Abstract: Methods and systems for assessing permeability, porosity, and production fluids pressure or analyzing reservoir fluids in connection with a drilling operation are provided, comprising collecting from a series of depths mud gas samples and reservoir cuttings of a minimum size; allowing the cuttings to evolve a headspace gas within one or more time delays to obtain headspace gas samples; determining within the time delay carbon isotope ratio differences between the mud gas and headspace gas samples; and identifying the depths having isotope ratio differences smaller than those from nearby depths.