Depositional and diagenetic controls on reservoir quality of microporous basinal lime mudstones (Aptian), United Arab Emirates

This petrographic and petrophysical study constrains parameters controlling porosity-permeability heterogeneity in unconventional microporous lime mudstones reservoir of the Lower Bab Member (Aptian) deposited in intrashelf Bab basin, Abu Dhabi, United Arab Emirates. Depositional texture and diagenetic processes control the petrophysical properties of the mudstones. The high concentration of coccoliths with relatively large intragranular pores causes increase in porosity and permeability. Diagenetic control on reservoir quality includes the degree of development and thickness of syntaxial micro-overgrowths around the micrite particles. Mudstones with well-developed and coalesced overgrowths, which are most common in the vicinity of stylolites, have the lowest porosity (5–8%) and permeability (0.01–0.1 mD) and narrowest pore throats (around 0.05 μm). Conversely, mudstones with relatively poorly developed syntaxial calcite micro-overgrowths have greater porosity (20–24%), permeability (0.3–2 mD) and pore-throat size (0.5 μm). Scattered moldic pores formed by the dissolution of aragonitic bivalves caused increase in porosity but not permeability of the wackestones. Combining petrographic and petrophysical properties revealed that depositional parameters and diagenetic processes resulted in three distinct unconventional microporous lime mudstone reservoir types.