Investigation of sedimentary facies and geochemical parameters of the Asmari Formation (Oligocene-Miocene) in the Shadegan Oil Field, Dezful Embayment, SW Iran

Carbonates of the Asmari Formation form the youngest reservoir rock of the Zagros Basin (Ghazban, 2007). Various studies have shown that sedimentary geochemistry can be used to evaluate palaeotemperature and palaeoclimate by reconstructing the chemical and isotopic content of the ancient seawater and/or diagenetic fluids (Winefield et al, 1996; Adabi, 2004; Adabi and Mehmandosti 2008; Crowe et al, 2013; Swart, 2015; Fallah-Bagtash et al, 2020; Omidpour et al, 2021). The present study is based on a combination of different data such as core analysis, thin-section petrography, trace-element and stable-isotope analysis of Asmari carbonates to recognize the original carbonate mineralogy and diagenetic environment. 

In this study, 524 thin sections prepared from core samples of well-11 were used to achieve the desired goals. All thin sections were stained with potassium ferricyanide and Alizarin Red-S to dolomite and calcite minerals identification (Dickson, 1965). The nomenclature for carbonate rocks used in this work is a combination of the terminology introduced by Dunham (1962) and Embry and Klovan (1971), which is based on textural aspects. Facies analysis and interpretation of the depositional environment was performed using by Burchette and Wright (1992) and Flügel (2010) schemes. Based on the detailed petrographic results, forty-five limestones and thirty-two dolomites from well-11 were carefully selected for trace elemental analysis. Elemental analysis carried out using atomic absorption spectrometry (AAS) in the geochemistry laboratory in the Ferdowsi University of Mashhad for the major and trace element determinations. Forty-five powdered of the limestone samples, that were previously used for the major and trace elements, were analyzed with a VG STRA Series II for oxygen and carbon isotopes at the G.G. Hatch Stable Isotope Laboratory, University of Ottawa.

The detailed thin-sections analysis of the carbonate samples resulted in the distinction of 26 microfacies types in a subsurface section of the Shadegan Oil Field along that have been deposited along a homoclinal ramp-type platform and is divisible into an inner ramp, mid ramp, outer ramp and basinal settings.The bulk-rock oxygen and carbon isotopic analyses of the Asmari limestones are compared with similar analyses of the Asmari Formation in the Dezful Embayment (Aqrawi et al, 2006) and of other Palaeogene-Neogene carbonates (Veizer et al, 1999). It can be deduced from Figure 10 that the δ18O and δ13C values correlate well with those found by Aqrawi et al. (2006) for the Asmari Formation, but show slightly lower δ18O and δ13C values than those mentioned by Veizer et al. (1999) for the Palaeogene-Neogene carbonates. The geochemical and isotopic data allow, in combination with the petrographic data obtained from thin-section analysis, recognition of the primary aragonite mineralogy and the evolution of the rock fabric, as well as a reconstruction of the diagenetic evolution, temperature, the nature of the percolating fluids, and the water/rock ratio or diagenetic system.  The input of the δ18O-enriched samples (-0.85‰) within the Anderson and Arthur (1983) formula gives a syn-sedimentary temperature of only 23 °C. Based on textural and geochemical features four types of dolomite (D1 to D4) were identified in the sedimentary succession of the Asmari Formation. D3 (with high values of iron and manganese) is more affected by diagenetic alteration than the D1. Due to oxidizing conditions, iron and manganese values in D1 (near-surface) are lower than the burial dolomites (D3), which formed under a more reducing state at the greater depth of burial (Tucker and Wright 1990; Hou et al, 2016).