FLUID HISTORIES OF MIDDLE ORDOVICIAN FAULT-FRACTURE HYDROTHERMAL DOLOMITE OIL FIELDS IN THE SOUTHERN MICHIGAN BASIN, USA

Dolomitized fault-fracture structures in the Trenton and Black River formations (TBR) are the type example for hydrothermalpetroleum reservoirs world-wide. However, fluid histories of these structures are only partially understood. Trenton and Black River reservoirs in the southern Michigan Basin are composed of fault associated, vertical dolomite bodies that are highly fractured and brecciated. Open spaces are partially to completely filled by saddle dolomite and less frequently by calcite cement. Cathodoluminescence microstratigraphies of void filling carbonate cements are not correlatable between oil fields. Fluid inclusion homogenization temperatures (T-h) measured in carbonate cements indicate two fluid endmembers: a warm fluid (similar to 80 degrees to 180 degrees C) and a hot fluid (180 degrees to similar to 260 degrees C). Increasing T-h proximal to the underlying Proterozoic Mid-Michigan Rift (MMR) suggest that the hot fluids emanated from the rift area. Included fluids are saline (16.1-49.4 wt. % NaCl equivalent), and salinity likely is sourced from overlying Silurian Salina Group evaporites. First melting temperatures (Tfm), interpreted as eutectic temperatures (T-e), of fluids range from -112 degrees C to -50 degrees C, indicating a complex Na-Ca-KCl brine; the expected composition of dissolved Salina salts. Lower Te proximal to the MMR suggest the rift as a source of additional complexing ions. C and O isotope values for carbonate cements are depleted with respect to delta O-18 (-6.59 to -12.46% VPDB) relative to Ordovician seawaters, and somewhat depleted with respect to delta C-13 (-1.22 to +1.18% VPDB). Equilibrium calculations from delta O-18 and Th values indicate that cement precipitating waters were highly evolved (+1.3 to +14.4% delta O-18% VSMOW) compared to Ordovician and Silurian seawaters (-5.5% delta O-18% VSMOW). Strontium isotope values indicate two fluid sources: Proterozoic basement and Late Silurian evaporites. Values of Sr-87/Sr-86 for cements in the Freedom, Napoleon, Reading, and Scipio fields (0.7086-0.7088) are influenced by warm water sourced from Silurian strata, and values for cements in the Albion, Branch County, and Northville fields (0.7091-0.7110) record continental basement signatures. Cement precipitating fluids in TBR oil fields likely have similar sources and timing. However, water-rock interactions along fault pathways modified source waters, giving each oil field a unique petrographic and geochemical signature. Fluid movement in TBR oil fields likely were initiated by reactivation of basement faulting during Silurian-Devonian tectonism.

Automated summary

Dolomitized fault-fracture structures in the Trenton and Black River formations are globally representative of hydrothermal petroleum reservoirs, with fluid histories only partially understood. The structures in the southern Michigan Basin are mainly composed of fault-associated dolomite bodies that are highly fractured and brecciated. The fluid inclusion homogenization temperatures and isotopic values of carbonate cements suggest multiple fluid sources and complex water-rock interactions in the oil fields.