An All-scale Analysis of Shale Pore Structure of Continental Shale in Ordos Basin, NW China

Abstract

Abstract

The Ordos Basin contains a large amount of shale gas in shale reservoir. Shale pore structure analysis is critical to understand the characters of shale reservoirs, and it has significant guiding significance to analyze the gas storage and flow mechanisms.

Fluid invasion experiments are widely used to analyze the characteristics of shale pore structure. In this research, we performed mercury intrusion, N₂ and CO₂ physisorption to analyze the pore size distribution (PSD), pore volume (PV) and specific surface area (SSA) of shale samples collected from Yanchang Formation in Ordos Basin, and make an effective combination of these methods to analyze an all-scale PSD. In addition, to comprehensively characterize the effect factors of pore structure, geochemical and mineralogical tests also have been carried out.

Geochemical analysis shows the shale samples to have an abundant TOC content, ranging from 0.47% to 11.44% with an average of 4.68%, indicating that the study area has abundant organic matter. The vitrinite reflectance (Ro) ranges from 0.68% to1.02%, which is lower than that of marine shale, showing that organic matter in the Ordos Basin is at oil–generation stage. Mineralogical analysis shows that the dominant minerals in the study area are quartz, feldspar, and clay minerals which mainly contain illite, chlorite, partly kaolinite, and scarce for montmorillonite. High-pressure mercury intrusion, and N₂ and CO₂ adsorption analysis can be effectively combined to analyze the all-scale (micro–meso–macropores) PSD with a limitation to 10μm. According to the all-scale PSD analysis, the PSD ranges mainly from 0.3 nm to 60 nm, and mesopores are dominant in shale samples, followed by micropores. Mesopores contribute the largest to PV and SSA, followed by micropores and macropores. Extremely low mercury withdrawal efficiency of shale samples manifest the poor connectivity. N₂ adsorption-desorption isotherms indicate that the plate-shaped and sheet-shaped pores are dominant in the study area. In addition, the controlling factors for the pore structure are Ro and the clay mineral content.

The analysis of characteristics of shale pore structure, has helpful and guiding significance to shale gas production in the Ordos Basin. This structure analysis results show that the shale reservoirs in the Ordos Basin are beneficial for the gas storage, but disadvantageous for gas flowing. Therefore, fracturing is necessary for the shale gas production.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016