A.A.M. Aqrawi1*, M. Keramati2, S. N. Ehrenberg1, N. Pickard3,
A. Moallemi2, T. Svånå 1, G. Darke1, J.A.D. Dickson4 and N.H. Oxtoby5
1 Statoil ASA, N-4035 Stavanger, Norway.
2 RIPI, NIOC, PO Box 1863, Tehran, Iran.
3 Cambridge Carbonates Ltd., Gullvivevagen 5, 756 55 Uppsala, Sweden; and Department of Earth Sciences, University of Wales, Cardiff CF1 3YE.
4 Department of Earth Sciences, University of Cambridge.
5 Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, Surrey.
* correspondng author: firstname.lastname@example.org
Dolomitisation is an important factor controlling reservoir quality in the Asmari Formation in many producing fields in SW Iran. Dolostones have higher average porosities than limestones. Petrographic and geochemical studies have been used to determine the causes of Asmari dolomitisation at the Bibi Hakimeh and Marun fields and at the Khaviz anticline. The formation is generally characterized by a large-scale trend of upward-decreasing accommodation. Basal strata were deposited under relatively open-marine, high-energy conditions, whereas the Middle to Upper Asmari succession was deposited in relatively protected settings with more frequent evidence of exposure and evaporitic conditions. There is a general upward increase in the abundance of both anhydrite (occurring as nodules and cement) and dolomite.
Two main types of dolomite fabric are recognised, reflecting the textures of the precursor limestones: (1) finely crystalline pervasive dolomite (commonly <20m) replacing mud-rich facies; and (2) combinations of finely crystalline replacive dolomite and surrounding areas of coarser dolomite cement (crystals up to 100m) in grain-supported facies. Fluid inclusion data indicate that finely crystalline dolomites formed at low temperatures (ca. <50°C), while the coarser dolomite formed at higher temperatures (50 -140°C).
Whole rock-carbonate oxygen and carbon isotope analyses of pure dolostone samples show no apparent correlation with either depositional or diagenetic textures: d18O is generally 0 to 2.7‰ PDB, and d13C is –1 to 4‰ PDB. The importance of evaporated seawater to Asmari dolomitisation is indicated by the ubiquitous occurrence of felty-textured anhydrite nodules in dolostone beds and the presence of high-salinity fluid inclusions in dolomite. The derivation of dolomitising fluids from contemporaneous seawater is supported by the general correspondence between age estimates derived from the strontium isotope composition of anhydrites and dolomites and those derived from stratigraphic considerations. This suggested synsedimentary dolomitisation.
Dolomitisation of the upper half of the Asmari Formation may have occurred as a result of two syn-sedimentary mechanisms: (1) by the reflux of evaporative brines concentrated in shallow lagoons or sabkhas, through immediately underlying strata (mainly during highstands); and (2) by the flushing of platform-top carbonates by basinal evaporated waters during lowstand/early transgression. Continued dolomitisation during deeper burial is supported by the presence of high-temperature fluid inclusions and iron-rich crystal rims. Dolomite within the lower part of the Asmari Formation probably mostly formed during burial as a result of compaction of, and fluid exclusion from, the underlying Pabdeh marls and shales.
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