Stability and cracking threshold depth of crude oil in 8000 m ultra-deep reservoir in the Tarim Basin

Abstract Crude oil gradually cracks into gas with increasing temperature and reservoir depth under geological conditions. Therefore, liquid oil is rarely preserved on a large-scale in ancient formations reaching deeper than 6000 m. Recently, an ultra-large oilfield with reserves of over 1 billion tons was discovered in the Ordovician carbonates in the ultra-deep layers of the Tarim Basin, NW China. This phenomenon altered the understanding of the cracking threshold depth of liquid oil. In this study, a large number of drilling and production datasets were used to analyze oil samples from 48 wells in depth range of 6500–8000 m through GC × GC-TOFMS. The results suggest that diamondoids and ethanodiamondoids indicative of high maturity and thermal cracking are rare and exhibit a low content in these oils. The oils are crude oils of normal maturity, which gradually increase with reservoir depth. According to the diamondoids and ethanodiamondoids related parameters, the oil maturity is calculated to be equivalent to the product of source rocks in a stage of 1.1%–1.2%Ro. By analyzing the evolution trend of these parameters, we propose the relationship between the cracking conversion rate of the hydrocarbon reservoir and its depth and temperature. The threshold temperature corresponding to a large amount of liquid oil cracking until its extinction is calculated as 210 °C. Large volumes of liquid oil can be preserved below this temperature. The depth limit of liquid oil preservation is likewise calculated, which provides a method to determine the fluid phase for deep oil and gas exploration.