Sedimentary Characteristics And Its Hydrogeological Significance Of Jurassic Zhiluo Formation In The East Of Ning Xia Province

Ordos Basin is the second largest basin in our country, and the largest basin contains three energy sources of petroleum, gas and coal. The east of Ning Xia Province is located in the western edge of Ordos Basin. Yan'an Formation is the Mesozoic coal-bearing strata in Jurassic in this area, and its overlying strata is Zhiluo Formation, which develops many sand bodies. The thick sand layers provide good storage spaces for groundwater. In order to mine the upper group coal of Yan'an Formation safely, it is of great significance to ascertain the characteristics and the water-rich situation of the sandstone-aquifer in Zhiluo Formation.By utilizing the principle and method of the sedimentology and the high-resolution sequence stratigraphy, fully collecting field outcrops and drilling and logging information, this paper systematically analyzes sedimentary characteristics of Zhiluo Formation in the east of Ning Xia Province and has divided the sedimentary system and identified the sedimentary facies in the study area, analyzed the Zhiluo Formation by the high-resolution sequence stratigraphy, established the sequence stratus frame, and has studied the sand body spreading regularity in this sequence stratus frame.The conclusion contains the follows:1. The vertical sedimentary sequence for Zhiluo Formation in the study area can be divided into braided river sedimentary system, braided river delta sedimentary system, meandering river sedimentary system and shore-shallow lacustrine sedimentary system in ascending order. This paper has identified the sedimentary facies,subfacies and microfacies in the study area.2. Based on principle and methods for base level cycle of the high-resolution sequence stratigraphy, the middle Jurassic Zhiluo Formation in the study area is divided into aⅡlevel sequence (SLSC1), threeⅢlevel sequences and tenⅣlevel sequences, respectively corresponding to a super long-term base-level cyclic sequence (SLSC1), three long-term base-level cyclic sequences (LSC1-LSC3) and ten mid-term base-level cyclic sequences(MSC1-MSC10), and has established the sequence stratus frame.3. This paper studies the sedimentary facies spreading regularity of Zhiluo Formation for the study area in the high-resolution sequence stratus frame, making the conclution that Zhiluo Formation sedimentary facies of the study area developed braided river facies-braided river delta facies-meandering river facies and shore-shallow lacustrine facies successively, reflecting that the western edge of Ordos Basin experienced tectonic cycle, which includes the tectonic decline and tectonic uplift.Furthermore, it is studied that the study area terrain roughly presents higher in the west and lower in the east, higher in the north and lower in the south, the sediment sources of Zhiluo Formation comes mainly from the northwestern and western edge on the higher topography,in the study area the sedimentary facies are distributed in N-S direction, mainly including sandy river skeleton, sand dam and silty sand-shale river alluvial flat, and the thick sand layers, formed by the sandy river and sand dam, provide good conditions for groundwater reservoir and development.4. According to the distribution characteristics of sand bodies, the sandstone aquifer water-rich capability of Zhiluo Formation in the study area is divided into three types: strong, middle or weak level, and districts.