A method of interpreting mud gas samples in a drilling operation of a target area. The method starts with obtaining a plurality of mud gas samples at a plurality of incremental depths of the target area. The plurality of mud gas samples are analyzed to find various isotopic data at each depth. The isotopic data of the mud gas samples are plotting on several charts to provide a visual representation of the isotopic data trends. The charts may include: depth versus ethane/methane; depth versus .delta..sup.13C per mil; and depth versus .delta..sup.13C and gas show. In addition, the percentage methane versus .delta..sup.13C is provided on a separate chart. This percentage methane versus .delta..sup.13C chart provides engineering and geological information such as the location of lithological hydrocarbon seals, barriers, and zones of good hydrocarbon communication (e.g., compartments). This engineering and geological information is then correlated with the other charts.