Organic matter pores in the chang 7 lacustrine shales from the Ordos Basin and its effect on reflectance measurement

Abstract

To quantify the pore characteristics of various macerals in Chang 7 lacustrine shales, macerals were effectively identified according to their optical and morphological characteristics, and the nanoscale pore structure of macerals was observed by scanning electron microscope. Meanwhile, the reflectances of different positions in the same pieces of vitrinite or solid bitumen with heterogeneous pores development were measured. The results showed that the average contents of sapropelinite, liptinite, vitrinite, inertinite and solid bitumen are 42.7%, 8.7%, 13.6%, 13.8% and 21.2%, respectively, which suggests that the source of the organic matter of the Chang 7 shales is a mixed source input. The organic pores of Chang 7 shales are enriched, and the pore shapes are mostly round or elliptical. The pore size of organic pores has a wide distribution, mainly concentrate in the range of 100–400 nm, and the average plane porosity of organic pores is 10.13%. The size order of the organic pores in various macerals is: solid bitumen < bituminite < alginite < vitrinite < fusinite < liptinite. The abundance order of organic matter pores of each maceral is as follows: alginite > fusinite > bituminite > solid bitumen > vitrinite > liptinite. OM pores are mainly contributed by bituminite, solid bitumen and fusinite. The plane porosity of bituminite increases with maturity. In the process of thermal evolution, the plane porosity of fusinite is distributed in the two ranges of 20%–28% and 1%–7%. The former is mainly the primary pores of the fusinite itself, and the latter is the secondary pores formed in the thermal evolution. As for the organic pores of other macerals, no obvious thermal evolution law was found. Meanwhile, the surface imperfections of vitrinite or solid bitumen is enhanced by the enrichment of organic pores (an increase in pore size or pore number), which may result in the underestimation of their reflectances.