Illite cement

Authigenic illite forms characteristic grainbridging cements, as seen by SEM. In very few samples, rims of illite of uniform thickness were found lining the pores (Fig. 14D) and contributing substantial amounts (2-5%) of cement, but in general illite cement, like chlorite, is negligible in Hibernia Field. 

An experiment was conducted using a soil called SI, an illitic clay containing chlorite from Michigan. Two sets of data showd the results of permeation of the regular soil, first with water and then with pure reagent grade heptane. The heptane caused the hydraulic conductivity of the regular compacted soil to skyrocket. About 8% cement was then added to the soil. 

It is generally agreed on the basis of TEM evidence (e.g., Rask et ai, 1997) and on the basis of chemical substitution in the several structural sites of illite (e.g., Lanson and Champion, 1991 Awwiller, 1993 Lynch et ai, 1997) that the illitization of smectite, like other replacement reactions in late diagenesis, proceeds through a dissolution/precipitation mechanism. Similar to other replacement minerals, illite also occurs as cements in sandstones as well as in shales, where it forms overgrowths on detrital illite particles and discrete crystals (cements) (e.g., Lanson and Champion, 1991 Rask et at, 1997). 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

The faulted, non-carbonate cemented sandstones which were sampled from Haltenbanken and Tampen Spur, show clear evidence of diagenetic modifications after the deformation had occurred. Feldspar dissolution, illite precipitation and stylolitization are examples of such diagenetic processes. The pre- and post-faulting diagenetic reactions have been demonstrated to be a useful tool for the purpose of dating fault movements relative to basin subsidence (Sverdrup and Bjprlykke, 1992 Saigal et al., 1995). 

As a result, the permeabilities in these domains within the formation become more uniform. Reduction in permeability in the more permeable domains improves the mobility ratio of waterflood. Premature breakthrough is thus reduced, and the efficiency of the waterflood is improved (Boston et al., 1969). Poorly cemented clay particles, such as kaolinite and illite, can become detached during aqueous flow, especially when flowing brines become fresher. 

In the Furado area there was no interruption in burial conditions caused by post-rift uplift, and the reservoirs remained at similar depths during this phase. Therefore, it is uncertain whether or not the mesogenetic constituents were precipitated during syn-rift or post-rift phases. Some C2 calcite, which engulfed and thus post-dated albite, chlorite, illite, quartz and trace amounts of pyrite, barite and sphalerite, is interpreted to have precipitated during this time interval. This C2 calcite is characterized by a chemical and isotopic composition similar to the carbonate cements formed during the syn-rift subsidence phase. 

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)... 

Sandstones in the central basin, with the highest initial porosities and permeabilities, were the first to experience extensive cementation. Presumably these were flow pathways and, considering the pore fluid volumes required for extensive cementation, it is not surprising that this relationship occurs. The basis for this observation is that sandstones with early high-volume cements are notably clay free (relatively low elemental Al content) compared with adjacent uncemented sandstones (Boles, 1989). The clay is a smectite-rich mixed-layer smectite/illite clay, believed to be detrital on the basis of its... 

Authigenic clay minerals identified by SEM/EDS include minor amounts of kaolinite, chlorite, illite and mixed-layer illite/smectite, which form grain coatings, platelets or fibrous cements. 

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