[1poster47-47] GG-9．地球化学

学生優秀発表賞エントリー

[G-P-11] Genesis and diagenetic pathway of dolomites in the lower Cretaceous carbonates offshore Abu Dhabi (United Arab Emirates)

*Mosekiemang Goitse¹、Yamamoto Kazuyuki²、Takayanagi Hideko^1,3、Asahara Yoshihiro ⁴、Montani Hiroki⁵、Alsabeai Jawaher⁵、Alshamsi Suad⁵、Yamamoto Kohshi⁶、Iryu Yasufumi^1,3 (1. Department of Earth Science,Graduate School of Science,Tohoku University,Sendai,Japan 、2. Technical Division INPEX Corporation,Tokyo,Japan、3. Advanced Institute for Marine Ecosystem Change (WPI-AIMEC), Tohoku University, Sendai, Japan、4. Department of Earth and Environmental Sciences, Nagoya University, Nagoya, Japan、5. ADNOC Offshore, Abu Dhabi, United Arab Emirates、6. Department of Earth and Environmental Sciences, Nagoya University, Nagoya, Japan.)

This research aims to understand the genesis and diagenetic pathway of dolomite in the Lower Cretaceous oilfield carbonates of Abu Dhabi (United Arab Emirates) based on its petrographical and geochemical signatures. The “A Formation dolomite” occurs as multiple stratiform beds associated commonly with evaporites (anhydrite nodules). Petrographic observation indicates that dolomite mainly occurs in mud-dominated facies, such as wackestone and mudstone deposited under a sabkha environment. This occurrence implies the dolomitization of precursor carbonates by hypersaline seawater immediately after the deposition at the intertidal to supratidal sabkha environment. This interpretation is supported by a high Na concentration of up to 5,000 ppm. The ⁸⁷Sr/⁸⁶Sr of the dolomite falls in a range of coeval seawater of the Lower Cretaceous. Hence, the dolomitizing fluid was originated from the evaporative seawater. The low δ¹⁸O values of the dolomite imply an overprint of the original geochemical signatures by diagenetic processes in burial settings. The dolomites were subjected to dedolomitization in burial settings, as proven by the depletion of trace elements and precipitation of calcite cements. The slightly increased Fe and slightly decreased Sr concentrations indicate that the dolomite interacted with another fluid such as an interstitial water in the buried depth. The deep burial diagenesis is confirmed by the high dolomite formation temperatures in a range of the reservoir temperature. This is because dolomite δ¹⁸O is largely reset by the ongoing dolomite-to-dolomite recrystallization, and the δ¹⁸O values no longer keep the initial values of evaporative dolomitization at the surface. The diagenetic pathway of the dolomite yielded different petrographic textures and, consequently, porosity and permeability of the reservoir rock. During the deep burial phase, geochemical compositions (δ¹⁸O and trace elements) were modified through dolomite-to-dolomite recrystallization, but there was no significant new dolomite formation as indicated by the initial seawater ⁸⁷Sr/⁸⁶Sr of the dolomite. Therefore, the main determining factor of the petrographic texture was the degree of dolomitization (= formation and supply of hypersaline water) in surface environments. As dolomitization progresses in mud-dominated facies, the permeability first increases as the fine-grained matrix micrite is replaced by larger dolomite crystals. However, as dolomitization progresses further, both porosity and permeability decrease due to dolomite cementation and to the replacive dolomitization. Therefore, the degree of evaporative dolomitization should not be too strong or too weak for the excellent reservoir rock properties of the “A Formation dolomite".