Model for how microbial methane generation can preserve early porosity in dolomite and limestone reservoirs

In some dolomite and limestone hydrocarbon reservoirs, protection from cementation is a primary factor in porosity preservation. We present a model in which methanogens (methane-producing microorganisms) produce methane gas (CH4[g]) that outgasses from solution in pore space, creating a two-phase system that reduces effective hydraulic conductivity (K), protecting pore space from cementation. Methanogens have been implicated in dolomite formation and can generate methane to fill pore space of a model near-surface carbonate sediment (37.5% primary porosity) with CH4(g) in 180 to 4650 yr, depending on nutrient levels. Gas generation results in occlusion of the aqueous phase from pore spaces and throats, creating a two-phase flow regime, reducing the effective hydraulic conductivity of model marine carbonate sand by about 50% (from 2.8 to 1.2 cm/day) in as little as 55 yr, therefore reducing aqueous fluid flow necessary for cementation. Despite rapid burial and complete cessation of methanogenesis, effective hydraulic conductivity could take more than 100,000 yr to return to its original value. If methanogenesis continues with burial, the effective hydraulic conductivity is reduced to zero (after 250 yr). Dolomites can preserve more primary porosity with depth than other carbonates. We propose that, in addition to increased structural resistance, a biogenic model exists for porosity preservation in dolomites that is linked to the activity of methanogens. This model represents specific end-member cases and illustrates the effect of methane buildup in relationship to the extent of reservoir diagenesis.