AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Advances in modeling biogenic gas generation, expulsion and migration.

Abstract

Biogenic gases represent more than twenty percent (20%) of the know gas reserves in the world, and some accumulations are very big (286 TCF, Urengoy). They are found everywhere, in conventional and unconventional areas, in shallow basins, but also in reservoirs as old as Cretaceous and deeply buried. Previous approaches that consisted to consider a first-order kinetic with low activation energies to mimic a maximum gas generation in a temperature window of 35-40C have proven there worthlessness as they almost predict the same whatever the geological conditions are. We developed a methodology to assess biogenic gas potential from Basin and Petroleum System Modeling simulators. It takes temperatures, pressures and porosity calculated through time by the Petroleum system modeling software (2D/3D), in addition to TOC and HI maps to calculate the amount of the biogenic gas generated and expelled. It also performs gas migration coupled with a thermogenic system. Here we refer to biogenic gas, the natural gas formed in sediments as a direct result of microbial activity. Since there is organic matter, biogenic gas is produced in a temperature window of 10-80C. We have assumed that the acetate fermentation was the main process to generate methane from organic matter. We used the CO2 kinetic defined by Deniau et al., 2005 to model this fermentation. Wellsbury et al., 1997 demonstrated that by heating surface coastal sediments with organic matter to reproduce the sediment burial increased by a factor of 1000 the concentration of acetate and also increased the microbial activity. This was confirmed by measurements on deep sediments. From these experiments, a microbial efficiency curve to generate acetate versus temperature was modified according to Zeikus & Winfrey, 1976 and is used in the model. The optimum temperature is around 35-40C. By calculating the acetate generation and its fermentation we get the generated biogenic gas. We have considered three mechanisms of retention that act sequentially. The first generated gas is adsorbed onto the kerogen internal surface. We used the Pepper and Corvi, 1995 equation that depends of pressure, temperature and the remaining TOC. The generated gas that is not adsorbed is dissolved into the water. We used the Yamamoto, 1976 equation that gives a maximum solubility that increases with depth and that essentially depends on pressure. The generated gas that has not been adsorbed nor dissolved is absorbed into the kerogen microporosity of the source rock that was fixed to a saturation threshold. Only the generated gas that was not adsorbed onto the kerogen internal surface or dissolved into the water or absorbed into the kerogen microporosity is expelled. We clearly see that the basin history of temperature, pressure, sedimentation rate and porosity have to be calculated through time and in a coupled manner to assess how much biogenic gas is generated and more importantly expelled from the source rock. This technique has been applied in real case studies which results will be presented.