The correlation between minerals composition and porosity evolution is an essential topic for black oxidation process. A series of minerals reactions change the shale porosity, in turn, the evolution of porosity, especially the “active” pores —enable oxygen and fluids flow into — determine the fluid-rock reactions areas and control the kinetic rate of minerals reactions [3, 4, 12, 30–36]. Previous studies have well established the minerals-porosity correlations based on the chemical oxidation mechanism, but as a basic oxidation mechanism, the role of biological effect is seldom to report.
In present study, two classic effects —acid solution and A. ferrooxidans—were applied to simulate the chemical and biological oxidized black shale process, respectively. The results suggest that the chemical and biological effects led to varied alterations of minerals composition and porosity evolution, mainly due to the different alter trends of TOC content.
The acid solution was proven to accelerate the organic matters oxidation [9, 10, 13], that can penetrate the decrease of TOC content in G2. But the A. ferrooxidans, to our acknowledgements, majorly promoted the pyrite oxidation [37–41], so it hard to directly explain the increase of TOC content in G3. Previous studies proposed that the A. ferrooxidans cell bodies will attach on the rock surface when oxidize the pyrite [37, 39], so the coverage of rod-like material on the shale surface (Fig. 5d) and the accumulation of elemental C (Fig. 6d) on the shale microsurface may be the attached A. ferrooxidans cell bodies. Liu et al.  also reported an accumulation of organic matters on the rock surface during A. ferrooxidans oxidized ore contained pyrite; Jin et al. proposed the addition of modern organic matters at the regolith surface was associated with the biological turbulence such as the plant leaves, animal feces and microbial carcasses . Therefore, the increase of TOC in G3 can be contribute to the attachment and dead A. ferrooxidans cell bodies.
Due to the ability of blocking rock pores and inhibiting fluid-rock interactions, the organic matter is generally recognized as a negative factor for porosity evolution [4, 10, 15, 43]. But researchers also reported a positive relation between the specific surface area and organic matters content [11, 16], probably since the different contributions of micropores and macropores to the specific surface areas and pore volume. Kuila et al and Fisher et al reported a double value of specific surface areas obtained by nitrogen absorption experiment than by high-pressure mercury intrusion test which can not measure the micropores [11, 15].
We found that the chemical effect mainly increases the amount of macropores, whereas the biological effect mainly increases the amount of micropores. As shown in Fig. 4, the ratio of micropores in the specific surface areas (more than 20%) is much more than in the pore volume (less than 5%), that can explain a more reduction of pore volume than the specific surface areas occurred in G3. However, these micropores may be “inactive”—where oxygen and fluid can not flow into, resulting in the lower fluid-rock reaction areas and rates. Therefore, we found that the most reduction of TOC in G2 accompanied by the most decrease of pyrite and increase of clay minerals content among groups. Whereas the increase of TOC in G3 resulted in less change in pyrite and clay minerals content than that in G2.
Pearson correlation coefficients for relationships between pores volume and minerals content indicated that the TOC content is positively correlated with micorpores and mesopores in biological group (R = 0.72, 0.97), whereas it negatively correlates with mesopores and macropores in chemical group (R= -0.93, -0.94) (Fig. 7).
In summary, we propose that the increase of TOC in biological group impacts the correlation between minerals alteration and porosity evolution during black shale oxidation. The probably pathway is as follows: Initially, A. ferrooxidans promotes the pyrite oxidation and porosity formation, so the minerals content and porosity characteristics are similar between chemical and biological groups. With the processing of oxidation, the attached and dead A. ferrooxidans bodies blocked the large pores and decrease the rock-fluid reaction areas, then slow down the minerals reaction rate in biological group.