ORDOVICIAN KARST WEATHERING CRUST AND RESERVOIRS IN CENTRAL AND EASTERN ORDOS BASIN

The formation of karst weathering crust is the major phase of the palaeo-karst evolution in the Ordovician limestone in the Ordos Basin. Based on the study of cores, thin sections and well logs from the central and eastern parts of the basin, combined with its specific hydrodynamic characteristics, this article takes the Well SH15 as an example to study the process of karstification. Three zones of weathering crust karst were recognized. They are, from the top to the bottom, vertical vadose zone, horizontal subsurface flow zone and bedrock subcritical flow zone. The process of palaeo-weathering crust karstification was discussed, and a model was established in this paper. The Ordovincian karst limestone is an important reservoir in the region. The characteristics of palaeokarst reservoirs and their governing factors are carefully studied based upon the data of lithofacies, palaeogeography, palaeogeomorphology and so on. Results show that the enrichment of natural gas is related to the development of weathering crust karst in this region. The conclusion is helpful to gas exploration in the Ordos Basin.