Micro-Characteristics of the Shale in the First Member of Silurian Longmaxi Formation in Southeastern Sichuan Basin, China

In the first member of Silurian Longmaxi Formation there are rich shale gas resources, and through series of exploration and production efforts, a great success has been achieved. The lithology of the first member of Longmaxi Formation varies vertically and the interval can be divided into three sub-sections. In Long 1(1) sub-section the main lithology is grey-black siliceous shale with the quartz content over 45%, and quartz often presents microcrystalline (1 similar to 2 mu m) and microcrystalline aggregation texture. The siliceous shale is also very rich in organic matters with an amount of 2-5%. But Long 12 and Long 13 sub-sections lithology are poor in organic Detailed measurement and statistical analysis of the micro-nano pore characteristic of first member in Longmaxi formation showed that large quantities of pores and fractures on the micro- and nano-scales exist. Furthermore, four types of pores could be summarized. Typically, the high TOC shale in the first member of Longmaxi Formation in Jiaoshiba area are organic pores dominated with the surface porosity of the organic matter of 10-50% (average 30%). Statistics revealed that most pores are small pores or mesopores according to the IUPC classification. Maximum pore throat radius in the matrix is 27.1 similar to 37.1 nm, with a medium range of 5.0 similar to 5.5 nm, which belongs to pore throat in nanosizes. The laboratory statistics showed that through the whole shale segment with rich organic matters, the average porosity is 4.78% and the average vertical permeability is 0.00065 mD. Vertically, the reservoir storage capacity is high at upper and lower parts but low at the middle part. The adsorption experiment of whole-diameter shale is carried out under the reservoir temperature (82 degrees C), and the Langmuir model is modified to fit the experimental result. Adsorption capacity is an increasing function of pressure. There is a significant difference in the low speed seepage mechanism between methane and nitrogen in shale. Under the same back pressure, the methane slippage factor is greater than that of nitrogen, and Klinkenberg permeability of methane was about half of that of nitrogen.