Sequence Stratigraphy of Silurian Black Shale and Its Distribution in the Southeast Area of Chongqing

Four sets of black shale in lower Cambrian,upper Ordovician-lower Silurian,lower Permian and upper Permian are distributed in south China,especially the lower Silurian shale widely distributed in the southeast area of Chongqing.Since the southeast area of Chongqing is the pilot test district of the shale gas exploration in China,the distribution of lower Silurian Longmaxi Formation black shale is considerably important to find the industrial shale gas.By means of correlation of litho-stratigraphy and fine description of outcrop sequence stratigraphy in the southeast area of Chongqing,we setup the sequence stratigraphic framework of Silurian and divide into 8 third-order sequences,i.e.ssq1,ssq2,ssq3,ssq4,ssq5,ssq6,ssq7,and ssq8.Based on the horizontal distribution of black shale,we determine northern and western depocenters of lower Silurian shale.There are two controlling factors for the sedimentary distribution of black shale as follows:(1) favorable sedimentary environment and palaeogeomorphology,(2) short-term relative sea-level change which constrained the total organic carbon(TOC) and kerogen type.These controlling factors are significant to confirm the favorable section of shale gas accumulation.