ILLITIZATION AND PALEOTHERMAL REGIMES IN THE MIDDLE ORDOVICIAN ST-PETER SANDSTONE, CENTRAL MICHIGAN BASIN - K-AR, OXYGEN-ISOTOPE, AND FLUIDINCLUSION DATA

Hydrocarbon reservoirs occur in the Middle Ordovician St. Peter Sandst one in the central Michigan basin at depths of 1.5-3.5 km and are diag enetically altered. Latest diagenetic cements include saddle dolomite, pervasive microcrystalline illite and chlorite, and quartz. A K-Ar an d O-18/O-16 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 deltaO-18 values between 12.7 and 16.9p arts per thousand SMOW. The deltaO-18 values of diagenetic quartz over growths range from 15.2 to 18.9parts per thousand. Fluid inclusion tem peratures in the quartz cement range from 70 to 170-degrees-C, reflect ing multiple generations of diagenetic quartz and/or precipitation ove r most of the diagenetic history. Reequilibrated fluid inclusions in t he saddle dolomite cement yield temperatures ranging from 90 to 150-de grees-C. A regionally significant episode of illitization occurred dur ing the Late Devonian-Mississippian. Temperatures of illite formation are indirectly estimated to be in the range of 125-170-degrees-C and m ost paleodepths of illitization are between 2.8 and 3.2 km. These resu lts imply that (1) illite formed from O-18-rich fluids, and (2) elevat ed geothermal gradients, i.e., greater than 34-degrees-C/km, existed i n the Michigan basin in the late Paleozoic. The K-Ar ages and the delt aO-18 values are not correlated to present depths of the samples or pa leodepths of illitization. Illites with young ages and low deltaO-18 v alues tend to be geographically distributed along the north-south bran ch of the buried Precambrian rift. The deltaO-18 values of the diagene tic quartz follow a similar trend. The spread of illite K-Ar ages and deltaO-18 values, and their geographic distribution, are best explaine d as reflecting abnormally high thermal regimes in the part of the bas in located above the presumably highly fractured basement along the ri ft.