Carbonate cements siderite

Carbonate cement content in the studied rocks varies from 0 to 45 vol. %, mostly forming the pore filling. The following carbonates were observed (Koztowska 2004) siderite, Fe-dolomite, ankerite and Fe-calcite. The term siderite corresponds to minerals from the isomorphic group FeCOs-MgCOs with 60-100 mol percent FeCOs. Most siderites fall into the interval siderite - sideroplesite (Fig. 3). 

Fig. 4. SEM picture.s of a siderite cemented interval in the Melke Formation (well 6506/12-6 depth 4197.6 mKB). Carbonate cementation contribute to a reduction in permeability. (A) Thin section in backscattered mode. (B) Authigenic siderite.

An increase in carbonate alkalinity in the NR subzone enhances the precipitation of carbonate cements with 6 0 compositions similar to oxic carbonates, but with a slight enrichment in Mn and Fe and depletion in C. Rhodochrosite and siderite precipitate in the MnR and FeR subzones of sediments containing large amounts of Mn- and Fe-oxides, respectively. Because the three subzones overlap, it is common to observe, such as in deep-sea sediments, that suboxic siderites and rhodochrosite are enriched in Mn and Fe, respec-... 

Well Depth (mRKB) Formation Quartz clasts K-feldspar clasts Plagio- clase clasts Mica clasts Heavy minerals Carbon- ate fossils Plant frag- ments Clay clasts Clay matrix Pyrite cement Authi- genic kaolinite Authi- genic illite Calcite cement Siderite cement Dolomite cement Quartz cement Porosity Grain size (mm)... 

Siderite occurs as scattered, small (10-15 pm), yellowish euhedral crystals attached to detrital grains or enclosed by later carbonate cements (Fig. 4A). The siderite has a relatively Mg-rich composition, up to 40 mol% relative to Fe (Boles, 1987), probably reflecting the Mg-rich composition of marine water (Mozley, 1989). Extensive cement zones or concretions of siderite have not been found, indicating that sideritization is not as common in this deep marine environment as in shallow marine and non-marine environments (e.g. see Mozley, 1989). 

Fig. 4. Photomicrographs of carbonate cements in sandstones of the San Joaquin basin. (A) Siderite rhombs (arrows) in pore space (dark areas). Well NCL 88-29, 2746.5 m (9010.8 ft). White bar is 0.25 mm. (B) Dolomite pore-filling sandstone from the central basin. Note high cement volume and undeformed detrital biotite (dark grains). Detrital grains are chiefly quartz and feldspar. Well NCL 88-29, 2717 m (8913 ft). White bar is 0.5 mm. (C) Calcite pore-filling cement from central basin. Note relatively high cement volume and partially crushed biotite. Well NCL 487-29,...

Diagenetic carbonate cement in reservoir sandstones of the Oseberg Formation (Brent Group) in the Oseberg field, Norwegian North Sea, occurs as disseminated siderite and ankerite, and as massively calcite-cemented intervals. Other diagenetic features include extensive feldspar dissolution and K-feldspar, quartz, kaolinite and dickite cements. Conditions of carbonate cementation are constrained on the basis of textural, geochemical and fluid inclusion evidence. 

Carbonate cement of variable mineralogy (siderite, calcite, dolomite and ankerite) is a common diagenetic feature in North Sea Jurassic reservoir sandstones (e.g. Saigal Bjorlykke, 1987 Walderhaug et al., 1989 Bjorlykke et al., 1992 Giles et al., 1992 ... 

Siderite is present in wells 30/6-7, 30/6-6 and 30/9-B26 in small amounts (1-5%). It occurs as intergranular rhombs or spherules disseminated throughout the sandstone, and often closely associated with detrital clays or micas (Fig. 6A). Siderite is an early diagenetic phase. It is never found postdating any of the other diagenetic phases, and is systematically engulfed by calcite in pores where both carbonate cements occur (Fig. 6A). 

Table 3. C and O isotopic analyses of carbonate cements (the dissiniinated wheat-seed siderite (Fig. lOA) of the Avalon Sandstone was not analysed isotopically)...

Carothers, 1992 Spiro et al., 1993 Morad et a ., 1994), The mineral is most suitable for the study of pore-water evolution during sediment subsidence because, unlike other carbonate minerals, siderite probably does not undergo recrystallization and isotope re-equilibration during burial diagenesis, as it has no unstable precursors or polymorphs (Curtis etai, 1975 Gautier, 1982 Pearson, 1985 Curtis Coleman, 1986). Therefore, stable isotope data of siderite cements can provide a powerful tool for the interpretation of diagenetic events in geological... 

Table 1. Derivation of the Tirrawarra Sandstone core samples. Mineralogical compositions were determined by semiquantitative bulk-rock XRD analysis. Siderite is the only carbonate cement present, but occurs in varying proportions...

Ra = R3 -h (10Rc)/0.9. The RC (residual carbon) term represents heavy bitumens or recycled kerogens not directly volatilized by pyrolysis, but that could be oxidized to CO2 or CO in the separate oxidation step (Lafargue et al. 1997). Flowever, the Kuparuk formation often contains cements, siderite among others, which decompose to CO2 or CO at temperatures reached in the oxidation step of Rock-Eval 6 analysis. Unacceptable variability in RC was observed in pyrolysis of Kuparuk samples, possibly because of decomposition of carbonate minerals. Therefore, a Y coefficient was adopted to correlate Rock-Eval 6 pyrolysis results to petroleum density, where F=(R1+R2)/ (R1 +R2 + R3). 

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