Framework grains

PassarettiM. L. and EslingerE. V. (1987) Dissolution andreUc textures in framework grains of Holocene sediments from... 

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. 

Fig. 12. (A) Type 3 (phreatic) tabular unit. Although there is good preservation of sedimentary structures, a tube that branches downwards is shown by the arrow. The hammer is approximately 18 cm long. (B) Type 3 (vadose) tabular unit. The arrows point to relict sedimentary structures. Divisions on the scale are in decimetres. (C) Spar-micrite microtexture from a type 3 (phreatic) unit. Framework grains are coated by displacive isopachous spar, and the space between is filled with micrite. (D) Micrite-spar microtexture from a type 3 (vadose) unit. Grains and groups of grains are coated with micrite, and the space between is filled with spar.

Waters that recharged the aquifer had undergone water-rock interaction, mixing oxygen values from meteoric waters with those derived from dissolution of 0-enriched minerals in the rock. Dissolution of framework grains, particularly volcanic rock fragments and feldspar, is common in the Zia Formation. 

COMPOSITION, PROVENANCE AND DIAGENETIC MODIFICATION OF THE FRAMEWORK GRAINS... 

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. 

Hematite occurs sparsely in the fine-grained flood-plain sediments, as tiny pigments that are either evenly distributed in the sediment or closely associated with infiltrated clay coatings around framework grains, and as alteration products of detrital Fe-bearing minerals such as Fe-Ti oxides. 

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. 

The climatic conditions and episodic flooding that characterized the depositional setting of the Lunde enhanced the infiltration of suspended clay particles and the formation of coatings around framework grains in the sandstones. Clay minerals formed by weathering processes in the hinterlaixl under arid to semi-arid climatic conditions would be expected to... 

The framework grains are composed predominantly of monocrystalline quartz (av. 49... 

The framework grain composition and palaeo-current analyses indicate that the source rocks for the Serraria Formation were mainly granitic-gneissic rocks, schists and quartzites of the Pre-Cambrian Complex bordering the northern sector of the Afro-Brazilian depression (Garcia, 1992) (Fig. 3). Other less important source rocks include a volcanic suite. 

The sandstones are well sorted and have a variable carbonate grain content (0-45% of the framework grains). The grain size decreases from medium... 

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. 

The Cretaceous fields are all similar in their framework grain composition and in the authigenic phases present (Table 2). There is one important difference to note, however, in comparing the two Dakota Formation producing fields. At Crooks... 

Fig. 3. Thin-section photomicrograph with partially crossed nicols of Tensleep sandstone from Big Sand Draw field, Wyoming, 2211 m (7255 ft), showing euhedral dolomite crystals < 1 gm (arrows) rimming framework grains. Scale bar= 38 gm.

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... 

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