combustible gas in the present process in an alternative embodiment the combustible gas may contain a small proportion 1 to 10 by volume of compounds which condense at the temperature and pressure conditions of the formation the condensed compounds e g methylamine ethylamine t butylamine and hydrocarbons have a condensation point of not more than 100 u00b0 c at ambient pressure and they are dissolved in the petroliferous deposits facilitating the combustion of the latter during the subsequent in situ combustion if the condensable compounds used for such purpose are hydrocarbons they must have a condensation point of at most 100 u00b0 c under ambient pressure conditions of about one atmosphere suitable condensable hydrocarbons for such purpose are all hydrocarbon of c 4 to c 7 such as alkanes alkenes and aromatics e g n butane isobutane n pentane isopentane hexane all of its isomers and heptane and all of its isomers benzene and toluene preferably normal pentane and isopentane hexane heptane and all of the isomers thereof the amount of condensable compounds present in the combustible gas injected into the formation is 1 to 10 preferably 2 to 8 and most preferably 3 to 5 by volume the condensable compounds.