Sandstone poikilotopic

Girard J.-P. (1998) Carbonate cementation in the Middle Jurassic Oseberg reservoir sandstone, Oseberg field, Norway a case of deep burial-high temperature poikilotopic calcite. In Carbonate Cementation in Sandstones. Distribution Patterns and Geochemical Evolution (ed. S. Morad). International Association of Sedimentologists, Oxford, vol. 26, pp. 285-308. 

In addition to dissolution, the destabilization of carbonate cements may result in recrystallization and replacement by other carbonates. Microcrystalline calcite and dolomite are sensitive to recrystallization at various burial depths. The recrystallization of dolomite has been reviewed by Mazzullo (1992). Burial recrystallization of micritic/microsparitic cements in sandstones may result in the formation of poikilotopic calcite (Saigal Bjorlykke, 1987). However, poikilotopic calcite is also a common primary cement in calcretes (e.g. Knox, 1977 Tan-don Narayan, 1981). Recrystallized calcite and dolomite are recognized as patchily distributed, coarsened crystals. In contrast, precipitational vari-... 

The framework grains in sandstones cemented by microcrystalline and coarse to poikilotopic calcite display a loose grain packing even when their replacement by carbonates is considered. Evidence... 

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. 

Barite occurs as scarce, large crystals (up to 2 mm) filling vugs and cracks and engulfing as well as replacing kaolinite and carbonate cements in dolocretes and calcretes (Fig. 14F). In the sandstones, barite occurs as a few poikilotopic and small crystals which cover, and thus postdate, chlorite rims around framework grains. 

Carbonate cementation in the Middle Jurassic Oseberg reservoir sandstone, Oseberg field, Norway a case of deep burial-high temperature poikilotopic calcite... 

Isotopically light poikilotopic calcite was also reported by Walderhaug et al. (1989) in the Jurassic sandstones of the Fensfjord Formation in the nearby Brage field. Late diagenetic calcite, which seems to be volumetrically minor in Veslefrikk, constitutes the main, and apparently only, calcite generation in the Oseberg field. 

The source of calcium involved in the formation of heavily cemented poikilotopic calcite intervals in marine sandstones of the North Sea has been a... 

Attention is focused on two Australian petroleum fields, located in different geological settings more than 2500 km apart the Gidgealpa Field in the Jurassic-Cretaceous Eromanga basin of Central Australia (Fig. lA) and the Angel Field, located in the Dampier sub-basin (Carnarvon basin) of Australia s North West Shelf (Fig. 1B). The case studies exemplify the occurrence of major poikilotopic carbonate-cemented zones in sandstones buried to depths between 1500 and 3000 m, namely the... 

In the Upper Namur Sandstone the poikilotopic calcite cement is absent in moderately to poorly sorted quartz arenites and feldspathic quartz aren-ites that contain abundant primary porosity and some secondary porosity. Only minor patches of siderite micrite and microspar are observed in these elastics, where euhedral quartz overgrowths are well developed, albeit not volumetrically significant (< 5-10%) (Fig. 14D). The detrital grains are dominated by tangential and long contacts, with rare sutured contacts. 

Last but not least, Stienstra (1992) cites fluid inclusion microthermometry data by Bone (1989) showing that poikilotopic calcite cements precipitated at temperatures between 85 and 120°C in the Jurassic Adori Sandstone (Fig. 2) of the Tantanna, Marana and Strzelecki Fields (Fig. lA) (Bone, 1989). In these fields, the calcite cement has the same petrographic and isotope characteristics (Fig. 8b) as the calcite in the Namur Sandstone of the Gidgealpa Field. 

Poikilotopic carbonate cements can reduce reservoir porosity in relatively clean, massive sandstones over large areas, of the order of at least 300 km (Angel Field). Based on log characteristics, major carbonate-cemented zones can attain a cumulative thickness of at least 165 m in marine sandstones (Dampier sub-basin, Carnarvon basin) and 110 m in fluvial sandstones (Eromanga basin). The total volume of carbonate cement in petroleum fields can approach 1 km, as exemplified by. the Angel Field case study. 

A major structural control on carbonate cementation is demonstrated in both the Angel and Gidgealpa Field areas. In both fields, poikilotopic carbonate cements concentrate in hydrocarbonbearing sandstones near the crest of the fields, along the axis of the main structural trends. 

Plate 1. (A) Medium-grained Avalon Sandstone completely cemented by early, poikilotopic ferroan calcite cement (C), which corrodes quartz (Q) grains (arrows). 0-35 well, 2191.5 m, scale bar = 0.1 mm. 

B) Boundary between calcite-cemented and uncemented horizons in Avalon Sandstone, showing no evidence of dissolution. Straight crystal faces of the poikilotopic calcite (left) at the boundary (centre) indicate the presence of a cementation front, rather than a dissolution front. Note that framework grains in the porous zone are coated with thin clay rims, which are absent in the cemented zone. The former is also slightly more compacted than the latter. Same locality as... 

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