Mineralogy and inorganic geochemistry of the Es4 shales of the Damintun Sag, northeast of the Bohai Bay Basin: Implication for depositional environment

Abstract

Abstract The Es4 shale, namely the fourth member of the Shahejie Formation deposited during the Eocene, is one of the most important hydrocarbon source rocks widely distributed in the Bohai Bay Basin. Twenty-nine samples with TOC ranging from 1.59% to 7.50% were gathered from the Es4 shale at the Damintun Sag. Compositions of mineralogy, major oxides, trace elements and REEs were analyzed to study their provenance, paleo-environment and tectonic setting. The Es4 shales show (La/Yb)N ratios ranging from 10.98 to 14.24 with an average of 12.45, and have similar C1-chondrite normalized REE\xa0+\xa0Y distribution to intermediate\xa0~\xa0mafic igneous rocks developed at the northeast NCC (North China Craton) during the Mesozoic and the Cenozoic. Weight ratios of Al2O3/TiO2, TiO2/Zr and Cr/Th of the Es4 shales show values of 12.76–18.81, 71.2–147.8 and 12.96–19.91, which reveal a common source of detritus from intermediate igneous rocks. The Es4 shales present values of K2O/Al2O3, ICV (Index of Compositional Variability), PIA (Plagioclase Index of Alteration) and CIA (Chemical Index of Alteration) locating at 0.07–0.21, 0.80–2.04, 39.82–83.85 and 44.76–76.15, respectively. These weathering proxies imply that the Es4 shales underwent moderate to intense weathering. Sensitive redox indicators, pyrite (1.7%\u202f~\u202f12.6%) and TOC (1.59%\u202f~\u202f7.50%), indicate a reduced depositional condition. TOC of the Es4 shales is positively correlated with U/Th and negatively correlated with (La/Yb)N, which indicates that organic matters were largely preserved at more reduced conditions and could be diluted by detritus influx. Absolute ratios of U/Th (0.11–0.25), V/Cr (0.80–1.46) and Ni/Co (2.42–4.40) suggested an oxic environment, which is contrary to the conclusion supported by high TOC content and existence of pyrite. These absolute ratios of U/Th, V/Cr and Ni/Co are probably inherited from parental rocks rather than reflecting redox conditions in this study. Shale samples showing higher TOC content are either accompanied with higher U/Th, lower Sr/Ba and lower (La/Yb)N, which demonstrates that organic matters are preserved in a deeper, fresh and reduced water environment. Plots of K2O/Na2O-SiO2, SiO2/Al2O3-K2O/Na2O, La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10 indicate a tectonic setting of oceanic island arc or continental island arc.