Formation and preservation of ultra-deep liquid petroleum in the Ordovician sedimentary succession in Tarim Basin during the neotectonic phase

The transformation of liquid petroleum to natural gases is accelerated at increasing depths and temperatures. Natural gas exploration predominantly focuses on deep layers. However, the ultra-deep petroleum (maximum burial depth of similar to 7110 m and reservoir temperature of similar to 166 degrees C) in the Ordovician sedimentary succession in Tarim Basin has been found to remain in a liquid phase without cracking, and their mechanisms of formation and preservation are unclear. This study performed integrated high-resolution comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC x GC-TOFMS), gas chromatography-mass spectrometry (GC-MS), stable carbon isotopes, and fluid inclusions. The similar molecular compositions and low delta C-13 values of n-alkanes ( 36.0 parts per thousand to 33.0 parts per thousand) suggest that the oils are derived from the Cambrian Yuertusi Formation, with moderate thermal maturity (0.8-0.9 %) without distinct cracking. The low variety and abundance of diamondoids, narrow distribution of diamondoid proxies (concentration ratios and isomerization ratios) as well as lack of thiadiamondoids and ethanodiamondoids support this interpretation. The main charging period was the Late Hercynian orogeny, as evidenced by fluid inclusions and the burial model. Abnormally rapid subsidence during the neotectonic phase (age < 5 Ma) in Tarim Basin led to an abrupt temperature rise, reaching the cracking threshold for crude oils. The uncracked oil could be attributed to a limited time of burial at high temperatures. The neotectonism activated the earlier fault systems, connecting source rocks and reservoirs, and facilitating the migration of trace amounts of gas from deeper layers. Therefore, the ultra-deep strata still have high exploration potential for liquid petroleum.