The comparative study of shale pore structure between outcrop and core samples of Ziliujing Formation shale from the northeastern Sichuan Basin in China

Abstract

The pore structure controls the storage and migration of geofluids within shale and ultimately determines its reservoir quality. Therefore, the characterization of shale pore structure has become more and more meaningful in geological engineering research. Due to the complicate sampling procedure and relatively high cost, the shale core samples are usually less in quantity and quite precious. Consequently, the shale outcrop samples, which were deposited in the similar sedimentary environment with shale core samples in the same layer, are always used to indicate reservoir characteristics. However, the pore structure similarities and differences between shale outcrop and core samples in the same layer is still unknown, which restricts the effective use of outcrop samples in shale pore structure characterization and requires detailed investigation. In this study, the pore structure of the outcrop and core samples of Jurassic Ziliujing Formation shale with high clay content from the Northeastern Sichuan Basin in China is quantitatively characterized by N2 adsorption and high-pressure mercury intrusion porosimetry (HPMIP) experiment and is semi-quantitatively revealed by field emission scanning electron microscope (FE-SEM) experiment. The results show that there is an obvious pore structure parameter difference between outcrop and core samples: the average specific surface area of outcrop samples is 8.47 times that of core samples, the average pore volume of outcrop samples is 1.83 times that of core samples; the average microfracture development level of outcrop samples is 1.43 times that of core samples, which is mainly caused by the higher microfracture development level of clay minerals in outcrop samples; the microfracture development direction is chaos in outcrop samples but relatively in order in core samples. However, there are still some pore structure similarities between outcrop and core samples: the whole-aperture pore structure distribution of outcrop and core samples has two similar distribution peaks at 15 ∼22 nm and 25 ∼42 nm; outcrop samples and core samples have similar pore space contributors: clay minerals (CM) pores > organic matter (OM) pores + quartz and feldspar (QF) pores while microfractures are mainly provided by clay minerals; the pore size distribution of OM and QF pores in outcrop samples were also similar to core samples. Overall, the pore structure of outcrop samples has certain indicative significance in terms of main pore size distribution interval, the relative contribution of different types of pores and the pore characteristics of OM, which shows the limited applicability of outcrop samples in shale pore structure characterization.