The portion of [[formation]] surrounding a well bore into which drilling fluid has penetrated. displacing some of the formation fluids. This invasion takes place in porous, permeable zones when the pressure of the [[mud]] is greater than that of the formation fluids. A mud [[mud cake|filter cake]] builds on the formation wall, limiting further invasion into the formation by [[mud filtrate]]. Directly behind the [[mud cake]] is a [[flushed zone]] from which almost all of the [[formation water]] and most of the hydrocarbons have been displaced by filtrate. The invasion process alters the distribution of [[saturation|saturations]] and other properties and, consequently, alters the values which are recorded on logs. The [[depth of invasion]] is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate. In oil-bearing zones, the filtrate may push a bank of formation water ahead of it to produce what is referred to as an [[annulus]].

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