Calcite cementation patchy

Poikilotopic calcite eement reveals evidence of substantial dissolution and creation of secondary porosity. In well 34/4-1 ealeite dissolution was accompanied by the precipitation of kaolinite. Evidence for calcite dissolution includes (i) the seat-tered, patchy corroded remnants (ii) similarity of eorroded shapes of framework grains in areas where ealeite is no longer present to those in areas eemented by calcite and (iii) the presence of replaeive calcite cement within the framework grains but not in adjacent pores. Sandstones subjected to partial cement dissolution contain undeformed ductile grains such as micas. 

Patchy or microconcretionary calcite cement, i.e. millimetre- to centimetre-sized specks of calcite cement with spacings that are also typically in the millimetre to centimetre range, seems to be less common than pervasively calcite-cemented concretions and layers, and has only been described from cores (Walderhaug Bjorkum, 1992). The cemented specks may apparently coalesce to form larger calcite-cemented volumes (Plate 4), but owing to the lack of outcrop data it is uncertain what shapes and sizes calcite-cemented volumes formed by the coalescence of calcite cement patches actually have. 

Plate 4. Patchy calcite cementation merging downwards to form an almost pervasively cemented interval. Mouth bar sandstone from the Middle Jurassic Oseberg Formation in the Veslefrikk Field, Norwegian North Sea. Width of core is 14 cm calcite-cemented areas are light grey. 

Calcite is the most abundant cement in the Namorado Sandstone, but is patchy at many scales of occurrence. Commonly, calcite forms discrete and tightly cemented concretions embedded in weakly... 

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