MIDDLE ORDOVICIAN HYDROTHERMAL DOLOMITE OIL FIELDS OF THE SOUTHERN MICHIGAN BASIN

Fault-fracture oil fields in the Trenton and Black River formations (TBR) constitute important petroleum reservoirs in the southern Michigan Basin and are the type example for “hydrothermal” petroleum reservoirs world-wide. The fields are structurally related and controlled by SE to NW trending fault systems. They include the Albion, Napoleon, Northville, Scipio, Stony Point, and several smaller fields. These oil fields share many characteristics in common with Mississippi Valley-type mineral deposits including dolomitized breccias, coarse crystalline carbonate cements, and sulfide mineralization.

Fluid inclusion homogenization temperatures (T_h) (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 T_h 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 (T_e) (-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 T_h values indicate cement-precipitating waters were highly evolved (+1.3 to +14.4‰ δ¹⁸O_VSMOW) compared to Middle Ordovician and Late Silurian seawaters (-7.5‰ to -5.5‰ and -6.5‰ to ‑3.5‰ δ¹⁸O_VSMOW 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.