Paleo-ocean redox environments of the Upper Ordovician Wufeng and the first member in lower Silurian Longmaxi formations in the Jiaoshiba area, Sichuan Basin

A study of paleo-ocean redox environments is important for understanding the deposition of black shale and has practical implications for shale gas exploration. Here, we selected a total of 52 shale samples from JY1 Well, the first shale gas well of commercial exploitation in China, to analyze the redox conditions of Upper Ordovician Wufeng (O(3)w) and the first member in lower Silurian Longmaxi shale (S(1)l(1)) in the Jiaoshiba area. Abundant pyrite framboids are observed in these units, with average framboid diameters ranging from 3.1 to 4.7 mu m and maximum diameter about 10 mu m. Analyses of redox-sensitive trace elements suggest the redox environment has evolved from an anoxic condition in the bottom of Member A to a dysoxic condition in the top of Member A, and to a dysoxic-oxic environment in Members B and C. Graptolite and radiolarian are discovered in these shale samples, indicating an oxygenated seafloor, which seems to be contradictory to the results from pyrite framboids and redox-sensitive trace elements. This contradiction is explained as follows: when the shale was deposited, the sedimentation was dominated by euxinic conditions; however, some oxygen may also occasionally migrate to the bottom water due to oxygen-deficient environment, deglaciation, and (or) strong upwelling of nutrient-enriched seawater, while the limited oxygen and anoxic environment is favorable for the preservation and accumulation of organic matter. Combined with the geological characteristics and redox conditions, Member A, especially the shales at the bottom of Member A, is expected to be the most favorable shale gas reservoir in the Jiaoshiba area.