Dating penecontemporaneous dolomitization in carbonate reservoirs: Paleomagnetic, petrographic, and geochemical constraints

Abstract

The predominant dolomitization in the Mississippian Debolt Formation, Western Canada sedimentary basin was hypothesized to be early in three gas fields, based on stable oxygen and carbon isotopic values and on strontium isotope ratios that are similar to postulated Mississippian dolomite values and seawater ratios, respectively. As the absolute age could not be determined by this method, paleomagnetism was used to place constraints on the age of the dolomitization. The magnetic analyses on Debolt specimens from the same wells revealed three magnetization components that could be tied to geologic/diagenetic events: (1) a low-temperature ( 350 degreesC) and coercivity (>80 mT) C component of primary or early diagenetic origin that is found mostly in the fine-grained limestone and dolomitic muds. The preservation of a primary or early diagenetic magnetization, combined with the preservation of primary isotopic values, indicates that little or no extrabasinal fluid flow is likely to have occurred. Thus, in this particular area, orogenically induced fluid flow cannot explain the presence of the Cretaceous B magnetization. Consideration of the potential methods for forming this B component suggests that it is likely an in-situ chemical remanence resulting from either hydrocarbon migration or dissolution and reprecipitation of Fe-rich minerals in the original pore fluids.