Characteristics and solution intensity of karst formations in Ordovician carbonates in the Halahatang area of the northern Tarim Basin

Fractured and vuggy reservoirs were found in several Ordovician carbonate formations in the Halahatang area of the northern Tarim Basin.The lithology of different formations and their vertical distribution were studied based on microscopic observations and geochemical testing of core samples.Dissolution tests of different types of carbonates showed that the contents of argillaceous matter and MgO could influence carbonate solubility.The intensity of dissolution decreased from pure limestones, dolomitic limestones, dolomites to muddy limestones.The Ordovician carbonates were classified into two classes, three types and five subtypes.The Yijianfang Formation and the first and second members of the Yingshan Formation are continuous limestones with strong karstification.The first member of the Lianglitage Formation and the third and fourth members of the Yingshan Formation are dolomitic limestones with medium karstification.The other formations were weakly karstified.Drilling results and the presence of "stringed beads" on seismic profiles indicated that the types of karst formation controlled the development of fractured and vuggy reservoirs.