1. A computer-implemented method for modeling reservoir property trends, the method comprising: storing in a computer storage device, well data, sampled from a portion of a reservoir volume of interest and corresponding to a reservoir property to be modeled, and a training image that describes expected trends of the reservoir property; utilizing a computer system having one or more processors configured to communicate with the computer storage device and execute one or more computer programs configured for performing operations comprising: discretizing a range of values of the reservoir property into at least two categories; discretizing the well data into the categories to create categorical well data, and discretizing the training image into the categories to create a categorical training image; applying a multiple-point statistics simulation using the categorical training image on a stratigraphic grid of nodes discretizing the reservoir volume to create a categorical property model conditioned to the categorical well data; creating for each category in the categorical property model, a sub-region comprising the nodes of the stratigraphic grid where that category has been simulated in the categorical property model; and applying variogram-based simulation in each sub-region to create a reservoir property model that reproduces the category histogram of each associated sub-region and models the reservoir property trends.