Petrophysical evaluation and geochemical evidences for recognition of original carbonat mineralogy of Ilam Formation in the Asaluyeh cross section and South Pars Gas Field

In this research, we tried to study the Ilam Formation using petrography (thin sections) and geochemical analysis (major and trace elements). Petrophysical studies have also been used to identify the characteristics and diagenetic environment, and to determine the sedimentary environment and evaluation of reservoir quality. 500 thin sections prepared from cutting samples in X and Y wells and 80 thin sections from Asaluyeh section have been studied to identify, evaluate facies and type of sedimentary environment beside diagenetic processes of The Dalan Formation. For classification and texture descriptions, the classification schemes of Dunham (1962) used. The method of Dickson (1965) used to differentiate calcite from dolomite, with Alizarin (Red-S) and potassium Ferosianid. Types of dolomites were identifies (Adabi, 2009). For description of the microfacies and interpretation of depositional environment model, Flügel (2010) scheme was used. Study of Petrophysical Properties of Ilam Formation in the South Pars Gasfield have been done based on petrophysical logs (gamma, sonics and acoustic), geological reports and reservoir data. Major and trace elements analysis and especially their diagenesis are the important part of carbonates studies. According to the major and trace elements studies and the ratio of these elements in the limestone samples of Ilam Formation, the original aragonite mineralogy in shallow parts and original calcite mineralogy in deeper parts of these deposits were interpreted. Comparison of the Ilam limestone samples with ranges that presented to aragonitic limestone of other fields and plotting the samples in this area confirmed the mixed aragonite-calcite mineralogy. High contents of Sr and Na, low Mn content and high Sr/Na ratio for aragonitic samples and low Sr/Na ratio for calcite samples show that the limestone samples have been stabilized by fluids in a closed to open diagenetic system. The shale zones are almost similar in both wells, and the shale volume has peaked at the lower boundary of both wells due to the high percentage of shale in the Lafan Formation. The sharp changes in the upper and lower limits of the Ilam Formation represents a change in the boundary of this formation with upper and lower formations.