Geologic characterization of the Asmari Formation, United Arab Emirates: the roles of depositional environment and diagenesis in determining the porosity and permeability properties of an important reservoir
Date
2022
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Publisher
University of Delaware
Abstract
The shallow marine carbonates of the Asmari Formation were deposited in an environment which varied from a deep lagoon to a reef-front margin with a local topographic restriction (Whittle et al., 1995). In the present study, we examined the depositional microfacies, biostratigraphy, and reservoir characteristics of three stratigraphic sections of the Lower Oligocene Asmari Formation in Al Ain, United Arab Emirates (UAE) to 1) determine the microfacies and the corresponding depositional environment within the Asmari Formation, and 2) to evaluate its microfacies characteristics and their quality as a reservoir for oil production. ☐ A multidisciplinary approach and an integration of sedimentology, biostratigraphy, geochemical and petrophysical analyses were used to achieve the research goal and answer the research questions. ☐ Ten microfacies were identified in the Senayah stratigraphic section: 1) lime mudstone, 2) packstone, 3) wackestone, 4) boundstone, 5) nummulitic packstone/grainstone, 6) coral algal grainstone, 7) coral algal boundstone, 8) coral algal wackestone/packstone, 9) coral algal packstone/grainstone and 10) reefal boundstone. These microfacies ranged from an open marine to mid-ramp with shallowing upward depositional trend, which implies a marine regression with the oscillations of the sea level. ☐ In the Mezyad stratigraphic section, three microfacies were distinguished: 1) coral algal boundstone, 2) molluscan packstone, and 3) oölitic grainstone. These microfacies were deposited in an inner ramp environment. Three microfacies were also recognized in the inner ramp carbonate environment preserved in the Malaqet stratigraphic section: 1) nummulitic grainstone, 2) highly fossiliferous wackestone/packstone with algal peloidal packstone, and 3) lime mudstone. ☐ The depositional model derived from the studied stratigraphic sections is closely aligned with that proposed by Whittle et al. (1995), though with some modifications. ☐ A total of forty-four species of benthic foraminifera belonging to twenty-five benthic foraminifera genera and two planktonic species belonging to one planktonic foraminifera genus were identified. The identified foraminifera mark the Early Oligocene and confirm the age of the studied section and were preserved within the shallow marine deposits. ☐ Porosity and permeability in the Asmari Formation ranged from 3.5% -19.98% and 0.11 mD - 9.17mD, respectively. The best reservoir microfacies within the Asmari Formation were found in the coral algal boundstone microfacies. The oil production from the grain-supported microfacies could also be improved with the hydraulic fracturing technique.
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Keywords
Asmari Formation, Early Oligocene, Foraminifera, Microfacies