Petrography, diagenesis and reservoir quality of the Triassic Fruholmen, Snadd and Kobbe formations, southern Barents Sea.
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Petrographic composition, diagenesis and reservoir quality of the Upper Triassic fluvial to marginal marine Fruholmen Formation, the Middle and Upper Triassic estuarine and coastal plain Snadd Formation, and the Middle Triassic estuarine Kobbe Formation have been examined in wells 7131/4-1 and 7222/11-1 located on the western Bjarmeland Platform and on the eastern Finnmark Platform in the southern Barents Sea. Twenty-nine thin-sections from the Fruholmen Formation, sixty-eight from the Snadd Formation and eleven from the Kobbe Formation were studied with a petrographic microscope, forty were point-counted, and six were studied with a cathodoluminescence microscope. The petrographic observations were compared with plug measurements of porosity and permeability.The Fruholmen Formation sandstones are moderately well and well sorted fine-, medium- and coarse-grained quartz arenites. The Snadd Formation sandstones are moderately well and well sorted and fine- and medium-grained, and the Kobbe Formation sandstones are mostly well sorted and fine-grained. Both the Snadd and Kobbe Formations comprise sublitharenites and lithic arenites with common to abundant metasedimentary rock fragments, but the Snadd Formation contains both K-feldspar and plagioclase, whilst the Kobbe Formation contains plagioclase only. These compositional differences between the three formations may largely be a result of the Snadd and Kobbe formations having had an eastern source area (Uralides), and the Fruholmen Formation a more southerly source area (Scandinavia), although a shift to a more humid climate in the Late Triassic may also have influenced sand composition.The Fruholmen Formation sandstones contain very little diagenetic cement, typically 1-2% quartz overgrowths, traces of pyrite cement, and occasionally 1-2% authigenic kaolin. The main diagenetic cement in the Snadd Formation is early diagenetic chlorite that occurs as grain coatings and more rarely as pervasive microporous pore-filling cement. Siderite is present in most Snadd Formation samples in amounts of 1-6%, and a few thin zones are strongly calcite-cemented. Up to 3% authigenic kaolin is commonly present, and traces of pyrite cement and quartz overgrowths occur. The Kobbe Formation sandstones contain the same diagenetic minerals as the Snadd Formation, plus a few albitic overgrowths on plagioclase. However, the total volumes of diagenetic minerals in the Kobbe Formation are typically very low, 1-5% in the point-counted samples.Diagenetic chlorite or poorly crystalline chlorite precursors may have formed from iron-rich colloidal material brought in by rivers and flocculated where fluvial waters mixed with marine waters. These colloids may also have been the main source for iron in early diagenetic siderite. Partly dissolved biogenic carbonate is still present in the Snadd and Kobbe formations, and carbonate fossils are probably the source of the calcite cement. The calcite cement engulfs and therefore postdates chlorite grain coats, siderite cement and authigenic kaolin. Quartz overgrowths were sourced from dissolution of quartz grains at stylolites evolved from clay laminae when temperatures reached 70-80°C.Reservoir quality is largely excellent in the Fruholmen Formation sandstones (25-32% helium porosity, 1 000-38 000mD permeability) because contents of detrital clay and diagenetic minerals are almost zero. Snadd Formation porosities are also in most cases high to very high, 26-36%, partly due to the chlorite coatings inhibiting quartz cementation. Permeabilities are mostly 100 to 5 000mD, but where microporous diagenetic chlorite fills the pore system permeabilities are very low to low, 0.1-15mD. The Kobbe Formation sandstones have been more deeply buried than the overlying formations, approximately 3.5km, content of soft components (detrital and authigenic clay, mica-rich rock fragments) is high, and compaction has therefore been severe. Porosities are consequently quite low, 15-21%, even in the best of the cored Kobbe Formation sandstones, and together with the fine grain size this results in low permeabillities, 1-20mD.Burial depth for the shallowest examined cores is only 0.4 and 0.56km, and present temperatures are around 30°C. The consolidated nature of these cores and the presence of quartz overgrowths that normally start forming at 70-80°C therefore suggest that the sandstones have been more deeply buried than at present. Lack of illitization of kaolin in the deepest samples indicates that they have not been subjected to temperatures above 130°C. Together with the degree of quartz cementation in the various examined samples this suggests uplift of around 1.5km in well 7131/4-1 and 1.7km in well 7222/11-1. Comparison of the present porosities in the quartz arenites of the Fruholmen Formation with the porosity depth trend for the Garn Formation also suggests 1.5km of uplift in well 7131/4-1.