Paleo-environment of the first member of Niutitang Formation on the southern margin of Huangling anticline, western Hubei province: a case study of well ZD-1

Abstract

The Lower Cambrian Niutitang Formation is one of the important targets for shale gas exploration in the western Hubei province. In view of the relatively weak research on the paleo-sedimentary environment of the 1st member of Niutitang Formation (Ꞓ1n1), the paleo-climate, paleo-redox conditions and paleo-productivity of the Ꞓ1n1 were comprehensively studied based on data collected from well ZD-1 on the southern margin of the Huangling anticline in Yichang area, and a model of the evolution of sedimentary environment was proposed. Compared with the organic-poor strata, the organic-rich strata have higher TOC content, much higher SiO2, U, V, Ni, Zn and Cu contents, and lower CaO content. The chemical index of alteration (CIA) shows that the Ꞓ1n1 has experienced weathering effect of moderate intensity. The climate was warm and humid when the organic-rich layers deposited, and became hot and wet when the upper layers deposited. The redox conditions of Ꞓ1n1 have experienced 3 stages, including euxinic in the lower part of organic-rich strata, dysoxic in the upper part of organic-rich strata, and oxic in organic-poor strata. In general, the study area developed in a weak-medium retention basin. The ratio of MoEF/UEF is usually 1-3 times than that of normal seawater due to the high content of Mo, showing much higher retention rate. The organic-rich strata have higher paleo-productivity, and there are 2 periods of rapid decline of paleo-productivity, which is directly related to the change of water from anaerobic to oxidized due to the decrease of sea level. According to paleo-climate, paleo-redox conditions and paleo-productivity characteristics, the Ꞓ1n1 can be divided into 3 evolution stages.

Keywords

• paleo-environment
• shale
• huangling anticline
• niutitang formation
• lower cambrian
• western hubei