The Origin and Diagenesis of Carbonate Microporosity and its Effects on Reservoir Performance

Abstract

Micron-size pores (“micropores”) are common in carbonate rocks. In subsurface reservoirs, they are known to lead to overestimation of recoverable hydrocarbon volumes. If column height is sufficient to charge micropores, common production methods (e.g., water-flooding) may bypass oil-charged micropores. If column height is insufficient to charge micropores, it can lead to the phenomenon of “low-resistivity pay,” where porosity logs read high, but resistivity logs read low despite oil-charged macropores due to masking by brine-saturated micropores. Not content to confound reservoir performance prediction, carbonate microporosity also reduces the longevity of concrete pavements. Work has shown that within limestone oil reservoirs these micropore reside mostly within a matrix of 0.9-9 micron, low-magnesium calcite crystals (“micrite”). This micrite shows a small number of morphologies (e.g., granular, clustered, fitted), which are correlated to porosity, permeability, and the size distributions of pore throats. Further work has shown that calcite microcrystals have been reported in carbonate reservoirs from all platform types, depositional environments, facies, lithologies, and grain types. Geochemical analyses suggest that most micrite is diagenetic. Bulk analyses of stable oxygen and carbon isotopes suggest it forms during shallow burial at temperatures 20-40°C warmer than early marine calcite cement precipitated in equilibrium with seawater. Bulk analyses of Mg/Ca and Sr/Ca are consistent with abiotic precipitation except in depositional chalks that show enrichment in Sr/Ca commonly found in biotic calcite. High-resolution analyses from a depositional chalk show that micrite crystals can display zoning with respect to Mg/Ca suggesting that bulk analyses may obscure a poly-phase history of diagenesis. Current work aims to elucidate the diagenetic pathways for micrite by searching for zoning in non-depositional chalks where the influence of meteoric water and poly-phase assemblage of depositional carbonates may affect the evolution of micrite. In addition, high-resolution cathodoluminescence microscopy will be used to further map the distribution of overgrowths on micrite crystals.

AAPG Datapages/Search and Discovery Article #90370 ©2020 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition Carbonate Reservoirs of the Middle East, Abu Dhabi, UAE, January 28-29, 2020