North Coles Levee reservoir calcite cements

Calcite cement is the dominant cement type in the central basin. Cemented zones can be visually recognized in cores and are from 10 cm to, in a few cases, more than 1 m thick (Boles Ramseyer, 1987). Cement zones cannot be easily traced between wells spaced as close as 100 m, suggesting that the intensely cemented zones are relatively isolated and discontinuous, certainly on a basin scale and in most cases on a reservoir scale. Most cement zones have not been studied in sufficient detail to establish growth patterns. A few detailed analyses of individual zones show that some have a composite history (i.e. variable isotopic compositions) on a scale of less than 0.5 m (e.g. cement zone at North Coles Levee, well NCL 488-29, 2621 m depth), whereas others show little variation (Schultz et al., 1989). Systematic growth patterns, such as are typical for concretions in shales (e.g. Raiswell, 1971 Boles et al., 1985) or in concretions that coalesce to form continuous cemented beds (Bjor-kum Walderhaug, 1990), have not been recognized in the zones studied to date. Apart from extensively cemented zones, calcite occurs as scattered crystals in many samples. 

Fig. 7. Sr content (ppm) and Sr/ Sr ratio in calcite cements from Coles Levee reservoirs. North Coles Levee data are from Schultz et al. (1989) and South Coles Levee data are unpublished analyses of J. Schultz (Table 2). Data show general increase of Sr content with decreasing Sr isotopic ratio, interpreted to be a result of plagioclase dissolution with increasing burial depth (see Schultz et al., 1989).

One would expect thermogenic carbon sources to become increasingly important as the sands are buried. However, the late carbonate cements at North Coles Levee (after about 4 Ma) have carbon isotopic values in a very restricted range (-2%o to -4%o), as if they were buffered by some process taking place in the reservoir (Fig. 9 in Wood Boles, 1991). Similarly, late calcite cements forming in the Rio Viejo, Yowlumne and San Emidio Nose fields (Table 2) also have relatively heavy carbon values, between -5 and 0. The timing of the buff-... 

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