Organic matter is the material that can transform and generate into hydrocarbons. In some phase, because of the burial sedimentation that can increase temperature and pressure in the basin and also from the bacterial acitivity. It can transform the organic matter that have biological content become more chemical to geopolymer then geomonomer.
On the process of organic matter preservation in sediment matterial, the microbial activity can degradate it, from all source. The factor that can increase the activity of microbial is conten of oxygen itself, in the environment. Indeed, oxygen availability is reflected by biological marker in sediment. So, the best environment for organic matter preservation is an environment that has low activity of bacteria which is cause by oxygen depleted.
Anoxic environment is reductive, where oxygen is very low. In this environment, aerobic bacterial activity was decrease by the deplection of oxygen. Despite of that, only anaerobic bacteria that effective to rework the organic matter. Lack of bioturbation slowed down the bacterial activity, which is limited diffusion of oxidant to the sediment. So that, the sediment is laminated and very ric in organic matter.
(image source : http://wetlandinfo.ehp.qld.gov.au/resources/static/images/assessment/threats/strssor-conceptual-models/organic-matter/phys-chem-state-organic-matter-in-water.jpg)
The environment that have anoxic condition is divided into 4 group (Demaisson and Moore, 1980), whish dinstinct by the accumulation of the organic matter, and that is :
- Large Anoxic Lakes
Permanent stratification of such lakes promotes development of anoxic bottom waters. These lakes are formed under warm equable climatic condition that favour lacustrine anoxia and, ultimately, dposition of source material that gives rise to non-marine oil; e.g. Lake Tanganyika.
- Anoxic Silled Basins
These are landlocked silled basins with positive water balance, such as the Baltic and Black Seas.
- Anoxic Layers
These are caused by upwelling where there is high surface biological productivity and, hence, the ocygen supply in deep water cannot match deman; e.g. Benguela current and Pperu coastal upwelling.
- Open Ocean Anoxic Layers
These are formed in ocygen minimum layers of the northeast Pacific and northern Indian oceans, far from the polar regions.
Philp, R. P., 1981, Diagenetic Organic Matter in recent Sediments and Environtments of Deposition, in BMR Journal Australian Geology & Geophysics, 6, 1981, 301-306.