Impact of Subsidence and Thermal History on Microbial Gas Generation in Ninilchik Field, Cook Inlet, Alaska

Hart, Eric; Nunn, Jeffrey

Cook Inlet, Alaska is a complex forearc basin setting which has produced prolific amounts of biogenic gas primarily from anticline structures such as the Ninilchik Anticline. Biogenic gas generation, like thermogenic gas, is controlled in part by the thermal history of organic rich sediments. Biogenic gas in the region is generated primarily by mesophilic methanogens as the coal layers pass through their habitable temperature range of 10-50 °C. Rapid accumulation of up to 30,000 ft of Tertiary sediments in Cook Inlet, comprised of braided stream alluvium and interbedded coal deposits suppresses the temperature gradient. This allows methanogenic bacteria to thrive at greater depths (4500 to 5000 ft) than in regions with higher geothermal gradients. Using well data from Ninilchik field to determine depth to tops of various formations and the number and thickness of interbedded coal layers, PetroMod basin modeling software is used to estimate temperature as a function of depth and time. These results are combined with a nonlinear relationship between temperature and methanogenic bacteria activity level to estimate timing and magnitude of biogenic gas generation from each identified coal layer. Model results show that syndepositional formation of anticlinal structures decreases subsidence on the anticlinal structures which holds coal-bearing layers within the optimal temperature regime for methanogenesis for almost 12 m.y. compared to 6 m.y. for an off structure well. While it was previously recognized that anticlines create closures for hydrocarbon accumulation, formation of anticlines synchronous with sedimentation gives rise to ideal temperature conditions and instigates maximum methanogenesis within the Cook Inlet basin. Model results also show that biogenic gas generation has occurred at a nearly constant rate over the last 15 Ma due to the even vertical distribution of coal layers. Within the Ninilchik field, variations in gas production may also be impacted by the amount of coal and size of the structural closure but time spent in the optimum temperature range seems to be the primary control. Mass balance calculations indicate that methanogenesis of less than 1% of the coal present is necessary to generate the observed gas production. Thus, Ninilchik field is a good candidate for enhanced coal bed methane by injection of fresh water and/or nutrients.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013