1. A computer-implemented method of interpreting well log data indicative of physical attributes of a portion of a subterranean formation comprising: partitioning, via computer, the well log data into i segments, each ith segment representing a respective contiguous portion of the extent of the logged well; defining, via computer, a membership function for each segment, the membership function defining a degree to which each segment belongs to a defined set; and determining, via computer, based on attribute values derived from previously interpreted depositional units, and the membership functions, a depositional type for each segment, converting, via computer, the well log data to shale volume data, wherein the shale volume data are represented as a series of numerical values V sh =s 1 , s 2 ,. .. , s i , where s i is an average of the shale volume data within the ith segment.