Elucidating Dolomitizing Conditions and Potential Recrystallization Using Textural and Geochemical Relationships, a Case Study From the Umm er Radhuma Formation, Qatar

Abstract

Geochemical proxies and crystal textures are routinely used to infer dolomitizing conditions. Petrographic and geochemical data from the Paleocene-Eocene Umm er Radhuma (UER) Formation in central Qatar support the hypothesis that different dolomite textures reflect separate dolomitizing events. Porosity and permeability data also show that dolomite textures control reservoir quality. Dolomite textures include nonplanar and planar-s mosaics, partially dissolved planar-e crystals with cloudy cores and clear rims (CCCR), and finely crystalline, planar -e fabric preserving (mimetic) dolomite. Although the UER dolomites occupy a narrow compositional range (49-51 mole% MgCO₃), powder XRD data distinguish three dolomite groups based on stoichiometry and cation ordering. Importantly, these groups correlate with the petrographically observed textures. Finely crystalline (x < 10 µm), planar-e mimetic dolomites are characterized by the lowest degree of cation ordering (0.36-0.50; x = 0.46) and stoichiometry (49.83-50.03 %Mg; x = 49.96%). Coarse (50-200+ µm) nonplanar and planar-s crystals comprising dolomite mosaics, in contrast, have the highest degree of cation ordering (0.73-0.88; x = 0.79) and moderate stoichiometry (49.87-50.13 %Mg; x = 50.05%). Planar-e dolomite crystals with CCCR also exhibit a high degree of cation ordering (0.63-0.95; x = 0.76) and the highest stoichiometry (50.77-51.03 %Mg; x = 50.87%). High-resolution SEM-EDS data corroborate the XRD-observed differences in dolomite stoichiometry between the textures. Petrographic and SEM-EDS observations also suggest that the CCCR dolomites have undergone recrystallization, whereby Mg-enriched rims formed at the expense of more Ca-rich cores. These results indicate that distinct environmental conditions influenced the resultant dolomite textures. Electron microprobe analysis is planned to further assess variations in the major cations (Ca, Mg) and trace elements (Fe, Mn, Sr). These data will allow more precise characterization of dolomite geochemistry, and will help better elucidate the conditions responsible for dolomitization of the UER.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019