1. A method of interpreting well log data indicative of physical attributes of a portion of a subterranean formation comprising: partitioning the well log data into segments, each segment representing a respective contiguous portion of the extent of the logged well; defining a membership function for each segment, the membership function defining a degree to which each segment belongs to a defined set; and determining, based on attribute values derived from previously interpreted depositional units, and the membership functions, a depositional type for each segment.