Novel tetracyclic terpanes in crude oils and source rock extracts in pearl river mouth basin and their geological significance

Novel tetracyclic terpanes X and Y namely C₂₄-des-A-oleanane and C₂₇ tetracyclic terpane were detected in crude oils and source rock extracts in Zhu 1 depression in Pearl River Mouth Basin by GC-MS analysis technology. These compounds are similar to oleanane in the structure, and their relative abundance in m/z 191 partial mass chromatogram has a good correlation with oleanane. Here compounds X and Y are considered to be derived from the des-A degradation of oleanoid precursor. The ratio of X/(X+C₂₄) and Y/(Y+C₂₄) increase with the increasing inputs of terrigenous organic matter in crude oils. Wenchang Formation middle-deep lacustrine source rocks with planktonic algae organic matter inputs and oil generating from it contain low abundance of compounds X and Y, while Enping Formation coal measures source rocks with terrigenous higher plants organic matter inputs and oil generating from it usually have higher compounds X and Y. In general when two terrigenous compounds C₁₉ tricyclic terpane and bicadinane-T are high in crude oils, there are also a corresponding high abundant compounds X and Y. Relative abundance of compounds X and Y is closely related to the source of organic matter and it can indicate the input of terrigenous organic matter.