Controls on the formation and evolution of multimodal pore network in lower cretaceous limestone reservoir, Abu Dhabi, United Arab Emirates

Petrographic and petrophysical analyses of a Lower Cretaceous limestone reservoir, Abu Dhabi, United Arab Emirates provide new insights into the formation and diagenetic evolution of bimodal pore network. The most important diagenetic controls include: (i) widespread development of microporosity by micritization of allochems and partial cementation by grain-rimming calcite during eogenesis at or below the seafloor, (ii) partial to complete dissolution of peloids and concomitant precipitation of equant calcite cement during early mesogenesis, and (iii) stylolitization and development of tension gashes and concomitant massive cementation of the macropores by coarse-crystalline calcite and partial cementation of the micropores by calcite micro-overgrowths around micrite during late mesogenesis. Lateral compression owing to ophiolite obduction in late Cretaceous allowed flow of hot basinal brines along stylolites and tension gashes, which resulted in the precipitation of saddle dolomite and pyrite. Abundant depositional interparticle macropores were preserved in the grainstones by sparse grain rimming, dogtooth-like calcite cement that reduced the effect of mechanical compaction. Micro- and macropores are well interconnected yet the moldic, fracture pores, and intraparticle micropores within peloids are less well connected with the pore system. Hydrocarbon emplacement in the naturally oil/mixed-wet carbonate reservoir both increases the fluid pore pressure within the pore spaces of the rock thus reducing overburden stress and effective compaction; and reduces nucleation sites for cement growth thus retarding chemical diagenesis. However, the hydrocarbon emplacement occurred gradually and selectively filled the crestal limestones from the top to bottom in a way that its effect on rate of diagenesis differ vertically and spatially across the study area. Constraining the formation, evolution, and distribution of multimodal porosity network, including abundant micropores and mesopores allows better detection and evaluation of low-resistivity pay zones in carbonate successions.