Abstract:

 Shales in Longmaxi Formation of the Lower Silurian developed in southeast Sichuan and northern Guizhou are rich in organic matters and are high quality hydrocarbon source rocks in marine strata of south China. These shales with great thickness are the source of shale gas and widely distributed in this area. Based on the data of 30 field outcrop cross sections and 12 wells, the facies marks are summarized in combination  with the studies on  lithology, element geochemistry, well logging and paleontology. And the sedimentary environment, sedimentological evolution and its effect on the hydrocarbon source rocks  are probed  into. The eight sedimentary types are identified including deep muddy shelf, turbidite deposit, shallow muddy shelf, shallow sandy and muddy shelf, shallow sandy shelf, shallow marlite and muddy shelf, shallow marlite shelf and reefbank complex on carbonate platform margin. The main sedimentary environment is deep muddy shelf.  Among them  the deep muddy shelf is the major sedimentary environment for the formation of the hydrocarbon source rocks of the Longmaxi formation. According to the research of the sedimentary environment it is concluded that the anoxic retention environment and the low sedimentary rate in early transgression stage of the Longmaxi sedimentation are the main controlling factors for the development of high quality hydrocarbon source rocks. This study is beneficial to the shale gas exploration of this area.

Key words: southeast Sichuan and northern Guizhou, Longmaxi Formation, sedimentary environment, source rocks, shale gas