An in situ method of producing hydrocarbon fluids from an organic-rich rock formation is provided. The method may include heating an organic-rich rock formation, for example an oil shale formation, in situ to pyrolyze formation hydrocarbons, for example kerogen, to form a production fluid containing hydrocarbon fluids. The method may include separating the production fluid into at least a gas stream and a liquid stream, where the gas stream is a low BTU gas stream. The low BTU gas stream is then fed to a gas turbine where it is combusted and is used to generate electricity.