The Wufeng-lower Longmaxi Formations are the main exploration targets of the Zhaotong shale gas demonstration area.Based on systematic tests of organic carbon

trace elements and REE (rare earth elements) upon samples from Well A in the area

as well as petrological and logging data

we analyzed the variations of geochemical parameters of relevant elements in black shale samples from Well A and tried to reveal the redox conditions of black shale and discuss the possible mechanisms of organic matter enrichment in the area.The results show that the sub-member I of the Wufeng-lower Longmaxi Formations in the area have the highest content of TOC

which is averaged at 3.13% and displays an upward decreasing trend.The enrichment of relevant elements is also observed to be connected with TOC content.Redox parameters such as trace elements and REE indicate that the Wufeng-lower Longmaxi Formations had once experienced anaerobic redox water body turning to a suboxidizing water body with the sub-member I in an anoxic-anaerobic environment

the sub-member Ⅱ in an anoxic environment and the sub-member Ⅲ in an anoxic -oxygen enriched environment.Among them the water body reducibility of layers I-1

I-3

and I-4

is stronger than that of layers I-2 and I-5.And there were still disparities in the forming mechanisms of redox environment for organic matter enrichment in black shale of the Wufeng and Longmaxi Formations against a high paleo-productivity in the area.For the Wufeng Formation

it was a stagnant water body that preserved the organic matter in an anaerobic-anoxic environment

and for the lower Longmaxi Formation

it was sea level rising that kept a rich organic matter in an anaerobic environment.The sub-member I at the Wufeng-lower Longmaxi Formations has the richest TOC and thus is regarded as the best target for shale gas development.