In which type of exercise would fat reserves be used to create atp

ATP is synthesizes in glycolysis, and through citric acid cycle.
In case of glucopenia (lack of sugars, after a long-term excerice), fat mobilization from adipose tissue is observed, followed by hydrolysis to fatty acids and glycerol.
Glycerol is used as a precursor in glucose recycling via gluconeogenesis: this accelerated gluconeogenesis requires energy.
Fatty acids are oxidized by beta-oxidation, pass through L-carnitine through the mitochondrial membrane (carnitine molecular shuttle) and via AcCoA into the citric acid cycle and even bring more energy that glucose burning but in the long run.