The Distribution of Triaromatic Steroids and Oil Group Classification of Ordovician Petroleum Systems in the Cratonic Region of the Tarim Basin, NW China

A practical method to estimate the abundance of each C-26-C-28 triaromatic steroid isomer was proposed and a triangular diagram can therefore be drawn to distinguish oil populations in the cratonic region of the Tarim Basin, NW China. The results indicate that oils discovered in this region can be divided into two petroleum systems. Oils derived from the Cambrian and Lower Ordovician source rocks show relatively high abundance of C-27 triaromatic steroids with the relative abundance of C-27 TAS being more than 25% of all TAS isomers. In contrast, the values are below 25% in oils and source rocks from the Middle-Upper Ordovician. The petroleum system definition is in agreement with previous studies using regular steranes, tricyclic terpanes, norcholestanes, and other molecular markers to distinguish oil groups. In addition, high thermal stability and greater resistance to biodegradation make triaromatic steroids more effective in distinguishing oil groups in the tectonic region of the Tarim Basin.