Petroleum signals from the deep Earth: Evidence from the world's deepest sedimentary rock borehole (11,156 m)

Traditional petroleum system models define a thermal limit for liquid hydrocarbon stability, commonly referred to as the oil “death line” at approximately 160°C–200°C. Beyond this threshold, petroleum is predicted to undergo rapid thermal cracking into gas, leaving behind inert pyrobitumen. This temperature-dominated framework has long underpinned interpretations of hydrocarbon preservation and the deep carbon cycle. However, new observations from Well X1 in the Junggar Basin challenge this paradigm. Drilled to a depth of 11,156 m entirely within sedimentary strata, X1 reveals the persistence of liquid-like organic matter under conditions far exceeding this conventional thermal limit. These findings suggest that hydrocarbon stability is not governed by temperature alone; rather, it is controlled by the combined effects of pressure and the duration of thermal exposure, which together regulate the evolution of deep organic matter.

Scientific drilling has repeatedly revised expectations of the deep subsurface.1 More recently, China’s Shendi Take-1 well in the Tarim Basin was completed at a final depth of 10,910 m, establishing an important regional benchmark for ultra-deep drilling in sedimentary basins.2 Early efforts, including the Mohole Project and the Kola Superdeep Borehole, revealed unexpectedly dynamic fluid systems and complex thermal regimes within crystalline basement rocks. However, these investigations largely bypassed sedimentary sequences, which constitute the principal long-term reservoirs of organic carbon. This limitation has now been addressed by the X1 borehole, which penetrates more than 11 km of continuous sedimentary succession and terminates within Middle Permian mudstones of the Xiazijie Formation (Figure 1; modified from Wang et al.3). As shown by the integrated geological framework and high-resolution seismic profiles in Figure 1, the deep structural architecture of the Junggar Basin is clearly imaged, with stratigraphic reflectors and fault systems continuously traceable into the ultra-deep domain below 10 km. Unlike previous ultra-deep drilling projects, X1 provides direct access to organic-rich source rocks, seals, and migration pathways under extreme pressure-temperature conditions, offering a rare opportunity to test maturity models that have previously relied on extrapolation.