Sequence stratigraphic division of Cambrian in western Hunan-Hubei and applications for petroleum exploration

The systematic analyses of outcrops and drilling profiles and the tests of reservoir samples and dark mudstones from Cambrian in the western Hunan-Hubei indicate that the Cambrian in the study area can be divided into 6 third-order sequences, and the sequence boundaries are mainly unconformity surfaces and lithologic transitional surfaces. Oolitic limestones/dolomites are the most favorable reservoirs in Cambrian. They mainly distribute in the transgressive system tract of sequence 5, and a few of them locate in the highstand system tract of sequences 4 and 5. The content of total organic carbon (TOC) varies obviously among dark mudstones in the Lower Cambrian. The dark mudstones rich in TOC distribute in the highstand system tract of sequences 1, while those in the upper formation have lower TOC content. The transgressive system tract of sequence 5 which deve-lops thick oolitic limestones/dolomites is the main target for normal exploration. The highstand system tract of sequences 1 which has higher TOC content is the main target for shale gas exploration.