New Evidence on the Stratigraphy and Chronology of 2 140～3 551.63 m Well 424 Section in Karamay Oilfield and “Basin−Mountain” Stratigraphic Comparison

The establishment and age determination of the stratigraphic sequence in the well is the key evidence for the division and correlation of rock stratigraphic units. This is significant for the correct establishment of the reservoir. The upper section of well 424 (2 140～3 178 m) in Karamay Oilfield is dominated by marine terrestrial clastic rocks. Sporopollen fossils, such as Noeggerathiops−idozonotriletes were obtained in the interbedded dark gray siltstone and gray−black fine sandstone in the depth of 3 176.94～3 178.44 m, indicating that they formed in early to middle stage of Late Carboniferous (Bashkirian−Moscovian); The lower section 3 178～3 551.63 m is mainly composed of volcanic breccia and intercalated lava. LA−ICP−MS zircon U−Pb age of 306 Ma was obtained from the amygdaloidal basaltic andesite at 3 340.50～3 343.50 m. It is considered that the upper terrestrial clastic rocks and the lower volcanic breccia are integrated contact and show obvious dichotomy characteristics. Accordingly, it is confirmed that the upper part (2 140～3 178 m) of the well 424 is comparable to the lower part of the stratotype section of the Aladeyikesai Formation in the Hala’alate Mountain, and the lower part (3 178～3 551.63 m) is comparable to the upper part of the Hala’alate Formation. This result not only subdivides lithostratigraphic unit of Well 424, but also makes up the deficiency of the unclear original stratigraphic contact relationship and old−new relationship between the stratotype sections of Aladeyikesai and Hala’alate formations in the mountain area.