Magnetic enhancement caused by hydrocarbon migration in the Mawangmiao Oil Field, Jianghan Basin, China

Magnetic parameters (volume-specific susceptibility κ, and hysteresis parameters and ratios) of 47 samples, collected from an oil-producing well (M36) and a dry well (M46) from the oil-bearing II-You Formation of Paleogene Xingouzui Group in the Mawangmiao Oil Field in China, were measured to address the secondary alteration of iron-bearing minerals associated with hydrocarbon migration. Our results indicated that both κ and magnetization (saturation magnetization Js and saturation isothermal remanent magnetization Jrs) of oil-bearing formation have been dramatically enhanced. Further grain size estimation reveals that the background samples (samples both in M46 and outside the oil-bearing formation in M36) contain coarser-grained magnetic particles (∼30 μm) of detrital origin. In contrast, the alteration of hydrocarbon produces finer-grained (∼25 nm) magnetic particles. The new constraints on grain sizes and its origin of the hydrocarbon-related magnetic particles improve our understanding of the mechanism of formation of these secondary finer-grained particles, even though the precise nature of this process is still unknown.