Provenance and diagenesis of the Upper Jurassic Qigu Formation sandstones in the southern Junggar Basin, NW China

Abstract

The Upper Jurassic Qigu Formation sandstones in the southern Junggar Basin typically have calcite cements. Two stages of calcite cements which were distinguished based on cathodoluminescence and casting body slice image increased rock pressure resistance to a certain extent, effectively prevented the quartz overgrowth, and are characterized by a style of filling of intergranular porosity that deteriorated the reservoir permeability. Result from the laser ablation inductively coupled plasma mass spectrometry indicates that the detrital zircons in the Qigu Formation sandstones are mostly magmatic in origin with U-Pb zircon ages of ∼300 and ∼160 Ma. These detrital zircons were largely derived from the Late Carboniferous to Early Permian and Late Jurassic magmatic rocks in the Tianshan area. All clastic rocks exhibit negative Nb, Ta, and Ti anomalies on primitive mantle-normalized elemental diagrams and high Al2O3/TiO2 ratios, implying the existence of a sedimentary source dominated by intermediate and felsic magmatic rocks. Our result indicates that the Late Jurassic volcanism may be extensively distributed around the Tianshan area. The diagenesis of calcite cements could decrease the Al2O3, TiO2, MgO, K2O, Hf, Th, and Zr contents and increase the MnO content, which may play an important role in variational contents of elements.