Dolomite cement diagenetic

The aims this study are to elucidate the factors controlling the distribution pattern, mineralogy and geochemical composition of calcite and dolomite cements, and of grain dissolution and kaolin-ization in the fluvial sandstones of the Serraria Formation, northeastern Brazil (Fig. 1). This unit provides an excellent opportunity to examine the influence of variations in the palaeogeographical setting, palaeoclimate and burial history on diagenetic processes. 

The patterns of diagenetic evolution recognized in this study allow discussion of the conditions for optimum porosity preservation and/or enhancement in the Serraria reservoirs. The best reservoirs of the unit occur in the Caioba area of the distal domain, where porosity was enhanced by dissolution of detrital feldspars and dolomite cement during telogenetic influx of meteoric waters. Similar conditions are expected for other structural blocks of the basin affected by post-rift uplift and erosion, or blocks bounded by major fault systems in which the Serraria Formation was relatively close to the... 

The absence of dolomite cement at sandbody bases and its abundance at sandbody tops, the reported organic carbon isotope signal in the rhombic ferroan dolomite and the mixture of pedogenic dolomite textures and burial diagenetic textures in the sandbodies suggests that options 1 and 4 together are probably responsible for, the distribution of dolomite in the Chaunoy sandbodies. 

Fig. 11. Paragenetic sequence for the Upper Angel Formation, Angel Field. Dolomite cementation was a relatively late, pore-filling diagenetic event that occurred, at least in part, synchronously with microfracturing and hydrocarbon migration (see Fig. IOC). Later-stage anhydrite cement precipitated in some Angel Formation sandstones at Angel-2 and, to a lesser extent, at Angel-4.

It is noteworthy that there is very little evidence of dedolomitization in the studied sand body at Ballycastle, in marked contrast to that described by Wang (1992) from reddened sandstones in the same succession. Given the interpretation that fracture-related cementation was Late Carboniferous in age, a possible implication is that flushing of unconformity-sourced oxidizing meteoric fluids in the Latest Carboniferous/Early Permian was impeded by the structural-diagenetic compartmentalization of the sand body, compared with those in which ferroan dolomite cement was more homogeneously distributed. 

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-... 

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

Spbtl C. and Pitman I. K. (1998) Saddle (baroque) dolomite in carbonates and sandstones a reappraisal of a burial-diagenetic concept. In Carbonate Cementation in Sandstones. Distribution Patterns and Geochemical Evolution (ed. S. Morad). International Association of Sedimento-logists, Oxford, vol. 26, pp. 437-460. 

Unlike calcite and dolomite, siderite rarely forms as an extensive pore-filling cement, but rather as discrete fine crystals, spherules and nodules scattered in the host sediments. Nevertheless, Baker et al. (1996) found that early diagenetic siderite concretions (0.5-2 mm) form up to 30% of Triassic sandstones and mudstones from eastern Australia. Laterally continuous siderite-cemented offshore shelf sandstone sheets (15 cm thick) occur in Upper Cretaceous sequences from Canada (McKay et ai, 1995). 

Carbonate cement of variable mineralogy (siderite, calcite, dolomite and ankerite) is a common diagenetic feature in North Sea Jurassic reservoir sandstones (e.g. Saigal Bjorlykke, 1987 Walderhaug et al., 1989 Bjorlykke et al., 1992 Giles et al., 1992 ... 

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