1. A method for determining a steam injection schedule for a set of subsurface regions of an oil field, the method comprising the steps of: a. determining thermal maturity for each subsurface region of the set; b. calculating a latent heat target for each subsurface region according to the determined thermal maturity therefore; wherein a Neuman rate calculator is employed to calculate the steam injection targets for thermally immature subsurface regions and wherein a heat maintenance rate calculator is employed to calculate the steam injection targets for thermally mature subsurface regions; c. calculating a steam injection target for each subsurface region according to the calculated latent heat target therefore; d. determining the availability of steam for injection to the subsurface regions; and e. calculating a steam injection schedule for each subsurface region according to the determined steam availability and calculated steam injection targets for all subsurface regions of the set.