Hydrocarbon Reservoirs

Abstract

A “reservoir” is defined as an accumulation of oil and/or gas in a porous and permeable rock — almost invariably of sedimentary origin. The conditions necessary (but not always sufficient) for the formation of such an accumulation are:

• 1. The presence in the sedimentary basin — in which the hydrocarbon accumulation is to form — of source rock;⁵ namely, rocks (generally shales) highly charged with organic material of vegetable and animal origins.

• 2. The transformation of this organic material into hydrocarbons.⁵ This occurs on a geological time scale of hundreds of thousands, or millions, of years, through the combined action of temperature, pressure, and the catalytic effects of certain components of the source rock, which cause the breaking down, or cracking, of the large organic molecules.

• 3. The migration of the hydrocarbons² out of the source rock, which is usually of extremely low permeability, into adjacent permeable strata (primary migration), and from here along permeable paths towards geological structures which might form suitable traps (secondary migration).

• 4. The presence, not too distant from the source rock, of a geological structure or rock configuration which is favourable to the entrapment and accumulation of the migrating hydrocarbons^1,4 (the trap).

• 5. A continuous impermeable stratum (cap rock) or other sealing structure, which prevents the further migration of the hydrocarbons into the overlying rock,^1,4 and their eventual seepage to surface.