RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 21, ES2001, doi:10.2205/2020ES000739, 2021

Anomalous porosity preservation in the Lower Cretaceous Nahr Umr sandstone, Southern Iraq

Abbas K. A. Mohammed


Core description, well logs data, petrographic analysis and scanning electron microscope technique were conducted to unravel factors controlling the preservation of high porosity up to 25% in deeply-buried sandstones > 4 km of the Lower Cretaceous Nahr Umr reservoir, southern Iraq. The Nahr Umr Formation composed of sandstone interbedded with shale, minor siltstones and streaks of limestone. The sandstones are arenites and range from fine to coarse, and poor to moderate sorted. Parallel lamination, planer cross-bedded and lenticular bedding are common sedimentary structures found in the formation, suggesting that the Nahr Umr deposited in fluvial-deltaic to the shallow-marine environment. Cementation by syntaxial quartz overgrowth was retarded by the presence of illite coats, which was formed by the illitization of the infiltrated smectite that formed during the deposition around the quartz grains. Microquartz coats in the form of quartz crystals probably further prevented the quartz overgrowths. Thus, the porosity of Nahr Umr preserved by the illite coats and microquartz crystals, whereas, the process of K-feldspar dissolution has created secondary porosity. The stylolite formation and the quartz-calcite replacement are the main sources of silica for the precipitation of quartz overgrowth.

Received 17 June 2020; accepted 23 August 2020; published 5 March 2021.

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Citation: Abbas K. A. Mohammed (2021), Anomalous porosity preservation in the Lower Cretaceous Nahr Umr sandstone, Southern Iraq, Russ. J. Earth Sci., 21, ES2001, doi:10.2205/2020ES000739.

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