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Lower Ordovician St. George Group Dolomite: Zoning Investigated by Using Secondary Ion Mass Spectrometer

Abstract

Dolomite is an important hydrocarbon reservoir but its origin and mechanism of formation remains a hot debate. Recrystallization to more stoichiometric dolomite is usually accompanied by characteristic textural and geochemical signatures. These factors are primarily studied using multiple populations of crystals, comparison of modern and ancient dolomites or from results of high temperature dolomite formation experiments. Representative dolomite samples of Lower Ordovician St. George Group carbonates were chosen for study. The study used multi-proxy high-resolution approaches to carry out imaging and elemental analyses of individual dolomite crystals viz: Scanning Electron Microscopy (SEM), cathodoluminescence (SEM-CL) and Secondary Ion Mass Spectrometer (SIMS). Data obtained were used to characterize the dolomites and also to constrain their condition of formation. Data from the concentric zones of the dolomites indicate that intensities of Cathodoluminescence (CL), Ultra Violet (UV) light and Blue (BL) light luminescence correlate with the concentration of activator elements (Mn2+ and Rare Earth Elements-Ce3+ and Y3+). Furthermore, data from SIMS show that there is coupled core to rim variations in Mg, Na, Sr, REE, Mn and Fe in the CL-zoned dolomites. This is interpreted to have occurred as a result of successive recrystallization in the ancient dolomites. It is note-worthy that the data obtained from SIMS is largely similar to those obtained from bulk solution analyzed by Inductively Coupled Plasma Mass Spectrometer. The main implication of this study is that recrystallization and episodes of dolomitization shown by multi-crystal populations is also apparent within dolomite crystals.