Sedimentary Environment, Clay Minerals and Diagenesis of Pabdeh Formation in its Type Section and Ziloee Oil Field (well No. 5 and well No. 8)

Field study, calcimetery and petrography of thin sections from Gurpi type section and petrography of cuttings from Ziloee oil field (well No. 5 and well No. 8) represent two carbonate members and two hybrid (mostly marl and in a few samples clay marl) members in these deposits. Microfacies analysis of the carbonate members implies 2 microfacies belong to open sea of a carbonate ramp model. Moreover, authigenic glauconite and phosphate and framboidal pyrite propose deposition of these sediments in an anoxic condition. XRD and SEM analysis of marl members provide existence of illite and chlorite as clay minerals by certain detrital source and smectite by digenetic and probability detrial source in the upper marl member and existence of quartz in lower member. Coexistence of these minerals proposes a temperate climate during deposition of the upper marl member. Semi-quantitative analysis of XRD data represents descending trend for relative amounts of illite and ascending trend for smectite during deposition of upper marl member of Gurpi Formation which implies deepening of the sedimentary basin and relative climate warming. The digenetic processes in these deep marine deposits are limited to cementation, illitization, hematization and fracturing. Moreover, studying SEM images evident autogenetic origin of smectite in two samples from upper cretaceous (under K/T boundary) which can form during fluid exchange with mafic minerals and/or detrial clay minerals such as detrial smectite, illite. The absence of digenetic smectite in two samples from Paleocene (over K/T boundary) can be related to change in basin condition in Zagros basin, this period of time. Also, studying relative abundance of K and Th in NGS logs of well number 8 in Ziloee oil field shows that clay minerals in this formation are illite and montmorillonite.