1. A method of estimating lithologic properties of a geological zone simulated by a computer, the method comprising: a) representing the geological zone using a grid having nodes; b) representing a lithologic property with the help of a categorical variable; and for each node in a set of target nodes of the grid to be scanned iteratively: c) estimating for each possible state of the categorical variable a value for a conditional probability of observing said state for the target node knowing that a respective state has been allocated to a node having an influence on the state of the categorical variable of the target node, on the basis of: univariate probabilities of observing a state of the categorical variable for the target node; and bivariate probabilities of observing two states of the categorical variable for two respective nodes of the grid; wherein the bivariate probability values that are used in estimating the conditional probability relate to the target node and to the node having an influence on the state of the categorical variable of the target node; so that for each possible state of the categorical variable, the corresponding conditional probability is as follows: p k 0 / k 1 , … ⁢ , k n = p k 0 ( 1 - n ) ⁢ ∏ i = 1 n ⁢ p k 0 , k i ∑ k 0 ′ = 1 n c ⁢ p k 0 ′ ( 1 - n ) ⁢ ∏ i = 1 n ⁢ p k 0 ′ , k i where: k 0 is an index of the possible state c k0 for the target node x 0 ; k 1 ,. .. , k n are the indices of the states c k1 ,. .. , c kn allocated to n the node having an influence on the state of the categorical variable of the target node, n being greater than or equal to 1; n c is the number of possible states of the categorical variable, n c being strictly greater than 1; p k 0 is the univariate probability for a node having the state c k0 ; p k′ 0 is the univariate probability for a node having the state c k′ ; p k 0 ,k i is the bivariate probability of observing the states c k 0 and c k i for the target node x 0 and a node x i having an influence on the state of the categorical variable of the target node target node x 0 ; and p k′ 0 ,k i is the bivariate probability of observing the states c k′ 0 and c k i for the target node x 0 and the node x i having an influence on the state of the categorical variable of the target node x 0 ; d) making a random draw from among the states of the variable, the draw being weighted by the conditional probabilities estimated in step c); and e) allocating the state drawn randomly in step d) to the target node.