The Prediction of Microporous Limestones Using Log-Derived Elastic Moduli. Example from the Arab Formation, Onshore Abu Dhabi

Abstract

The Lower Arab D Member of the onshore United Arab Emirates is a thick succession of homogeneous low-energy mid-ramp carbonate mudstones with minor interbeds of storm sourced cm-scale skeletal-rich floatstones. The mudstones are dominated by matrix-hosted micropores and rare moulds and vugs. This leads to a variability of poor to very good porosity and poor to rarely good permeability within the pore system. Thin-section observations from six wells in a sour gas reservoir demonstrate these variations are linked to micron-scale heterogeneity in the micritic fabric. In particular, anhedral compact micrites with coalescent intercrystalline contacts generate small and poorly connected micropores, while polyhedral to subrounded with facial to subpunctic intercrystalline contacts result in well-developed micropores. To aid with the prediction in the microporosity variations, an approach using log-derived Young’s Modulus has been used. This approach has used Young Modulus (YME) along with porosity data in two wells to explore the relationship between micritic fabric, porosity, permeability and elastic properties. Through the evolution of micritic fabric from anhedral compact to polyhedral / subrounded, Young's Modulus decreases with increasing porosity, indicating a decrease in the overall stiffness. Based on these two learning wells, specific porosity and YME cut-offs have been identified to distinguish the various micrite fabrics. Subsequently, the cut-offs have been successfully tested in four other wells used as a blind test for the vertical prediction of the micritic fabrics, in which accurate predictions reached up to 90%. Following these results, porosity and YME cut-offs have been used to produce the first model of the distribution of the various micritic fabrics at the field-scale. These results have a fundamental impact on the understanding of the sedimentological and diagenetic heterogeneity of microporous limestones and can be used as predictive tools at the well and field-scales.

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