A study of limestone cores from the Upper Shu'aiba Member (upper Aptian) in an oilfield in northwestern Oman shows examples of a cementation phenomenon that appears to be widespread in microporous carbonate reservoirs and fundamental for understanding their porosity evolution during burial diagenesis. The studied reservoirs have total porosity of 4 to 36, which consists mainly of "micropores," defined here as pores not distinctly visible in a petrographic thin section (thus generally <30 microns diameter). In these mud-rich rudstones, floatstones, packstones, wackestones, and mudstones, calcite cement preferentially filled the larger pore spaces, while micropores remain relatively open in the more porous rocks. The present wide range in total porosity results mainly from varying microporosity of mud matrix and grains. Lack of corroded pore boundaries militates against formation of micropores by late dissolution.
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