Facies, sequence framework, and reservoir quality along a Triassic carbonate ramp: Kangan Formation, South Pars Field, Persian Gulf Superbasin

The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution. Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences K52 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KSla, KSlb, and KSlc) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.

Automated summary

This study explored the impact of depositional facies and diagenetic features on reservoir quality in the Lower Triassic Kangan Formation of the Persian Gulf Superbasin. The results identified different carbonate-evaporite facies and hydraulic flow unit distributions, with reservoir quality improving seaward.