The Sarvak Formation (late Albian–early Turonian) is one of the major oil and gas reservoir units in southwestern Iran and was deposited in a wide carbonate ramp marine setting. Three major transgressive–regressive sequences are interpreted in the upper part of the Sarvak Formation in the Dehluran Field. Sequence boundaries are recognized from facies shifts and diagenetic effects related to sea-level fall, whereas maximum flooding episodes are indicated by deep-water facies characterized by abundant bioturbation and high gamma-ray log responses.
Diagenesis is the main controlling factor on reservoir quality and plays both a constructive and a destructive role. Permeability is decreased by cementation, which fills primary porosity and disconnects pore throats, while compaction decreases porosity by establishing tighter intergrain contacts. Conversely, the dissolution of unstable minerals (mainly aragonite) is the major process that improves porosity and then permeability by enlarging pores and pore throats. Dolomitization, when associated with dissolution, creates the best reservoir intervals in the Sarvak Formation, although this is not a widespread phenomenon.
These diagenetic processes are controlled by sea-level fluctuations, and thus the sequence stratigraphic development. Dissolution, early cementation and exposure-related dolomitization took place during falling sea-levels, mainly in the uppermost, regressive parts of the major sequences. Dolomitization is recognized in the transgressive and in the regressive systems tracts. Stylolitization and fracturing are independent of sea-level fluctuations and are most likely to have formed through a combination of compaction and tectonic events.
- 2006 EAGE/Geological Society of London