Writers : Kavoosi,Mohammad Ali
Refference : The first international applied geological congress; Department of geology Islamic azad university- Mashhad
Publishing Year : 2010

Abstract :

The Kopet Dagh Basin in northeast Iran contains giant Khangiran and Gonbadli gas fields. This study deals with the main hydrocarbon reservoir of Upper Jurassic (Oxfordian-Tithonian)
Mozduran Formation, which is composed mainly of limestone and dolomite, with minor amounts of marl/shale, siliciclastics and evaporites. The objective of this study is carbonate mineralogy of
the Mozduran Formation. Thin sections were stained by alizarin-red S to detect dolomitization of grains and cements. Regarding diagenetic products and their diagenetic environments, selected
samples were observed with a cathodoluminescent microscope (Nikon CL, CCL 8200) at the Research Institute of Petroleum Industry (R.I.P.I). Detailed field studies, petrographic investigations and facies analyses of eight surface sections and four wells, led to the recognition of several facies that define deep basin, fore-shoal, shelf margin, lagoonal, tidal flat and coastal plain facies belts, which deposited on a rimmed-shelf and a carbonate ramp during Oxfordian and Kimmeridgian respectively.
Our petrographic and CL studies indicate, lack of leached ooids (Oxfordian) with preserved radial and concentric fabrics indicates primary low-Mg calcite mineralogy. In contrast, at Khangiran and Gonbadli gas fields, Kimmeridgian ooids are aragonitic. In the Mozduran Formation, pay zones are located in Kimmeridgian deposits. Reservoir facies are composed of tubiphytes boundstones, tubiphytes packstone and grainstone, microbialite boundstone, dolomitized ooid grainstone/packstone and dolostones. Mentioned facies stack aggradationally and were cemented by marine cements with isopachous, fibrous and acicular fringe fabrics, assign a windward margin and high permeability of sediments at the Khangiran-30 well location.
In the mentioned intervals due to hypersalinity and presence of sulfide ions concentration, aragonite tends to form in preference to calcite. Kimmeridgian ooids in the basin show both original aragonite and high-Mg calcite mineralogy that have been completely dolomitized, so the original fabric has been destroyed, but ghosts of spherical shapes characterize ooid grains.
Despite the fact that ooids have been dolomitized, marine cements of mentioned carbonates at Khangiran field show excellent preservation. Some cements (isopachous) show mimetic dolomitization. Increased hypersalinity, evaporite precipitation and consequently an increase in the Mg/Ca ratio, along with increasing high temperature (inferred from increasing evaporites), led to the preferential formation of aragonite to calcite. The aragonite mineralogy could be a result of evaporite precipitation and consequently an increase in Mg/Ca ratio. In southern
outcrops of the Kopet Dagh Mountain Range, ooids in ooid gainstones with quartz nuclei have preserved radial and concentric fabrics. It shows there is a probable relationship between siliciclastic input and calcite mineralogy. The presence of preserved ooids with radial and concentric cortexes in shallow-water settings which are near to a siliciclastic source, together with the formation of aragonitic ooids accompanied with evaporites in the gasfields, suggest that the mineralogy was probably controlled by salinity variations.

Subject List : Carbonatite Rocks