PALEO-OCEAN REDOX ENVIRONMENTS OF BLACK ORGANIC-RICH SHALES IN FULING SHALE GAS FIELD, 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 form JY1 well, the first shale gas well of commercial exploitation in China, to analyze the redox conditions of upper Ordovician Wufeng (O₃w) and the first member in Lower Silurian Longmaxi shale (S₁l¹) in Fuling shale gas field. Abundant pyrite framboids are observed in these units with averaged framboids diameters ranging from 3.1 μm to 4.7 μm and the maximum diameter about 10 μm. Analyses of redox-sensitive trace elements suggests redox environment has evolved from anoxic condition in the bottom of Member A to dysoxic in the top of Member A, and to 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 condition; 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 reservoirs in Fuling shale gas field.