I don't have a reference for a curve, but I do have one for a single point:
Table 6A-1. Composition of Crude Oil Carried by EPC Pipeline (p. 5 of 9, about 14 rows down)
http://www.epa.gov/region6/6en/xp/lppapp6a.pdf
You'll have to dig around for or contact EPA to determine what the actual...