Tar sand deposits are widely distributed over the United States of America. These deposits vary from small to very large, and some of the latter may prove to be of economic significance. Curiously, virtually all of the largest known deposits occur in Utah, and this state is believed to possess reserves of bitumen totaling 4.2 Gm3 (27 billion barrels) Ritzma 1979. However, only a small portion of the Utah reserves ate of sufficiently shallow depth ;to permit recovery by the techniques currently used in Alberta, Canada. The majority will therefore have to be recovered by some in-situ technique. However, such an in-situ process must by some means reduce the very high viscosity, greater than 10 4 mPa.s(10 4 cp) of the bitumen.