Review of the Stochastic Nature of Reservoirs

A reservoir in the subsurface is in itself intrinsically deterministic. It is there, it has potentially measurable, deterministic properties and features at all scales (if we could only excavate every part of it) and it is the end product of many complex processes (sedimentation, erosion, burial, compaction, diagenesis, ...) which operated over millions of years to form it. So why do we still speak of the stochastic nature of reservoirs if they are all actually deterministic? The word stochastic has its origin in the Greek adjective stochastikos which means skilful at aiming or guessing. As we shall see later, reservoir description ultimately is a combination of observations (the deterministic component) and formalized, educated aiming (geology, sedimentology) or guessing (the stochastic component). Today we think of a stochastic phenomenon or a stochastic parameter as something which is characterized by the property that its observation under a given set of circumstances does not always lead to the same observed outcome (so that there is no deterministic regularity) but rather to different outcomes in such a way that there is statistical regularity.