5.2.1. Impact of TOC content
The enrichment of organic matter in shale is closely related to the sedimentary environment. Correlation analyses between sedimentary environment parameters and the content of Total Organic Carbon (TOC) reveal that redox-sensitive parameters (U/Th, V/Cr, Ni/Co, Mo(EF), δU) and a paleoproductivity indicator (P/Ti) are positively associated with TOC levels (Fig. 7a, b, c, d). In contrast, detrital influx indicators (Zr, Ti) and paleoclimate proxies (CIA, Sr/Cu) demonstrate an inverse relationship with TOC content (Fig. 7e, f). These findings underscore that anoxic conditions and elevated paleoproductivity are critical drivers in the accumulation of organic matter. Reducing conditions are conducive to the preservation of organic material, whereas high paleoproductivity fosters its proliferation, collectively influencing the organic carbon content and, by extension, the resistivity and polarizability of the rock matrix.
TOC content exhibits a negative correlation with resistivity (Fig. 7g), indicating that the resistivity of LMXF shale decreases with increasing TOC content. This trend is inconsistent with the classical positive correlation between TOC and ∆logR models. A similar trend has been documented in previous studies of Yangtze LMXF shale (He et al., 2017) and Qingshankou Formation shale (Jia et al., 2021). The analysis suggests that LMXF shale is in an over-mature stage, where organic matter thermally decomposes into a gaseous state, forming organic matter pores. TOC content shows a positive correlation with porosity (Fig. 7h). The bottom shale of LMXF has a higher TOC content, resulting in more organic matter pores (Fig. 8a). Proliferation of pores facilitates the emergence of an interconnected and porous network (Delle et al., 2018). An increase in pores, on one hand, shortens the electrical circuit of the rock, leading to a decrease in resistivity when liquid enters the pores. On the other hand, the increase in pores results in a larger surface area, activating organic functional groups under high temperature, leading to an increase in cation exchange capacity (Woodruff et al., 2017). Therefore, the negative correlation between TOC content and resistivity in the highly over-mature shale of the study area is mainly attributed to the presence of organic matter pores.
5.2.2. Influence of mineral composition and properties
Previous studies have indicated that some major oxide groups can represent the mineral composition, such as SiO2, Al2O3, Fe2O3, and CaO + MgO, which can represent the content of quartz, clay minerals, pyrite, and carbonate minerals in shale (Ross et al., 2009;He et al., 2022༛Li et al., 2023).
U/Th, V/Cr, Mo (EF), and δU show positive correlations with SiO2 (R2 = 0.63, R2 = 0.66, R2 = 0.79, R2 = 0.49, respectively, Fig. 9a, b). P/Ti also exhibits a positive correlation with SiO2 (R2 = 0.54, Fig. 9c). The Al-Fe-Mn ternary diagram is commonly used to determine the source of silica, where the ratio for purely hydrothermal quartz is 0.01, and biogenic quartz is close to 0.6 (Yamamoto, 1987). All samples from the LMXF plot within the bio-origin range (Fig. 9d), indicating the influence of biota on quartz in LMXF shales. In the reducing environment at the shale's base, there is a proliferation of biota, including graptolites and radiolarians (Fig. 8b, Fig. 8c). Additionally, SEM image and excess silica calculations reveal that the quartz in the bottom shale of the LMXF exhibits a flocculent structure (Fig. 8d), akin to that observed in the Barnett Shale (Milliken et al., 2012). This structure is primarily biogenic, suggesting a deeper water column at the bottom where biota remains preserved in the reducing environment. Subsequently, under certain temperature and pressure conditions, the remains undergo transformation to form biogenic quartz (Peltonen et al., 2009). SiO2 shows a negative correlation with resistivity (R2 = 0.70, Fig. 9e), indicating that biogenic quartz not only enhances the brittleness of the rock but also strengthens the shale matrix, thereby resisting compaction during diagenesis and maintaining certain porosity. This results in a positive correlation between porosity and SiO2 (R2 = 0.64, Fig. 9f). When conductive fluids are present in the pores, the shale's resistivity decreases.
