Dolomitization and porosity evaluation of khurmala, gara anticline, Dohuk area, Kurdistan region, Iraq

Abstract EN

The Khurmala Formation (Late Paleocene – Early Eocene) is well exposed at the southern limb of Gara mountain, Dohuk area, Kurdistan Region of north Iraq.

The thickness of the Formation is 91 m and generally consists of buff to grey, medium to thick-bedded stromatolitic dolostone alternating with thin beds of buff to dark gray shale or marlstone.

Shale interlayers increased in thickness upwards.

The studied section has been divided into three units based on shale ratio.

The focus of this study is to investigate the type and origin of the dolomite and its impact on the porosity using field observation, petrologic and petrographic investigations with SEM microscopy.

The common dolomite types include (1) Unimodal, cryptocrystalline to fine crystalline dolomite of nonplanar mosaic.

This type is associated with the relics of the stromatolites and commonly recognized in the upper part.

(2) Unimodal fine to medium crystalline, planar-e to Planar-s dolomite mosaic, which is associated with dolomite horizons showing ghosts or relics of benthic foraminifera, algae, ostracods and other bioclasts.

The other types are less common and include (3) Porphorotopic dolomite and (4) Bimodal medium to fine, planer-e to planar-s dolomite mosaic.

The latter is associated with dolostone horizons display important moldic and vug porosity.

Molds, vugs and intercrystalline porosity are noticed in all types but with variable percentages.

Fenestral porosity is common in the stromatolitic dolomite of type one.

The vertical distribution of the different type of dolomite and its porosities indicate cyclic alternation of the studied Khurmala section which implies effective eustatic control on deposition and dolomite diagenesis.

The dolomitization resulted from a possible combined effect of seawater refluxion and sabkha evaporation mechanisms.

Keywords: Khurmala; Dolomitization; Porosity; Iraq; Dohuk