Paleokarst of the Lianglitage Formation related to tectonic unconformity at the top of the Ordovician in the eastern Tazhong Uplift, Tarim Basin, NW China

Abstract

The Lianglitage Formation (also named Lianglitag Formation) is an important hydrocarbon reservoir in the Tarim Basin; the reef and shoal facies of this formation have previously been extensively investigated. The paleokarst of the formation related to a tectonic unconformity at the top of the Ordovician (Tg5) has, however, not been widely investigated. In this paper, we undertake a comprehensive study of the paleokarst by an integrated analysis of seismic data, well‐logging data, and core data. Several geomorphologic units that formed at the end of the Ordovician period are identified, including a fault‐thrust uplift high, inter‐uplifts sag, uplift platform, and fault‐thrust anticline; these units are controlled by west–east and northwest–southeast striking faults. The thickness of erosion is estimated to be 200 to 700 meters. The paleokarst is characterized by large and small caves, dense vugs, and dissolution fractures. Large caves predominantly developed in exposed areas and on the slopes. Dense vugs and small caves, features that contribute to premium hydrocarbon reservoirs, are generally tens of meters away from the unconformity, whereas dissolution fractures, predominantly filled by mud and silts, are situated in close proximity to the unconformity and faults. Four karst zones can be identified from vertical profiles: weathered crust (1–2 m thick), vertical infiltration zone (tens of meters thick), phreatic zone (tens to hundreds of meters thick), and a bottom zone that is largely unaffected by karstification. The karst of the Lianglitage Formation at the end of the Ordovician was mainly controlled by tectonic movement, paleogeomorphology, lithofacies, faults, and fractures. The karstification process is summarized in a karst model.