Abstract
Pore structure is a major factor affecting the storage space and oil-bearing properties of shales. Mineralogy, organic matter content, and thermal evolution complicate the pore structures of lacustrine shales. In this study, the porosity evolution of organic-matter-rich shales from the Cretaceous Nenjiang Formation in the Songliao Basin, Northeast China, are investigated using thermal simulation experiments and in-situ scanning electron microscope analysis. Three findings were obtained as follows: 1) The pore system of shales from the Nenjiang Formation is dominated by inter-granular dissolution pores of plagioclase and intra-granular pores of illite-smectite mixed layers. Few organic-matter pores are observed. 2) New pores developing during thermal evolution are primarily organic matter pores and clay mineral pores, with diameters greater than 18 nm. Clay mineral pores with diameters of 18–50 nm are the principal contributors to porosity at temperatures between the low maturity stage and the oil-generation window, and organic matter pores with diameters of greater than 50 nm comprise the majority of pores generated between the gas-generation window and the high-/over-mature stages. 3) Porosity increases continuously with maturity, and the pore system varies at different maturity stages. Porosity evolution is controlled by illite content and organic matter abundance. Total pore volume correlates positively with illite content but negatively with organic matter abundance. These findings could provide guidance on shale oil evaluation in the Songliao Basin and assist in the ‘sweet-spotting’ of lacustrine shale systems across China.
Highlights
As a new field of oil and gas exploration, research on shales has been a focus of attention for more than a decade
Zhang et al (2017) found that OM pores in Wufeng-Longmaxi Formation shale are mostly micropores and mesopores, smaller than 50 nm. 3) the clay minerals content, total organic carbon (TOC), and thermal evolution are major influences on pore structure (Zhu et al, 2016b; Wu et al, 2015, 2019; Cui et al, 2012; Kang, 2020; Duan, 2017; Xu J. et al, 2020; Hou et al, 2021a)
XRD mineral analysis shows that the minerals in the Nenjiang Formation (NJ Fm) shales undergo different changes with increasing temperature during thermal evolution (Table 1; Figure 3)
Summary
As a new field of oil and gas exploration, research on shales has been a focus of attention for more than a decade. In terms of static characterization of pore structures, research has concentrated on using various methods to evaluate shale pore types and determine their sizes and controlling factors In terms of dynamic evolution, the previous research has mainly focused on two aspects: 1) Geological comparison of shales with different mineral compositions, different OM abundances, and different thermal evolution maturity, using scanning electron microscopy, nitrogen adsorption, and other methods to evaluate the quantitative differences between pores (Curtis et al, 2012a). In terms of dynamic evolution, the previous research has mainly focused on two aspects: 1) Geological comparison of shales with different mineral compositions, different OM abundances, and different thermal evolution maturity, using scanning electron microscopy, nitrogen adsorption, and other methods to evaluate the quantitative differences between pores (Curtis et al, 2012a). 2) High-temperature and high-pressure physical simulation of the evolution of low-mature shales using physical simulation apparatus, focusing on determining the characteristics of pore evolution during thermal evolution (Cui et al, 2012; Jiale et al, 2021; Wu et al, 2015; Zhen and Wei, 2018; Wu et al, 2019)
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