Palaeoenvironment reconstruction, diagenetic overprint and geochemistry of the Upper Cretaceous Sarvak Formation in the north of Dezful Embayment, south-west of Iran

AbstractThe Sarvak Formation, with a total thickness of 1566 m and major limestone lithology in the Ahvaz and Mansouri oil fields, is studied to determine its palaeoenvironment, diagenetic overprint and geochemistry. In the studied wells, the formation conformably overlies the Kazhdumi Formation and is overlain by the Ilam Formation. The petrographical studies led to identifying twelve facies precipitated in four major sub-environments, including inner-, middle-, and outer ramp and an intra-shelf basin on a homoclinal ramp-type setting. Dissolution, compaction, and cementation are the main diagenetic alterations that changed the primary chemical composition and the reservoir property of the Sarvak Formation. These diagenetic processes occurred in phreatic marine, meteoric, and burial realms. Sedimentary geochemistry of major and trace elements, including Ca, Mg, Fe, Mn, and Sr, along with O and C stable isotopes, reveal aragonite as the original carbonate mineralogy of the Sarvak Formation. The depositional environment in the lower Sarvak with the predominant shallow open marine and stratigraphic succession with no evidence of exposure, change to stratigraphic succession with more shallow sub-environments (lagoon and bioclastic-shoal), and the evidence of exposure such as cementation and extensive dissolution, which led to depletion in Sr, δ18O, and δ13C and enrichment in Fe and Mn during sea level fall in the inner ramp microfacies. The Fe and Mn cross-plot with a positive trend shows the effects of diagenetic phases in minor phreatic marine and mainly meteoric realm, all confirmed by petrography. The Sr/Ca ratio has the most correlation to the modern tropical warm-water shallow marine that could confirm the primary aragonite mineralogy; which is supported by the predominance of carbonate mud in the identified facies, primary isopachous fibrous marine cements, and extensive dissolution evidence. The cross-plot of Fe versus Sr and Mn versus Sr/Mn ratio suggests deposition in mainly an oxygenation state that experienced low to high water/rock interaction during different depositional sequences. The δ18O and δ13C co-variation and their comparison with the other fields of the Sarvak conducted in different regions show precipitation in an isotopic equilibrium in the carbonates with the Upper Cretaceous seawater, since calculating salinity (Z=130) and temperature (T= 26°C) of the Sarvak Formation confirm the above statement.Keywords: Diagenesis, Sedimentary geochemistry, Water/rock interaction, Upper Cretaceous, Sarvak Formation. Introduction:The Albian–Turonian Sarvak Formation (Motiei 1993), which forms a part of the Bangestan Group the second most important reservoir succession in the Zagros Basin and Persian Gulf (Esrafili-Dizaji et al. 2015; Assadi et al. 2016). This succession is equivalent to Mishrif, Ahmadi and Rumaila in Saudi Arabia, Natih in Oman, Derder in Turkey, and Mishrif in Iraq, which was deposited on a shallow carbonate platform along with interashelf basins on the passive margin of the Arabian Plate (Ziegler 2001; Piryaei et al. 2011; Alsharhan 2014). Various studies conducted on the geochemistry of carbonates reveal its adequacy in determining palaeoclimate and original mineralogy, defining the rate and type of diagenetic alteration, distinguishing diagenetic realms and trends as well as the stratigraphic position of depositional sequence stratal surfaces and boundaries (Adabi and Asadi Mehmandosti 2008; Crowe et al. 2013; Fallah-Bagtash et al. 2020; Omidpour et al. 2021). In this study, regarding the oil industry needs for exploration and production and based on petrographical and geochemical analysis, the facies, paloeoenvironment, diagenetic characteristics and geochemistry of the Sarvak Formation are studied in the northern Dezful Embayment, southern Iran. Material & MethodsThe facies analysis and diagenetic characteristics of the Sarvak Formation in two wells in the Ahvaz and Mansouri oil fields are studied using 900 thin sections obtained mainly from core samples. The nomenclature for carbonate rocks used in this study is the terminology introduced by Dunham (1962) and its modified classification by Embry and Klovan (1971). The facies analysis and interpretation of the depositional environments were conducted using the standard microfacies of Burchette and Wright (1992) and Flügel (2010). All thin sections were stained with potassium ferricyanide and Alizarin Red-S to distinguish carbonate minerals (Dickson 1965). The geochemical studies are done using analyzing trace and major elements including Ca, Mg, Fe, Mn, and Sr along with O and C stable isotopes of carbonate rocks both in the Exter University of England on 57 samples from the Ahvaz Oil Field and 43 samples from the Mansouri Oil Field. The trace and major elements are analyzed using Agilent 5110 VDV Inductively Coupled Optical Emission Spectrometer (ICP-OES) with 0.02 mmol/mol precision for Mg/Ca ratio, 0.4 µmol/mol for Sr/Ca ratio, 0.02 mmol/mol for Fe/Ca ratio and 0.006 mmol/mol for Mn/Ca ratio. For stable isotopes, the analysis is done using SerCon 20-22 Gas Source Isotope Ratio Mass Spectrometer (GS-IRMS) based on the Copenhagen University Standard (LEO, Carrara Marble) and with the precision of ±0.08‰ for δ13C and 0.28‰ for δ18O. Discussion of Results & ConclusionThe Sarvak Formation, with dominant limestone lithology and 831 and 735 m thickness, respectively in Ahvaz and Mansuri Oil Fields, conformably overlies the Kazhdumi Formation and is unconformably overlain by the Ilam Formation. The petrographical studies led to determinate five facies association deposited on a low angle ramp type setting along with an intrashelf basin. Diagenetic studies reveal the impact of marine, unconformity-related meteoric, and shallow and deep burial diagenesis that mainly caused intense compaction, dissolution and cementation. These main diagenetic features changed the nature of the associated facies regarding the reservoir property of the Sarvak Formation. The impact of main diagenetic features along the Sarvak succession improved the understanding of the elemental, and O and C stable isotope analyses. The geochemical analysis as well as petrographical evidences such as diversity of fauna and flora and primary marine diagenetic features confirm the original aragonite mineralogy for the Sarvak carbonate, which was formed in a warm and humid condition on a shallow marine platform. The analyzed δ18O and δ13C, along with the variable concentration of Sr, Fe and Mn suggest maintenance of the primary composition along with alteration in burial realm and mainly meteoric diagenesis. The cross-plot of trace element and O and C stable isotopes indicate that the Sarvak carbonate rocks altered in the open to close diagentic system during different depositional stratigraphic sequences depending on the exposure of the carbonate platform and its scale and duration