A mud gas analyzer and a well-site system using the mud gas analyzer are described. The mud gas analyzer includes at least one degasser adapted to extract gas from drilling mud passing through a flow path formed at least partially by a drill string within a well and an annulus positioned between an exterior surface of the drill string and a formation surrounding the well at a well-site, at least one gas analyzer adapted to interact with the gas extracted from the drilling mud. The gas analyzer determines an isotopic composition of a gas liberated from the drilling mud and generates a sequence of signals indicative of ratios of isotopes of a gas species liberated from the drilling mud. The computer system includes a processor adapted to execute logic to cause the processor to access information indicative of the well's and drilling tool's geometry and mud flow rates or access information from any other source providing particular locations of the well at which mixing of fractions of a gas species occurred including formation gas and recycled gas, receive the sequence of signals, and to calculate and log isotopic characteristics of the gas species entering the drilling mud at particular locations of the well within a geological formation.