Ferroan calcite cement

The less common late ferroan calcite cement is associated with a minus-cement porosity of <15% (Fig. 13C), indicating that it was precipitated after significant mechanical compaction. It consists of pervasive sparry calcite, which completely filled the remaining reduced intergranular pore space. Fracture-filling calcite cement occurs in grain fractures (Fig. 13D) and rock fractures. It is common below the Avalon Sandstone. 

Replacement dolomite forms euhedral to subhedral rhombs up to 85 pm in diameter in the clay matrix of sandstones and in early ferroan calcite cement. 

Pyrite occurs both as early and late cement in Hibernia Field. Early pyrite is present in early calcite cement, fossil fragments and siderite nodules. Late pyrite is found in intergranular pores and fractures. The association of pyrite with a porous zone in Hibernia Sandstone and its absence in adjacent ferroan-calcite cemented sandstone suggests that this pyrite formed after the dissolution of the ferroan calcite. 

The Avalon/Ben Nevis Sandstone is an example of the depositional environment having affected subsequent sandstone diagenesis. The dominantly finegrained, marine to marginal marine and coastal sandstones (Sinclair, 1993) and interbedded shales are rich in calcareous fossils and probably provided the source of much of the early calcite cement. Early ferroan calcite cement in the Avalon/Ben Nevis... 

Secondary porosity contributes more than half (6.7%) to the total porosity (average.of 11%, excluding tight-sandstone zones) of the Catalina Sandstone. Of the available drill cores from wells 0-35 and K-18, 45.7 and 17.5%, respectively, are completely cemented by early ferroan calcite. The difference between the two wells may be due to the fact that the sandstone beds are thicker in well K-18. Where the early ferroan calcite is absent, mechanical compaction and quartz overgrowths have reduced the primary porosity further. In K-18, the framework grains are largely coated by a micritic-calcite rim, approximately 25 pm thick, which prevented silica cementation. Dissolution of the early ferroan calcite cement contributed most of the... 

Porosity reduction due to recementation af ter the dissolution event is generally small (1-2%) and involved kaolinite, quartz, pyrite and ferroan cal-cite and dolomite cements. Locally, as much as 10% of late cements have been precipitated. Kaolinite usually reaches no more than trace amounts, although locally up to 6% has been observed. Late quartz overgrowths are negligible. Late pyrite generally does not exceed 1% however, locally it may occlude all available pore space. Late ferroan calcite cement has completely filled the available pores at a few levels, but the total porosity loss by this cement and by late ferroan dolomite is not significant. 

The history of porosity development at the various reservoir levels in Hibernia Field clearly underscores the role of carbonate cements in delaying irreversible porosity loss in sandstone reservoirs. Early non-ferroan and ferroan calcite cements, precipitated at burial depths shallower than 2000 m, are widespread in oilfields around the world (e.g. Lindquist, 1977 Blatt, 1979 Loucks etal., 1984 Olaussen et a/., 1984 Bjorlykke et a/., 1986 Imam Shaw, 1987 Kantorowicz et al., 1987 Saigal Bjorlykke, 1987 and many others) and are typical as first major cements for a group of cement parageneses (Franks Forester, 1984). Four different sources of this early calcite cement have been considered in the literature (i) carbonate dissolu-... 

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. 

Carbonate cement/replacement in sandstones of the Denver Basin is most commonly ferroan calcite based on staining techniques. Dolomite may occur, but is volumetrically unimportant except in the Fox Hills Formation where dolomite (mean of 6.7vol%) is more common than ferroan calcite (mean of 4.5vol%). Ferroan calcite averages 2.8vol% for sandstones of the Middle Pierre Shale compared with less than 1 vol% for dolomite. Sandstones in the two Codell cores are quite different in terms of the amount of ferroan calcite cement, but average 2.1vol% with no dolomite. The Dakota J Sandstone has only trace amounts of ferroan calcite and dolomite cement. The Lyons Sandstone contains trace amounts of calcite and no dolomite. 

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