Widespread dolomite recrystallization and porosity modification of upper Permian Zechstein carbonates, Symra discovery, Utsira High, Norwegian North Sea

Blinkenberg, Kasper H.; Rameil, Niels; Hodgskiss, Malcolm S.W.; Polonio, Israel; Riber, Lars; Gianotten, Ingrid Piene; Roberts, Nick M.W.; Lepland, Aivo; Stemmerik, Lars. 2024 Widespread dolomite recrystallization and porosity modification of upper Permian Zechstein carbonates, Symra discovery, Utsira High, Norwegian North Sea. Marine and Petroleum Geology, 170, 107064. 10.1016/j.marpetgeo.2024.107064

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Official URL: https://doi.org/10.1016/j.marpetgeo.2024.107064

Abstract/Summary

The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1016/j.marpetgeo.2024.107064
ISSN:	02648172
Date made live:	04 Dec 2024 15:39 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/538505

Item Type:

Publication - Article

Digital Object Identifier (DOI):

10.1016/j.marpetgeo.2024.107064

ISSN:

02648172

Date made live:

04 Dec 2024 15:39 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/538505