--> Pore Structure and Fractal Characterization of Transitional Shales: A Case Study of Sha-3 Member Shales Within Shahejie Formation in Qikou Sag, Huanghua Depression, NE China

AAPG ACE 2018

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Pore Structure and Fractal Characterization of Transitional Shales: A Case Study of Sha-3 Member Shales Within Shahejie Formation in Qikou Sag, Huanghua Depression, NE China

Abstract

Investigation of pore structure and fractal characterization were mainly concentrated on marine or lacustrine shales in south China, related researches on transitional shales were scarce by contrast. This study will contribute to understand the pore systems of transitional shales and have a practical significance on shale gas exploration of Huanghua depression with a low development level at present. Pore structure and fractal characteristics of 15 shale samples from the Shahejie formation were investigated by X-ray diffraction, organic geochemical analysis , dual beam focused ion beam scanning electron microscope (FIB-SEM), and Low temperature N2 adsorption experiments. Fractal dimensions D1 at relative pressure 0-0.5 and D2 at relative pressure 0.5-1 were calculated respectively using Frenkel-Halsey-Hill (FHH) method based on N2 adsorption data, D1 ranged from 2.13-2.63 with an average of 2.33 and D2 ranged from 2.46-2.78 with an average of 2.51. FIB-SEM observation results showed that 15 shale samples were dominated by interpores and intrapores, OM pores were rarely found. The relationships between mineral constituents, TOC content, pore structure parameters and fractal dimensions (D1 and D2) were discussed. The results showed that mineral composition of shale samples were dominated by clay, follwed by brittle minerals including carbonate and quartz, which was different from marine or lacustrine shales. TOC content ranged from 2.0% to 2.5% with an average value of 2.2%, Ro ranged from 0.5% to 2.5%, and most of Ro were greater than 1.2%, reflecting a good shale gas potential. Fractal dimensions (D1 and D2) exhibited positive correlation with clay content on account of the flocculent structure of clay minerals with tremendous specific area, but fractal dimensions (D1 and D2) showed no apparent relationship with TOC content due to limited OM pores within the transitional shales of which the organic matter type was prevailingly humic. Both D1 and D2 showed a linear positive correlation with specific surface area, whereas a linear negative correlation with pore size, along with that D1<D2, indicating that the heterogeneity of pore structure was mainly contributed by smaller pores. In conclusion, fractal analysis is a valid method to characterize the pore structure and adsorption capacity of transitional shales, and clay minerals rather than OM contribute the most to adsorption capacity of transitional shales.