the oxygen is consumed in the processes of aerobic decomposition of the organic matter. Moreover, the ongoing deposition of mineral particles, especially clay and silt grains, isolates the already deposited organic matter-enriched layer with its biomass of feeding bacteria from the external environment, in particular from any better oxygenated bottom water.
In the conditions of emerging anoxic conditions, the bulk of the organic matter is decomposed by anaerobic bacteria. Due to bacterial activity and increasing redox fronts, hydrogen sulfide appears in upper layers of the sediments. This represents an additional factor that limits the activity of burrowing animals and, consequently, oxygen penetration into the sediment as a result of bioturbation. Combined with ongoing sedimentation of clay minerals, this effectively isolates the organic layer from oxygenated zone and remineralization is confined to anaerobic processes.
The processes of aerobic and anaerobic organic matter decomposition mark the beginning of the second stage of crude oil formation, namely that of diagenesis.
Further reading:
- Zawisza, L., 2009. Geologia Naftowa. Kraków, January 2009. Last accessed: March 2014. Available at: wiertnictwo.w.of.pl/pliki/naftowa/naftowa2.pdf
- Tissot, B.,P., Welte, D., H., 1984. Petroleum Formation and Occurrence. Second Revised and Enlarged Edition. Springer- Verlag Berlin Heidelberg 1978 and 1984. DOI: 10.1007/978-3-642-87813-8
author: Katarzyna Dybkowska
