Producing natural gas and mineral oil
Biomass – millions of years oldNatural gas and oil form over hundreds of millions of years from dead organic material that has accumulated on the bottoms of seas, lakes and swamps. Oil is formed primarily from dead microalgae, or phytoplankton, while coal and natural gas derive mainly from land plants. Especially large amounts of biomass accumulate in warm regions with lush vegetation or prolific algal growth. Dead biomass normally decomposes in water where it is broken down by bacteria into carbon dioxide and water. This process consumes oxygen. With the input of large amounts of biomass, oxygen can be completely depleted by the bacteria. This creates oxygen-free zones where decay no longer occurs. Thus, over time, packages of biomass several hundred or thousand metres thick can accumulate on the sea floor. Whether natural gas or mineral oil is formed from the biomass depends primarily on the temperatures at these depths.
Plankton cooked down to oilOil deposits formed through a series of consecutive processes. First, the phytoplankton accumulated on the sea floor. Together with fine rock and clay particles washed into the sea from the mountains and flatlands, the algal biomass was transformed into an organic-rich sludge. Over periods lasting many millions of years, so much of the organic sludge was deposited on the sea floor that, due to its enormous weight, it gradually changed to claystone and was finally compressed and hardened into a clay-rich shale. Even today it can be said that, to some extent, in these porous shale layers at depths of 2000 to 4000 metres and at temperatures between 65 and 120 degrees Celsius the transformation of biomass into oil is taking place. This temperature range is called the “oil window”. Just like in a chemistry laboratory, the biomass here cooks down into a broad range of chemical compounds that are composed exclusively of carbon and hydrogen, and are thus referred to as hydrocarbons. Crude oil is therefore a mixture of hundreds of different compounds that are initially separated in refineries or split into smaller molecular chains. The splitting process is referred to as “cracking”. Not only are fuels such as petrol and diesel produced from the crude oil. Other products of the refineries include ethylene gas and propylene gas. The tiny hydrocarbon molecules of ethylene and propylene, which contain only a few atoms, are used in plastics production and many other applications. The shale and other rocks in which oil forms are called oil source rocks. They contain up to 20 per cent organic material. Over millions of years the source rocks have gradually been compressed by the sediment and rock layers being deposited on top of them, resulting in the formation of oil. As increasing amounts of oil formed, more of it escaped from the source rock and rose slowly through the overlying rock and sediments. In some areas it even reached the surface. Near the northern German city of Celle, for example, tar pits formed naturally, containing a black liquid which, historically, has been used as lamp oil, a lubricant, and even as a natural remedy.
- Oil reservoirs were formed whenever the upward travel of the oil was blocked by impermeable materials such as salt or clay layers. If these impermeable layers were underlain by a porous reservoir rock such as sand or limestone, it acted as a sponge, causing the oil to accumulate. Specialists call these formations underground trap structures. In addition to oil and other hydrocarbons, the porous rocks also contain large amounts of water, which has to be separated out during production of the oil. Because the continents, as a result of continental drift, have been moving for millions of years or more, the ancient seas in which the shales were formed no longer exist today. At a time around 120 million years ago, South America and Africa began to break apart. Initially a small tropical sea surrounded by land was formed in this process, in which a large volume of biomass was deposited. This sea then expanded to become the South Atlantic Ocean. Today the ancient sediments of the former tropical sea lie off the coasts of South America and West Africa.
- 1.15 > 300 million years ago extensive clubmoss and horsetail forests were common. The plants were several metres high, much taller than today. Coal and natural gas were formed from them.
Peat Peat is defined as soil that contains more than 30 per cent organic material. This consists of partially decayed plant remains that could not be completely broken down in oxygen-free, standing swamp water.
