Deposits of sedimentary basins

The majority of fossil fuels are found within sedimentary basins whose formation can be related directly or indirectly to plate motions. In addition to the sedimentary environment, quite stringent conditions are necessary for the development and preservation of these resources.

There are four principal criteria which must be met for the development of petroleum and gas, hereinafter referred to as hydrocarbons: layers rich in organic matter within the sedimentary succession; a source of heat applied for a time sufficient for the maturation of organic materials into hydrocarbons; permeable pathways which allow movement of the hydrocarbons; and a porous reservoir whose top is sealed by impermeable capping beds.

The main source of the disseminated organic matter, or kerogens, in sediments is plankton. The abundance of plankton is controlled by climate, the quantity of nutrients available, and water body geometry. The first two factors are latitude dependent, and the majority of oil basins originate at low latitudes. The latitude is obviously affected by the north-south component of plate motion, while the plate configuration at any given time determines water body geometry. Organic material is especially abundant along continental margins where there is major river runoff into large deltas.

The preservation of kerogens requires conditions which are not oxidizing. These are achieved along continental slopes where the production of organic matter exceeds the availability of free oxygen to convert them to carbon dioxide, and in closed anoxic basins. It follows that the shales and mudstones produced in such environments are the most common source rocks as they have the ability to absorb kerogens and remove them from the effects of free oxygen.

The temperature experienced by the kerogens after burial is critical, and depends on the local geothermal gradient. Temperatures of 70-85 °C are required to develop liquids and 150-175°C for dry gas. It is also important that a critical exposure time to these temperatures is exceeded, so the basin must be free from tectonism and uplift during this period.

After formation, the hydrocarbons undergo primary migration from the fine-grained source rocks and secondary migration as they concentrate and accumulate in a reservoir of high porosity. Migration occurs because of the buoyancy of the hydrocarbons, and it follows that all hydrocarbon accumulations are allochthonous. There are several types of oil trap, including anticline, fault, stratigraphic, unconformity, and lithological traps, which have the effect of providing a capping to the reservoir with an impermeable cover which prevents further upward movement.

Plate tectonics controls the locations of reservoirs in that it is responsible for the formation and preservation of the sedimentary basins in which hydrocarbons are generated and trapped. These include:

1 intracratonic basins formed by hotspot activity, Paris and Michigan basins;

2 basins associated with continental rifting, e.g. the Gulf of Aden, Red Sea;

3 aulacogens (Section 7.1), e.g. the North Sea;

4 passive continental margin basins, e.g. the Gabon Basin;

5 ensialic backarc basins, e.g. the Oriente Basin of Ecuador and Peru;

6 marginal seas, e.g. the Andaman Sea;

7 accretionary prisms, e.g. the coastal oil-fields of Ecuador and Peru;

8 forearc basins, e.g. the Cook Inlet of southern Alaska;

9 pull-apart basins associated with strike-slip faults (Section 8.2), e.g. the Los Angeles Basin, western USA (Moody, 1973);

10 foreland basins (Section 10.3.2) of orogens, e.g. the Aquitaine Basin, southwest France,

11 tensional basins associated with indentation tectonics (Section 10.4.6), e.g. southern Asia and Tibet.

Not only can plate tectonics create the habitat of hydrocarbon deposits, it can also explain why certain regions are particularly rich in these deposits. A large proportion of the Earth's hydrocarbon reserves are located in the Middle East, and the evolution and preservation of these deposits has been a consequence of a specific pattern of plate interactions (Irving et al., 1974).

During Mesozoic and early Cenozoic times two large embayments existed on the continental shelf of the Afro-Arabian continent on the southern side of the Tethys Ocean (Figs 13.5, 13.6). Such embayments around the Tethys, which also included the Gulf of México and the Persian Gulf, may have been connected via the proto-Mediterranean Sea, or Tethys-Atlantic seaway, which was situated at low latitudes. At about 100 Ma the rate of spreading of the seaway increased, maximizing the development of hydrocarbon source rocks because of the formation of extensive, warm, shallow seas to which were supplied large quantities of nutrients from the spreading center. When the seaway subsequently began to close following the development of a subduction zone at its north margin, the geometry of the plate movements was such as to protect the Persian Gulf from major tectonism. This arose because the rapid northerly motion of the Indian Plate absorbed most of the energy associated with the collision with the Eurasian Plate. The Gulf of México was similarly protected by northeastward motion of the Greater Antilles.

Coal is a combustible sedimentary rock containing in excess of 50% by weight of carbonaceous material. It is formed by the decomposition, compaction, and diagenesis of an accumulation of terrestrial and freshwater plant debris. Coals thus appear in the geological record from Devonian times when the first plants appeared.

In order to prevent the total destruction of the vegetable matter by biochemical decomposition, very wet conditions are required to stop the decay by the accumulation of toxic waste products. The conditions under which this process occurs are controlled by climate and topography. Normally a warm, wet climate is required to promote luxuriant growth, and this should be under the condition of constant standing water. Although, in regions of high rainfall, peat forms in upland areas, it is rarely preserved due to the erosion experienced in this environment. The prime conditions for coal formation are those of flat, low-lying ground invaded by swamps with stagnant water. The slow sinking of these regions preserves the organic layers by progressive burial.

The process of coalification refers to the physical and chemical changes experienced by the organic matter after burial in response to rising temperature and pressure. On compression, water and volatiles are expelled and the deposit becomes enriched in carbon. The degree of coalification is reflected in coal of different ranks, varying from the low rank lignite to high rank anthracite.

Plate tectonics affects coal formation in that it controls the latitude of a region (Section 3.4) and creates the environments necessary for the preservation of organic matter, of which the most important are passive continental margins (Section 7.7). Deltas formed on such margins produce the most favorable conditions for coal formation, and swamps can develop on a regional scale. Present day examples include the Niger, Amazon, and Mississippi deltas, and ancient examples the Carboniferous coals of North America and northwest Europe. Intracratonic deltas, such as the Rhine, are similarly productive and are likely to be preserved due to their stable surroundings. Coal deposits are also found in aulacogens and ensialic backarc basins. The tectonism associated with collisional orogens provides an environment whereby coals increase in grade by high-pressure metamorphism.

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