Model for How Microbial Methane Generation can Preserve Early Porosity in Dolomite and Limestone Reservoirs Geohorizons

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• November 20, 2018
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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 (CH 4 [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 CH 4 (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 …