The significance of 24-norcholestanes, triaromatic steroids and dinosteroids in oils and Cambrian–Ordovician source rocks from the cratonic region of the Tarim Basin, NW China

• Published in: Applied geochemistry 2012-08, Vol.27 (8), p.1643-1654
• Main Authors: Li, Meijun, Wang, T.-G., Lillis, Paul G., Wang, Chunjiang, Shi, Shengbao
• Format: Article
• Language: English
• Subjects: Bacillariophyceae; biomarkers; Dinophyceae; early development; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Geochemistry; oils; petroleum; phytoplankton; Pollution, environment geology; rocks; sediments; steroids
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2012.03.006

► Ten source rocks and 42 oils from Tarim Basin are analyzed for steroids investigation. ► Cambrian rocks and oils have abnormally high contents of dinosteranes, norcholestanes. ► Two oil systems can be distinguished on the basis of less common steroids in Tarim. ► Oils with Cambrian origin were further discovered and may have promising prospect. ► Dinoflagellates might have flourished in Tarim Basin during the Cambrian. Two oil families in Ordovician reservoirs from the cratonic region of the Tarim Basin are distinguished by the distribution of regular steranes, triaromatic steroids, norcholestanes and dinosteroids. Oils with relatively lower contents of C28 regular steranes, C26 20S, C26 20R+C27 20S and C27 20R regular triaromatic steroids, dinosteranes, 24-norcholestanes and triaromatic dinosteroids originated from Middle–Upper Ordovician source rocks. In contrast, oils with abnormally high abundances of the above compounds are derived from Cambrian and Lower Ordovician source rocks. Only a few oils have previously been reported to be of Cambrian and Lower Ordovician origin, especially in the east region of the Tarim Basin. This study further reports the discovery of oil accumulations of Cambrian and Lower Ordovician origin in the Tabei and Tazhong Uplifts, which indicates a potential for further discoveries involving Cambrian and Lower Ordovician sourced oils in the Tarim Basin. Dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24-norcholestanes for dinoflagellates and diatoms. Therefore, the abnormally high abundance of these compounds in extracts from the organic-rich sediments in the Cambrian and Lower Ordovician and related oils in the cratonic region of the Tarim Basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the Tarim Basin during the Cambrian Period. Steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms, as well as define petroleum systems.