Deepest oil in Asia: Characteristics of petroleum system in the Tarim basin, China

The petroleum exploration well Luntan-1 (LT1) located in the Tabei Uplift, Tarim Basin, China reached vertical depth 8882 m and is now the deepest well in Asia. A significant petroleum flow (134 m3 of oil and 45,900 m3 of gas) was obtained from the Cambrian Wusongger Formation perforated in the interval 8203–8260 m, with a pressure of 90.8 Mpa and interval logging temperature is 162 °C. The well LT-1 is the deepest producer of oil from Paleozoic reservoirs in the world. According to the analysis of the effect of acidizing fracturing reservoir reconstruction, the Lower Cambrian Wusongger Formation has low pump stop pressure, large pressure drop, fast deceleration, and good reservoirs and the lithology are limestone dolomite, lime-bearing dolomite, and dolomite. The source rock is the Lower Cambrian Yuertusi shale containing significant amount of total organic carbon (2–30 wt%) and located in the late oil/early gas maturity window (vitrinite reflectance 1.4–1.7%). Through GC × GC-TOFMS analysis of this oil showed low concentration of diamondoids, which indicates lack of thermal cracking of oil to gas. Thiadiamondoids, typically indicative of thermochemical sulfate reduction, were not detected. Using kinetic calculations, we estimated the temperature threshold for thermal cracking of well LT1 oil to gas at ~210–220 °C. Based on the thermal evolution and burial history of the Tarim Basin, the depth threshold for the disappearance of liquid oil is ~9000 m. Low geothermal gradient (19.6 °C/km) and rapid subsidence after petroleum accumulation jointly contribute to the preservation of liquid petroleum in ultra-deep strata in the Tarim Basin. Our findings suggest that the favorable exploration area in ultra-deep, high-temperature and high-pressure Cambrian reservoirs in the Tarim Basin exceeds 20,000 km2 and may contain over 1 billion tons of oil resources. Furthermore, the significant discovery of oil in the well LT1 provides important insights for global exploration of Paleozoic ultra-deep oil.