Accumulations of Organic Matter

It is well documented that oil accumulations are of organic origin and formed from organic matter in sediments. Methane can be formed inorganically and is found in the atmosphere of several other planets, but inorganic methane from the interior of the earth is likely to be well dispersed and thus not form major gas accumulations in the earth’s crust. The organic matter from which petroleum is derived originated through photosynthesis, i.e. storage of solar energy (Fig. 1.1). Sunlight is continuously transformed into such energy on Earth but only a very small proportion of the solar energy is preserved as organic matter and petroleum.

The oil and gas which forms in sedimentary basins each year is thus minute in comparison with the rate of exploitation (production) and consumption. In practice petroleum must therefore be regarded as a non-renewable resource even though some petroleum is being formed all the time. Most of the organic materials which occur in source rocks for petroleum are algae, formed by photosynthesis. The zooplankton and higher organisms that are also represented grazed the algae and were thus indirectly dependent on photosynthesis too. The energy which we release when burning petroleum is therefore stored solar energy. Since petroleum is derived from organic matter, it is important to understand how and where sediments with a high content of organic matter are deposited.

The total production of organic material in the world’s oceans is now 5×1010 tonnes/year. Nutrients for this organic production are supplied by erosion of rocks on land and transported into the ocean. The supply of nutrients is therefore greatest in coastal areas, particularly where sediment-laden rivers discharge into the sea. Plant debris is also supplied directly from the land in coastal areas. Biological production is greatest in the uppermost 20–30 m of the ocean and most of the phytoplankton growth takes place in this zone. In clear water, sunlight penetrates much deeper than in turbid water, but in clear water there is usually little nutrient supply. At about 100–150 m depth, sunlight is too weak for photosynthesis even in very clear water. Phytoplankton provides nutrition for all other marine life in the oceans.

Zooplankton feed on phytoplankton and therefore proliferate only where there is vigorous phytoplankton production. Organisms sink after they have died, and may decay so that nutrients are released and recycled at greater depths. Basins with restricted water circulation will preserve more organic matter and produce good source rocks which may mature to generate oil and gas (Fig. 1.2a, b). In polar regions, cold dense water sinks to great depths and flows along the bottom of the deep oceans towards lower latitudes.

This is the thermal conveyor belt transporting heat to higher latitudes and it keeps the deep ocean water oxidizing. In areas near the equator where the prevailing winds are from the east the surface water is driven away from the western coast of the continents. This generates a strong upwelling of nutrient-rich water from the bottom of the sea which sustains especially high levels of primary organic production (Fig. 1.3). The best examples of this are the coast of Chile and off West Africa. Through photosynthesis, low energy carbon dioxide and water are transformed into high energy carbohydrates (e.g. glucose):

CO2 + H2O → CH2O (organic matter) + O2

Phosphorus (P) and nitrogen (N) are the most important nutrients, though the supply of iron can also be limiting for alga production. It is this process of photosynthesis, which started 4 billion years ago, that has built up an atmosphere rich in oxygen while accumulating reduced carbon in sedimentary rocks as oil, gas and coal. Most of the carbon is nevertheless finely divided within sedimentary rocks, for example shales and limestones, in concentrations too low to generate significant oil and gas.