A Universal Classification Scheme For the Microcrystals That Host Limestone Microporosity

Abstract

Abstract Microporosity is recognized as a significant concern in Phanerozoic-age limestone reservoirs throughout the world because its presence can severely complicate hydrocarbon evaluation and production. Numerous studies have documented the occurrence and abundance of micropores, their physical appearance, and diagenetic origins in specific reservoirs, but no published study has been systematic and global in its approach. Presented here is a global assessment of limestone microporosity that is based on scanning electron microscope, crystal size distributions, helium porosimetry, and mercury-injection capillary-pressure data collected from 12 microporous limestone reservoirs spanning a broad range of geologic ages, depositional facies, and burial depths. Results indicate that the vast majority of limestone microporosity is hosted within a framework of low-magnesium calcite microcrystals. In general, crystal size distributions are narrow and positively skewed. Compiled data indicate that > 99% of all microcrystals measure between 0.5 and 9.0 µm in diameter with a mode at ca. 2.0 µm. Based on microcrystal shape, packing, and edge density, microporous limestones can be classified into three major textural classes, which we term granular, clustered, and fitted. Granular textures, which include subhedral and euhedral subclasses, are characterized by a framework of unconsolidated rhombic to polyhedral microcrystals that exhibit straight to curvilinear boundaries and intermediate crystal edge densities. Clustered textures, which include loose and fused subclasses, are characterized by a framework of irregularly shaped microcrystals with relatively high densities of discontinuous, curvilinear crystal edges. Fitted textures, which include partial and fused subclasses, are characterized by a dense mosaic of slightly larger microcrystals with mostly curvilinear boundaries and relatively low edge densities. These textural classes have unique mean crystal sizes, average pore-throat radii, bulk porosities, and permeabilities. Furthermore, textural classes and their characteristic pore-throat radii fall along a single log-normal transform in porosity–permeability space. Based on these relationships, three petrophysical microporosity “types” are distinguished. Type 1 is associated with granular subhedral crystal fabrics and typically exhibit relatively large average pore throat radii (ca. 0.7 µm) as well as high porosity (> 20%) and permeability (1–20 mD). Type 2 includes granular euhedral and clustered fabrics, which most commonly exhibit intermediate average pore-throat radii (ca. 0.2 µm), porosity (10–20%), and permeability (0.1–1 mD). Type 3 includes fitted crystal fabrics, which are generally characterized by smaller average pore-throat radii (ca. 0.06 µm), and relatively low porosity (< 10%) and permeability (< 0.1 mD).