Dolomite cementation

Since we have no direct information about the chemistry of the Fountain fluid, we assume that its composition reflects reaction with minerals in the evaporite strata that lie beneath the Lyons. We take this fluid to be a three molal NaCl solution that has equilibrated with dolomite, anhydrite, magnesite (MgCC>3), and quartz. The choice of NaCl concentration reflects the upper correlation limit of the B-dot (modified Debye-Hiickel) equations (see Chapter 8). To set pH, we assume a CO2 fugacity of 50, which we will show leads to a reasonable interpretation of the isotopic composition of the dolomite cement. 

We can predict the oxygen and carbon isotopic compositions of the dolomite produced by this reaction path, using the techniques described in Chapter 19. Figure 25.4 shows the compositions of calcite and dolomite cements in the Lyons, as determined by Levandowski et al. (1973). The calcite and dolomite show broad ranges in oxygen isotopic content. The dolomite, however, spans a much narrower range in carbon isotopic composition than does the calcite. 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

Oil in the Cretaceous Dakota sandstone, a shallower aquifer than the Lyons, has migrated laterally as far as 150 km into present-day reservoirs (Clayton and Swetland, 1980). In contrast, oil has yet to be found in the Lyons outside the deep strata where it was generated. The formation of anhydrite and dolomite cements may have served to seal the oil into reservoirs, preventing it from migrating farther. 

Leach, D. L., G. S. Plumlee, A. H. Hofstra, G. P. Landis, E. L. Rowan and J. G. Viets, 1991, Origin of late dolomite cement by C02-saturated deep basin brines evidence from the Ozark region, central United States. Geology 19, 348-351. 

The timing of the dolomitization of carbonate rock bodies and emplacement of dolomite cements has been one of the more controversial aspects of the "dolomite problem." Most of the basic factors controlling dolomite formation, where were discussed in Chapters 6 and 7, also apply to dolomite formation during the later stages of diagenesis. However, the extended periods of time, the solution compositions likely to be encountered, and the elevated temperature and pressure that occur during deep burial provide highly favorable conditions for dolomite formation. 

Mitchell J.T., Land L.S. and Miser D.E. (1987) Modem marine dolomite cement in a north Jamaican fringing reef. Geology 15, 557-560. 

Underclay and overburden sediments are typically clays with about 80% clay and 20% silt and muds with about 50% clay and 50% silt. The most distinct chemical variation in the overburden is the compact siderite- and dolomite-cemented concretion zone, 4.5 m above the base of the Kinneman Creek lignite. 

Rowan, E. L., 1986, Cathodoluminescent zonation in hydrothermal dolomite cements Relationship to Mississippi VaUey-type Pb-Zn mineralization in southern Missouri and northern Arkansas. in Hagi, R. D., ed.. Process Mineralogy VI, Metallurgical Society, p. 69-87. 

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. 

Our results indicate that kaolinite distribution in the Lunde Formation is not strictly controlled by the Kimmerian uplift and erosion. This is due partly to the formation of kaolinite during eodiagenesis and partly to the strong relationship between kaolinite abundance and detrital composition of the sandstones, particularly the original amounts of feldspars and mud intraclasts. Pervasive kaolinite formation, coupled with dissolution of calcite and dolomite cements, has been substantial in well 34/4-1. In well 34/7-A-3H sandstones, the top of which was buried deeper below the unconformity than that of well... 

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. 

Distal Domain (Caioba area) (dolomite-cemented)... 

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... 

Wireline log analysis has been used to define porosity and mineralogy (with three components quartz, dolomite and shale), and these data have been used to derive permeability. They will be used subsequently to assess dolomite cement distribution within the reservoir. 

Chaunoy sandstones of the same depositional facies are cemented by both quartz and dolomite (Fig. 3B). Quartz cement forms approximately equal thickness overgrowths whereas dolomite cements tend to fill pores (Fig. 3B) (Cade et al., 1994). These two different cement morphologies haye profoundly different effects upon the pore network. 

Amount and distribution of dolomite cement in the Chaunoy sandstone... 

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