Estimating Hydrocarbon Gas Fluxes Through Sedimentary Basins with Chemical Proportions and Stable Isotopes Fractionations

Abstract

Gases in sedimentary basins have two main origins. They can derive from deeper formations (mantle, deeper crust) or be produced in s i tu via thermal cracking or biological activity. Interactions between gases and sediments result in qualitative and quantitative changes that influence the mass balance of gases in sedimentary basins and have the potential to affect our understanding of global geochemical cycles. Estimating a gas budget in sedimentary basins has two basic interests: a better control of gas accumulation, with an energetic target, and an important clue in the global carbon budget in the Earth, with an environmental target. In the majority of studied sedimentary basins, the mass of accumulated hydrocarbon gas in reservoirs is smaller by one or more order of magnitude than the estimate for hydrocarbon generated in source rocks. Causes for this poor yield may result from various processes taking place during genesis, expulsion, migration, but it is more likely the result of alteration and leakage from the sedimentary rocks. As gas caprocks are never absolute seals at geological time scales, leakage from reservoirs may be the main explanation for the poor yield of gas accumulation. Leakage may have considerable weight on the proportion of reduced carbon from sedimentary basins oxidized to CO2 and recycled back to the atmosphere. A natural gas reservoir has then to be seen as an accumulation due a delay during gas transfer from its source to the atmosphere, rather than an absolute trap, not allowing the gas molecules to go further up.