Implications of geothermal history on microbial gas generation and coalbed methane production in the Ninilchik Field, Cook Inlet basin, Alaska
Master of Science (MS)
Geology and Geophysics
Coalbed methane (CBM) production operations atop the Ninilchik anticline of Cook Inlet, Alaska pose intriguing questions regarding the nature of biogenic gas generation in a thermally complex forearc basin setting. This thermal complexity stems, in part, from its up to 30,000 ft thick Tertiary section, comprised of braided stream alluvium and expansive, interbedded coal deposits. Rapid accumulation of cold, glacial strata is documented as being capable of regionally suppressing the elevated heat flux values expected within a forearc basin. In addition, beds with high organic content, such as the coal layers, act as natural insulators from basinal heat flux and possibly cause a further increase to the thermal gradient (i.e. subsurface temperatures may be encountered at comparatively deeper depths relative to a basin with a more thermally conductive lithology). According to Head et al. (2003), 80 °C is the generally accepted maximum temperature that methanogenic bacteria can withstand before being pasteurized inside CBM reservoirs; therefore, the basin’s geothermal history should play a pivotal role in determining which formations have sourced the highest amounts of methane and on what timescale this generation occurred. To investigate these questions, Ninilchik field’s thermal and geohistories were calculated using the PetroMod® basin modeling software package to measure the changes in each layers’ temperature with respect to time and depth. From these results, a sensitivity analysis of the controlling factors for biogenic gas generation (methanogenesis) was conducted to test the hypothesis that rapid Tertiary sedimentation has outpaced the basinal heat flux and is in fact the controlling variable for Ninilchik’s natural gas potential. Results show that regionally, within the deepest parts of the basin that exhibit the highest sedimentation rates, this hypothesis is accurate. Locally, however, it is found that relative anticlinal uplift decreases both subsidence and sedimentation rates, and holds the coal-bearing layers within the optimal temperature regime for extended periods of time. While it is previously understood that the anticlines create vast closures for hydrocarbon accumulation, it is concluded that, above all, relative anticlinal uplift gives rise to the most ideal reservoir conditions and instigates maximum methanogenesis within the Cook Inlet basin.
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Hart, Eric M., "Implications of geothermal history on microbial gas generation and coalbed methane production in the Ninilchik Field, Cook Inlet basin, Alaska" (2012). LSU Master's Theses. 2012.
Nunn, Jeffrey A