Le, H.N., Phi, N.D., Nguyen, T.B.H., Nguyen, T.T., Ha, T.H., Nguyen, T.Q. and Pham, T.D.H. 2015. Geochemical characteristics of oligocene coal and coaly mudstones in Blocks 06, 07 and 12, Nam Con Son basin. Petrovietnam Journal. 11, (Nov. 2015), 20 - 29.