ABSTRACT
Hydrocarbon reservoirs occur in the Middle Ordovician St. Peter Sandstone in the central Michigan basin at depths of 1.5–3.5 km and are diagenetically altered. Latest diagenetic cements include saddle dolomite, pervasive microcrystalline illite and chlorite, and quartz. A K-Ar and 18O/16O study of the fine-grained authigenic illite in 25 samples from 16 wells covering a large area within the basin yields K-Ar ages ranging from 367 to 322 Ma and δ18O values between 12.7 and 16.9‰ SMOW. The δ18O values of diagenetic quartz overgrowths range from 15.2 to 18.9‰. Fluid inclusion temperatures in the quartz cement range from 70 to 170°C, reflecting multiple generations of diagenetic quartz and/or precipitation over most of the diagenetic history. Reequilibrated fluid inclusions in the saddle dolomite cement yield temperatures ranging from 90 to 150°C.
A regionally significant episode of illitization occurred during the Late Devonian-Mississippian. Temperatures of illite formation are indirectly estimated to be in the range of 125–170°C and most paleodepths of illitization are between 2.8 and 3.2 km. These results imply that (1) illite formed from 18O-rich fluids, and (2) elevated geothermal gradients, i.e., greater than 34°C/km, existed in the Michigan basin in the late Paleozoic.
The K-Ar ages and the δ18O values are not correlated to present depths of the samples or paleodepths of illitization. Illites with young ages and low δ18O values tend to be geographically distributed along the north-south branch of the buried Precambrian rift. The δ18O values of the diagenetic quartz follow a similar trend. The spread of illite K-Ar ages and δ18O values, and their geographic distribution, are best explained as reflecting abnormally high thermal regimes in the part of the basin located above the presumably highly fractured basement along the rift.
