Catalytic capacity evolution of montmorillonite in in-situ combustion of heavy oil

•The catalytic capacity evolution of Mt in ISC of HOO was explored.•The thermal transformation process of Mt had a small effect on the LTO of heavy oil.•Mt IW dehydration process enhanced its catalytic role in the HTO of heavy oil.•Mt dehydroxylation process weakened its catalytic role in the HTO of heavy oil.•The overlapping process had no positive catalytic effect on the HTO of heavy oil.