1. n. 
Water that occurs naturally within the pores of rock. Water from fluids introduced to a formation through drilling or other interference, such as mud and seawater, does not constitute formation water. Formation water, or interstitial water, might not have been the water present when the rock originally formed. In contrast, connate water is the water trapped in the pores of a rock during its formation, and may be called fossil water.
2. n. 
Water in the undisturbed zone around a borehole. The resistivity and other properties of this water are used in the interpretation of measurements made in the borehole or from the surface. Although formation water normally is the same as the geological formation water, or interstitial water, it may be different because of the influx of injection water.