| The Jurassic Yan'an Formation is one of the main coal systems in the Ordos Basin.In recent years,mine flooded accidents have been a problem restricting coal mining.The pore structure is the microscopic characterization of the penetrated sand layer,which not only provides the necessary reservoir space for the fluid,but also affects the rate.This restricts the enrichment of the water body and seriously affects the safety production of coal fields.Studying on the characteristics of pore structure is the basis of understanding,evaluating,protecting and reforming the penetrated sand layer.This article is based on the core observation of the coring wells in the west of the Xinjie town,Inner Mongolia,and through the conventional and casting thin section,particle size analysis,SEM,mercury injection test,grain analysis and other testing means.With reservior geology,sedimentary petrology,sequence stratigraphy and other disciplines as the guidance,and based on the study of stratigraphic division and sedimentary microfacies,this article discuss the relationship between the pore structure and the water-richness of the sand-bearing layer.The petrological characteristics,diagenesis,physical characteristics,pore evolution,pore structure,pore type division of the sandstone in the Zhiluo Formation were analyzed and studied systematically.And the distribution of the structural characteristics of the sandstone in the study area is plottedThe results show that the Zhiluo formation can be divided into two segment,and the lower part is Zhi 1 member,the upper is Zhi 2 member.In addition each segment has two sub-layers.Zhi 1 member is mainly braided river deposition,and Zhi 2 member is the meandering river deposition.The lower sub-layer sandstone of Zhi 1 member and Zhi 2 member is coarser,and the upper sub-layer is mainly fine sandstone,siltstone and mudstone.In the study area,the sandstone is mainly the medium-coarse and medium-fine feldspar sandstone.The content of the impurity is low and the cement is high,which is mainly kaolinite,chlorite,calcium and siliceous.Diagenesis mainly manifests as compaction,cementation and dissolution,in which the pore loss rate of compaction is about 49%,the cementation is about 14.5%,and the contribution of dissolution to the present face rate is about 30%.The sandstone in the Zhiluo Formation is in the early diagenetic stage B-middle diagenetic stage A.The pore type of sandstone is mainly composed of the intergranular pore,feldspar and cutting dissolving pore.The throat types mainly include variable section shrinkage,sheet,bent sheet and bundle shape.The pore structure is more complicated.In the lower sub-layer of Zhi 1 member,there are middle and large pore,fine and tiny throats.In the lower sub-layer of Zhi 2 member,the middle pore and fine throats is development.According to the fractal characteristics,the structure of the large pore in the lower sub-layer of Zhi 1 member is relatively simple,and the structure of the middle and small pore is more complex.The structure of the middle and small pore in the lower sub-layer of Zhi 2 member is relatively simple and evenly distributed.The factors affecting the pore structure include deposition,diagenesis and tectonism,in which the diagenesis is the main reason for the undevelopment of the throat and uneven distribution.Analyzing the pore structure type of the sandstone,selecting the characteristic parameters with good correlation,and using the combination of macroscopic and microscopic way,the Zhiluo Formation sandstone is divided into four kinds.And the pore structure distribution map is drawn.The distribution of pore structure shows that the pore structure is better in the river channel,and the sand dams of the lower sub-layer of Zhi 1 member and Zhi 2 member is more developed.Combined with the distribution of pore structure and the affecting factors,the water rich distribution in the southeastern part of the research area can be well delineated,which provides a good guideline for the safety production in coal mine. |
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