Framework grains cement

Carbonate cements either indirectly enhance or deteriorate the reservoir properties of sandstones. Enhancement of reservoir properties occurs when (i) appreciable volumes of carbonate cements are dissolved, causing the formation of secondary porosity and (ii) small amounts of carbonate cement are evenly distributed in the sandstones to support the overburden weight and prevent the collapse of framework grains and consequent elimination of primary porosity. Souza et al. (1995) demonstrated that a few per cent of dolomite cement is sufficient to prevent the collapse of Aptian reservoir sandstones from Brazil despite the high content of ductile lithic fragments. 

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. 

Early precipitation of calcite cement is indicated by high (>25%) minus-cement porosity and the absence of other cements, except minor siderite and quartz cements and clay-mineral coatings. Framework grains enclosed by early calcite cement are loosely packed and often appear to be floating as a result of marginal replacement. Precipitation of the early calcite cement occurred after certain burial because some grain breakage had already taken... 

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... 

DC, dissolved cement DFG, dissolved framework grains DRC, dissolved replacive cements F, fracture porosity PP, primary porosity SP, secondary porosity ToT.P, total (thin-section) porosity. 

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... 

Intergranular porosity is the dominant pore type observed in thin sections from the cores examined in this study. The amount of porosity that can be attributed to dissolution of framework grains in the Norphlet is volumetrically insignificant (Table 2). Although partial dissolution of early anhydrite is noted, no evidence for dissolution of significant volumes of pore filling cements was found in this study. 

Pressure solution and quartz cement to a great extent depend on a number of geological variables and in particular on grain size of the quartz and temperature. The influence of other parameters like grain sorting, clay cement content, composition of framework grains, presence of early cement and burial history of the reservoirs also have to be considered. The volume of quartz dissolved by pressure solution denotated as OQ or overlay quartz and the volume of quartz cement (QC) are expressed as a percentage of the overall volume of the detrital quartz (DQ) (Fig. 4.11). In this context, the utilization of the same approach by Houseknecht (1984), i.e. of the ratios OQ/DQ and QC/DQ, excludes in the sandstones considered any influence of the variability of the volume of detrital quartz on the volume of dissolved quartz and quartz cement. A quantitative evaluation of quartz cement and pressure solution was carried out on the basis of a study of thin sections under the optical microscope and with the aid of cathodoluminescence. 

Not only is the shallowest formation (Fox Hills) plagioclase-rich, but it also has the most porosity attributable to plagioclase dissolution. The Codell Sandstone is highly argillaceous with low permeability, which has retarded the dissolution of framework grains. In all sandstones, secondary porosity attributable to dissolution of K-feldspar is limited to trace amounts. The third form of secondary porosity, which is dissolution of calcite cement and replacement, is sparse (maximum of 1.5vol%) and absent in most samples. Trace amounts of secondary porosity also formed by the dissolution of lithic fragments. 

With continuous and progressive burial, a sandstone enters the intermediate burial zone at approximately 80°C (zone of intense diagenesis, 80 to 110°C Table 1). In this interval, a potential hydrocarbon reservoir accommodates important porosity enhancing reactions early-formed carbonate cements may be dissolved (or later carbonate cements inhibited), and aluminosilicate framework grains (both feldspar and lithic) may be dissolved. Thus, in this zone, porosity can be preserved or significantly enhanced. 

The dissolution of framework grains and early dolomite cement has significantly enhanced porosity in many Latrobe sandstones. Within these sandstones, about 2 to 10% porosity is estimated to have been created by dissolution. The proportion of dissolution porosity relative to total porosity appears to increase with increasing burial. Above 2000 m (6500 ft), only minor enhancement of porosity was observed. With increasing depth, and... 

It is concluded that the major diagenetic events modifying porosity within the Latrobe sandstones were (1) mechanical compaction and early dolomite cementation, (2) dissolution of dolomite and framework grains, and (3) deep cementation and recementation with quartz, kaolinite, and Fe-carbonate. Thus, the diagenetic history and porosity evolution of these sandstones can be summarized as follows (1) the first significant event was early porosity destruction by mechanical compaction and IGV preservation by early car-... 

Interest in the creation of secondary porosity by the dissolution of framework grains and cements is not new. Early contributions from Heald and Larese (1973), Rowsell and DeSwardt (1974) and Parker (1974) recognized... 

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