Logging identification of the Longmaxi mud shale reservoir in the Jiaoshiba area, Sichuan Basin

Abstract

Compared with conventional gas reservoirs, shale gas reservoirs are not sensitive to petrophysical properties, making it much difficult to identify this kind of reservoirs with well logging technologies. Therefore, through a comparison of the logging curves of the Lower Silurian Longmaxi marine shale in the Jiaoshiba area, Sichuan Basin, it is found that the mud shale on conventional log curves generally features high gamma ray, high uranium, low thorium, low kalium, relative high resistivity, high interval transit time, low neutron, low density and low photoelectric absorption cross section index, while on elements logging curves, it features an increase of silicon content and a decrease of aluminum and iron content. Based on the logging response characteristics of mud shale, the logging curves most sensitive to shale, gamma ray, neutron and density logging were selected and overlaid to identify mud shale effectively. On the basis of qualitative identification, the density logging value can identify the non-organic-rich mud shale from organic-rich mud shale, because the former has a density of 2.61–2.70 g/cm3, while the latter has a density of less than 2.61 g/cm3. The identification results agree well with the results of field gas content test, TOC experiment, and gas logging, so this study can provide reference for the logging interpretation.

Keywords

• Sichuan Basin
• Jiaoshiba area
• Silurian
• Mud shale
• Shale gas
• Logging response
• Reservoir identification
• Overlay technique
• Intersection method