--> Unraveling the Impact of Sea-Level Changes and Early Diagenesis on the Reservoir Quality of Eocene Carbonates in Qatar

AAPG ACE 2018

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Unraveling the Impact of Sea-Level Changes and Early Diagenesis on the Reservoir Quality of Eocene Carbonates in Qatar

Abstract

Eocene carbonates of central Qatar reflect deposition in a shallow sub-tidal to restricted marine setting. Because these rocks have not been extensively buried, they offer an ideal opportunity to examine the effect of significant near-surface diagenesis on reservoir quality. A 135 m research core drilled in 2016 penetrated (i) the Umm er Radhuma Fm. (UER), characterized by dolomitized sub-tidal to peri-tidal carbonates with palygorskite-rich cycle caps; (ii) the Rus Fm., characterized by a lower unit comprised of carbonates and evaporites, and an upper unit characterized by completely recrystallized limestone and mimetic dolomite; and (iii) the Dammam Fm., characterized by dolomite and partially recrystallized fossiliferous limestone. The dataset includes observations from thin section, SEM-EDS, and C and O isotope data from >250 rock samples. In the UER, δ18O (-2.1 to +1.6‰, x= -1.04‰) and δ13C (-2.3 to +0.6‰, x= -0.26‰) values are interpreted to reflect dolomitization in normal marine fluids. Subsequent dolomite dissolution and replacement of detrital smectite by palygorskite near the cycle tops correlates with poorer reservoir quality. Later stage silica cements partially replace dolomite in the lower UER and pore-occluding gypsum cements are observed in the upper UER. Despite strongly negative δ18O values (x=-10.6‰), Upper Rus limestones are interpreted to have been exposed and intensively recrystallized in meteoric fluids as evidenced by the abundance of microcodium and large (≤500 µm) equant calcite crystals. Strongly negative δ13C values in the Upper Rus (x= -8.2‰) can be explained by isotopically light pedogenic soil gas. The recrystallized limestones have low porosity (10%) and variable permeability (0.01-10 mD). The overlying bed of mimetic dolomite exhibits a transition back to typical marine δ18O values (x= -0.7‰), as well as increases in porosity (>40%) and permeability (≤1000 mD). Negative δ18O values (x= -9.8‰) are observed in the overlying Dammam, and abundant HMC biomolds, partially recrystallized fossils, and microcodium suggest a second phase of subaerial exposure. Partially recrystallized limestones of the Dammam have lower porosity (1-15%) and permeability (.01-0.3 mD). These observations highlight the importance of (i) fluctuating sea-level on early, pre-burial diagenesis, and (ii) the impact of early diagenesis on reservoir quality.