The reservoir characteristics and their controlling factors of Callovian-Oxfordian carbonates in Amu Darya Basin

Abstract

The Amu Darya Basin in northeastern Turkmenistan is one of the most important hydrocarbon basins in central Asia.According to drilling core data and analysis,it was determined that the Callovian-Oxfordian carbonates in the Amu Darya basin were deposited by platforms and characterized by the fore-edge slope. Six main facies were identified:( 1) evaporation platforms;( 2)restricted platforms;( 3) open platforms;( 4) platform-edge reef-banks;( 5) platform fore-edge slopes; and( 6) the basin. The reefs and shoals on the inner slope and edge of the platforms and platform fore-edge slopes were the most favorable reservoirs. The microfacies and lithology play a direct role in controlling the reservoir types and properties. The massive reef limestones and moderate to thick-layered shoal grainstones in the high energy environment consisted mainly of primary pore dominated reservoirs and fissure-pore reservoirs. Grainy micrites,Cryptophyta limestones,and micritic limestones in the low energy environment were mainly found in contained in fissure reservoirs with very poor matrix properties. The honeycombed exploration and exploitation reservoirs consisted of reef and shoal limestones within different facies zones. After comprehensive analysis,it is concluded that the microfacies,lithology,and diagenesis were the primary controlling factors behind the formation of the Callovian-Oxfordian carbonate reservoirs.