The distribution and carbon isotopic composition of unusual polycyclic alkanes in the Cretaceous Lengshuiwu Formation, China

In this paper, several series of unusual polycyclic alkanes have been identified in Cretaceous mudstone extracts and the bitumen of sandstones from the Geyang Basin, Jiangxi Province, Peopleʹs Republic of China. The mass spectra of these compounds show that they are tricyclic to octacyclic alkanes. The hexacyclic and heptacyclic alkanes are the more abundant and have average δ13C values of −34.8‰ and −36.2‰, respectively, which are close to those of most of the hopanoids (−35.6‰) indicating that they may be derived from similar microbial precursors. In contrast, the δ13C values of pristane and phytane are isotopically heavier (about −28.7‰ and −29.6‰, respectively). The high ratios of gammacerane to C30 hopane (0.2–0.5) suggest deposition in a brackish lacustrine environment. The existence and distribution patterns of these compounds in the rock extracts and the bitumen are of great importance for oil–source correlation in this area.