Layer upon layer of peatAs a rule, natural gas is formed from terrestrial vegetation that once grew in flat coastal areas or in near-coastal swamps in subtropical and tropical climates. In swampy areas, peat usually formed first. Due to numerous cycles of sea-level rise and fall over the millennia, these wetland areas were repeatedly flooded. Fine sand and clay particles that were transported from the land to the sea were then deposited upon the submerged peat layers. When the water retreated with falling sea level, land plants colonized the areas again, allowing a new peat layer to form. Over millions of years, the rising and falling of sea level created a layer-cake sediment pattern in which sandy and clayey layers alternated with thick peat layers. Ideal conditions for the formation of peat were present in large regions of central and northern Europe and in North America during a period from 290 to 315 million years ago. At that time these regions lay close to the equator. They were located in a warm, tropical zone, and were rich in vegetation. Not until later did these land masses drift several thousand kilometres northward to their present position. With time, the layer-cake structure of alternating peat and clay layers was also covered by new sediments and compacted by their enormous weight. However, no oil was formed from the old peat layers, but first lignite and later hard coal. At a depth of 4000 to 6000 metres and temperatures between 120 and 180 degrees Cel- sius, natural gas formed in the coal over many millions of years. For the formation of natural gas, higher tempe- ratures are required than for oil.
- As a rule, natural gas contains around 90 per cent methane. This is accompanied by other gas-phase hydrocarbons such as ethane, propane and butane, as well as non-flammable gases such as carbon dioxide and nitrogen. An additional component is hydrogen sulphide, which has to be removed from the natural gas before it can be used. Hydrogen sulphide can convert to acid when the gas is burnt, which can lead to corrosion in power plants and heating systems. Natural gas with an especially high content of hydrogen sulphide or carbon dioxide is called acid gas. If this is to be used it must undergo extensive cleaning. Natural gas also migrates gradually out of the source rock. If it is not trapped by dense rock layers then, like oil, it can rise all the way to the Earth’s surface. The “eternal fires” in Iran are fed by rising gas and condensate, and were presumably lit initially by a lightning strike. There are many places around the world where fires fed by underground gas are still burning. Many of these were venerated by ancient cultures and have become sacred sites. Wherever underground trap structures were present, the natural gas, just like oil, could accumulate in reservoirs. Generally, the accumulations are only considered to be reservoirs when they are large enough and the rocks permeable enough to make production of the hydrocarbons economically feasible. This is equally true for both gas and oil. Gas or oil accumulations that are too small to be economically produced, however, occur much more frequently in nature.
Natural gas and oil trappedSpecialists distinguish different kinds of reservoirs in which large amounts of natural gas or oil have accumulated. Typical reservoir types include: ANTICLINE: An anticline is an arching structure of rock layers, a kind of underground hill. It is formed when dense rock layers undergo pressure from the sides caused by movement of the Earth’s crust. When the anticline is composed of impermeable rocks, the rising gas and oil can accumulate there, as in an inverted bowl. FLANK OF A SALT STOCK: Salt stocks are large underground accumulations of solid rock salt that can be as much as thousands of metres thick. If an impermeable rock layer (a trap structure) abuts on the flank of a salt stock, then ascending oil and gas will be trapped between the rock layer and the flank, because the salt is also impermeable. UNCONFORMITY: An unconformity arises at locations where rock layers abut obliquely, or at an angle to one another. Unconformities are formed by lifting, subsidence, or squeezing of rock packages that are subsequently overlain by younger sediments. If these overlying sediment layers are impermeable, ascending gas and oil can accumulate and concentrate in the underlying rock packages. CORAL REEFS:In many instances, natural gas and oil collect in very porous limestone that has formed from ancient coral reefs. SALT STOCK OVERHANG: Some salt stocks are mushroom-shaped with a wide dome at the top, which forms a kind of umbrella, known as the overhang. Gas and oil can accumulate beneath this. Salt stock overhangs are mainly the result of immense underground pressure. Salt rises because it is less dense than the overlying strata. It bulges upward into domes or the mushroom-shaped overhangs. These movements are referred to as salt tectonics. >