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Abstract

AAPG Bulletin, V. 102, No. 1 (January 2018), P. 129-152.

Copyright ©2018. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/0327171524717016

Geochemical, petrographical, and petrophysical evaluations of a heterogeneous, stratiform dolomite from a Barremian oil field, offshore Abu Dhabi (United Arab Emirates)

Kazuyuki Yamamoto,1 Gary Ottinger,2 Osama Al Zinati,3 Hideko Takayanagi,4 Koshi Yamamoto,5 and Yasufumi Iryu6

1INPEX Corporation, Akasaka Biz Tower, 5-3-1 Akasaka, Minato-ku, Tokyo 107-6332, Japan; [email protected]
2Zakum Development Company, P.O. Box 46808, Abu Dhabi, United Arab Emirates; [email protected]
3Zakum Development Company, P.O. Box 46808, Abu Dhabi, United Arab Emirates; [email protected]
4Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aobayama, Sendai 980-0856, Japan; [email protected]
5Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-0814, Japan; [email protected]
6Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aobayama, Sendai 980-0856, Japan; [email protected]

ABSTRACT

A stratiform dolomite is developed in the Barremian carbonates of offshore Abu Dhabi, United Arab Emirates. Although the average thickness is only 1.1 m (3.6 ft), it is an important drilling target characterized by heterogeneous porosity–permeability values in the studied oil field. We discuss the origin and development of the dolomite based on geochemical, petrographical, and petrophysical evaluations using 81 cores recovered from the field. The δ13C values of the dolomites are relatively high (>5‰), and 87Sr/86Sr mostly fall in the range of Barremian seawater. The thin stratiform geometry and the geochemical signatures of the dolomite suggest early dolomitization just below the seafloor that was driven by the diffusion of Mg2+ from the seawater on top of a shallow platform. Lateral changes in the degree of dolomitization seem to be controlled by the permeability of the precursors. Dolomitization progressed further in packstone precursors (higher permeability) than in wackestone precursors (lower permeability), which led to greater permeability improvement in the packstone by replacement of fine matrix with larger dolomite crystals. Subsequently, late burial dolomite cementation occurred during oil migration, which preferentially affected the higher-permeability early dolomite, and areally, it progressed more in the flank of the field because of the delayed oil charge there. Dolomite-to-dolomite recrystallization became dominant after the oil emplacement. The recrystallization has been continuing under current burial temperatures (>100°C [>212°F]), fully resetting δ18O values and modifying the Fe and Mn concentrations of the dolomites, but has not significantly affected their petrography and petrophysics in the oil leg.

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