The Origin of Dolomite in the Asmari Formation (Oligocene‐lower Miocene), Dezful Embayment, Sw Iran

Abstract

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 <20μ) replacing mud‐rich facies; and (2) combinations of finely crystalline replacive dolomite and surrounding areas of coarser dolomite cement (crystals up to 100μ) 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: δ18O is generally 0 to 2.7‰ PDB, and δ13C 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.

Biostratigraphy and paleo-ecological implications in microfacies of the Asmari Formation (Oligocene), Naura anticline (Interior Fars of the Zagros Basin), Iran

“…Recently published research on the biostratigraphy and facies analysis of the Asmari Formation are: Seyrafian and Hamedani 2003;Seyrafian and Mojikhalifeh 2005;Vaziri-Moghaddam et al 2006;Aqrawi et al 2006;Amirshahkarami et al 2007;Ehrenberg et al 2007;Hakimzadeh and Seyrafian 2008;Laursen et al 2009;Mossadegh et al 2009;Rahmani et al 2009;Sadeghi et al 2009;Vaziri-Moghaddam et al 2010;Van Buchem et al 2010;Allahkarampour Dill et al 2010.…”

Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

“…Textural relationships and chemical composition suggest an early diagenetic origin for microcrystalline dolomite, which could be similar to the grain-coating dolomite reported in this paper. In the condensed episodes (dolomite in yellow beds, Figure 11), the dolomite overgrowths show δ 18 O values comparable to detrital dolomites and overgrowths of the turbidite arenites (from -8.5 to -6.3‰ δ 18 O, Table 4), which could indicate that cementation Engineering, 2009Engineering, , 7, A39 doi: 10.2202Engineering, /1542Engineering, -6580.1881 10 by dolomite continued during deeper burial (Hendry et al 2000;Aqrawi et al 2006;Machent et al 2007). V VI I.…”

The Control of fracturing and dolomitisation on 3D reservoir property distribution of the Asmari Formation (Oligocene-Lower Miocene),

“…Deformation during successive Alpine orogenic pulses has resulted in the formation of various fracture systems within these anticlines, which have influenced their reservoir quality of the Asmari Formation, in particular when dolomitised (Wennberg et al 2006(Wennberg et al & 2007Aqrawi et al 2006). …”