Abstract
The phase or state of natural gas during migration determines how much is expelled and accumulated. Phases of natural gas migration include water-solution, oil-solution, free gas and diffusion which evolve vertically in a sedimentary stratigraphic section. Gas expulsion modelling must be based on an evaluation of the migration phase. First of all, the depth limits of each migration phase are determined, then the expelled gas quantities in each range are computed individually, and finally added together. For oil-related gas, the oil-solution phase is the most important migration phase; therefore, the expulsion modelling of oil-related gas can be treated, to a considerable degree, as the modelling of oil expulsion. A method for direct modelling of oil expulsion is presented. The concept of equilibrium concentration is particularly important for gas diffusion modelling. In this paper, methods of modelling natural gas expulsion are presented through some specific examples such as South China Sea Basin, Bohai Gulf Basin of east China and Sichuan Basin of southwest China.
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