MIDDLE ORDOVICIAN HYDROTHERMAL DOLOMITE OIL FIELDS OF THE SOUTHERN MICHIGAN BASIN
Fluid inclusion homogenization temperatures (Th) (79ºC to 258ºC) for carbonate cements from six oil fields and one unproductive well indicate two fluid end members: a warm fluid (<180ºC) and a hot fluid (>180ºC). Decreasing average Th values distal from the Proterozoic Mid-Michigan Rift in southeastern Michigan suggest that the hot fluids emanated from the rift area in the underlying basement. Fluids are saline (16.1 to 49.4 wt. % NaCl equivalent) and likely ultimately sourced from Salina Group (Silurian) evaporites. Eutectic temperatures (Te) (-50ºC to -112ºC) suggest a complex Na-Ca-KCl brine.
Strontium isotope values of carbonate cements (0.7086 to 0.7110) are consistent with two fluid sources: Proterozoic basement and late Silurian evaporites. Carbonate cements are depleted with respect to δ18O (-6.59 to -12.46‰VPDB) and somewhat depleted with respect to δ13C (-1.22 to +1.18‰VPDB). Equilibrium calculations from δ18O and Th values indicate cement-precipitating waters were highly evolved (+1.3 to +14.4‰ δ18OVSMOW) compared to Middle Ordovician and Late Silurian seawaters (-7.5‰ to -5.5‰ and -6.5‰ to ‑3.5‰ δ18OVSMOW respectively).
Hydrothermal fluids in TBR oil fields likely have similar sources and timing. However, cathodoluminescence cement stratigraphies indicate that water-rock interactions along fault pathways modified source waters, giving each oil field a unique petrographic and geochemical signature. Reactivation of basement faulting and fluid movement in TBR oil fields likely occurred during Silurian-Devonian tectonism.