Rare earth elements and yttrium (REY) distribution pattern of lower Cambrian organic-rich shale in Yichang area, Western Hubei Province, South China, and source of carbonate minerals

Abstract

Critical breakthroughs of Cambrian shale-gas exploration have been achieved in the lower Cambrian organic-rich Shuijingtuo shale in the Yichang area, Western Hubei Province, South China. High carbonate content (at times >50 wt%) of the lower Shuijingtuo Formation could have facilitated shale-gas retention, but its source remains uncertain. In this study, a 100-m-long, continuous core interval from well EYY1 (2970 to 3070 m) covering the lower Cambrian Yanjiahe and Shuijingtuo Formations in the Yichang area was characterized using whole-rock rare earth elements and yttrium (REY), total organic and inorganic carbon (TOC and TIC), and major and trace-element analyses, and cathodoluminescence (CL) imaging. Shales and carbonates in Yanjiahe and the organic-rich lower Shuijingtuo Formations at Yichang area were imprinted with REY signatures derived from oxic-seawater or basinal hot brine, in which the enrichment factor of total REY contents (ΣREYEF = (ΣREY/Al) sample/(ΣREY/Al) PAAS) correlated positively to TIC contents and negatively to aluminum contents, indicating that carbonates serve as both carriers of hydrogenous REY and diluent of terrigenous REY signatures. Two kinds of carbonate minerals were distinguished under CL. Dolomite with bright-red-orange CL luminescence in upper Yanjiahe Formation was responsible for whole-rock HREE-depletion, and its association with abundant barite and silt-sized terrigenous silicate minerals likely indicate alteration of basinal hot brine in low-sea-level conditions and provide evidence supporting a depositional hiatus after deposition of the Yanjiahe Formation in the Yichang area (∼526–521 Ma). Calcite particles with dark-orange CL luminescence were the carrier of oxic-seawater-like REY features in the organic-rich shale deposited under anoxic conditions (indicated by high ratios of (U/Th) wt and (Corg/P) mol) in the lower Shuijingtuo Formation. These calcite particles could be allochthonous and perhaps formed in an oxic environment on the carbonate platform and transported to the adjacent anoxic, deepwater basinal environment. The enrichment of calcite leads to an elevation in rock brittleness, which is favorable to shale-gas exploration in the lower Shuijingtuo Formation in the Yichang area.