A large amount of deep oil has been discovered in the Tazhong Uplift, Tarim Basin whereas the oil source is still controversial. An integrated geochemical approach was utilized to unravel the characteristics, origin and alteration of the deep oils. This study showed that the Lower Cambrian oil from well ZS1C (1x) was featured by small or trace amounts of biomarkers, unusually high concentration of dibenzothiophenes (DBTs), high δ34S of DBTs and high δ13C value of n-alkanes. These suggest a close genetic relationship with the Cambrian source rocks and TSR alteration. On the contrary, the Middle Cambrian oils from well ZS1 (2a) were characterized by low δ13C of n-alkanes and relatively high δ34S of individual sulfur compounds and a general "V" shape of steranes, indicating a good genetic affinity with the Middle-Upper Ordovician source rocks. The middle Cambrian salt rock separating the oils was suggested to be one of the factors responsible for the differentiation. It was suggested that most of the deep oils in the Tazhong Uplift were mixed source based on biomarkers and carbon isotope, which contain TSR altered oil in varied degree. The percentage of the oils contributed by the Cambrian-Lower Ordovician was in the range of 19-100% (average 57%) controlled by several geological and geochemical events. Significant variations in the δ34S values for individual compounds in the oils were observed suggesting a combination of different extent of TSR and thermal maturation alterations. The unusually high DBTs concentrations in the Tazhong-4 oilfield suggested as a result of mixing with the ZS1C oil (1x) and Lower Ordovician oils based on δ34S values of DBT. This study will enhance our understanding of both deep and shallow oil sources in the Tazhong Uplift and clarify the formation mechanisms of the unusually high DBTs oils in the region.
Bibliographical noteFunding Information:
This study was funded by the Natural Science Funding Council of China (Grant Nos. #41473047 , #41173061 ), the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Nos. PRP/indep-1-1101 and PRP2009-02 ), Natural Science Research Council of China (China 973 National Key Research and Development Program 2011CB201102 ) and the Specialized Research Fund for the Doctoral Program of Higher Education ( 20120007110002 ). Alon Amrani thanks the Israeli Science Foundation (ISF) Grant Number 1269/12 for partial support of this study. The authors thank Zhao Hong, Chen Zulin and Mao Zhichao for excellent experimental assistance, and Lu Yuhong for help with sample collections. We are grateful to Prof. Chen Jianfa for allowance of publication of the carbon isotope of the natural gas from Well ZS1C. Dr. L.R. Snowdon, the Editor-in Chief and Dr. Kenneth Peters, the Associate Editor, are particularly acknowledged for the primary review of this manuscript. Dr. Luc Fusetti and Dr. Jianping Chen are sincerely acknowledged for helpful comments on an earlier version of this manuscript.
© 2014 Elsevier Ltd.
- Compound specific carbon/sulfur isotope
- Mixed source oil
- Quantitative assessment