The research team, led by Xin Wang of the SINOPEC Petroleum Exploration and Production Research Institute together with colleagues from the China University of Petroleum in Beijing, focused on wells in the Changning, Luzhou, Da’an, Zigong, Yongchuan, Dazu and Hejiang areas of the Southern Sichuan Low-Steep Structural Belt. The Longmaxi Formation, deposited roughly 440 million years ago in a deep-water shelf to semi-enclosed stagnant marine basin, hosts 20 to 60 meters of organic-rich black shale. Anoxic bottom waters during deposition preserved abundant organic matter, and subsequent tectonic events including the Caledonian, Indosinian, Yanshanian and Himalayan orogenies subjected different parts of the basin to distinct burial and thermal histories. That spatial variation in heating, the study shows, is central to the electrical puzzle.

To classify the anomalies, the researchers introduced a dimensionless resistivity enhancement index, Zg, which compares the resistivity of gas-bearing shale intervals with that of adjacent non-gas-bearing dry intervals. Using this relative measure rather than absolute resistivity alone, they divided the shales into three categories: ultra-low-resistivity shale with Zg below 1, low-resistivity shale with Zg between 1 and 2, and conventional-resistivity shale with Zg above 2. The classification revealed striking regional patterns. Basin-margin wells in western Changning and Changning are overwhelmingly dominated by ultra-low-resistivity shale, with enhancement indices all below 0.5, while basin-interior wells in Luzhou, Yongchuan and Dazu show considerable divergence among individual stratigraphic sublayers, mixing conventional, low and ultra-low-resistivity intervals within single wells.

The link between resistivity and gas content proved equally revealing. When resistivity falls below 5 ohm-meters, most samples carry less than 2 cubic meters of gas per ton of rock, and gas content barely changes with resistivity. Between 5 and 15 ohm-meters, gas content rises rapidly with increasing resistivity, marking the most sensitive range for evaluating low-resistivity shales. Above 15 ohm-meters, most samples show medium to high gas contents, though the trend flattens. This means ultra-low-resistivity shales generally hold poorer gas-bearing capacity and higher exploration uncertainty, a practical warning for drilling programs that rely on resistivity logs alone.

Armed with an unusually broad experimental toolkit, the team then probed the physical basis of the conductivity. They combined well logging with geochemical analysis, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy, carbon dioxide and nitrogen adsorption, high-pressure mercury intrusion porosimetry, fractal analysis and laboratory electrical tests. Scanning electron microscopy revealed abundant organic pores ranging from 5 to 200 nanometers, including honeycomb-like pore networks tens of nanometers across, bubble pores up to 500 nanometers, and slit-like clay intercrystalline pores of roughly 0.5 to 2.0 micrometers. In some samples, carbonized organic matter had visibly bent and deformed, becoming filled with minerals or bitumen and losing most of its porosity.

Pore-size distributions told a consistent story. The shales show three dominant pore-size peaks, at 0.5 to 0.6 nanometers, 2 to 3 nanometers, and 10 to 10,000 nanometers. As resistivity decreases, total pore volume falls, driven mainly by the loss of mesopores and macropores. Ultra-low-resistivity shale has the dominant pore system shifted to 10 to 20 nanometers, low-resistivity shale to 10 to 50 nanometers, and conventional-resistivity shale to 1,000 to 10,000 nanometers. Micropores and small mesopores contribute more than 65 percent of the total specific surface area, and ultra-low-resistivity shale shows the least developed pore network of all, with its macropore surface area amounting to just 0.970 square meters per gram, or 2.899 percent of the total. The researchers interpret this meso-to-macropore loss primarily as a consequence of maturity-driven organic matter graphitization and the associated collapse or closure of organic pores.

Fractal analysis of nitrogen adsorption data from 16 samples added a geometric dimension to the picture. The fractal dimension D1, which describes pore-surface roughness, ranged from 2.5708 to 2.7200, while D2, which captures the structural complexity of the accessible pore network, ranged from 2.7685 to 2.8813, indicating strong heterogeneity. Resistivity increases with D1, because rougher surfaces impede the transport of charged particles, but decreases with D2, because more complex networks can enhance connectivity and open conductive pathways. Low-resistivity shales generally show higher D2 and lower D1 values than conventional shales, a combination that favors fluid retention and ionic conduction.

