The Effect of Post Extinction Facies on Reservoir Compartmentalization of Kangan and Dalan Formations in Central Persian Gulf

The post extinction facies are known as unusual deposits in sedimentary record. This study focuses on the role of these facies in reservoir behavior of the carbonate-evaporite series of Kangan and Dalan formations in central Persian Gulf. Sedimentological studies in three wells in the studied field show that these facies could be recognized with the unusual bloom of thrombolites through the field. Increase in seawater dissolved carbonates, caused by the demise of most carbonate-secreting organisms, produces frequent and huge cements immediately after the Permian time. This was coupled with agitation in sedimentary environment caused by sea-level fall at the end-Permian time. The nature of the thrombolites as well as the precipitation of these cements reduces both porosity and permeability of these strata. In spite of this fact, analysis of initial reservoir pressures and gas volume percentages as well as gas carbon isotopes reveals strong similarities between two reservoirs and so this unit could not be considered as a static reservoir compartmentalizer. The linear trend of reservoir initial pressures with increasing depth and also cluster analysis of gas volume and carbon isotope data confirms this interpretation. Regarding low porosity and permeability and also sedimentological characteristics of this unit, the post extinction anachronistic facies of the base of Kangan Formation will act as a dynamic reservoir compartmentalizer at the time of production. This situation separates the Kangan and Dalan formations into two distinct reservoir units from reservoir pressure point of view. Regarding such variations, the initial dynamic reservoir predictions will change and this will have a strong effect on future economic situations of the field.