1. Introduction
Shale gas is a self-generated, self-stored, and self-enveloped unconventional natural gas gathered in the dark or high carbon mud shale. The shale gas exploration and development in North America has changed the energy structure of the world and has become extremely vital. Shale gas resource is rich in our country [1], especially the Paleozoic Marine black shale generated in the Sichuan Basin, which has a huge potential in terms of shale gas resource, and the dragon stream group of shale in the Lower Silurian series is considered as the most favorable layers for the development of shale gas. There is a humongous gas reserve in Neijiang-Luzhou-Yibin region. Shale gas exploration has gained high-yield commercial production in the Longmaxi Formation in the Changning-Weiyuan region of the Sichuan Basin [2] and Jiaoshiba region [3].
Carbon isotope is one of the most important indicators of both natural gas and gas. It is a universal phenomenon. The composition of carbon isotope in shale gas is in inversion with the carbon number distribution patterns all over the world. For example, shale gas that is widespread worldwide possesses carbon isotopic composition of the inversion occurrence with the carbon number distribution patterns. Examples are as follows: The four main shale gas production regions are the Barnett, Woodford, Haynesville, and Fayetteville in America and the Appalachian shale gas production region in Canada. They comprehended phenomenon [4], [5], [6], [7] and [8] of gaseous hydrocarbon carbon isotopic inversion confirms that most of the carbon isotope inversion are related to the production of shale gas, which can indicate the “core” region of shale gas [5], [6] and [8]. The reason behind carbon isotope inversion of the dragon stream group in Sichuan Basin still lacks systematic research, and it requires further investigation to find out whether it is related to the shale gas production.
The geochemical gas research of shale gas offers an important and direct basis for the forming mechanism of shale gas especially the contribution of the secondary cracking of crude gas content on shale gas, the tectonic activity on the preservation, dispersion of shale gas, and the gas content in shale and the development of air source. This paper analyzes the chemical composition and carbon isotope of the dragon stream group of the Silurian system in the Changning and Weiyuan regions in the south of the Sichuan Basin. Geochemical features of the shale gasses from the two regions were compared with one another as well with the shale gas from Barnett, America. The origin of hydrocarbon and non-hydrocarbon gasses and the factors and significance of the shale gas isotope inversion in the area are to be discussed.