The invention provides for in situ processing of a hydrocarbon reservoir which includes selecting in the reservoir a matured steam chamber and an operating steam chamber generally adjacent to and in fluid communication with the matured steam chamber. The operating steam chamber is undergoing or is capable of undergoing SAGD processing. A non-oxidizing gas is injected into the matured steam chamber to form a pressurized non-oxidizing buffer zone between the matured and operating steam chambers. An oxygen-comprising gas is also injected into the matured chamber for maintaining pressure within the matured chamber so as to reduce or avoid fluid leaking or cross-flow from the operating chamber into the mature chamber while continually performing SAGD in the operating chamber.
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