Centimeter-Scale Early Calcite Cementation Heterogeneity Resulting in Interlaying of Collapsed and Uncollapsed Ooid Layers Producing Intense Variation in Porosity Creation, Destruction, and Preservation; Permian San Andres Formation, West Texas

Abstract

A type of pore network seen in the Permian San Andres cross-bedded ooid limestones in the Permian Basin shows layers of open oomoldic pores where the interparticle pores are well cemented alternating with layers of collapsed ooids where most of the former oomoldic pores are cemented as well as the interparticle pores. This carbonate pore system has very strong reservoir heterogeneity at the centimeter scale. Porous, open oomoldic pore layers can have high porosities, but associated very low permeabilities. The cemented collapsed layers form small scale permeability barriers between the low permeability layers. The paragenesis that produces this heterogeneous pore network consists of several stages. Very early differential cementation controlled by layering (possibility grain size) produced alternating moderately cemented layers and uncemented to weakly cemented layers in what are interpreted as originally aragonite ooid sands. The initial cement was derived from dissolution of the aragonite. Continued ooid nuclei dissolution created an unsupported, weak “egg shell” texture in the uncemented to weakly cemented layers. These unsupported layers proceeded to collapse. The ooids in the layers that were moderately cemented also underwent nuclei dissolution, but the early cemented rim produced a rigid framework preventing collapse. The dissolved calcium carbonate from the aragonite ooid dissolution reprecipitated in the interparticle pores leaving the moldic pores cement free. Continued dissolution cemented nearly all the interparticle pores.

The early cement rim around the ooids are not readily apparent in plain light under the microscope. Mercury-vapor UV fluorescent light is necessary to show the several stages of cementation in the samples. The earliest stage of interpore cement, that varied in abundance between layers, was a bladed (bright-yellow under UV light), finely-crystalline calcite cement. The later and final pore-filling calcite cement crystallized as slightly coarser medium-crystalline equant calcite (dark under UV light). Without UV light, the calcite crystals appear similar to typical early meteoric-water calcite cementation. However, the cementation history is more complicated and occurred in several stages that varied between thin-bedded layers of ooids that promoted very early differential compaction. The resulting reservoir within this facies will have poor quality because of the lack of permeability.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018