Microfacies and sequence stratigraphy of the Qom Formation; Goylar section; Southwest of Zanjan province

Sedimentary environment and sequence stratigraphic studies of the Qom Formation as a reservoir rock is of utmost importance in terms of the potential for hydrocarbon resources. Bozorgnia (1966) divided the Qom Formation into nine members (a, b, c1, c2, c3, c4, d, e, f) in the age of Rupelian to Burdigalian, which the National Stratigraphic Committee of Iran accepted. This research is focused on paleoenvironmental interpretations and sequence stratigraphy of the Qom Formation in the Goylar stratigraphic section, which is located northwest of the structural zone of Central Iran.

A total of 64 thin sections of hard samples of Qom Formation in the Goylar section have been prepared. Due to the similarity of foraminifera in the Qom and Asmari formations, the biozonation of the Asmari Formation, which is documented by Wynd (1965), Adams & Bourgeois (1967), and Laursen et al. (2009), has been used in this research. Determination of planktonic foraminifera has been done only by studying thin sections. To recognition of the facies and sedimentary model of the Qom Formation, studies of Reed (1995) and Flugel (2010) have been used, and to undertake sequence stratigraphy, the model of Hunt & Tucker (1993, 1995) is used.

Qom Formation in the Goylar section is 155 meters in thickness. Qom Formation includes thick-bedded limestone, thin to medium bedded sandstone, shale and marl, reef-coral limestone, and green marl and argillaceous limestone alternation. According to the lithological characteristics, Qom Formation in this area can be equivalent to member f. A total of 24 genera and 18 species, including 17 genera and species of Benthic foraminifera: Borelis melocurdica, Psuedoilthonella richelli, Peneroplis evolutus, Peneroplis thomasi, Heterolopa dutemplei, Asterigerina rotula, Valvulina sp.1, Valvulina sp.2, Pyrgo sp.1., Amphistegina spp., Spiroloculina sp., Quinqueloculina sp., Austrotrillina sp., Miogypsina sp., Lenticulina sp., Elphidium sp.1., Rotalia sp., Nodosaria sp., Textularia sp.; and 7 genera and 13 species of planktonic foraminifera with an axial incision in thin sections including Globigerinoides primordius, Globigerinoides subquadratus, Globigerinoides triloba, Globigerinoides immaturus, Paragloborotalia mayeri, Paragloborotalia spp., Globigerina praebulloides, Globigerina sp., Globigerinella obesa, Globorotalia archeomenardii, Globorotalia sp., Praeorbulina transitoria, Bolivina sp.According to paleontological studies and based on the stratigraphic distribution of foraminifera, Borelis melocurdica-Borelis melo melo Assemblage Zone (Burdigalian, Early Miocene) has been recognized certainty for the Qom Formation. Facies analysis, including variety in fauna features, facies geometry, and sedimentary textures, has produced seven microfacies related to the four facies belts of the lagoon, reef, slope, and open marine. Deep Shelf/Slope Facies Belt: Mf.1. Planktonic foraminifera wackestone-packstone, Mf.2. Bioclast Miogypsina packstone, and Mf.3. Amphistegina coral corallinacea packstone; Shoal/ Reef deposits facies Belt: Mf.4. Coral Boundstone, and Mf.5. Bioclast bryozoan corallinacea Packstone, and Lagoonal facies Belt: Mf.6. Bioclast Packstone, and Mf.7. Porcelaneous foraminifera Packstone.Consequently, of lacking the sediment gravity flows, turbidity facies, and presence of the reef facies deposits (25 percent of the Qom deposits (including coral boundstone, algae, and bryozoan, sedimentation of the Qom formation occurred likely on an open shelf. Facies distribution chart demonstrates that the beginning and closure of the Qom Formation sedimentation occurred significantly in the continental slope environment (without considering the possible erosion phenomenon). Based on sequence stratigraphic studies, five depositional sequences and six-sequence surfaces have been separated (the first and sixth sequence surfaces being of the type-1). Comparing the sequence levels of this study with the sequences identified in other regions of the Central Iran Basin and the European Basin shows a good correlation.