Petrography and diagenetic history of the Shurijeh Formation (Late Kimmeridgian–Hautrivian) at Estarkhi section, Kopet-Dagh Basin, NE Iran

Abstract

AbstractThe Shurijeh Formation is a Late Jurassic–Early Cretaceous (Late Kimmeridgian–Hauterivian) siliciclastic rock unit of the Kopet-Dagh Basin, up to 392 m thick. In the Estarkhi section, it rests disconformably on the Mozduran Formation (Oxfordian), and is disconformably overlain by carbonate rocks of the Tirgan Formation (Barremian–Aptian). The sandstones are predominantly subarkose, sublitharenite, feldspathic litharenite, and litharenite with minor quartzarenite, rich in quartz and feldspars, sedimentary and igneous rock fragments (plutonic), and rarely metamorphic fragments. Based on petrological and geochemical studies, minor diagenetic events in the early diagenetic stage include cementation by calcite and iron oxide. Deep burial diagenetic events were dominated by compaction, cementation (silica, dolomite, chlorite), fracturing, dissolution, pressure solution, and albitization. Minor late diagenetic events include dissolution and cementation (calcite, iron oxide). Based on diagenetic events, a temperature of >80°C during burial diagenesis is suggested for Shurijeh Formation. The existing porosity is secondary, resulting largely from the dissolution of feldspar and carbonate cement, with some fracture porosity. The porosity and permeability of 11 core plugs average 7.78% and 4.84 md, and the point count of 38 thin sections shows an average porosity of 10.05%. The good porosity and permeability of the middle part of the section predict an acceptable reservoir potential for this formation in the study area.Keywords: Kopet-Dagh, Shurijeh Formation, diagenesis, porosity, Cretaceous Introduction The Kopet-Dagh Basin is the most important zone due to the existence of the best outcrops of Jurassic and Cretaceous rocks in NE Iran and also its high potential of petroleum system elements. The Kopet-Dagh Basin formed after the closure of the Hercynian Ocean following the Early Cimmerian orogeny. The siliciclastic Shurijeh Formation comprises important gas reservoirs in the Kopet-Dagh Basin. The Shrijeh Formation with 392 meters thick, in the Estrakhi area, rests disconformably on the Mozduran Formation, and is disconformably overlain by the Tirgan Formation. In this paper, the petrography, interpretation of diagenetic history, thermal history, porosity, and permeability of the Shurijeh sandstones in the Estarkhi area are presented. Material & MethodsThe one stratigraphic section was logged graphically, and 60 fresh sandstone samples were systematically collected, from which 38 thin sections were made. Petrographic modal analyses were made using a Nikon E400 Pol microscope, with 500 point counts on 38 selected samples using the Gazzi-Dickinson method to identify grains, cement types and proportions. The porosity and permeability were determined on core plugs from 11 sandstone samples using standard methods. The samples were vacuum-dried at 60°C and analyzed using a permeameter for permeability in nitrogen (air), and for porosity using porosimeter in helium gas, under a pressure of 120 to 380 psi, at a temperature of 180°C, and the amount of gas output was measured. Porosity percentages were also estimated using 500 counts on 38 thin sections, stained with blue epoxy. Closely similar porosity measurements were obtained using these two methods. Six polished thin sections were studied to determine the composition of mineral components. Discussion of Results & Conclusion Based on field and laboratory studies, three terrigenous lithofacies of different grain sizes including conglomerate, sandstone, and shale and minor carbonate facies have been identified. The sandstones are fine- to medium-grained and grain-supported, with some coarse-grained and well-rounded components. Based on the angularity, sorting, and matrix content, most sandstones are mature, sub-mature, and rarely supermature. The sandstones are predominantly subarkose, sublitharenite, feldspathic litharenite, and litharenite with minor quartzarenite, rich in quartz and feldspars, sedimentary and igneous rock fragments, and rarely metamorphic fragments. The Shurijeh sandstones experienced diagenetic events that included cementation, alteration, compaction, and fracturing, dissolution and albitization. The predominant cement are silica and carbonate, iron oxide, clay minerals, with minor authigenic minerals such as K-feldspar overgrowth, albitization, and rutile. The cement occupy inter- and intragranular spaces, form veins and fill fractures, and vary from microcrystalline to coarsely crystalline in the calcite cement type. Iron oxide cement is present throughout the Shurijeh sandstones as an alteration product and cement. Clay minerals are less widespread than other types of cements, but chlorite is the main clay mineral. The sandstones show variable degrees of mechanical and chemical compaction especially prominent where early cement are lacking. Grain contacts include elongate and concavo-convex, point contacts in rare cases, and sutured contacts that indicate intergranular pressure solution and deformation at a more advanced stage. Quartz and feldspar grains have been intensively fractured, but the fractures have been largely healed through silica cementation, allowing the grains to maintain their integrity. Dissolution is prominent in the sandstones. Detrital K-feldspar, quartz, and carbonate cement all show evidence of partial to complete dissolution.According to petrological and geochemical studies, minor diagenetic events in the eodiagenetic stage include cementation. Mesodiagenetic events were dominated by cementation, compaction, grain microfractures, dissolution and replacement, pressure solution, the formation of K-feldspar overgrowth and albitization and rarely rutile. Minor telodiagenetic events include dissolution and cementation. Thus, based on diagenetic events, a temperature of >80°C during burial diagenesis is suggested for Shurijeh Formation. Existing porosity is secondary, resulting largely from the dissolution of feldspar and carbonate cement, with some fracture porosity. The porosity and permeability of 11 core plugs average 7.78% and 4.84 md, and the point count of 38 thin sections shows an average porosity of 10.05%. The bulk of the porosity is secondary.

Keywords

• kopet-dagh
• shurijeh formation
• diagenesis
• porosity
• cretaceous