Sedimentology
Ali Hossein Jalilian
Abstract
The Mesozoic sequence in the Zagros Region (SW Iran) consists dominantly of carbonates especially dolomite that play the role of reservoir in many oil and gas fields of the region. To investigate the function of sedimentary and diagenetic processes in the formation of Triassic-Neocomian dolomites of ...
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The Mesozoic sequence in the Zagros Region (SW Iran) consists dominantly of carbonates especially dolomite that play the role of reservoir in many oil and gas fields of the region. To investigate the function of sedimentary and diagenetic processes in the formation of Triassic-Neocomian dolomites of the easthern High Zagros, this study focuses on petrography and geochemistry of the carbonates crop out in the Khaneh Kat anticline. Field and laboratory data led to the recognition of four different dolomitization models within the studied carbonates. The Lower-Middle Triassic and Upper Jurassic dolomites are medium to thick-bedded and composed of fine to medium-grained relatively ordered dolomite. The stoichiometric chemistries and heavier oxygen isotope ratios of these dolomites as well as the considerable presence of evaporites indicate the favorable conditions for the formation of early dolomite due to downward-percolating of concentrated evaporate brines (reflux model). The Upper Triassic dolomitic portion of the Zagros consists of medium to coarse-crystalline, massive-bedded dolomites with relatively light isotopic composition. Mineralogy and geochemistry of these dolomites indicate a high temperature origin as well as their relationship to diagenetic processes, including recrystallization in a deep-subsurface environment (burial model). In the Upper Jurassic portion of the succession, dolomites are ...
A. Bavi Ovaydi; M. H. Adabi; A. Sadeghi; H. Amiri Bakhtiar
Abstract
Dolomitic limestone and dolomite comprise the most part of Ghorban member of the Sachun Formation (Paleocene-early Eocene) in Ghareh Anticline section in southeast Shiraz. Geochemical and petrographic studies of these deposits indicate that these dolomites have formed in marine, meteoric and burial diagenetic ...
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Dolomitic limestone and dolomite comprise the most part of Ghorban member of the Sachun Formation (Paleocene-early Eocene) in Ghareh Anticline section in southeast Shiraz. Geochemical and petrographic studies of these deposits indicate that these dolomites have formed in marine, meteoric and burial diagenetic environments. According to these studies, three types of dolomites have been recognized including dolomicrite (less than 16 µm), dolomicrosparite (16-62 µm) and dolosparite (more than 62µm). Evidences such as fenestral porosity, evaporate minerals (gypsum and anhydrite), intraclast and lack of fossil show that dolomicites have been deposited in supratidal environment. Dolomicrosparites which been formed due to recrystalization of dolomicrites and replacement of micrite, have higher amount of Fe and Mn, and lower amounts of Na, Sr and Mg. Quantities and contributions of the above mentioned elements are related to more diagenetic effect on dolomicrites and formation of these dolomites in medium to deep burial diagenesis environment. Dolosparites have been seen in three forms. The first form includes euhedral crystals of dolomite that have been replacement of red algae and micrite. Second form of dolosparites that consist of crystals with syntexial overgrowth, luminescence with zones consisting of light and dull bands and fluids inclusion are pore filling cements which form in burial diagenetic environment. Euhedral-subhedral, porous, destructive fabric with sucrose texture Dolosparites (the third form) in Sachun-Jahrum formations boundary have been interpreted as meteoric-marine mixing zone environment product.
F. Karimzadeh; M. H. Adabi
Abstract
Carbonate and siliciclastic sedimentary sequences of Sorkh Shale and Shotori Formations ( Lower and Middle Triassic) with a total thickness of 870 m , have a gradational and conformable contacts, at Behabad region of Bafgh in Central Iran. At the beginning of this sequence, shale, ...
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Carbonate and siliciclastic sedimentary sequences of Sorkh Shale and Shotori Formations ( Lower and Middle Triassic) with a total thickness of 870 m , have a gradational and conformable contacts, at Behabad region of Bafgh in Central Iran. At the beginning of this sequence, shale, siltstone and sandstone gradually change into dolomitic carbonates of tidal flat sub-environment. Thick layered or massive sequence of mainly dolomitic rocks of Shotori Formation, are transitionally overlain by a limestone member namely Espahak limestone. Based on petrographic studies, four different types of dolomites (based on shape, crystal size and crystal boundaries), have been recognized. Dolomite type 1 (dolomicrite) formed during the first stage of sedimentation and under surface temperature, this is very early diagenetic dolomite. The other types, having coarse crystal size, formed during shallow to deep burial environments. Change of composition and temperature of dolomitizing fluids at several stages of diagenesis (early to late ) led to formation of different types of Shotori dolomites. On the basis of geochemical studies (elemental analysis such as Ca, Mg, Sr, Na and stable isotopes such as ,), formation of these dolomites occurred in a reducing environment and increasing temperature. The source of Mg is sea water for dolomite type 1, but for the other types of dolomites, Mg provided was by clay minerals diagenesis and basinal brine. The results of XRD and XRF on chlorite and montmorillonite clay minerals of Sorhk Shale Formation show a decrease in MgO content from 36% and 84% respectively compared to standard clay minerals. This reduction is due to diagenesis of clay minerals that exist in shales and Mg was originated from overlain layers. Measurement of organic carbon contents of dolomites and limestones showed that dolomites have several times more organic carbon than limestones. This information confirm that limestones rich in organic matter, are more susceptible to dolomitization. Microbial structures such as stromatolites, in the Shotori Formation have major role in trapping of organic matters. The calculated paleotemperature for the formation of early dolomites is about and for late diagenetic dolomite is .
