地质科技通报 (Jan 2024)
Geochemical characteristics and geological significance of Sinian crude oil from Well Tashen 5, Tahe Oilfield, Tarim Basin
Abstract
Objective The Well Tashen 5 that was completed in 2021 has a burial depth of 9 017 m and is currently the deepest drilling well in Asia. It is also the first hydrocarbon breakthrough in the dolomite sectionof Upper Sinian at a depth of 8 780-8 840 m in the Tarim Basin. This is a major discovery in the field of ultradeep dolomite after the breakthrough of the Cambrian subsalt from the Well Luntan 1. Methods To better understand the hydrocarbon generation process, the organic geochemical characteristics of the Sinian condensate and the Upper Ordovician heavy oil in the Well Tashen 5 and its neighboring Cambrian volatile oil and Ordovician condensate were compared. Results The n-alkane series of both the condensate and the chloroform extracted from dolomite are well distributed in the Sinian of the Well Tashen 5, indicating the domination of anoxic environment during the deposition of organic matter. Both biomarker compounds and carbon isotope distributions in the Sinian hydrocarbons of the Well Tashen 5 are similar to those of the Ordovician normal oils-ultraheavy oils and condensate oils of the Yakla region, implying the same hydrocarbon source between the Well Tashen 5 and the Yakla region. Equivalent vitrinite reflectance (Rc) in the Sinian hydrocarbons of the Well Tashen 5 is 1.7%, which is higher than the Rc of the Upper Ordovician heavy oil and Cambrian light oil but is comparable with condensate of nearby regions and belongs to the high maturity stage.It is reasonable to suggest that, the Sinian hydrocarbons of the Well Tashen 5 were mainly sourced from the overlying Lower Sinian Yurtus Formation that also was the seal rock layer. The high-maturity condensate was generated by the Yurtus Formation in the depression area during the Late Himalayan Period and was transported into the structure traps rich in Upper Sinian dolomites along the slope. Conclusion The breakthrough of Sinian hydrocarbons in the Well Tashen 5 has discovered a new hydrocarbon-bearing layer, displayed the process of ultradeep hydrocarbon formation, and indicated a new exploration direction for ultradeep exploration in the Tarim Basin.
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