An estimate or measurement of organic matter density is required for converting between the weight percent of total organic carbon (TOC) and the volume percent of organic matter for wireline log calibration; it is therefore important to recognize when significant changes in organic matter density occur. A method is presented for calculating organic matter density from measurements of crushed-rock dry grain density and Soxhlet-extracted TOC. I have investigated the thermal evolution of organic matter by tracking changes in the intrinsic density of organic matter as a function of thermal maturity. Organic matter density shows two step increases that correspond to the generation of liquid hydrocarbons in the oil window (up to similar to 1.2% vitrinite reflectance [R-o]) and the conversion of organic matter to graphitelike carbon (more correctly, "turbostratic carbon") at high thermal maturity (>4% R-o). Profound structural changes of organic matter may, in part, determine the maturity limits of source-rock tight liquids and shale-gas plays, particularly at high thermal maturity, where gas is hosted within the organic matter hosted pore system.