P. Craig Smalley

An improved analytic solution is presented that describes the dissipation of abnormal pressures via Darcy flow out of a compartment formed of high-permeability rock capped top and bottom by lower permeability rock. The solution tends to the solutions presented by previous workers in the limit of (1) a thick compartment and thin barrier and (2) a thick barrier and a thin compartment. The importance of including fluid compressibility when analyzing hydrocarbon reservoirs is demonstrated. Previous workers in this field have only included the effect of bulk rock compressibility and have neglected fluid compressibility. The solution is applied to pressure compartments on the scale of a typical hydrocarbon reservoir and overpressured regions on the basin scale. It is shown that the pressure gradient in a typical hydrocarbon reservoir compartment will return to the hydrostatic gradient over timescales of the order of hours or days, but that abnormal pressures are likely to dissipate over periods of tens of thousands to hundreds of thousands of years. Thus, any reservoir compartment with a different pressure from its neighbors at discovery can be considered as an independent unit during production. However, abnormal pressures on the basin scale may take tens or hundreds of millions of years to dissipate. There is thus no need to invoke zero-permeability seals or capillary-pressure seals in order to explain the existence of abnormal pressures over geological time.

Oil emplacement retarded the rate of quartz cementation in the Brae Formation deep-water sandstone reservoirs of the Miller and Kingfisher fields (United Kingdom North Sea), thus preserving porosity despite the rocks' being buried to depths of 4 km and 120°C. Quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs. Important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time (>15 m.y.). The earlier the hydrocarbon fill, the greater is the degree of porosity preservation. Failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg, potentially leading in turn to poor decisions about the need for and placement of downflank water injectors. During exploration, the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement.