Yuan Xie, Chengdu Institute of Geology and Mineral Resources (China)
Jian Wang, Chengdu Institute of Geology and Mineral Resources (China)
Xinsheng Jiang, Chengdu Institute of Geology and Mineral Resources (China)
Guangcai Hou, Xi'an Institute of Geology and Mineral Resources (China)
Yonghe Wang, Xi'an Institute of Geology and Mineral Resources (China)
Guoshi Deng, Chengdu Institute of Geology and Mineral Resources (China)
The Ordos Basin in North China abounds in oil-gas, coal, and groundwater. Due to the lack of surface water, the groundwater leaving in the Lower Cretaceous are now considered as the local important water sources. However, there are complicated variations in the regional distribution of the groundwater quality. So the groundwate assessment and exploitation may be seriously constrained. This paper has discussed in detail for the first time the influence of the Early Cretaceous sedimentary facies and palaeogeography on the groundwater quality leaving in the Lower Cretaceous, on the basis of many field outcrops, well logs and hydrogeological data. The Early Cretaceous Ordos Basin experienced two sedimentary and palaeogeography evolution phases(Yijun-LuoheiúHuachi-Huanhe phase and LuohandongiúJingchuan phase) from desert facies to lake facies in response to two arid-semiaridiúhumid climatic cycles. During these periods, 1300 m-thick Lower Cretaceous clastic rocks were formed as a giant groundwater system. The aquifer system may be divided, from bottom to top, into Yijun-Luohe, Huachi-Huanhe and Luohandong-Jingchuan water-bearing complexes, of which the former two are widely developed with tremendous thickness whereas the latter is only restricted to the western and northern margin the basin. During the Yijun-Luohe stage of the first sedimentary and palaeogeography evolution phase, clearly constrained by arid climates and other conditions, the overall palaeogeographic framework of the basin displays rivers in the northwestern part, deserts in south-central part. Into the Huanhe stage, due to tectonic subsidence and humid climates, the basin is overall characterized by the presence of rivers in the north and lakes of high salinity in the south. Into the Luohandong stage of the second evolution phase, because of arid climates, the basin totally displays rivers in the northern margin and deserts in the southwestern margin.With humid climates and differential uplift in the Jingchuan stage, the basin is characterized by rivers in the northern margin and lakes in the western margin. The Early Cretaceous climate, palaeogeography, sedimentary facies and their evolution and distribution led to the very complex regional distribution of the salt content of sedimentary rocks and conditions of groundwater circulation. Under the combined influences of the sedimentary environmental conditions and other hydrogeological conditions, today the distribution of quality of the groundwater leaving in the Cretaceous are complicated, totally showing the groundwater total dissoloved solids is generally lower in the north and higher in the south, moreover lower in the east and higher in the west, and lower in the lower parts and higher in the upper parts in the south of the basin. That is, groundwater quality is better in the north than in the south, and is good in the east and poor in the west and good in the lower part and poor in the upper part in the southern basin.