Analyses of the relationship between lithology and shale gas accumulation for the Wufeng Formation to Longmaxi Formation in the west of Hubei province:Acase study of the Erhongdi 1 well

Shale gas exploration is relatively slow in the west area of Hubei province.According to current exploration results,the Wufeng Formation and the Longmaxi Formation in this area show a good prospect for shale gas exploration.However,the division of lithofacies and the relationship between lithofacies and shale gas accumulation in this area have not been quantitatively subdivided.The Fulin shale gas field and the Weiyuan Changning national demonstration zone successfully commercialized in the past indicates that compared with other strata commercially,the first member of Longmaxi Formation is provided with the best potential of exploration and development.In order to reveal the petrophysical characteristics of the Wufeng Formation and the Longmaxi Formation and scientifically forecast the distribution of high quality shale,the lithofacies of the first member of Longmaxi Formation is divided quantitatively based on the research of the Erhongdi 1 well.Through a series of geological analyses (collecting and observing drilling core,classifying graptolite biostratigraphy and dividing sedimentary facies) and experimental tests (testing TOC,major trace elements,mineral composition and gas content,and scanning electron microscope) of the Erhongdi 1 well,the study was conducted in two aspects:① From lithology,mineralogy,geochemistry,paleobiology,electrical characteristic and other aspects,the first member of Longmaxi Formation is recognised as shallow deep water shelf facies and deep water shelf facies,afterwards,the lithology of the first member of Longmaxi Formation in this well is quantitatively classified through the features of TOC,quartz,clay minerals and graptolite.Therefore,the shale of the first member of Longmaxi formation can be further subdivided into eight lithofacies.Siliceous shales with high content of carbon,silica and graptolite fossils and radiolarian siliceous shales containing high content of carbon,silica and graptolite fossils are the most advantageous lithofacies of them.② The relationship between different lithofacies and trace elements,reservoir space and gas content is studied.Each lithofacies influent the accumulation of shale gas by TOC and mineral composition.Siliceous shales with high content of carbon,silica and graptolite fossils and radiolarian siliceous shales containing high content of carbon,silica and graptolite fossils have the greatest influence on the gas content of a shale gas field.