1. A method for estimating the type of hydrocarbons in a formation surrounding a borehole drilled with an oil-based mud, comprising: (a) providing a mathematical model for a fluid mixture of native hydrocarbons and oil-based mud in the formation; (b) providing NMR signals from one or more locations in the formation; and (c) estimating the type of native hydrocarbons in said one or more locations in the formation based on the provided mathematical model and the provided NMR signals.