Impact of Biogenic Methane Generation on Formation of Dolomite Reservoirs

Kenward, Paul¹, Luis Gonzalez¹, Robert H. Goldstein¹, Jennifer A. Roberts¹ (1) University of Kansas, Lawrence, KS

Preservation of early porosity is one way in which a dolomite hydrocarbon reservoir can be formed. In some dolomite reservoirs, protection from cementation may be the main factor in porosity preservation. Here we report on the experimental investigation of mechanisms employed by methanogenic microbes in mediating the nucleation of dolomite. Modeling of the system shows that biogenically produced CH4 exsolves in porespace, creating a two-phase system that significantly reduces permeability, protecting pore space from cementation. In batch precipitation reactors containing a mixed consortium of methanogens, microbial biomass increased from 1 x 105 to 7x 107 cells ml-1 coincident with an increase of P-CH4 from 0.025 to 0.3 after 300 days of incubation. The methanogenic population within these experimental vessels yielded approximately 46 fmoles of CH4 per cell per day. Given a trap, a dilute population of methanogens (103 cells ml-1) within a cubic meter of marine sediment at a kilometer of depth (100 bars), with 50% porosity, could generate methane at a rate of 0.4 cm3 1000yrs-1, filling half of the available pore space in less than 0.7 Ma. Shallower depths and more abundant populations of methanogens will increase the rate of methane generation considerably, up to 36cm3 1000yrs-1 at 105 cells ml-1. As, methane generating microbes have been associated with the formation of dolomite, it is possible that some porous dolomite reservoirs owe both porosity preservation and dolomite mineralogy to the metabolic activity of microbes. This possibility makes identifying the controls on distribution of biogenic dolomite an important factor in hydrocarbon exploration.