Ferroan carbonate cement

Mixed-layer days gradual conversion. Kaolinite development. Detrita quartz coating mainly by authi-genic chlorite. Slight quartz overgrowths. Some non-ferroan carbonate cement precipitation or replacement. 

Several porosity-destroying reactions also characterize this zone. For example, late ferroan carbonate cements are potential reaction products in this zone, and may significantly occlude porosity (Boles 1978). Also, there is a wide variety of aluminosilicate reaction products (e.g., kaolinite, illite, chlorite, and quartz) that can form in this zone as a result of aluminosilicate dissolution. The imbalance between porosity-enhancing or-preserving reac-... 

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-... 

Fig. 21. Distribution of porosity and carbonate cements in the Catalina Sandstone (= B Sandstone), 0-35 well. The sandstone beds are largely cemented with early ( ) ferroan calcite, except for a few centimetres above shale interlayers, where cement dissolution has generated significant secondary porosity.

Choquette, P.W. (1971) Late ferroan dolomite cement, Mississippian carbonates, Illinois basin, USA. In Carbonate Cements (Ed. Bricker, O.P.). Johns Hopkins University Studies in Geology, 19, 339-346. 

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. 

A systematic and sequential set of carbonate reactions characterizes most clastic source/reservoir rock systems during progressive burial. Typically, this sequence of carbonate reactions with increasing thermal exposure (depth of burial) is as follows (1) formation of early carbonate cements that preserve IGV (2) dissolution of the early carbonate cements (3) formation of late carbonate cements (usually ferroan), again preserving IGV and (4) if temperatures are high enough, dissolution of the late carbonate cements. This carbonate reaction sequence is responsible for windows of opportunity for porosity enhancement (positive porosity anomalies). The zones of volumetrically important porosity enhancement are the result of carbonate dissolution. 

The absence of dolomite cement at sandbody bases and its abundance at sandbody tops, the reported organic carbon isotope signal in the rhombic ferroan dolomite and the mixture of pedogenic dolomite textures and burial diagenetic textures in the sandbodies suggests that options 1 and 4 together are probably responsible for, the distribution of dolomite in the Chaunoy sandbodies. 

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