Origin and quantitative source assessment of deep oils in the Tazhong Uplift, Tarim Basin

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 (is an element of(1x)) was featured by small or trace amounts of biomarkers, unusually high concentration of dibenzothiophenes (DBTs), high delta S-34 of DBTs and high delta C-13 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 (is an element of(2a)) were characterized by low delta C-13 of n-alkanes and relatively high delta S-34 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 delta S-34 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 (is an element of(1x)) and Lower Ordovician oils based on delta S-34 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. (C) 2014 Elsevier Ltd. All rights reserved.