Transition Zone Dolostones (TZD) and Reservoir “Sweet Spots” in the Kangan-/Dalan (Khuff) Formation, Iran
Tore Amund Svånå1, Arnstein Waldum2, Arild Eliassen3, Joanna Garland3, Stephen Neville Ehrenberg3, and Arild Eldøy3
1Statoil ASA, Harstad, Norway
2Statoil, Ranheim, Norway
3Statoil ASA, Stavanger, Norway
Depositional evolution, diagenesis and reservoir properties are studied in a mixed warm water limestone/dolostone platform succession of the Kangan-/Dalan (Khuff) Formation (Kazanian - early Induan) in Iran. Core description combined with thin section and petrophysical analysis form the basis for this study.
Lithology is linked to variations in depositional setting: Limestones form in periods with open marine circulation, while restricted hypersaline conditions are required for early formation of dolostones (seepage-reflux model). The abundance of associated evaporites depends on degree of restriction and climate. These variations affect reservoir quality, but late diagenetic alteration can disturb this pattern.
Both the described succession and examples from other areas show that in interbedded limestone/dolostone deposits, the best reservoir properties tend to occur in the “transition zone dolostones” (TZD), near the contact to over- and underlying limestones. Dominant lithology is typically macrocrystalline dolomite with good intercrystalline porosity and little porosity plugging anhydrite. Away from the limestone contact the reservoir quality tends to decrease with smaller and more tightly cemented dolomite crystals, increasing abundance of porosity plugging anhydrite and late diagenetic replacive anhydrite cement.
A process model to explain these relationships involves the balance between open marine conditions and influence of dolomitizing fluids and evaporites that are generated in landward restricted settings. In the TZD the balance between these factors is favourable for formation of good reservoir properties, while more intense dolomitization and evaporite plugging occur in proximal dolostones away from these zones.
Similar links between TZD reservoir sweetspots, primary depositional environment and diagenesis are found in several other investigated successions.
AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece