The formation of oil and gas requires sufficient sedimentary organic matter, appropriate temperature, pressure, time, bacteria and other physical, chemical and biochemical conditions.
Organic matter: organic matter is the material basis for the formation of oil and gas. The organic matter that can form oil and gas is mainly aquatic organisms, such as algae, or the cuticular wax and pollen of higher plants. Organic matter is generally preserved in mudstone or shale.
Temperature: temperature is the most important factor in the formation of oil and gas. In sedimentary basins, the temperature at which organic matter begins to generate oil is usually between 60 ° C and 120 ° C, which varies with the burial time of organic matter. The end temperature of liquid oil generation is generally not higher than 170 ℃, and the generation of natural gas is accompanied by the generation of liquid oil. The end temperature is higher than that of liquid oil, but generally not higher than 230 ℃. In other words, the formation of oil and gas only occurs in a limited temperature range. Too low or too high temperature is not conducive to the formation of oil and gas.
Time: time also plays an important role in oil and gas generation. Time and temperature can complement each other, but temperature takes the first place. At very low temperatures, time usually doesn't work, that is, if the organic matter is at very low temperature, it can't generate hydrocarbon no matter how long it goes through.
Pressure: with the increase of the burial depth of the sediment, the thickness of the overlying strata increases, and the temperature and pressure of the sediment increase. The increase of pressure will promote the chemical reaction. However, the effect of pressure on oil and gas generation is relatively small. In some cases, it can promote oil and gas generation, but in some cases, it can inhibit oil and gas generation.
Bacteria: for oil and gas generation, the most significant is anaerobic bacteria. Bacteria play an important role in the decomposition and transformation of organic matter. At the same time, when the organic matter is decomposed at low temperature, the biogas mainly composed of methane can also be produced.
Catalyst: catalyst is a chemical reaction accelerator. It is generally believed that clay minerals can play a catalytic role in the process of oil and gas generation and cracking.
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