Timing of deep oil accumulations in the western part of the northern Tarim Basin: Records from fluid inclusions and carbonate U–Pb dating

Liquid oils in deeply buried reservoirs are usually subjected to thermal cracking by the prolonged time-temperature effect during burial. However, large quantity liquid oils are being produced from the Cambrian reservoirs in the Tarim Basin at a depth of over 5500 m. Previous studies suggest that a late oil charging event should have been responsible for the preservation of liquid oil, but the evidence from fluid inclusions is questionable. In this study, carbonate U–Pb dating following a systematic fluid inclusion analysis is applied to constrain the timing of oil charging in the Cambrian reservoir in the Kalayuergun structural belt in the western part of the Tabei Uplift. The results show that the diagenetic minerals of the Cambrian Xiaqiulitage Formation include the dolomite matrix, anhedral dolomite, euhedral dolomite and calcite. Secondary oil inclusions were detected as trails in the dolomite matrix while primary oil inclusions are distributed along growth zones of calcite. Both of these inclusions are featured by multiple fluorescence colors in single oil inclusion assemblage, which are interpreted to be the results of trapping fractionation or post-entrapment alteration. The homogenization temperatures of oil inclusions with different fluorescence colors in the same assemblage are in the same range of their coeval aqueous inclusions, denoting oil inclusions are formed in a single trapping event and the oil should have been gas saturated. Projecting the lowest homogenization temperature of the aqueous inclusion onto the burial history, yielding an age of around 450 Ma which is very close to the U–Pb age of calcite (440.5 ± 9.6 Ma), corresponding to the first oil generation peak of the Yuertusi Formation during the Late Ordovician to Early Silurian. Considering this, the early generated oils migrated upwards to the shallower reservoir through fault planes together with the low heating rate of the Tarim Basin prevented the oil in place from thermal cracking. This study demonstrates the effectiveness of integrating carbonate U–Pb dating and fluid inclusion methods to investigate deeply buried reservoirs in the Tarim Basin, which can be applied to similar reservoirs in the world.