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Spectrum of Carbonate Nano- and Micropores and their Origin


Nano- and micropores are common in carbonate reservoirs as either the sole pore type or in combination with macrospores and fracture pores. Micropores are defined generally as pores with diameters < 10 microns or pore radii < 0. 5 microns. Nanopores are less than a micron in diameter. These small pores can form in limestones and dolostones. The pores occur between crystals that are commonly less than 1 to 6 microns in limestone, but in dolostone the crystals can be much larger. This size difference is related to multiple contrasting origins of these pores. Limestone nano- and micropores generally originate in three ways. In chalks they are related to the original depositional fabric as interpartilce and intraparticle associated with coccoliths and pelagic allochems. In Mg-calcite-rich sediments they form by the transformation of unstable Mg-calcite allochems (e.g., larger forams and red algae) to calcite. The pores are restricted to the distribution of the allochems. In mud mixtures of aragonite, Mg-calcite, and calcite the origin is both depositional and diagenetic as aragonite dissolves and reprecipitates as microrhombic calcite, Mg-calcite transforms to microrhombic calcite, and original calcite allochems are generally unaffected. In dolostones nano-and micropores are associated with extensive dolomitization and the nearly complete occlusion of pores. In some dolostones the nano- to micropores pores are inherited form the precursor microporous limestone. A very late process of nano- and micropores formation is subaerial weathering of both limestones and dolostones. Weathering can create extensive micropore formation with porosities above 30 percent. In some cases pulverulite forms (micron-scale calcite or dolomite power). This is a dissolution process as evidenced by dissolution and corrosion of crystals. This form of micropores could be preserved at unconformities as enhanced porosity zones. Organic-matter nano- to micropores in solid bitumen in organic-rich argillaceous limestones (e.g., Eagle Ford Group) are very common as well in cleaner carbonates with solid bitumen filled pores. Only in organic-rich limestones do organic matter pore form an effective pore system. Because nano- and micropores have multiple origins, their lateral and vertical distribution will reflect their origin. Therefore, one must not just describe the nano- to micropores, one must also define their origin.