Cyclic solvent process for in-situ bitumen and heavy oil production

A process for recovery of hydrocarbons in a production fluid from an underground reservoir of said hydrocarbons, the process comprising of: (a) injecting a viscosity reducing solvent of a fraction of said hydrocarbons into said reservoir at a pressure in the reservoir of above a liquid/vapor phase change pressure of a fraction of said solvent; said pressure in said reservoir also being sufficient to cause geomechanical formation dilation or pore fluid compression, and then, (b) allowing said solvent to mix with said hydrocarbons under pore dilation conditions, and then, (c) reducing the pressure in said reservoir to below said liquid/vapor phase change pressure of at least said fraction of said solvent thereby evincing solvent gas drive of said fraction of said hydrocarbons from said reservoir; and then, (d) repeating steps (a) to (c) as required.