Hydrocarbon-linked chemical remagnetization in the Upper Ordovician Zhaolaoyu Formation, southern margin of the Ordos Basin: constraints from paleomagnetism and geochemistry

Remagnetization in sedimentary basins is commonly regarded as “noise” that overprints primary remanence. However, if its acquisition timing and underlying mechanism can be constrained, remagnetization can instead be exploited as a physical archive of burial history, tectonic deformation, clay-mineral transformation, and hydrocarbon-related fluid activities. With the growing importance of hydrocarbon resource evaluation, magnetization resetting associated with organic-matter maturation and hydrocarbon migration has gradually become a major focus of remagnetization studies. To explore the relationship between remagnetization and hydrocarbon activity, we targeted the Upper Ordovician Zhaolaoyu Formation in the Fuping area along the southern margin of the Ordos Basin and carried out an integrated investigation combining petrographic observations, rock-magnetic experiments, paleomagnetic analyses, and organic geochemical measurements.

The results showed that magnetite was the dominant magnetic carrier, and stepwise demagnetization isolated a stable characteristic remanent magnetization. The corresponding paleomagnetic pole matched the Early–Middle Triassic segment of the apparent polar wander path of the North China Block, indicating a Triassic remagnetization. Anisotropy of magnetic susceptibility results indicated a primary sedimentary compaction fabric, and together with previous high-resolution Sr-isotope studies showing no signature of tectonically derived fluids in the section, these observations effectively ruled out remagnetization driven by subsequent tectonic fluids. Optical microscopy showed heterogeneous iron-oxide infillings within microfractures, and SEM further revealed spherical magnetite developed within microfractures; fluorescence microscopy also indicated that organic matter was predominantly hosted within microfractures. Collectively, these microscopic observations suggested that the spatial distribution of authigenic magnetite may be linked to the presence of organic matter. Notably, based on the commonly used parameter (NRM–TOC), natural remanent magnetization (NRM) showed a significant positive correlation with total organic carbon (TOC), further supporting an association between remagnetization in the Zhaolaoyu Formation and hydrocarbon activity. Meanwhile, we introduced the hydrocarbon generation potential (P_g) and the ratio of effective specimen number to total specimen number (N₀/N), and established quantitative relationships between P_g and NRM, as well as between N₀/N and TOC to evaluate the relationship between magnetic records and organic matter. Both relationships showed positive correlations.

In summary, the Zhaolaoyu Formation records an Early–Middle Triassic chemical remagnetization event associated with organic-matter maturation. This interpretation is consistent with previous hydrocarbon-generation modeling results for the study area. This study provides key constraints on the hydrocarbon-generation evolution of Ordovician source rocks along the southern margin of the Ordos Basin and, for the first time in the Ordos Basin, verifies the feasibility and applicability of using remagnetization as a tool to constrain hydrocarbon activity. In addition, the two parameter sets proposed in this study (NRM–P_g and N₀/N–TOC) provide new quantitative metrics that can be applied to explore similar remagnetization mechanisms in other stratigraphic intervals and sedimentary basins.