Illitization and paleothermal regimes in the Middle Ordovician St. Peter Sandstone, Central Michigan Basin: K-Ar, Oxygen Isotope, and fluid inclusion data
- BRGM, Orleans (France)
- Western Michigan Univ., Kalamazoo, MI (United States)
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 {sup 18}O/{sup 16}O 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 {delta}{sup 18}O values between 12.7 and 16.9% SMOW. The {delta}{sup 18}O values of diagenetic quartz overgrowths range from 15.2 to 18.9%. Fluid inclusion temperatures in the quartz cement range from 70 to 170{degrees}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{degrees}C. A regionally significant episode of illitization occurred during the Late Devonian-Mississipian. Temperatures of illite formation are indirectly estimated to be in the range of 125-170{degrees}C and most paleodepths of illitization are between 2.8 and 3.2 km. These results imply that (1) illite formed from {sup 18}O-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 {delta}{sup 18}O values are not correlated to present depths of the samples or paleodepths of illitization. Illites with young ages and low {delta}{sup 18}O values tend to be geographically distributed along the north-south branch of the buried Precambrian rift. The {delta}{sup 18}O values of the diagenetic quartz follow a similar trend. The spread of illite K-Ar ages and {delta}{sup 19}O 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.
- OSTI ID:
- 54891
- Journal Information:
- AAPG Bulletin, Vol. 79, Issue 1; Other Information: PBD: Jan 1995
- Country of Publication:
- United States
- Language:
- English
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