Karstification of reef-bank facies carbonate and its control on pore distribution: A case study of Upper Ordovician Lianglitage Formation in eastern Tazhong area, Tarim Basin, NW China

Abstract

The karstification type and characteristics and its control on pore distribution in reef-bank carbonate rock in the Upper Ordovician Lianglitage Formation (O3l) in eastern Tazhong area were studied using cores, thin sections and well logging data. The area experienced two types of karstification: syndepositional karstification and early diagenetic karstification near surface. During the depositional stage of reef-bank complex, meteoric diagenetic lens with lots of selective dissolution pores were formed by the syndepositional karstification, they were developed in the middle and upper parts of reef-bank sedimentary cycles, and a single porous/cavity layer has a thickness of 10–30 m, increasing the absolute porosity by 3%–4%. After depositional stage, sediments were uplifted and exposed without experiencing burial diagenesis, the depositional topography was influenced by early diagenetic karstification near surface and turned into karst paleotopography and formed four karstification zones within 100 m below the top of the Upper Lianglitage Formation. The surface karstification zone and radial flow karsitifiction zone have favorable porous connectivity, increasing the absolute porosity by 4%-8%. These two types of karsitifiction are continuous in diagenesis stage and the formed porosity is successive in distribution. The favorable space-time coupling is the key reason for effective pores being largely developed in the O3l3–O3l1 members. Key words: pore distribution, karstification, reef-bank facies, Lianglitage Formation, Ordovician, eastern Tazhong area