Unraveling the parameters controlling diagenetic evolution in relatively shallow-buried, syn-tectonic ramp limestones (Upper Cretaceous), United Arab Emirates

Abstract

This study utilizes petrographic, geochemical, and fluid-inclusion microthermometric analyses to unravel the controls on the distribution of diagenetic alterations of shallow-buried Upper Cretaceous syn-tectonic, foreland, ramp limestones of the United Arab Emirates. The diagenetic alterations are linked to porewater evolution during the tectonic evolution of the basin, as well as to the depositional facies and sequence stratigraphy. Limited improvement of reservoir quality was promoted by dissolution of allochems and formation of moldic pores. Porosity reductionensued mostly from: (i) cementation by meteoric waters and by hot basinal/hydrothermal brines with wide ranges of δ¹⁸O_V-PDB (−6.3 ‰ to −3.2 ‰) and precipitation temperatures (T_h ≈ 65–125 °C; salinity ≈ 16–22 wt% NaCl eq), and (ii) mechanical compaction of ductile peloids formed by micritization of allochems. Micritization, notably prevalent in the transgressive lagoon and upper ramp slope packstones, as well as in the regressive shoal grainstones below marine-flooding surfaces, led to the development of abundant microporosity. Porosity was preserved in some of the grainstones by partial calcite cementation (rims, as well as scattered equant crystals and syntaxial overgrowths), which supported the framework against mechanical compaction. Dissolution of aragonitic allochems and concomitant cementation by equant calcite was prevalent in regressive shoal grainstones during repeated episodes of subaerial exposure and meteoric-water incursion as a consequence of the syn-tectonic deposition. The δ¹³C_V-PDB (−11.6 ‰ to +13.2 ‰; mostly −2‰ to +2 ‰) suggests derivation from marine porewaters and/or dissolution of the host limestones and, in some cases, from methanogenesis of organic matter and methane oxidation. The lack of systematic differences in porosity and permeability between the limestones from the crest and the flanks of the field is attributed to the accomplishment of most diagenetic alterations before oil emplacement.