Effects of inherent pyrite on hydrocarbon generation by thermal pyrolysis: An example of low maturity type-II kerogen from Alum shale formation, Sweden

To resolve the controversy of iron sulfide effect on hydrocarbon generation of kerogen, a confined anhydrous pyrolysis experiment was performed on an Alum kerogen before and after the removal of kerogen-associated inherent pyrite within a temperature range of 336 degrees C to 600 degrees C at 50 MPa. The effect of kerogen-associated inherent pyrite on hydrocarbon generation and the reaction mechanism were explored by comparing the results from pyrite-contained kerogen with pyrite-removed kerogen. The thermal transformation of pyrite into pyrrhotite gradually increased with increasing temperature, particularly in the range of 432 degrees C - 528 degrees C, accompanied by the formation of nascent sulfur. The bitumen/heavy hydrocarbons (C14+) yield was lower in the pyrite-contained (imbedded) kerogen due to strong bonding interaction between pyrite and kerogen. The temperature of peak liquid hydrocarbon (C6-14) production in the pyrite-contained kerogen was nearly 50 degrees C lower than that of the pyrite-removed kerogen, indicating that inherent pyrite accelerated the generation of liquid hydrocarbons. The wet gas yield (Sigma C2-5) decreased significantly for the pyrite-contained kerogen above 432 degrees C, caused by the hydrogen abstraction of additional nascent sulfur. Moreover, the presence of inherent pyrite significantly increased the iso- to normal paraffin ratios (iC(4)/nC(4), iC(5)/nC(5)).