The MGIL technique delivers detailed downhole isotopic logging that provides geoscientists with unprecedented geochemical perspective on geological environments and information about hydrocarbon sources and accumulation history. Stable carbon isotopes provide critical additional data for identifying exploration targets, selecting frac and completion zones, monitoring production allocation to optimize well performance, and determining maturity and type of hydrocarbon kitchen. For environmental monitoring, MGIL helps identify possible leakage zones, classify gas in aquifers, and provide preservation of evidence. The technique is inexpensive, easily accommodated on drilling rigs, and samples the same gas stream used for conventional mud gas analysis using existing mudlogging equipment. Gas samples are collected using IsoTubes or other gas-tight vessels from the mudlogging gas flow line. MGIL maximizes information obtained from each well and assists in developing accurate exploration and production prognoses by correlating isotope-geochemical data with productive well data, making it invaluable for both conventional wells and unconventional plays like shale gas.