Sedimentary environment of the Lower Cambrian Mufushan Formation in the Lower Yangtze region: Evidence from whole-rock geochemistry

The Mufushan Formation of Lower Cambrian is the most significant hydrocarbon source rock for shale gas in the Yangtze Platform. No exploration breakthrough has been achieved so far in the Lower Yangtze area, compared to the Middle and Upper Yangtze areas. Recently, the well of GD-1 has been completed, for which the Early Cambrian Mufushan (MFS) Formation is completelyly cored. Geochemistry of calcareous/carbonaceous mudstone of Early Cambrian Mufushan (MFS) Formation are carefully investigated for paleo-environment, provenance and tectonic settings. The samples of MFS are characterized by enriched large ion lithophile elements and depleted high field strength elements and transition elements. The analysis results show that the total REE concentrations of MFS mudstones vary from 14.81 to 107.47 ug/g. The Chemical Index of Alteration (CIA) ranges from 64.84 to 78.81. And the A-CN-K plot indicate that the source rocks has undergone a moderate weathering. In the Th/Sc versus Zr/Sc plot, most samples are located in the area between basalt and felsic igneous rocks, with negligible sedimentary recycling. Both the Al₂O₃/TiO₂ ratios and TiO₂/Zr ratios indicate an intermediate-felsic igneous provenance. The Cr/V ratios and La/Th-Hf diagrams also suggest that most of the materials are derived from intermediate-felsic rocks.