A subsurface thermally autogenous reactor and method has downgoing and upgoing flow passages connected at the bottom to form a U-tube and in heat exchange relation to each other. The downgoing and upgoing flow passages each have an upper heat exchange section and a lower reaction section. An air injection system injects air into the downgoing flow passage between the heat exchange and reaction sections. The cross sectional area of the upgoing heat exchange section is greater than the cross sectional area of the downgoing heat exchange section to balance the downgoing and upgoing flow velocities. The downgoing heat exchange section has multiple tubes to increase the heat transfer area.