Chlorite cement

Figure 14 Partially crushed foraminifer with chambers filled by chlorite cement (chi), kaolinite (k), and pyrite (py) cements. Reactions evident in these anomalously large pores are indicative of reactions that may also proceed in smaller pores throughout the shale. Frio Fomiation, Oligocene, South Texas. Backscattered electron image.

Calcite is the dominant cement in all of the sandstones except the chlorite cement-rich Ran-zano. The absence of volumetrically significant cements other than carbonate precludes a definitive placement of carbonate cementation within a progression of diagenetic events. No petrographic evidence marks any particular group of concretions as temporally distinct from another. IGV provides a crude estimate of burial at the time of cementation, and suggests that most of the concretions formed after considerable compaction. 

Sheet silicates (Q ) with significant isomorphic replacement of Si by AF+ or Fe +. These were decomposed by poly(acrylic add) to silica gel. The chlorite, thuringite, formed a strong cement but was much affected by water. 

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. 

Kaolinite and chlorite occur both in sandstones and shales. It is clear that in sandstones these minerals are primarily authigenic. They form largely as cements, but also as grain replacements. While these minerals may constitute a few percent locally, they usually amount to less than a percent... 

Chlorite in sandstones forms as both early (—20 °C) and late cements and grain replacements (Grigsby, 2001). As described in Section 7.07.4.5, early chlorite may exert a profound effect on porosity in late diagenesis as a result of its inhibitory effect on quartz cementation. 

Pyrite averages 0.2 vol%, and only in a few samples forms up to 1.3 vol%. It shows two occurrence habits (i) fine crystals (< 2. im) or framboids scattered in kaolinized or chloritized detrital clays and micas, or engulfed by coarse carbonate cements (Fig. 15E) and (ii) coarsely crystalline (up to 200 pm across), intergranular replacive cement. 

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. 

Sandstones with potentially better porosity preservation are characterized by (i) coarser grain size and better sorting (ii) lower tendency to host extensive eogenetic carbonate cement than the finer sediments, which are more represented by well 34/4-1 samples and (iii) chlorite rims evolved from the infiltrated clay coatings, which are more abundant in coarse-grained sands which inhibited precipitation of pore-occluding quartz and carbonate cements. 

Fig. 14. (A) Scanning electron micrograph of prismatic quartz outgrowths covering and engulfing chlorite rims (B) optical photomicrograph of a sandstone extensively cemented by quartz overgrowths which contain bitumen inclusions crossed polarizers.

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. 

Calcite cementation in bioclastic hybrid and lithic arenites of the Bismantova-Termina succession is pervasive along layers and concretionary horizons. Cementation in the hybrid shelf arenites was mostly precompactional and began with marine calcite rims, syntaxial overgrowths on echinoderms, K-feldspar and dolomite overgrowths, chloritic clay rims, framboidal pyrite and heulandite, followed by... 

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. 

Authigenic chlorite occurs as thin grain coatings and as fibrous cements, in both the Avalon/Ben Nevis and Catalina Sandstones, but rarely in the deeper reservoirs of Hibernia Field. It is one of the earliest cements in Hibernia Field, preceding early quartz cement. 

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