Novel C26 tetracyclic degraded bicadinanes in oil sands from the Haitoubei–Maichen Sag of the Beibuwan Basin, South China Sea

Three C26 tetracyclic terpanes were detected in the saturated hydrocarbon fractions of oil sand samples especially rich in bicadinanes W, T and R from the Oligocene Huizhou Formation in the XW-1 well in the Haitoubei–Maichen Sag of the Beibuwan Basin. Their mass spectra show molecular ions of the three tetracyclic terpanes at m/z 358, and their key fragment ions at m/z 343 (M+-15) and m/z 315 due to loss of an isopropyl group. These characteristics are very similar to that of C30 cis–cis–trans (W) and trans–trans–trans (T) bicadinanes. In addition, the relative abundances of the C26 tetracyclic terpanes show an excellent positive correlation with the C30 bicadinanes W and T in 27 oil sand samples. Therefore, the three C26 tetracyclic terpanes are tentatively identified as degraded bicadinanes, which like bicadinanes, may indicate input from dammar resins and can be used to discriminate the origin of crude oils.