The Effect of Dolomitization on the Petrophysical Characteristics of the Kangan Formation Using Petrography and Geochemical Analyses

Abstract

The reservoir rock of South Pars Gas Field, the largest gas accumulation in the world, is mostly limited to Dalan and Kangan Formations, which are of Late Permian to Early Triassic in age. The Kangan Formation is nearly 193 m thick and consists of limestone, dolomite, anhydrite dolomite, and thin shaly layers from the bottom to the top, respectively. Both dolomite and limestone are susceptible lithologies for developing a high quality hydrocarbon reservoir unless the diagenesis processes have destroyed the petrophysical characteristics. In the current article, the effect of diagenesis processes on carbonates and dolomites of Kangan Formation were studied through petrography and geochemical analyses. The studies demonstrated the extensive penetration of meteoric fluid within the deposited sediments, which caused widespread moldic and vuggy pores with developments of the meteoric environment cements, including inter-granular bladed cements and equate cements. The cements might grow in the preceding time of the limestones compaction. Permeability in the limestone of the aforementioned formation is highly dependent to porosity, geometrical shapes of pores, shape and size of grains, quantity and quality of distributed cements in the reservoir rock, and texture of crystalline dolomites, which are the most important parameters for interpreting the performance of a rock that direct fluids within the fractures and pores. On the other hand, in most of the dolomites the amount of permeability is not directly dependent on total porosity and size of crystals and the relations of pores via throats control the ability of rock for transmitting fluids. In this regard, the large crystalline dolomites with large values of inter-crystalline porosity have larger permeability compared to the dolomites with micrite and microsparite crystals. Also, the euhedral and subhedral dolomites were studied in which the euhedral dolomites showed a greater permeability increment with respect to subhedral type.

Keywords:

• dolomitization
• geochemical analyses
• permeability
• petrography
• porosity

ACKNOWLEDGMENTS

The authors gratefully acknowledge the Pars Oil and Gas Company and all of the staff of the oil and gas director in Asalouyeh, especially Mr. Milad Razami and Mr. Iman Soleymanpour, for their kind assistance and help during this research.