Development of Abnormal and Subnormal Pressures in Reservoirs Containing Bacterially Generated Gas

Abstract

Burial of reservoirs containing a bacterially generated gas phase may lead to pressures that are above, equal to, or below hydrostatic. The pressures that develop depend on the way in which the gas-bearing volume is connected to, or isolated from, its surroundings. Where a noncompacting volume is totally isolated, subnormal pressures will develop. This approximates the situation in chalks, which have high porosities but low permeabilities and often contain underpressured gas. Low pressures develop because the thermal pressuring of the gas with depth occurs at a lower rate than the increase in hydrostatic pressure and because the gas dissolves in the water. When the gas-bearing volume is freely connected to surrounding, normally pressured sediments, the gas pressures remai hydrostatic, but the volume of gas steadily diminishes as it dissolves in the pore water. If gas occupies 40% of the pore volume at 1,000 ft (305 m), it will all be in solution by 4,600 ft (1,402 m) and a gas phase will no longer exist. Water must flow in to replace the dissolving gas or the rock must compact. This is the situation for normally compacting shales. When the gas is in an isolated but variable volume system (such as a compacting shale), pressure rises above hydrostatic due to sediment loading. This mechanism likely has produced many of the shallow sands charged with high-pressure gas that are a drilling hazard in many offshore areas.