Mixing zone origin of [sup 13]C-depleted calcite cement: Oseberg Formation sandstones (Middle Jurassic), Veslefrikk Field, Norway
- Unocol Science and Technology Division, Brea, CA (United States)
Carbonate cement is one of the more volumetrically important diagenetic components of sandstones. The sandstones of the fan delta facies of the Oseberg Formation (Brent Group) in Veslefrikk field contain abundant carbonate cemented zones, commonly comprised of 30-40% carbonate by volume. The cement is predominantly poikilotopic, ferroan calcite, with minor amounts of ankerite and siderite. Petrographic evidence for high porosities at the time of calcite precipitation indicate that cementation occurred under shallow burial conditions, probably less than 500 m. Crosscutting textural relations demonstrate that calcite cementation was predated and postdated by tectonic stresses that produced both grain fractures and through-going fractures, respectively. In the non-calcite cemented parts of the formation major compaction, pressure solution, feldspar dissolution, and cementation by quartz and clays postdated calcite cementation. The oxygen isotopic composition of the calcite averages -8.5[per thousand] (PDB), and shows large variations within individual calcite cemented zones (up to 10[per thousand] in [delta][sup 18]O). The depleted and variable oxygen isotopic composition, in conjunction with evidence for shallow precipitation, indicate calcite growth occurred in a mixing zone environment, variably influenced by meteoric water. Isotopic variation within many individual cemented zones is symmetric about the center of the zones, indicating that growth of the cemented zones was from the center outwards in a concretionary fashion. The high Fe and Mn content of the calcite (avg. 1.3 mol% Fe, avg. 0.4 mol% Mn) and its association with disseminated pyrite indicate that porewater was reducing during precipitation. Carbon isotopic data (avg. [delta][sup 13]C = -15.7[per thousand] PDB, range = -3 to -31[per thousand]) show that major amounts of organic carbon, at least partly derived by sulfate reduction of biogenic methane, were incorporated in the calcite.
- OSTI ID:
- 6705728
- Journal Information:
- Geochimica et Cosmochimica Acta; (United States), Vol. 58:12; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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