COMPOSITION AND ORIGIN
Natural gas is a hydrocarbon gas mixture consisting primarily of methane (CH4) (more than 90%). Other hydrocarbons concentration is lower (ethane, propane, butane and higher molecular weight hydrocarbon impurities). Natural gas also contains small amounts of carbon dioxide (CO2) and nitrogen (N2), while sometimes one can find impurities of helium, hydrogen sulphide, argon, hydrogen, mercury and other vapours. The origin, type and content of these impurities in natural gas depend on the type of source rock, on the impact of igneous or hydrothermal processes in lithosphere and on natural gas migration processes.
Underground natural gas deposits have been identified at the depths from several meters to 5 thousand meters, sometimes under the pressure above 300 bar, and temperatures above 180°C, depending on the deposit depth.
Gas deposits in permeable rock beds (in which cavities gas can be located), surrounded by impermeable beds (so they prevent gas migration and hold it “trapped” in permeable rock beds). It is most commonly found as gas cap in oil deposits, however, clear gas fields without oil presence are not rare.
Natural gas is widely spread in sedimentary rocks, and can be found in eruptive rocks. It can be traced in rocks formed in all geological periods, however, it can be found more frequently and in larger volumes in rocks from the period of more intensive life on the earth (Cambrian period – Ordovician period, more than 500 million years ago).
The formation of natural gas and its accumulations has not yet been definitely established. One of the theories is the organic theory of natural gas formation. This theory is based on the identified organic materials in natural gas, necessary for hydrocarbons formation, and the explained chemical processes of organic matter maturation and hydrocarbons formation. However, hydrocarbons, particularly methane, have been traced deep in granite massifs with no trace of organic material, so one should not reject the inorganic theory of natural gas formation.
Long before our era, the Chinese used natural gas to light their temples bringing natural gas by bamboo pipes, so it is therefore the first example of organised natural gas production and transport. In the modern time, the first example of natural gas use was by the end of 19th ct in the USA – in Fredonia (New York State) for heating apartments. The wider use started in 1884, when natural gas was brought to Pittsburgh by a 23 km long gas pipeline, where it was used for lighting, heating and thermal processes. Until 1950 the USA was almost the only natural gas producer, then came Russia, Canada, the Netherlands, Great Britain, Norway, Germany, Romania, Italy, Mexico, Venezuela, Algeria, Nigeria, Indonesia, Malaysia, and lately the Middle East countries.NATURAL GAS MARKET OF THE REPUBLIC OF CROATIA
Beside coal, natural gas is the only primary form of energy which can be used directly. It burns providing greater efficiency than other fuels, so its use in households has been increasing, for heating and cooling, in technological processes, for the production of thermal and electric energy, and as a raw material in chemical industry, primarily petrochemical industry.
Natural gas is transported in gaseous state by pipelines, or liquefied in special vessels for liquefied natural gas; rarely in special thermo-insulated tanks in rail or road traffic.
In our region natural gas was discovered in 1917 in Bujavica. In 1918 the use of natural gas for lighting started, while its use in industry started in 1926. More organised exploration of natural gas started after the World War II. The discovered reservoirs provided conditions to construct the first main gas pipeline from Janja Lipa to Zagreb in 1954. Further discovery of gas fields was accompanied by the construction of the main gas pipelines network in the entire northeast Croatia, up to the current length of 2410 km. At the point Rogatec – Hum na Sutli our system has been connected to the Slovenian, that is, the European system. This is the intake point of imported natural gas.