Process for in SITU upgrading of heavy hydrocarbon

The invention relates to a method for in situ upgrading of heavy hydrocarbon during a "huff and puff" or cyclic pressurizing and production technique. A process for in situ upgrading of a heavy hydrocarbon includes the steps of (a) positioning a well in a reservoir containing a heavy hydrocarbon having an initial API gravity of less than or equal to about 8; (b) injecting a light solvent into the well at reservoir conditions so as to provide an upgraded hydrocarbon in the reservoir, the upgraded hydrocarbon having an improved API gravity greater than the initial API gravity; and (c) producing the upgraded hydrocarbon from the well.