Intergranular volume cementation

Burial carbonate cementation occurs subsequent to considerable compaction, leading to a successive decrease of both intergranular volume (IGV) and of 5 0 of the carbonate. However, in some basins, carbonate cementation may occur by ascending hot basinal brines to shallow depths (Sullivan et al.,... 

Fig. 16. Plot of intergranular volume (%) versus cement (%) for Lower and Middle Lunde sandstones with an intergranular volume 40% (see Houseknecht, 1987).

Few fluid inclusion data have been published for calcite cement in shallow marine sandstones, and homogenization temperatures in calcite may possibly be reset (Barker Goldstein, 1990). However, the measurements reported by Saigal Bjorlykke (1987) are in the range 56-68 C and thus compatible with calcite precipitation prior to deep burial. Finally, the high intergranular volumes found in many calcite-cemented sandstone samples (Table 1) also indicate relatively early calcite precipitation. 

M. Cervarola, M. Modino and Macigno formations contain only small amounts of calcite cement, because intergranular volume (IGV) was reduced to <10% before cementation took place. 

Fig. 15. Intergranular volume versus calcite cement amounts in the Namorado Sandstone, based on the diagram of Houseknecht (1987)i...

Fig. 9. BSE image of a fine-grained, moderate to poorly sorted back-barrier marsh sample completely cemented by SI (white). Note the serrated nature of some quartz grains and the very high intergranular volume (>50%), which suggest the replacement of part of the margins of quartz grains by siderite cement. Sample F4-9441, Fly Lake 4, 2877.6 m. Scale bar = 500 pm.

Fig. 14. Intergranular volume (IGV) versus total cement for Norphlet sandstones from Mobile Bay. The diagonal lines represent intergranular porosity. The majority of samples plot in the shaded area representing IGV < 26%, indicating that intergranular pressure solution has played a significant role in reducing IGV.

We shall now propose an approach for the evaluation, in the Saharan reservoirs, of the relative importance of the process of compaction and cementation in the reduction of porosity by presenting correlation plots of intergranular volume (VIG) vs. cement (Fig. 4.16). It is generally held that under the conditions of sediment accumulation on surface the VIG of well-graded sandstones is about 40%. This VIG or porosity can only be reduced by compaction, a mechanical process reducing VIG to 30%. Any further reduction by chemical compaction or pressure solution is a specially important process. The intergranular porosity of a sandstone is a function of the volume preserved after compaction and of its (the VIG s) portion filled by cement (Fig. 4.16). The inter-... 

It is usual for heavy concretes to have a certain structure. After selecting quantitative mixed proportions for producing dense packed coarse aggregates, the intergranular volume must be filled by cement-sand... 

The formation of carbonate cements typically preserves the intergranular volume (IGV) present at the time of cement formation with subsequent progressive burial, the IGV will be anomalously high. If the carbonate cement is later destabilized and the carbonate is dissolved, the sandstone will be characterized by enhanced porosity, a positive porosity anomaly. Enhanced porosity, or a positive porosity anomaly, is defined herein as porosity at a specific depth that is greater than the porosity predicted at that depth by a compaction/depth curve for the corresponding sandstone lithology (see Sclater and Christie 1980 Pittman and Larese 1991). 

Mineral precipitation within primary intergranular pore spaces leads to cementation. The existence of cements demands elemental mobility at least on the scale of a thin section, because cements reduce porosity that was present at the time of deposition. During early diagenesis of unconsolidated sediments, displacive cementation may lead to cement volumes in excess of 45%. In late diagenesis the upper limit of cement volume is constrained by the IGV at the time of cement emplacement. 

High cement volumes characterize many calcite cements from the basin margin sediments, and these presumably formed at shallow burial depths before significant compaction (Fig. 4E). Intergranular cement volumes are of the order of 30% to more than 40% in these samples. However, most cements from the basin margins have volumes indicating that they formed after some compaction and, like the central basin, cementation is a process that extends through much of the burial history (al-... 

Ankerite occurs as disseminated intergranular rhombs in wells 30/6-7, 30/6-8 and 30/9-1. It was identified by XRD and EPMA (Table 1). Ankerite rhombs ate often precipitated within exfoliated mica flakes or on detrital surfaces of adjacent quartz grains (Fig. 6B). Ankerite abundance does not exceed a few percent, except in sample 30/9-1 2781.0, where it reaches 33% of the total rock volume. This is the only occurrence of abundant ankerite cement found in the samples investigated in this study. 

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. 

Fig. 19. Results of compaction and quartz cementation models calculated using quantitative petrographic data, Touchstone and the thermal history shown in Figure 16. (a) Calculated versus observed IGV 4% tolerance, (b) Calculated versus observed quartz cement volume 3% tolerance, (c) Calculated versus observed intergranular porosity 4% tolerance.

The calculation of the hydrological parameters necessary for establishing the above-mentioned silica balance appears to be a rather complicated problem as intergranular pressure solution leads to a dynamic reduction in rock mass and volume and frequently also to a lowering of the porosity. These parameters have been simulated for different petrophysical and geochemical conditions. The first results show that for the large amount of secondary silica observed to become exported, 0.5 x io -o.5 x lo cm of water is required for every cm of the Cambrian sandstones of the Oued el-Mya Basin. If we assume, on the other hand, that the quartz cement in the Ordovician sandstones from Ahnet Mouy-dir resulted within the rocks themselves from pressure solution then the water flux necessary was small or virtually nil and this formation approached an isochemical system. 

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