Paleoenvironmental, diagenetic, and eustatic controls on the Permo-Triassic carbonate-evaporite reservoir quality, Upper Dalan and Kangan formations, Lavan Gas Field, Zagros Basin

Permo-Triassic (P-T) successions in Iran are known as one of the most extensive carbonate-evaporite sequences in the world, holding prolific hydrocarbon accumulations, such as the gas fields in the Zagros Basin. This study addresses the impact of paleoenvironmental changes across the P-T extinction boundary, variations in eustatic level and diagenesis on the carbonate reservoir quality of the Permo-Triassic Upper Dalan and Kangan formations in the Lavan Gas Field (offshore Zagros). A total of eight lithofacies have been defined and interpreted, representing the shallower parts (inner ramp to shoal) of a carbonate homoclinal ramp. The diagenetic features show the mixing influence of marine, meteoric and burial processes, with prevalence of multistage dolomitization and late meteoric-related imprints. The main dolomitization processes are interpreted to be penecontemporaneous and associated with continuous seawater reflux and high evaporation within the intertidal environment. Combined analysis of facies and petrophysical properties has been utilized to define six rock types, including RT4 (grainstones from oolitic shoals) as the best reservoir, whereas RT1 (supratidal anhydrite) is acting as the main cap rock within the studied carbonate-evaporite sequence. Multiscale characterization of lithological and petrophysical properties suggest that the Upper Dalan Formation has a single reservoir unit (D1), whereas the Kangan Formation may be divided into two reservoir units (K1 and K2). The best reservoir quality is promoted by dissolution and dolomitization associated with late Transgressive Sequence Tracts (TST) to early Highstand Sequence Tracts (HST) sequences, while low reservoir quality corresponds to late HST sequences accompanied by pore-occluded cements. Petrophysical analysis suggest better porosity-permability for the Upper Dalan than for the Kangan Formation. This observation is closely associated with the different nature of lithofacies, where the Kangan Formation contains more fine-grained carbonate rocks, and the higher intensity of porosity-grain dolomitization in the Upper Dalan Formation. This study provides a better understanding on reservoir quality prediction across the P-T boundary and the effect of eustatic fluctuations and diagenesis in controlling porosity-permeability evolution with similar settings elsewhere.