Disclosed are methods of modelling a subsurface region. One method comprises defining a plurality of variables for describing said subsurface region, each variable having a corresponding, domain set neighbourhood set, and conflict set. The conflict set comprises conflict variables, which are previously assigned variable which reduces the domain set of its corresponding variable. The method comprises further sequentially performing for each variable: a) selecting an unassigned variable; b) assigning a value to the selected variable from its domain set; c) assessing whether a constraint is satisfied. The selected variable's corresponding conflict set is updated to include any previously assigned variables comprised within the selected variable's corresponding neighbourhood set when said constraint is assessed not to be satisfied at step c). Another method comprises foreseeing dead end variables before they are simulated.