Comprehensive two-dimensional gas chromatography (GC x GC) coupled to time-of-flight mass spectrometry (TOFMS) is currently one of the most powerful analytical tools that can separate mixtures of organic matter at a high resolution. Here we apply this method to study biodegraded oils from China, including the identification of individual compounds that cannot be clearly separated using conventional one-dimensional gas chromatography with mass spectrometry (GC-MS). A C-27 tetracyclic terpane (i.e., C-27-17, 21-secohopane) is particularly prominent in crude oils, and this study tentatively addresses the distribution and geological and geochemical significance of this compound. The analysis of crude oil from Well Y1900, in the Biyang Depression of eastern China, indicates that numerous long-chain tricyclic terpanes can be detected between C-19 and C-25 and high abundances of 25-norhopane series compounds, ranging from C-26 to C-31. These unknown terpanes labeled X-1, X-2 and X-3 can also be separately identified. Compared to X-1, X-2 and X-3 are in relatively low concentrations. As such, this X1 compound was discussed in focus. It is tentatively identified as a C-27 tetracyclic terpane or C-27-17, 21-secohopane from its mass. The origin of this compound was tentatively identified by comparison with oils from other areas in China, and its abundance is likely to relate to organic input and the paleoenvironment of formation. The concentration of this element also correlates well with the level of biodegradation. This suggests that this X-1 compound is a new geochemical compound that can be used to identify crude oils with marine/lacustrine origins and to determine the biodegradation degree of crude oils and possibly even organic matter in source rocks. (C) 2014 Elsevier Ltd. All rights reserved.