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An SEM-Based Descriptive Classification Scheme for Limestone Microporosity


Proposed here is a textural classification scheme for limestone microporosity based on scanning electron microscope (SEM), crystal size distribution (CSD) and mercury-injection capillary-pressure (MICP) data from a global survey of microporous carbonate reservoirs that span a broad range of geologic age, depositional facies, burial depth, and geographic location. The vast majority of limestone microporosity is hosted within low magnesium calcite (LMC) micro-crystals. CSD are coarse skewed with 90% of micro-crystals falling between 0.5 – 7.0 μm with a mean of 2.0 μm. The proposed microporosity classification is based on micro-crystal shape, size, orientation, packing, and interfacial boundary geometry. Microcrystalline textures can be grouped into three dominant fabrics, which we term granular, clustered, and fitted. Granular textures are characterized by a loose framework of euhedral to subhedral micro-crystals that are randomly oriented. Crystals are smooth sided and boundaries are characterized by punctic and serrate contacts. Clustered textures are characterized by a loose framework of agglutinated, randomly oriented micro-crystals. Discrete micro-crystals exist but are regularly stacked together and overgrown. Crystal contacts are serrate with boundaries occurring between irregular/convex crystal faces. Fitted textures are characterized by a dense mosaic of anhedral micro-crystals. Crystal faces are scalloped and boundaries are interlocking. Different microcrystalline textures result in a variety in pore sizes and geometries, which are reflected in variable porosity and permeability values. MICP data indicate that pore throat sizes for samples dominated by microporosity fall in a fairly narrow range. In carbonate rocks where micropores are the dominant pore type, pore throat size distributions correlate with microcrystalline textures observed in SEM. Granular euhedral or cluster morphologies exhibit narrow size distributions with intermediate porosity and permeability values. Fitted crystal fabrics display broader MICP profiles that are shifted toward smaller pore throat sizes and have correspondingly lower porosity and permeability. Granular subhedral crystal fabrics also display broader MICP profiles but are shifted toward larger pore throat sizes and higher porosity and permeability values than the granular euhedral or clustered morphologies.