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Preferential Calcite Cementation of Macropores in Microporous Limestones


A study of limestone cores from the Upper Shu'aiba Formation (Lower Cretaceous) in an oilfield in northwestern Oman shows examples of a phenomenon that may be widespread and characteristic of microporous limestones. The studied reservoirs consist of mud-rich rudstones, floatstones, packstones, wackestones, and mudstones that are presently at their maximum burial depth of 1.4-1.5 km at 80°C. Total porosity varies from 5 to 35% and consists mainly of ‘micropores,’ whether this term is defined as pores <30 microns diameter (not visible in thin section) or pores with throats <2 microns radius (based on MICP analysis). Previously abundant macropores have mostly been filled by coarse-blocky calcite cement, so the present wide range in total porosity results mainly from varying microporosity of mud matrix. The conclusion that the micropores have remained relatively open and cement-free in the more porous rocks is supported by the common observation of very small cement-free visible pores (such as chambers within orbitolinids and small molds) immediately adjacent to larger former pores filled by coarse calcite cement. Lack of corroded-looking pore boundaries militates against formation of the smaller pores by late dissolution. Observations from other microporous limestones in Middle East oilfields as well as North Sea chalks indicate that the cement-pore-size relationships observed in these Oman cores may be a general characteristic that develops in limestones with ‘chalky porosity’ during burial diagenesis. Recent studies of quartz cementation in sandstones can be applied to explain preferential cementation of the limestone macropores as the result of interfacial energy effects that increase the solubility and decrease the precipitation rate of very small crystals growing in tiny pore spaces. These results have important implications for understanding the evolution of capillary pressure characteristics in microporous carbonate reservoirs.