Origin and Evolution of Microporosity in the Arab Formation (Upper Jurassic), Abu Dhabi, United Arab Emirates

Abstract

The Upper Jurassic deposits in the Middle East host significant hydrocarbon reserves, which are stored within both macroporosity and microporosity. However, the key controls on microporosity distribution need further exploration. This research provides an integrated model linking diagenesis, depositional facies, and sequence stratigraphy to the evolution of reservoir quality in the Upper Jurassic Arab D reservoir in Abu Dhabi, United Arab Emirates, providing key insights into the distribution and heterogeneity of both pore types. This study is integration of petrogrohy, petrophysical, Digital Rock Physics (DRP), and geochemical modeling that allow a deeper understanding of the distribution of microporosity within a sequence stratigraphic framework.

Key findings highlight that grain-supported limestones in highstand system tracts (HST) and lowstand wedges (LSW) exhibit better reservoir quality due to enhanced micro- and macropore connectivity. Secondary pores formed by the dissolution of aragonitic skeletal fragments (e.g bivalve and forams) during sea-level falls. However, deviations in porosity-permeability trends arise from isolated moldic and vuggy pores and abundant intragranular microporosity. In contrast, transgressive systems tract (TST) limestones have poorer reservoir quality due to matrix-supported textures, mechanical compaction of abundant peloids, and extensive micrite coalescence from calcite micro-overgrowths, sourced by intensive stylolitization. More pervasive stylolitization in mud-supported TST limestones is linked to higher clay mineral content compared to HST limestones.

Geochemical modeling indicates that calcite precipitation is influenced by (i) higher flow rates in grainstones, enhancing cementation, (ii) elevated temperatures reducing calcite solubility, and (iii) PCO₂ fluctuations in near-surface environments (e.g., intertidal zones) through evaporation increases CO₃²⁻ concentration, leading to calcite precipitation. In burial diagenesis, calcite can still precipitate from acidic, oversaturated fluids at high temperatures, promoting stylolite-sourced calcite micro-overgrowth around micrite, especially in TST limestones.

A Digital Rock Physics (DRP) approach was utilized, integrating multifractal analysis with focused ion beam–scanning electron microscopy (FIB-SEM) and mercury injection capillary pressure (MICP) to quantify and constrain the heterogeneity of mud- and grain-supported limestones. The results indicate that grain-dominated samples exhibit a more heterogeneous pore structure, with the highest non-uniformity degree (Δα) values observed in MICP experiments.

Date of Award	7 May 2025
Original language	American English
Supervisor	Mohammad Alsuwaidi (Supervisor)