A method for treating a petroleum reservoir to recover synthesis gas and mobilized petroleum therefrom, the reservoir containing petroleum and water, the method comprising the steps of: a. providing at least one production well in the reservoir, the at least one production well situated in an upper part of the reservoir; b. providing at least one heating well in the reservoir beneath the at least one production well for treating a lower part of the reservoir with heat; c. using the heat, heating a portion of the reservoir directly beneath the at least one production well to a temperature sufficient to cause at least one of gasification, water-gas shift and aquathermolysis reactions to occur within the portion of the reservoir, the at least one of the reactions involving at least one of the petroleum and the water; d. allowing the at least one of gasification, water-gas shift and aquathermolysis reactions to produce synthesis gas from the at least one of the petroleum and the water in the portion of the reservoir, the synthesis gas comprising hydrogen gas and mobilizing a portion of the petroleum to form the mobilized petroleum; e. allowing the synthesis gas to rise toward the at least one production well as the mobilized petroleum accumulates in the portion of the reservoir; f. producing at least a portion of the synthesis gas to surface through the at least one production well; characterized in that g. operating the at least one heating well as an at least one additional production well to produce a portion of the mobilized petroleum to the surface through the at least one additional production well simultaneously with production to the surface of the portion of the synthesis gas through the at least one production well. a. providing at least one production well in the reservoir, the at least one production well situated in an upper part of the reservoir; b. providing at least one heating well in the reservoir beneath the at least one production well for treating a lower part of the reservoir with heat; c. using the heat, heating a portion of the reservoir directly beneath the at least one production well to a temperature sufficient to cause at least one of gasification, water-gas shift and aquathermolysis reactions to occur within the portion of the reservoir, the at least one of the reactions involving at least one of the petroleum and the water; d. allowing the at least one of gasification, water-gas shift and aquathermolysis reactions to produce synthesis gas from the at least one of the petroleum and the water in the portion of the reservoir, the synthesis gas comprising hydrogen gas and mobilizing a portion of the petroleum to form the mobilized petroleum; e. allowing the synthesis gas to rise toward the at least one production well as the mobilized petroleum accumulates in the portion of the reservoir; f. producing at least a portion of the synthesis gas to surface through the at least one production well; characterized in that g. operating the at least one heating well as an at least one additional production well to produce a portion of the mobilized petroleum to the surface through the at least one additional production well simultaneously with production to the surface of the portion of the synthesis gas through the at least one production well.