CIA shows a positive correlation with Al2O3 (R2 = 0.46, Fig. 10a), indicating that paleoclimate conditions had a significant influence on the formation of clay minerals. Clay minerals are more susceptible to weathering in warm and humid environments. Resistivity exhibits a positive correlation with Al2O3 (R2 = 0.34, Fig. 10b), while porosity demonstrates a weak negative correlation with Al2O3 (R2 = 0.33, Fig. 10c). This suggests that the increase in resistivity with higher clay mineral content might be influenced by porosity. Shales with high clay mineral content are mainly developed in the upper part of the LMXF. SEM observations reveal poorly developed clay mineral porosity (Fig. 8e), indicating that during later diagenesis, the relatively lower compressive strength of clay minerals restricts the preservation of porosity. Less developed and poorly connected pores in clay-rich shales result in decreased ion exchange, leading to higher resistivity. Meanwhile, the polarization rate shows a weak negative correlation with Al2O3 (Fig. 10d), indicating that the polarizability decreases gradually with the increase of clay minerals of the LMXF. This phenomenon is mainly because the induced polarization effect of shale is related to the double electric layer on the surface of rock particles. When the clay content is high, the compaction effect results in smaller pore throat radii, which reduces the specific surface area (Nei et al., 2018; Ma et al., 2020; Tan et al., 2023), thereby weakening the double electric layer-induced polarization characteristics of the shale.
The Fe2O3 content ranges from 3.21–6.64%, with visible clusters of pyrite in the core (Fig. 8f), predominantly appearing as pyrite framboids at the microscopic level (Fig. 8g). U/Th, V/Cr, Mo(EF), and δU show positive correlations with Fe2O3 (R2 = 0.65, R2 = 0.45, R2 = 0.35, R2 = 0.67, respectively, Fig. 11a, b), indicating a generally low pyrite content and its formation being closely related to redox conditions. The particle size of pyrite framboids in shales can indicate changes in depositional environments (Wilkin et al., 1996). In the lower siliceous shales of the LMXF, pyrite framboids particles are generally smaller than 5 µm and are more numerous (Fig. 8g, h). Conversely, in the middle to upper shale sections, the particle size of pyrite framboids exceeds 5 µm (Fig. 8i), suggesting that the reduced conditions at the bottom are conducive to pyrite formation. A trend of increasing pyrite content with rising TOC content is evident (Fig. 11c), indicating that both TOC and pyrite are influenced by the redox environment. Resistivity shows a negative correlation with Fe2O3 (R2 = 0.72, Fig. 11d), and porosity exhibits a positive correlation with Fe2O3 (R2 = 0.52, Fig. 11e), suggesting that pyrite can influence resistivity changes in a dual manner. On one hand, pyrite, being a conductive mineral, shortens the conductive path when subjected to an electric current, leading to lower resistivity at higher pyrite content. On the other hand, increased pyrite content aids in pore preservation, with SEM images revealing numerous intercrystalline pores within pyrite (Fig. 8h), which can facilitate the movement of conductive fluids. Additionally, there is a positive correlation between pyrite content and polarization rate (R2 = 0.69, Fig. 11f). Under the electromagnetic field, the pyrite particles and the electric charge in the pore fluid undergo surface polarization (Misra et al., 2015). Under the effect of surface polarization, the dispersion characteristics will be enhanced, resulting in an abnormal polarization phenomenon. The content of pyrite in the anoxic environment is higher, and the interface polarization is obvious.
CIA, Sr/Cu, and Cao + MgO show positive correlations (R2 = 0.58, R2 = 0.33, Fig. 12a), indicating that carbonate minerals are easily influenced by paleoclimate, being more prone to weathering in a humid and warm environment. Resistivity exhibits a positive correlation with Cao + MgO (R2 = 0.45, Fig. 12b), and porosity shows a negative correlation with Cao + MgO (R2 = 0.37, Fig. 12c). This suggests that when the content of carbonate minerals is higher, porosity decreases, leading to higher resistivity. This phenomenon is primarily due to the higher content of plastic minerals such as clay minerals in the middle to upper shale sections. Although carbonate minerals can develop dissolution porosity during later diagenesis (Nie et al., 2019), these pores may be later filled, resulting in poor pore connectivity and hindering the flow of conductive ions.