David A. Barnes

Illitization and paleothermal regimes in the Middle Ordovician St. Peter Sandstone, central Michigan Basin; K-Ar, oxygen isotope, and fluid inclusion data

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 ^dgr18O values between 12.7 and 16.9^pmil SMOW. The ^dgr18O values of diagenetic quartz overgrowths range from 15.2 to 18.9^pmil. Fluid inclusion temperatures in the quartz cement range from 70 to 170°C, reflecting multiple generations of diagenetic quartz and/or precipitation over m st 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 ^dgr18O values are not correlated to present depths of the samples or paleodepths of illitization. Illites with young ages and low ^dgr18O values tend to be geographically distributed along the north-south branch of the buried Precambrian rift. The ^dgr18O values of the diagenetic quartz follow a similar trend. The spread of illite K-Ar ages and ^dgr18O 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.

Sedimentology and diagenesis of the St. Peter Sandstone, central Michigan Basin, United States

The Middle Ordovician St. Peter Sandstone occurs at depths between 1600 and 3600 m in the central Michigan basin. Integration of conventional core and wireline log studies indicates that the formation consists of, from base to top, (1) up to 250 m of sandstone deposited in intertidal and supratidal sand flats with associated dolomitic lagoonal deposits, and shallow subtidal shoreface environments, (2) sandstones deposited in subtidal shoreface to upper offshore environments, and, (3) dolomitic and argillaceous sands deposited in a storm-dominated epeiric sea. Sandstone composition is closely related to depositional environment: quartz arenites occur in higher energy littoral facies, whereas feldspathic and carbonate-rich sands occur in predominantly lower energy shelfal f cies. Important modification of primary mineralogy and porosity occurred during diagenesis. A generalized, basinwide model for paragenesis includes (1) early calcite marine cement, (2) syndepositional dolomitization, (3) widespread precipitation of quartz and K-feldspar overgrowths, (4) pervasive replacement of early carbonate by burial dolomite, (5) local dissolution of unstable framework grains (e.g., K-feldspar) and carbonate cement, (6) growth of authigenic clay, and (7) pressure solution and additional precipitation of quartz overgrowths. Authigenic clays apparently formed along with or subsequent to economically significant secondary porosity. The different pathways of sandstone diagenesis observed in the St. Peter Sandstone are largely dependent on primary textures and composition inherited from the environment of deposition rather than other factors, such as depth of burial or position in the basin. The relationships among primary textures and composition, diagenesis, and reservoir sandstone properties is useful for prediction of reservoir quality in the St. Peter Sandstone in the Michigan subsurface. Weakly cemented sandstone reservoirs characteristic of high-energy shoreface and shallow offshore depositional environments have the best reservoir quality in the basin. Clay-cemented sandstone reservoirs deposited on offshore shelves have the poorest reservoir quality. Incompletely quartz-cemented sandstone reservoirs, comm n in intertidal/supratidal facies, have intermediate reservoir quality.