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)).

Automated summary

This study investigated the effect of inherent pyrite on hydrocarbon generation of kerogen through a series of experiments. The presence of pyrite in kerogen resulted in lower yield of liquid hydrocarbons and higher peak temperature for liquid hydrocarbon production. In addition, the thermal transformation of pyrite decreased the wet gas yield and increased the iso- to normal paraffin ratios.