Influence of substrate morphology on reef diagenesis: Examples from the Zechstein Limestone formation (SE Wolsztyn Ridge, W Poland). (February 2022)

• Record Type: Journal Article
• Authors: Fheed, Adam; Świerczewska, Anna; Strzelecki, Piotr Jan; Radzik-Hotloś, Natalia; Gebus-Czupyt, Beata
• Is Part Of: Marine and petroleum geology. Volume 136(2022)
• Publication Date: 2022-02
• Subjects: Reefs -- Substrate morphology -- Diagenesis -- Sedimentary environments -- Southern Permian Basin -- Zechstein -- Reservoir -- Facies; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2021.105442
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Reefs may be formed in the middle parts of sedimentary basins and grow on tectonically uplifted substrate blocks. In this paper, the strata of the Central European reef system of the Upper Permian Zechstein Limestone (Ca1) formation located in West Poland are evaluated. The formation has produced natural gas since the early nineties of the past century. The reefs grew on an uplifted horst built of Carboniferous rocks, known as the Wolsztyn Ridge. As individual sub-blocks originally showed diversified altitudes, four zones suspected to represent initial sedimentation conditions are distinguished: (1) basin zone, located close to the margins of the WR; (2) lower reef zone; (3) transitional zone and (4) the upper reef zone . Clear diagenetic differences between particular zones are observed. The basin zone strata are characterized by a strongly recrystallized mosaic of dolomite but show a limited extent of anhydrite cementation. The lower reef zone rocks, covered by thick Zechstein evaporites have been dolomitized and anhydritized most intensively, probably by means of the downward seepage of heavy brines, as suggested by the stable isotope data. It is assumed that the process was focused on the calm-water facies due to their low primary porosity and permeability. This mechanism was also probably responsible for the dolomitization of Ca1 strata in the remaining zones, where lower δ 18 O values in dolomite reflect some temperature increase on the final isotopic signature of rocks. Transitional zone carbonates appear to be almost purely calcitic with only a minor dolomite admixture. They contain various types of calcite cement, including marine and meteoric-phreatic phases, reflecting an intermediate position of the substrate and optimal reef growth conditions. Marine phases are also commonly recognized in the upper reef zone. Owing to a significant elevation of the substrate in this zone, the corresponding strata might have experienced some impact of karstification, presumably taking place well before the deposition of the Zechstein evaporites. It is concluded that the morphology of the substrate in tectonically-influenced areas strongly controls the reef diagenesis, and thus can affect the distribution of hydrocarbon accumulations. Highlights: The morphology of the substrate influences reef diagenesis and should be considered in hydrocarbon exploration. The downward seepage of brines can cause dolomitization and anhydritization of reef rocks developed on uneven substrates. Brine seepage-related dolomitization may be intensified in mud-supported facies of small thickness, overlain by evaporites. The carbonate build-ups of the highest topographic position may become karstified and penecontemporaneously dolomitized. Porous reefs formed at intermediate altitudes are areas of intensive calcite cementation in marine and meteoric conditions.