A mud gas analyzer for use at a well site, wherein the well site includes a flow path (50; 202, 206) for drilling mud formed at least partially by a drill string positioned within a well and an annulus positioned between an exterior surface of the drill string and a formation surrounding the well, comprising: at least one degasser adapted to extract gas from the drilling mud, wherein the at least one degasser includes a first degasser (61-2) and a second degasser (61-1), the first degasser positioned to extract gas from the drilling mud being injected into the drill string, and the second degasser (61-1) positioned to extract gas from the drilling mud coming out of the annulus and downstream of to the first degasser (61-2) at least one gas analyzer , adapted to analyze the gas and generate a sequence of signals indicative of ratios of isotopes of gas species of gas extracted from the drilling mud, wherein the at least one gas analyzer is a first gas analyzer (62-2) adapted to analyze gas extracted by the first degasser (61-2), and a second gas analyzer (62-1) adapted to analyze gas extracted from the second degasser (61-1), and wherein signals to be generated by the first gas analyzer (62-2) are indicative of a first level of retention of gas species of the gas in the drilling mud prior to reinjection of the drilling mud into the drill string, and signals to be generated by the second gas analyzer (62-1) are indicative of a second level of retention of the gas species of the gas in the drilling mud upon exiting from the annulus ; and a computer system including a processor adapted to execute logic to cause the processor to, receive the sequence of signals, and to calculate and log isotopic characteristics of gas entering the drilling mud at particular locations of the well. at least one degasser adapted to extract gas from the drilling mud, wherein the at least one degasser includes a first degasser (61-2) and a second degasser (61-1), the first degasser positioned to extract gas from the drilling mud being injected into the drill string, and the second degasser (61-1) positioned to extract gas from the drilling mud coming out of the annulus and downstream of to the first degasser (61-2) at least one gas analyzer , adapted to analyze the gas and generate a sequence of signals indicative of ratios of isotopes of gas species of gas extracted from the drilling mud, wherein the at least one gas analyzer is a first gas analyzer (62-2) adapted to analyze gas extracted by the first degasser (61-2), and a second gas analyzer (62-1) adapted to analyze gas extracted from the second degasser (61-1), and wherein signals to be generated by the first gas analyzer (62-2) are indicative of a first level of retention of gas species of the gas in the drilling mud prior to reinjection of the drilling mud into the drill string, and signals to be generated by the second gas analyzer (62-1) are indicative of a second level of retention of the gas species of the gas in the drilling mud upon exiting from the annulus ; and a computer system including a processor adapted to execute logic to cause the processor to, receive the sequence of signals, and to calculate and log isotopic characteristics of gas entering the drilling mud at particular locations of the well. <EOS>