Geochemical characteristics of shales from Upper Carboniferous Yanghugou formation in Weiningbeishan area, China: Implication for provenance, source weathering and tectonic setting

Abstract

The Upper Carboniferous paralic succession in Weiningbeishan area has been a preliminary target for shale gas exploration in a new round of strategic mineral prospecting operations (2021–2035). However, the provenance, source weathering, paleoclimate conditions and tectonic setting of this succession have not yet been studied. The mineral compositions and geochemical characteristics of shale samples from ZK03 well in the east of the area are investigated to constrain the setting of the region and to establish the tectonic and sedimentary evolution model for this period. Yanghugou shales are a clay dominated lithotype, but an advantageous condition of interbedded sandstone and coal seams enables an improvement in the hydraulic fracturing capacity of these shales. The shales are relatively enriched in Al, Fe, Ti, Sc, V, Co, Ga, Nb, Cs, Th, U, Zr and Hf relative to UCC, but depleted in Si, Na, Cr, Ni and Rb. The rare earth elements (REEs) display an enrichment of light rare earth elements (LREEs) and a depletion of heavy rare earth elements (HREEs) relative to chondrite; a distinct negative Eu anomaly is also present. The plot of Zr/Sc versus Th/Sc and the index of compositional variability (ICV) values suggest that the shales are deposited in the first sedimentary cycle and are not the results of sedimentary reworking. The Al2O3–CaO*+Na2O–K2O (A–CN–K) ternary diagram and the chemical index of alteration (CIA) values combined with the cross plots of C-value versus CIA and K2O/Al2O3 versus Ga/Rb imply that the parent rocks of shales underwent a strong degree of weathering under a humid and warm paleoclimate. Multiple geochemical proxies indicate that Yanghugou shales are mainly originated from diorite and granite (a felsic-intermediate igneous source), formed in a continental island arc environment that were mainly prevalent in the Beidasha, Nuoergong, Yamatu and Langshang regions in Alxa block. Under the influence of transgression of paleo-tethys ocean and warm and humid late Carboniferous climate, high quality source rocks (TOC>1.5%) are developed in Weiningbeishan area.