Evolution modes of interbedded weathering crust karst: A case study of the 1st and 2nd members of Ordovician Yingshan Formation in EPCC block, Tazhong, Tarim Basin

Abstract

The evolution of interbedded weathering crust karst on the top of the Ordovician Yingshan Formation were described based on the geological background, oil and gas show while drilling, core, log imaging and seismic reflection features in the Tazhong area. The study shows that: Filling features of dissolved caves near unconformities depend on the hydrodynamic strength, which, together with the vertical lithofacies, determines the development degree of the interbedded karst. Karst units are classified into several first order units, namely highland, sub-high, slope, and sub-sag, and some secondary units such as dissolved peak, cliff, platform, and dissolved mound, stretching like a strip from northwest to southeast on the whole. The surface weathering zone developed only in the north of the EPCC block, the vertical vadose zone has different thickness in south and north, while the seasonal variation zone changes to horizontal vadose zone from south to north. Sedimentary microfacies affects karst development, but it is not the deciding factor of karst cycle classification, the main factors resulting in the karst difference between south and north are structural uplift, climate, and ancient hydrological conditions. Three cycles and eight secondary cycles are identified in the interbedded weathering karst in Member Ying1-Ying2 in the EPCC block, among them, the third cycle, including the fifth to the eighth secondary cycles, remains, and the fifth and eighth secondary cycles show obvious “different facies” features from south to north. Key words: Tazhong, EPCC block, Ordovician, interbedded karst, karst cycle, evolution mode