The most striking finding concerns the fate of organic matter itself. Because Lower Paleozoic marine shales lack indigenous vitrinite, the team measured solid-bitumen reflectance and converted it to an equivalent vitrinite reflectance, Ro,eq. When Ro,eq exceeds roughly 2.9 percent, shale resistivity drops sharply and inversely with total organic carbon content, following a negative logarithmic trend. Below that maturity level, the correlation between organic carbon and resistivity is weak or absent. Transmission electron microscopy made the transition visible at the atomic scale: the NX202 sample from western Changning displayed abundant ordered layered carbon structures with clear lattice fringes and ring diffraction patterns, evidence of advanced graphitization, while the Yongchuan sample showed only disordered carbon with no obvious lattice structure. XPS analysis quantified the trend, with the graphitic carbon proportion reaching a maximum of 24.68 percent in the NX202 kerogen and a minimum of 8.51 percent in a Zigong sample. Samples containing more than 15 percent graphitic carbon generally showed resistivity below 10 ohm-meters. The mechanism is physical: as aromatic carbon structures condense and order into graphite-like domains, delocalized pi-electron systems emerge that can carry current through the rock. The authors caution that the 2.9 percent value is an empirical division for their sample set, not a universal threshold.

At lower maturities, other conductors take over. Clay minerals enhance ionic conduction through cation exchange, electrical double layers and bound water, though correlations between clay content and resistivity proved weak, with determination coefficients of only 0.0713 to 0.3227. Pyrite showed stronger local effects, with determination coefficients reaching 0.4514 to 0.6754 in some wells, but the mineral’s influence depends on whether it forms interconnected framboidal aggregates, laminae or continuous bands rather than isolated grains. Given its low abundance, pyrite alone cannot span the rock; instead, connected pore water and clay-associated water films appear to bridge spatially separated pyrite grains, clay surfaces and graphitized organic matter into a composite conductive network. Formation water emerges as the primary ionic medium: average water saturation in low-resistivity intervals exceeds 40 percent in most wells, reaching 63.48 percent at NX202 and 51.19 percent at N211, compared with 33.06 percent at the conventional-resistivity well Ning-215. Laboratory electrical tests showed that resistivity rises steeply once water saturation drops below about 40 percent, as continuous water films break down, while above 50 percent saturation interconnected water films keep resistivity low.

Synthesizing these observations, the team proposes a maturity-dependent genetic model. Below Ro,eq of about 2.9 percent, low resistivity is governed mainly by ionic conduction through formation water and clay-mineral surface conductivity, with pyrite playing a secondary role and fractures promoting local water enrichment. Above that maturity, organic matter graphitization becomes increasingly important, and ordered graphitic carbon domains provide electron-conductive pathways that, together with clay-water ionic conduction and pore-structure evolution, drive resistivity down. The authors emphasize that the model is qualitative, since the conductivities of individual phases, formation-water chemistry and pressure effects were not independently quantified. Even so, the framework gives exploration geologists a practical rule of thumb: in highly to overmature basins, a low resistivity reading may signal not water but rock that has been cooked into a semiconductor, and reservoir evaluation methods must adapt accordingly to avoid writing off some of the richest shale gas intervals.

Subject of Research: Conductive mechanisms and genetic model of low-resistivity marine shales in the Longmaxi Formation, Sichuan Basin

Article Title: Influencing factors and genetic model of low-resistivity shales: a case study of the Longmaxi Formation, Southern Sichuan Basin, China

Article References: Wang, X., Cao, Z., Hu, Z., Jiang, Z., Hao, R., Shi, H., Gao, W., & Wang, D. (2026). Influencing factors and genetic model of low-resistivity shales: a case study of the Longmaxi Formation, Southern Sichuan Basin, China. Frontiers in Earth Science, 14, Article 1950939. https://doi.org/10.3389/feart.2026.1950939

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1950939

Keywords: low-resistivity shale, Longmaxi Formation, Sichuan Basin, organic matter graphitization, shale gas, pore structure, formation water, pyrite, clay minerals, thermal maturity, well logging, X-ray photoelectron spectroscopy