The organic shales in the Lower Silurian Longmaxi Fm in Fuling area

the Sichuan Basin are proved to be shale gas reservoirs of high quality.Nevertheless

they have relatively strong vertical heterogeneity

directly influencing its gas content and reservoir fracability.An isochronal sequence stratigraphic framework is first established.Then

the heterogeneities of the shale are analyzed in respects of petrological and mineralogical compositions

the content of pyrite

sedimentary structures and organic matter abundance within this framework.The shales deposited during TST are characterized by authigenic siliceous minerals and pyrites of sedimentary origin

obvious lamellation

high TOC and special positive relationship between TOC and siliceous mineral content.The shales deposited in the early HST feature in the occurrence of terrigenous silts

small wavy lamination and lenticular bedding

higher clay content than that of TST

moderate TOC

and the existence of both authigenic and secondary pyrites.The shales deposited during the late HST feature in even higher clay content but lower silt content

low TOC and the existence of both authigenic and secondary pyrites.The factors influencing shale heterogeneity are discussed in respects of sedimentology

paleo-marine productivity and redox conditions and they mainly include the influx rate of terrestrial deposits

paleo-marine productivity

redox conditions and paleo-current.