Portal de Periódicos da CAPES

Microporosity in micritic limestones: a review

1989; Elsevier BV; Volume: 63; Issue: 3-4 Linguagem: Inglês

10.1016/0037-0738(89)90132-2

1879-0968

Stephen O. Moshier,

Geochemistry and Elemental Analysis

Microporosity is extensively developed in many ancient, subsurface micritic limestones, including significant petroleum reservoirs. While these limestones may be described as “chalky-textured”, the rocks represent the depositional environments of shallow carbonate platforms and are not true chalk deposits. SEM studies reveal numerous types of micropores in micritic limestones, including both primary and solution-enhanced intercrystalline micropores, as well as micromolds, microvugs and microchannels. Intercrystalline micropores in carbonate matrix are typically 5–10 μm wide and are commonly responsible for > 20% porosity and > 1 md permeability. Secondary micropores commonly have diameters of less than 64 μm. During early lime-mud diagenesis, calcium carbonate is transferred from metastable carbonate particles to micrite and microspar (calcite) cements growing in adjacent micropores. Complete cementation produces a mosaic texture of anhedral micrite- or microspar-size crystals. Microporosity may be retained if cementation ceases shortly after the stabilization process is complete, resulting in a porous framework texture of micro-rhombic calcite. Uplifted Cenozoic carbonates with chalky texture do not serve as appropriate analogs for all micritic limestones with abundant intercrystalline microporosity. In fact, meteoric alteration beneath exposure surfaces commonly results in cementation and occlusion of intercrystalline microporosity in matrix material. As great amounts of freshwater flush through the deposit, metastable skeletal fragments are leached and the lime mud appears to act as a sink for calcium carbonate cementation. This is evident in matrix with mosaic texture in Pleistocene limestones beneath Holocene lagoonal sediments in the Bahamas and in a subsurface Miocene buildup that were studied. Moreover, marine stable-isotope signatures preserved in porous matrix with framework texture indicate that matrix lithification proceeded in a somewhat closed system of high rock/water ratio, isolated from outside sources of calcium carbonate; this is evident in reservoirs composed of thick microporous Miocene and Lower Cretaceous carbonates that were studied. Precursor mineralogy and rock/water ratio during diagenesis appear to be important factors in the textural evolution of recrystallized carbonate muds.