Late Paleozoic Remagnetization and Its Carrier in the Trenton and Black River Carbonates from the Michigan Basin

Abstract

Paleomagnetic study of subsurface samples of the Upper Ordovician Trenton and Black River Limestones collected from six industrial wells in the southern part of the Michigan Basin yields a well-clustered characteristic direction (mean Dec/Inc of 170.6°/ - 10.9°, $$\alpha_{95} = 7.6^\circ$$) after rotation of random declinations by aligning a present day field overprint with geographic north. The corresponding paleomagnetic pole at 52.4°N 111.0°E is similar to those of the remagnetized carbonates in Ontario and New York State, suggesting that the Michigan formations were also remagnetized in the late Paleozoic. Rock magnetic characterizations suggest that the characteristic magnetization is carried by single and pseudosingle domain magnetite grains. Very fine rounded single crystal magnetite grains, believed to be products of chemical precipitation, observed by scanning and scanning transmission electron microscope (SEM/STEM) are inferred to be the carrier of the remagnetization. Paired carbon isotope analyses of bulk carbonate and anhysteretic susceptibility ($$X_{arm}$$) of cuttings from two wells were examined to test the hypothesis that oxidation of hydrocarbons may lead to formation of authigenic magnetite. Oil saturated horizons and overlying strata in a producing well exhibit lower $$X_{arm}$$ intensities than the equivalent horizons in an adjacent non-producing well. Little correlation between $$X_{arm}$$ and $$\delta^{13}C$$ is observed in carbonate-rich lithologies due to the masking of the hydrocarbon related cement compositions by the host carbonate. In contrast, carbonate-poor shale horizons overlying hydrocarbon-rich strata in the producing well exhibit strong correlative peaks in $$X_{arm}$$ and $$\delta^{13}C$$. The overall lower $$X_{arm}$$ intensities in the hydrocarbon-producing well suggest that it is not merely the presence of hydrocarbons which promotes authigenic magnetite formation. Rather, the magnetite/hydrocarbon relationship more likely records processes associated with hydrocarbon migration and interaction with oxidizing subsurface fluids in areas peripheral to sites of accumulation.