Multiple drain method for recovering oil from tar sand

A thermal method is described for recovering normally immobile hydrocarbon oil from a subsurface tar sand deposit. The procedure comprises: (a) establishing at least one substantially vertical production bore hole extending from the surface of the earth to at least the bottom of said subsurface formation; (b) providing a plurality of bore holes extending downwardly from the surface of the earth through the tar sand formation to substantially the bottom thereof and then substantially horizontally at or near the bottom of the tar sand formation and converging radially inward to each bore hole, each radial bore hole containing a perforated or slotted tube; (c) continuously injecting steam downwardly through the perforated or slotted tubes whereby the steam discharges through the perforations or slots and into the tar sand formation to reduce the viscosity of the normally immobile oil, with a substantial proportion of the steam being injected into the formation via the portion of each tube extending downwardly through the tar sand formation whereby the steam reduces the viscosity of the normally immobile oil over an area extending substantially between the perforated tube and the top of the tar sand formation with this viscosity reducing area expanding radially and moving axially inwardly toward the vertical production bore hole thereby creating an expanding generally conical-shaped production chamber; and (d) draining the less viscous oil and steam condensate thus obtained downwardly by gravity to the bottom of the production chamber and then through the horizontal tubes into the bottom of the vertical production bore hole for collection.