Factors shaping reservoir architecture in the Jurassic Arab carbonates: A case from the Persian Gulf

Abstract

The Upper Jurassic Arab Formation of the Ferdowsi Oil Field represents deposition in a high-energy sub-equatorial carbonate ramp. Prevailing arid climate and sea-level fluctuations resulted in four shallowing-up 3rd order sequences, starting with porous and permeable reservoir rocks and ending with low permeable evaporites (local sealing units), causing vertical compartmentalization in the field. Repeated successions of shoal, lagoon, and peritidal facies with shoaling-upward cycles in various orders formed variable reservoir zones. Reservoir quality was primarily controlled by depositional setting, but post-depositional processes were also important in enhancing/creating porosity, especially in the lagoonal facies. Marine cementation by development of rigid framework resulted in the preservation of primary poroperms values even after burial. In addition, evaporite capping led to further primary reservoir preservations by creation of a hydro-pressurized system. Six petrophysical groups (PGs) are distinguished in the studied interval by considering primary and secondary controls on pore type and size distribution. Accordingly, high-quality PGs are distributed in the lower parts of the sequences, and the least permeable PGs in the upper parts. The spatial distribution of each PG is justified by its position within the sequence stratigraphic framework of the field and thus petrophysical and general reservoir architecture of the field could be established. Consistent with this reservoir rock classification, the Arab B and A Members have higher reservoir quality than the underlying Arab C and D Members in this field. Accordingly, the general reservoir architecture in this field is mainly shaped by arrangements and features of depositional sequences.