Distribution Patterns of Pentacyclic Triterpenoid Hydrocarbons in Low‐Mature Source Rocks from Eastern China

Abstract

Abstract: Four typical distribution patterns of pentacyclic triterpenoid hydrocarbons (types A‐D) are distinguished in the low‐mature source rocks from eastern China. Type A has a relatively high content of pentacyclic triterpenes. It exists in immature sediments and the distribution and abundance of triterpenes vary with the maturity of the sediments. An unknown C30 triterpene (UCT2) has also been detected in very shallow sediments. This compound is very unstable and disappears rapidly with the increase of depth. Type B is characterized by a relatively high amount of 17α(H), 21β(H)‐30‐homohopane. This kind of distribution pattern is common in coals and terrestrial sediments of low maturity. Type C has a relatively high content of diahopane and neohopane series. The analysis shows that this distribution pattern may have an indirect relationship with the input of higher plants despite its microbial source. There are C30—unconfirmed triterpane (UCT2) and a relatively high content of C35 hopane in type D. The distribution patterns of pentacyclic triterpenoids are varied under influences of biological sources, diagentic and sedimentary environments, thermal maturity etc. Therefore, they can provide useful information about the source rocks. Based on geochemical characteristics of the source rocks, the authors postulated that the type B and type C patterns are mainly related to suboxic‐anoxic environments. Type D indicates the reducing to anoxic environment, in which C30‐UCT3 may be related to some specific biological source in saline environments. The precursors and thermal evolution of some triterpenoid hydrocarbons are also discussed herein.