South Pars / North Dome Gas-Condensate Field - Reservoir Characteristics

Reservoir Characteristics

This largest non-associated gas reserve of the world is hosted by the Upper Dalan–Kangan (Upper Khuff equivalent) Permo-Triassic carbonate–evaporite successions. Detailed characterization of these strata in the South Pars field has shown that the reservoir properties are a function of both sedimentary and diagenetic processes at the field scale. Facies analysis of the studied units indicates that the sediments were deposited in the inner regions of a homoclinal carbonate ramp and were subsequently subjected to shallow diagenesis and minor burial. The vertical distribution of the facies shows cyclic patterns that impact reservoir quality. The rock type classes have been grouped on the basis of the dominant pore spaces, and have enabled distinct fields to be identified. This approach has demonstrated a relationship between poroperm values and rock type groups. Diagenetic overprinting has significantly affected the reservoir properties.

Although the original poroperm heterogeneities in the studied reservoir are inherited from the Upper Dalan–Kangan palaeoplatform, they have been modified strongly by diagenetic overprinting. Consequently, tentative correlation may be possible between facies types and reservoir properties based on diagenetic effects. Therefore, for precise characterization of the Upper Dalan–Kangan reservoir properties it has been necessary to integrate both the depositional history and diagenetic features. Based on the thin section studies from this field, pore spaces are classified into three groups including depositional, fabric-selective and non-fabric selective. Stable isotope studies confirm the role of diagenesis in reservoir quality development. Integration of various data show that different diagenetic processes developed in two reservoir zones in the Kangan and Dalan formations.

While dolomitisation enhanced reservoir properties in the upper K2 and lower K4 units, lower part of K2 and upper part of K4 have experienced more dissolution. Integration of RQI, porosity-permeability values and pore-throat sizes resulted from mercury intrusion tests shows detailed petrophysical behavior in reservoir zones. Though both upper K2 and lower K4 are dolomitised, in upper K2 unit non-fabric selective pores are dominant and fabric destructive dolomitisation is the main cause of high reservoir quality. In comparison, lower K4 has more fabric-selective pores that have been connected by fabric retentive to selective dolomitisation. Generally, the early diagenetic overprints of marine origin include micritisation, marine cementation and early stages of compaction. The petrological features of this realm include fibrous cements, skeletal micritisation, orientation of the elongated grains, early anhydrite cementation and slight deformation of grains. The diagenesis stages continue with meteoric diagenesis during which metastable skeletal and non-skeletal grains were dissolved, generating secondary fabric- and non-fabric selective porosity. Grading recrystallisation and chemical compaction indicate a burial diagenetic environment and its petrographic characteristics include coarse mosaic calcites with undulatory extinction, stylolites, sutured and concave-convex contacts between grains Depositional environment summary, diagenetic imprints, uranium- thorium geochemistry, carbon, oxygen and Sr isotope excursions, faunal distribution, chemostratigraphy and correlation and exact determination of Permian-Triassic boundary (PTB) in South Pars Field could be find in Tavakoli and Rahimpour-Bonab paper. Results of this study indicate that gradual U decrease to the PTB in Kangan–upper Dalan extinction horizon reflects a gradual decrease of the U content of shallow seawater. These considerations indicate that even though the redox conditions of depositional environments in various areas fluctuated, they indicate a general decline in oxygenation, and may point towards anoxic conditions in the Late Permian, though shelf environments were more oxygenated than deeper waters