A method for calculating the volume of various predetermined organic end-members in samples of rock at various depths in oil reservoir rock is utilized to produce one or more graphic displays that are use to interpret the data to identify, e.g., tar mats, in order to improve the efficient production of hydrocarbons from the well. Data is collected from the samples by known pyrolysis and compositional modeling methods; additional data is obtained by elemental analysis to determine weight percentages of C, H, N, S and O in the selected end-members and characterization of physical properties of representative samples of the reservoir rock, e.g., from core samples; the data is then processed in accordance with the method to provide a series of data points used to produce the graphic displays for visual interpretation.