Carbon isotope analyses of gases extracted from drilling muds while drilling in the Western Canada Sedimentary Basin (WCSB) can be used to create carbon isotopic depth profiles. These profiles provide essentially continuous data through the stratigraphic section, offering a unique opportunity to study the in-situ gases in various rock matrices. Carbon isotope and molecular compositions of Jurassic - Cretaceous mud gases have been examined from ten depth profiles in the undisturbed WCSB. The isotopic profiles are surprisingly complex, showing numerous inflections and deviations towards increasing and decreasing carbon isotope values (13C) and wetness index with depth that suggest a correlation with the stratigraphic framework and can be explained in terms of the origin and alteration of the gases. However, the gas isotope geochemistry must be incorporated and applied in a multidisciplinary approach in order to gain a better understanding of causes of variations. The discernible degree of correlativity of carbon isotope trends between the WCSB wells are likely to be related to the presence of major gas compartments bounded by stratigraphic surfaces, compartmentalization of the gas being strongly influenced by stratigraphic variations. The majority of these boundaries act as effective barriers to gas migration. Mudgas geochemistry is best employed in conjunction with petrophysical analysis and conversion into mineralogy, for defining details of transition zones and reservoir compartments. Combined evidence suggests that isotopic variability of WCSB gases is only partly induced by source maturity at one single location. The main shifts of carbon isotope ratios are likely to be related to the physical properties of the rocks, differences between organic precursors (type II versus type III kerogen), total organic carbon (TOC) content, gas biodegradation and mixing. The present thesis demonstrates that the carbon isotopic mud gas profiles represent a powerful tool that provide information about the compartmentalization of the gas, the effectiveness of low permeability barriers, the origin, alteration and maturity of gases, and the regional gas dynamics. Mudgas geochemistry proves to be one part of the puzzle in the investigation of regional gas dynamics, and should be integrated with geological information, lithostratigraphic-, and sequence stratigraphic information, petrographic information and geophysical data.

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