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Early Cambrian shale is an important petroleum source rock around the world. Because of little drilling data and poor seismic data, until recently, organic matter enrichment of the Lower Cambrian Yuertusi and Xishanbulake formations shale is still an enigma in the Tarim Basin, northwestern China. Total organic carbon (TOC), major and trace element data of Cambrian shale samples from five boreholes have been analyzed to decipher the mechanism of the organic matter enrichment. The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents (3.2%–19.8%, on average 11.0%) than those from the eastern basin (2.2%–10.2%, on average 4.5%). The paleoproductivity proxies (Ba, Ba/Al, P/Al) in the western restricted platform are much higher than those in the eastern basin. The trace element indicators such as V/Cr, Ni/Co, Mo–TOC and MoEF–UEF suggest an anoxic environment across the basin, but a more restricted environment in the western intraplatform. The paleoproductivity rather than anoxic condition and hydrothermal activity are concluded to have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales; the restricted environment was favorable for paleoproductivity and preservation of organic matter. [ABSTRACT FROM AUTHOR]

[Display omitted] • This study provides a Cambrian bio- and C isotope stratigraphic framework for the eastern Tarim Basin. • The palaeogeographical connections of basins with the open ocean have a significant impact on basin seawater 87Sr/86Sr values. • The eastern Tarim Basin was not well connected to the open ocean until Furongian. Cambrian was a critical period in Earth's history, marked by rapid diversification of early animals and significant changes in oceanic conditions. Marine sediments, particularly carbonates, have been used to study variations in the global carbon cycle, seawater redox, weathering input, etc. In search of new archives of Cambrian paleo-seawater chemistry, we conducted an integrated study of Cambrian C and Sr isotope stratigraphy using the carbonates of the Xingdi section in the Kruqtagh region of the eastern Tarim Basin. Based on reported paleontological data, the C isotope record is comparable to the Cambrian global seawater C isotope variation, providing a feasible stratigraphic framework for the Cambrian strata in the eastern Tarim Basin. A sharp δ13C increase at the top of the Xidasan Formation probably corresponds to the recovery of the Redlichiid–Olenellid Extinction C isotope Excursion. Additionally, a negative C isotope excursion in the lower part of the Mohuosan Formation and a positive C isotope excursion at the bottom of the Tuersaketagh Formation likely correspond to DrumIan Carbon isotope Excursion and Steptoean Positive Carbon Isotope Excursion, respectively. However, the Sr isotope curve in the eastern Tarim Basin decoupled from global seawater Sr isotope variations during Cambrian Series 2–3 until Furongian, indicating that the eastern Tarim Basin was not well connected with the open ocean until Furongian. It also highlights a significant impact of paleogeographical connections of basins with the open ocean on their seawater Sr isotopic signal. [ABSTRACT FROM AUTHOR]

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. • Well Luntan-1 in the Tarim Basin in China discovered effective ultra-deep petroleum system. • Oil accumulation is sourced from the Lower Cambrian Yuertusi shale and sealed by the Middle Cambrian evaporites. • Although under high temperature and pressure conditions, the light volatile oil is not cracked due to short residence time. • There is significant oil potential in ultra-deep Paleozoic reservoirs in the Tarim basin. [ABSTRACT FROM AUTHOR]