Geochemical Constraints on the Origin of Dolomite in the Ordovician Trenton and Black River Limestones, Albion-Scipio Area, Michigan
GRANATH, VICTORIA C., Conoco, Inc., Ponca City, OK
The Albion-Scipio and Stoney Point oil fields in south-central Michigan produce from porous, vuggy dolomite in long linear trends associated with faulting in otherwise tight limestones. 87Sr/86Sr ratios for undolomitized limestones (0.70796-0.70824) fall within the range for Middle Ordovician seawater given by the Burke curve and indicate they have undergone little alteration since deposition. (87)Sr/(86)Sr ratios
for the matrix and dolomite cements in the reservoir horizons (0.70834-0.70898) are more radiogenic and overlap the range in composition for Late Silurian seawater. Thus, dolomitization might have resulted from brines dominated by Silurian seawater components.
Fluid inclusion geothermometry and oxygen isotope compositions of the dolomite indicate formation under maximum burial conditions. The average homogenization temperature for matrix and vug lining dolomite is 115 degrees C (n = 73). Later baroque dolomite cement homogenization temperatures are 133 degrees C(n= 11). O(18) composition of the matrix dolomite (-7.3 to-9.1% PDB) and the dolomite cements (-8.0 to-10.7% PDB) are also consistent with deep burial dolomitization. Average freezing temperatures are -29 degrees C (n = 72) and -36 degrees C (n = 19), respectively, indicating fluids are calcium- and magnesium-rich brines. 0(18) and H(2) compositions of fluids in fluid inclusions are similar to data for Michigan basin brines and suggest some interaction with heavy oxygen probably from siliciclastics.
These data indicate dolomitization in the Albion-Scipio area occurred under burial conditions from hot brines dominated by a Late Silurian seawater component. The brine may have been slightly modified either through mixture with another basinal fluid or interactions with siliciclastics during its circulation in the basin.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)