Non-cracked oil in ultra-deep high-temperature reservoirs in the Tarim basin, China. (January 2018)

• Record Type: Journal Article
• Authors: Zhu, Guangyou; Milkov, Alexei V.; Chen, Feiran; Weng, Na; Zhang, Zhiyao; Yang, Haijun; Liu, Keyu; Zhu, Yongfeng
• Is Part Of: Marine and petroleum geology. Volume 89:Part 2(2018)
• Page Start: 252
• Page End: 262
• Publication Date: 2018-01
• Subjects: Ultra-deep reservoirs -- Oil -- Adamantane -- Tarim basin; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2017.07.019
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Thermal stability of liquid petroleum in the subsurface is closely linked to reservoir temperature. Most oil accumulations occur at temperatures <120 °C. Oil cracks into gas at temperatures >150–160 °C leading to the dominance of gas condensate and free gas accumulations in ultra-deep high-temperature reservoirs. The recently drilled Fuyuan-1 exploration well (northern Tarim basin) produced high-quality non-cracked single-phase (black) oil from a carbonate reservoir located at maximum depth 7711 m and temperature 172 °C. This is the deepest oil discovery in China to date and among the deepest in the world. The oil density (0.825 g/cm 3 at 20 °C or API gravity 40°), relatively low gas/oil ratio (135 m 3 /m 3 or 758 scf/bbl), low variety and abundance of adamantanes as well as lack of thiaadamantanes and dibenzothiophenes indicate that the oil was expelled from a source rock at moderate thermal maturity and has not been cracked. The molecular and isotopic composition of oil-associated gases are consistent with this interpretation. We suggest that the oil remained uncracked because the residence time at temperatures >150-160°C was relatively short (<5 my based on 1D modeling) and apparently insufficient for cracking. We conclude that there is potential for finding unaltered liquid petroleum in other high-temperature reservoirs with relatively low geothermal gradient and recent burial in the Tarim basin and around the world. Highlights: Evidence of non-cracked oil from high-temperature reservoirs are presented. Oil is preserved due to short residence time at high temperatures. Single-phase unaltered oil can be present in ultra-deep high-temperature reservoirs.