Carbonate cements distribution

One of the key problems in describing the distribution of cement is the cost (in terms of time and money) of acquiring the data. Petrographic data are usually collected at a far lower density than core analysis data (if at all), are harder to quality-control and are highly operator-dependent. In this paper we describe a way to assess carbonate cement distribution in sandstones using petrophysical logs (hereafter known as wireline logs). We use this method to... 

Seismic mapping shows that carbonate cement distribution broadly follows the structural trend of the GMI anticline, in a SSW-NNE direction (Figs lA and 21). The carbonate cements extend over an area up to 7.5 km wide and at least 20 km long, covering an area of 150 km. Comparison of the carbonate cement isopach map with a depth structure contour map for the top Toolachee Formation (McIntyre et al., 1989) demonstrates that the cements concentrate near the crest of the Gidgealpa Field in the Lower Namur Sandstone. The high amplitudes produced by the carbonate cementation decrease downflank, and phase out completely in more off-structure regions of the fi eld. 

Morad, S. 1998. Carbonate cementation in sandstones distribution patterns and... 

An extensive literature exists on the occurrence of early carbonate precipitates in marine sediments, where they are generally termed cements. Included in this literature are books devoted solely to carbonate cements (e.g., Bricker, 1971 Schneidermann and Harris, 1985) and numerous reviews (e.g., Milliman, 1974 Bathurst, 1974, 1975 Harris et al 1985). Many investigations have been largely descriptive in nature, focusing primarily on the distribution, mineralogy, and morphology of the cements. Here we will briefly summarize the major aspects of these observations, and we will concentrate on the chemical aspects of the formation of these precipitates. 

Fuchtbauer H. and Hardie L.A. (1976) Experimentally determined homogeneous distribution coefficients for precipitated magnesian calcites Application to marine carbonate cements. Ann. Mtg. Geol. Soc. Amer. 8, 877. 

Meyers W.J. (1978) Carbonate cements Their regional distribution and interpretation in Mississippian limestones of southwestern New Mexico. Sedimentology 25, 371-401. 

Beckner J. R. and Mozley P. S. (1998) Origin and spatial distribution of early vadose and phreatic calcite cements in the Zia Formation, Albuquerque Basin, New Mexico, USA. In Carbonate Cementation in Sandstones. Distribution Patterns and Geochemical Evolution (ed. S. Morad). International Association of Sedimentologists, Oxford, vol. 26, pp. 27-52. 

Pierson, B.J. Shinn, E.A. (1985) Cement distribution and carbonate mineral stabilization in Pleistocene limestones of Hogsty reef, Bahamas. In Schneidermann, N. Harris, P.M. (Eds) Carbonate Cements. Special Publication 36. Tulsa, OK Society of Economic Paleontologists and Mineralogists, pp. 153-168. 

Factors that control the geochemistry, abundance and distribution of carbonate cements are of prime importance in the understanding and prediction of porosity-permeability variations and in tracing the geochemical evolution of pore waters during the burial of sandstones and associated sediments. Moreover, the stable isotopic composition of nearsurface, eogenetic carbonates (e.g. in soil profiles) provides important clues to the palaeoclimatic conditions (e.g. Ceding, 1984). 

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. 

When carbonate cements are subjected to physicochemical conditions that vary considerably from those under which they formed, they may dissolve and re-precipitate at various scales. Carbonate dissolution and the creation of secondary porosity may occur during eodiagenesis or telodiagenesis or in response to progressive burial. Eogenetic secondary pores may survive subsequent burial and compaction in sandstones that have been subjected to early overpressuring or hydrocarbon emplacement, or if dissolution is incomplete, and leave evenly distributed remnants of carbonate cement. 

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