1. n. [Geology]

A body of rock whose fluid saturation, porosity and permeability permit production of groundwater.

2. n. [Geology]

A water-bearing portion of a petroleum reservoir with a waterdrive.

3. n. [Drilling]

Any water-bearing formation encountered while drilling. Drillers often are concerned about aquifers and are required to take special precautions in the design and execution of the well plan to protect fresh water aquifers from contamination by wellbore fluids. Water in aquifers can flow into the wellbore, contaminate drilling fluids and cause well control problems.

See related terms: brine,  surface casing