Lithofacies-based study of the structure of the aquifer system of the Cretaceous Ordos basin

based on lithologic, lithofacies, porosity and logging studies, the aquifer system in the Cretaceous Ordos basin may be classified into two subsystems, i.e. the single structure of the desert plateau in the north and the multiple structure of the loess plateau in the south. The structure characteristics of the two subsystems are notably different: the northern subsystem is a uniform aquifer from the top downward, marked by vertical overlapping and association of aquifers with large and moderate water storage capacities and no regional continued and steady aquifuge. By contrast, the southern subsystem is a separated hydrologic layer, characterized by overlapping of aquifers and aquifuges and pronounced vertical hydrogeological layering. The types of the northern and southern aquifer structures of the basin are markedly different. The water storage capabilities of the aquifers are classified. Altogether 7 aquifers with a large storage capacity, 7 aquifers with a moderate storage capacity and 2 aquifers with a small storage capacity are distinguished. Comparing the north and south of the basin, the aquifers in the north have a high porosity and a. The groundwater mainly concentrates in the north because the porosity in the north is higher and the storage capacity is larger than those in the south. In the north, the porosity changes from higher to lower and the water storage capacity turns from larger to smaller; so the groundwater mainly concentrates in the shallow and middle levels. In the south, the Luohandong formation in the upper part and the Luohe Formation in the lower part have higher porosities and larger water storage capacities, while the porosity and storage capacity in the Huanhe Formation in the central part are relatively poor; so the groundwater mainly concentrates in the first two aquifers.