Propensity and Probability in Depositional Facies Analysis and Modeling

Abstract

Probabilistic models of geologic phenomena have often been criticized for lack of realism because they do not resemble the “picture” of geology in geoscientists’ minds. A geologic process is a single case, while conventional probabilistic methods assume a certain frequency of events. This is called “the problem of application” of the probability theory to physical sciences by Reichenbach. Propensity concept was proposed by Popper in the late 1950s, as an alternative interpretation of probability, to solve this problem with an emphasis of single cases in quantum physics. Similarly, a reservoir or a geologic process is a single case as there are no two identical reservoirs or processes in nature. The peculiarity of a geologic process as a single case often causes controversies regarding how the probability theory should be applied to geosciences. In this paper, Popper’s propensity concept is introduced to describe geologic depositional environments and facies sequences. As a matter of fact, a geologic process is simultaneously indeterministic and causal. Such a phenomenon has been shown to be well described by the propensity concept in other scientific domains and social applications. Representation of a depositional-facies conceptual or sedimentological model or facies sequences using propensity allows quantitative integration of descriptive geology with facies frequency data at wells. Such integration can produce facies probabilities that convey the descriptive geology. Subsequently, these facies probabilities can be used to constrain stochastic modeling and make a geologic model more realistic.