Sm-Nd isochron dating and geochemical (rare earth elements, 87Sr/86Sr, δ18O, δ13C) characterization of calcite veins in the Jiaoshiba shale gas field, China: Implications for the mechanisms of vein formation in shale gas systems

The Wufeng and Longmaxi organic-rich shales host the largest shale gas plays in China. This study examined the petrography, rare earth element (REE) and other trace-element geochemistry, Sm-Nd geochronology, and isotope geochemistry (Sr-87/Sr-86, delta O-18, delta C-13) of fracture-cementing minerals within core samples of the Wufeng and Longmaxi Formations from the Jiaoshiba shale gas field in order to (1) characterize the mineral phases occurring in the veins (mineralized fractures); (2) determine the ages of the calcite by the Sm-Nd isochron dating method; (3) understand the sources of calcite-precipitating fluids; and (4) explore the possible mechanisms responsible for calcite vein formation in shale gas systems. The fractures hosted in the Longmaxi Formation are mineralized with quartz as the predominant fracture cement, and calcite as an intracementation phase postdating the earlier quartz cement. In contrast, the fractures hosted in the Wufeng Formation are dominantly mineralized by calcite, which occurs either as the only cement present or as a cement phase predating later quartz cement. Calcite veins within the Longmaxi Formation have a Sm-Nd isochron age of 160 +/- 13 Ma and 6 13 C values of -4.71%c to -3.11%c, delta O-18 values of 17.1%17.4%0, and Sr-87/Sr-86 values of 0.72437-0.72869. Calcite veins within the Wufeng Formation yielded a Sm-Nd isochron age of 133 +/- 15 Ma and are characterized by delta C-13 values of -2.29%c to -1.03%c, delta O-18 values of 17.3%c-17.7%c, and Sr-87/Sr-86 values of 0.72202-0.72648. The similarity between Sr-87/Sr-86 values of the calcite and those of their respective surrounding host rocks (0.72670-0.72875 of the Longmaxi shales; 0.72030-0.72648 of the Wufeng shales), combined with relatively depleted delta C-13 and uniform fluid delta O-18 isotopic features, indicates that the calcite-precipitating fluids within the Wufeng and Longmaxi Formations were derived largely from their respective surrounding host-rock sources. REE data equally indicate that the distinguishable Eu anomalies (6.20-19.35; 4.45-11.91), Y anomalies (1.03-1.50; 1.44-1.70), and Y/Ho ratios (28.80-39.16; 38.86-45.18) of calcite veins within the Longmaxi and Wufeng Formations were controlled by their respective surrounding host rocks. The Sm-Nd isochron ages and fluid inclusion data of fracture cements suggest that fracture opening and calcite precipitation in composite veins within the Wufeng and Longmaxi Formations were triggered by gas generation overpressurization.