SEDIMENTARY FACIES OF THE ORDOVICIAN MA 5₄ IN THE WESTERN PART OF JINGBIAN PLATFORM

The Odors Basin is one of the major hydrocarbon-bearing basins in China with the Ordovician carbonate rocks as the major gas reservoir. The Member Ma5₄ of the Ordovician is a main producing reservoir in the west Jingbian Gas Field of the Basin. As a carbonate dominated reservoir, it is critical to study the reservoir properties for better exploration and development. This paper is devoted to the sedimentary facies of the Ma5₄ member of the west Jingbian Gas Field on the basis of previous studies and the comprehensively use of the present materials including drilling cores,logging and testing data, and the mapping method upon single factor analysis and multifactor analysis. Types of sedimentary facies and their distribution pattern are carefully studied. The Ma5₄ was deposited in a low energy environment of a restricted platform under a dry and hot climate and the sea level was low then. The platform was even exposed to air sometimes. It can be further classified into five microphases:dolomitic flat, gypsum dolomite flat, gypsum-bearing dolomite flat, gypsum dolomite flat and pelitic dolomite flat. There was shifting of microfacies and subfacies from Ma5³ ₄ to Ma¹5₄ while the sea level rose relatively.