Regionally extensive, overpressured natural gas pools within the St. Peter Sandstone (Middle Ordovician) of the Michigan basin are bounded by rock types that include 3-7-m-thick zones of low-permeability, low-porosity carbonates or diagenetically banded quartz sandstones. Replacive dolomite from an approximately 5-m-thick carbonate interval in the east-central portion of the Michigan basin has very low ^dgr¹³C values that systematically decrease from approximately -5^pmil at the top to -10^pmil (PDB) at the base. ^dgr¹⁸O values for the replacive dolomite also decrease systematically with depth from approximately 27 to 23^pmil (SMOW). These data suggest an upward decrease in isotope exchange between the replacive dolomitization fluid and the precursor rock (i.e., the system was rock dominated at the top of the carbonate interval), which implies upward, cross-formational movement of the dolomitizing fluid. Fluid-rock interaction modeling suggests that the dolomitizing fluid had a total dissolved carbon (TDC) content of approximately 4000 ppm and a ^dgr¹³C value of -27^pmil, which indicates that the carbon was primarily derived from organic diagenesis. Sr isotope and major element data suggest that this dolomitizing fluid had a modified seawater origin.

Carbonate intervals in the St. Peter Sandstone contain dolomite cement in quartz sandstone laminations and fractures that have ^dgr¹³C values that closely mimic the large ^dgr¹³C variations of the adjacent replacive dolomite with depth. This correlation indicates that the replacive dolomite controlled the ^dgr¹³C value of the carbon dissolved in the parent fluids of the later dolomite cement. Moreover, these data suggest that the parent fluids of the dolomite cement were not confined to a common fracture network, but flowed pervasively through the carbonate intervals. The ^dgr¹³C values of dolomite cement in diagenetically banded quartz sandstone intervals from three locations in the central portion of the Michigan basin range from -9 to -4^ mil and are relatively invariant at a particular locality; therefore, the TDC of the dolomitizing fluid in the central Michigan basin is interpreted to have contained only about 20-30% organic carbon. The fact that all dolomites analyzed in the St. Peter Sandstone have much lower ^dgr¹³C values than carbonates in adjacent formations indicates that dolomitization and the formation of hydraulic seals were related to organic matter diagenesis.