Depositional Environmental Analysis of Shally Units of Pabdeh-Gurpi Formation and Clay Minerals Effect on Wellbore Stability, Aghajari Oil Field

Shally units are one of the most problematic parameters in the process of drilling oil reservoirs. The current study is related to the instability of the wellbore due to the presence of clay minerals bearing shally units of the Pabdeh-Gurpi formations in Aghajari oilfield. Investigation and identification of shales were done using the XRF laboratory and NGS logs. The results showed that the NGS method is sensitive and applicable to identify Illite mineral merely. According to Fe2O3 content, the upper part of the oxidant conditions, but towards the Gurpi, is dominated by reduction condition. The role of other elements in the formation of clay minerals was discussed. Based on the amount of iron, manganese and vanadium, the samples are classified as the Quinby_Hunt and Wilde class lll (non-sulfide resuscitation but Eh recovery and medium to low pH). The range of changes in the Th/U ratio (4-1.5) signified an environment with changes from sea to intermediate states.