Pyrite thermal decomposition in source rocks

Sulphides, mainly pyrite, are very common minerals in source rocks for oil and natural gas. Pyrite thermal transformation is very complex process, depending on many factors affecting the decomposition reactions. Thermal decomposition of pyrite contained in bulk rock is even more complex due to the possible interactions with organic matter present in the source rock. In the paper the reactions of pure pyrite decomposition under different atmosphere were presented. Moreover the decomposition of pyrite in kerogen and bulk source rock were studied with TG/DSC methods. The evolved gases derived from combustion and pyrolysis processes were studied by FTIR and QMS methods. The results show that SO₂ is the only gas detected during pyrite decomposition. Pyrite content in a rock sample is detectable over 10% contamination; below this value the sulphide presence can be masked by thermal decomposition of other rock components. In case of combustion process of source rock containing pyrite sulphur dioxide is released in temperature range of 400–430 °C. When the source rock is pyrolyzed, then pyrite decomposition, reflected by SO₂ evolution, begins near temperature of 400 °C, and is extended in time. The release of sulphur dioxide from pyrolysed kerogen as a result of pyrite decomposition is detectable in the range of 400–480 °C.