Kalkreuth and Dr P. K. Mukhopadhyay for vitrinite reflectancem easurementsT. he valuablet echnicala ssistance of M. Derichs. LJ. Disko. F. Leistner and A. Putral is gratefully acknowledged. This paper benefited from critical reviews by Dr R. Pelet (IFP;
Rueii-Malmaison;
France) and Dr K. Peters (Chevron Oil Field Research Company;
La Habra;
U.S.A.);