ABSTRACT Microporous carbonate rocks typically record complex diagenetic histories driven by microbial activity and fluid interactions within multiple diagenetic environments. The relative roles of meteoric and burial diagenetic processes in pore evolution remain debated, particularly within Cretaceous carbonate systems of the Middle East, as does the potential for porosity development during burial and after compaction. This study addresses these issues within the Cenomanian‐aged Mishrif Formation in Field X, Qatar. Prior work emphasised the importance of meteoric diagenesis beneath the Top Mishrif Unconformity (TMU) as the main control on pore formation. However, new clumped isotope and U–Pb geochronology data, combined with advanced petrographical analysis, reveal a more complex, longer‐lived history of fluid–rock reaction, extending from the Cenomanian to the Neogene. This work shows that microporosity was enhanced immediately following deposition by micritisation and boring of organisms by endolithic algae on the sea floor. During Turonian platform emergence at the TMU, dissolution of aragonitic components by freshwater formed secondary mouldic and vuggy porosity and karstic cavities. These pores were then occluded by calcite cement during shallow burial (clumped isotope temperatures of 49.2 ± 6.9°C), from the Maastrichtian until the late Eocene (U–Pb dates of 48.3 to 33.1 Ma). The ingress of basinal brines associated with hydrocarbon migration, driven by fault reactivation and tilting during the formation of the Qatar Arch resulted in a change in fluid composition. This initially led to the growth of micrite microcrystals and continued pore‐filling cementation, reducing porosity. Although petrographically similar, these younger cements record precipitation from isotopically distinct fluids, reflecting evolving diagenetic conditions during burial. Cementation persisted within the transition zone until the Neogene, when porosity was further enhanced by leaching around fractures and stylolites, and cementation was terminated. These findings add new evidence for porosity generation in the mesogenetic realm, away from the influence of surface fluids, and demonstrate how fluid–rock reaction can be maintained within carbonate reservoirs, even during hydrocarbon charge.
Bitault et al. (Tue,) studied this question.