Diagenesis of a Lower Cretaceous Limestone Reservoir in a Giant Oilfield, Abu Dhabi, United Arab Emirates: Comparison Between the Crest and Flanks

Abstract

Petrographic, stable isotopic and fluid inclusion analyses were conducted on a Lower Cretaceous limestone reservoir, onshore Abu Dhabi, United Arab Emirates to compare the diagenetic processes in the oil zone (i.e. crest) versus water zone (i.e. flanks) of a giant oilfield anticline. The near-surface and burial diagenetic processes across the structure were mediated by modified marine pore waters and include micritization of the allochems, mechanical compaction, cementation by calcite (circumgranular rim, overgrowths around echinoids and pore-filling equant spar) and rhombic dolomite, dissolution of the peloids, partial dolomitization and incipient stylolitization. Diagenetic processes during tectonic compression and concomitant oil migration were mediated by flux of hot basinal brines charged with organic-acids. These processes, which are more extensive in the flanks than the crest, include cementation by saddle dolomite, equant calcite, and kaolin. Diagenetic processes, which postdate the tectonic compression phase and oil emplacement in the oil zone, occurred during rapid deep burial and include extensive stylolitization and cementation by coarse-crystalline calcite in the water zone. This calcite cementation, which was mediated by geochemically evolved marine pore waters, accounts for the lower porosity and permeability of the water zone than the oil-filled crest. This study demonstrates that diagenesis and its impact on reservoir quality evolution across the field can be constrained when linked to: (i) the timing of generation, migration and emplacement/saturation of oil, and (ii) burial-tectonic evolution of the basin and related geochemical evolution and episodic fluid flux.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018