Formation of Source Rocks

All marine organic material is formed near the surface of the ocean, in the photic zone, through photosynthesis. For the most part this is algae. Some phytoplankton are broken down chemically and oxidised and some are eaten by zooplankton. Both types of plankton are eaten by higher organisms which concentrate the indigestible part of the organic matter into fecal pellets which may be incorporated into sediments. Plankton is made up of very small organisms which sink so slowly that they are in most cases almost entirely degraded (oxidised) before they reach the bottom.

Pellets, on the other hand, are the size of sand grains and sink more rapidly, and this organic matter is more likely to be preserved in the sediments. On the bottom, organic matter will be subjected to breakdown by micro-organisms (bacteria). It will also be eaten by burrowing organisms which live in the top portion of the sediments. The activity of these organisms contributes to reducing the organic content of the sediments because most of the organic matter is digested when the sediment is eaten. Bioturbation also stirs up the sediments, exposing them more to the oxygen-bearing bottom water.

However, if the bottom water is stagnant, the lack of oxygen and the toxicity of H2S will exclude most life forms. The resultant lack of bioturbation will thus preserve more organic matter in the sediment together with perfect, undisturbed, lamination. Stagnant, or anoxic, conditions are defined by an oxygen content of the Atlantic experienced stagnant bottom conditions during such periods. Limited water circulation in semi-enclosed marine basins due to restricted outflow over a shallow threshold is a common cause of stagnant water bodies (Fig. 1.2a).

The Black Sea is a good example. In response to an abundant freshwater supply from rivers and a relatively low evaporation rate, a low salinity surface layer leads to density stratification in the water column and a consequent reduction in circulation. In basins with little precipitation and where there is net evaporation, the surface water will have higher salinity and density than the water below it, and will sink down.

This circulation brings with it oxygen from the surface and can give oxidising bottom conditions with little chance for organic matter to survive to form source rocks. Lakes or semi-enclosed marine basins often have a temperature- or salinity-induced density stratification so that oxygenated surface water does not mix with water in the deeper part of the basin. This leads to anoxic conditions and a high degree of preservation of the organic matter produced in the surface waters.

This aspect is therefore of considerable interest in exploration for petroleum in freshwater basins, particularly in Africa and China. The open oceans have normally had oxygenated water, but during the Cretaceous most of the Atlantic Ocean is believed to have been stagnant during so-called “anoxic events”, and substantial amounts of black shale were deposited in the deeper parts of the ocean during these periods.