Datapages, Inc.Print this page

ABSTRACT: Microporosity in Micritic Marine Cement: A Permian Example

P. M. Harris

Thin-section and SEM (scanning electron microscope) examination of coarse dolograinstone samples of Permian age (Grayburg-lower Queen formations) from an oil field in southeast New Mexico reveal a unique first-generation micritic cement. The cement is formed of micron-size dolomite crystals that commonly occur within a clotted fabric. The cement lines interparticle and fenestral pores in a skeletal-intraclast grainstone-rudstone, often thickening and bridging the pores at grain contacts to produce a texture that is reminiscent of marine vadose (beachrock) textures described from modern examples. This diagenetic setting is consistent with depositional textures and the stratigraphic position of the samples within the upper parts of a shoaling sequence.

The micritic cement is overlain by a bladed to blocky dolospar that lines and partly fills pores and is believed to be a marine phreatic cement. The micritic, bladed, and blocky cements are all strikingly similar in texture and distribution to Mg-calcite cements described from Pleistocene limestones of Hogsty Reef, Bahamas.

Petrography indicates that a significant amount of effective microporosity (micron-size intercrystalline pores) was present in the micritic cement. The micropores are now filled with degraded, immobile oil and scattered pyrite crystals that impart a distinctive brown-black color to the cement in hand sample and thin section. This example of effective microporosity within an early-phase carbonate cement generation, subsequently dolomitized, shows a possible scenario for the establishment of a localized microenvironment in which subsequent diagenesis (oil entrapment and pyrite formation) has significantly altered the appearance in samples. This same diagenesis, if occurring on a larger scale, would also possibly alter the response of downhole logs though the host rock.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990