DOLOMITIZATION AND ANHYDRITE PRECIPITATION IN PERMO‐TRIASSIC CARBONATES AT THE SOUTH PARS GASFIELD, OFFSHORE IRAN: CONTROLS ON RESERVOIR QUALITY

Abstract

Dolomitization and related anhydrite cementation can complicate the characterization of carbonate reservoirs. Both processes have affected the Permo‐Triassic Upper Dalan – Kangan carbonates, the main reservoir at the South Pars gasfield, offshore Iran. The carbonates were deposited in a shallow‐marine ramp or epeiric platform and, according to previous studies, underwent intense near‐surface diagenesis and minor burial modification. Detailed petrographical and geochemical analyses indicate that dolomitization and anhydrite precipitation can be explained in terms of the sabkha/seepage‐reflux models. The early dolomites then re‐equilibrated or re‐crystallized in a shallow burial setting. Evaluation of poroperm values in different reservoir intervals indicates that replacive dolomitization in the absence of anhydrite precipitation or with only patchy anhydrite has enhanced the reservoir quality. Where anhydrite cement is pervasive and has plugged the rock fabric, poroperm values are significantly decreased.As emphasized in previous studies and confirmed here, dolomitization and anhydrite cementation, together with original facies type, are the major factors controlling reservoir quality in the Dalan – Kangan carbonates at South Pars. When associated with minor anhydrite cementation, replacive dolomitization has enhanced reservoir quality by increasing permeability. However, porosity in fabric‐retentive dolomite was apparently inherited from the precursor rock and therefore reflects the original depositional environment.Low‐temperature dolomitization is commonly fabric‐selective and partially fabric‐retentive. Whole rock stable isotope thermometry indicates that fabric‐destructive dolomites in the reservoir rocks formed at temperatures above 22°C, whereas fabric‐retentive dolomites and associated anhydrites formed in surface and near‐surface conditions. Fabric‐destructive dolomite or dolomite neomorphism post‐date fabric‐retentive dolomite and continued to form in deep burial conditions (∼1400m). These observations may explain why fabric‐retentive dolomite and anhydrite fabrics are traversed by stylolites.