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The Gigas beds were studied in a subsurface area adjacent to the northern subcrop limits of the Lower Saxony basin, west of the Weser River. Fossiliferous beds were deposited, in supersaline epicontinental seas as the basal beds of a major saline cycle. Isopach maps reflect a pattern of low-thickness sills and near-shore areas adjacent to basins of greater thickness. Shale and sulfates predominate in the basins. Bioclastic and oolithic calcarenites and micrites form most of the shallow-water deposits.
Early diagenetic protodolomite replaced more than 50 per cent of the total sedimentary calcium carbonate. The extent of dolomitization varies from a trace to complete replacement. Dolomitization increases: (1) as the northern boundary of the Lower Saxony basin is approached, (2) as thickness decreases, and (3) as clay content increases. The three parameters are interdependent.
Dolomitization of the shallow-water carbonates postdates initial lithification. Here, protodolomite replaced calcium carbonate volume per volume, but due to dolomitization part of the calciclastics have been dissolved, creating voids. Dolomitization of basinal carbonates predates lithification and created no porosity. In both environments, protodolomite originated
before the conversion of aragonite to calcite and before tectonic fracturing. The magnesium content of protodolomite ranges from 41.5 to 47.5 mol. % MgCO3. It is influenced by depositional environment and the calcite/dolomite ratio of the individual sample. Dolomitization affects first the high-clay matrix, second the low-clay matrix, third the aragonitic calciclastics, and last the calcitic calciclastics. The original aragonite content in the carbonates is extremely important for the material balance during diagenesis.
Large scale replacement of protodolomite by calcite, encountered in one oil field, is related to weathering prior to a transgression. Intensive late diagenetic cementation by mostly calcite occurred before and following oil migration.
Dolomites with high diagenetic porosity are the main reservoir rocks. Undolomitized calcarenites with original high intergranular porosity form a secondary reservoir rock type. The reservoir properties of the Gigas beds change markedly in isofacies and isodolomite areas as a result of the variable intensity of the late diagenetic cementation. The occurrence of reservoir rocks is determined by the local coincidence of favorable facies with favorable diagenesis.
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