Secondary Pore Development Pattern And Formation Mechanism Of He 8 Member In Daning-Jixian Area Of Ordos Basin,China

The He 8 Member of the Shihezi Formation in the Daning-Jixian area on the southeast edge of the Ordos Basin has a broad potential for tight gas exploration,and the pore type is mainly secondary pore space,and it is important to explore the development characteristics of secondary pore space in the He 8 Membe to find high-quality reservoirs.In this paper,based on the comprehensive analysis of logging,logging,core data and other data,combined with field investigation,core observation,cast thin section,cathodoluminescence,scanning electron microscopy,X-ray diffraction and other technical means,we have systematically analyzed and studied the type of secondary pore space,development law and formation mechanism of He 8Membe reservoir in Ordos Basin,and obtained the following understanding:(1)Through rock thin section and scanning electron microscope identification and other testing methods,it is determined that there are various types of rock formation such as dissolution,cementation and compaction in He 8 Member of the study area,and the secondary pore types are mainly feldspar dissolution pores,and after quantitative calculation of rock formation,it is found that compaction is the main cause of reservoir densification,and the secondary pores formed by dissolution are important factors to improve the reservoir conditions.Based on the morphology of authigenic minerals,dissolution filling,account relationship and Imon mixed layer ratio under the microscope to determine the established diagenetic evolution sequence in the study area,and analyze the characteristics of secondary pore development in each diagenetic stage,the results show that the middle diagenetic stage A is the secondary pore generation stage,and the middle diagenetic stage B has a large amount of cement filling pores.(2)The organic acids in the sandstone of the He 8 Member are derived from the coalderived hydrocarbon rocks of the lower Shan 2 Member and the carbonaceous mudstone of the Shan 1 Member;the pore types formed by dissolution are mainly feldspathic pores;the calcite carbon and oxygen isotope results indirectly indicate that the acidic fluids dissolving the soluble minerals are organic carboxylic acids with high activity intensity.(3)The sedimentary environment is a prerequisite for the development of secondary pore space,and the microphase sand body of the submerged diversion channel is a high-quality reservoir development area;the greater the thickness of the sand body,the more secondary pore space is preserved in the later stage,and the filling of carbonate and other cement is mostly in the top and bottom part of the sandstone,so the sand thickness is positively correlated with the physical properties;the mud content affects the entry of organic acids into the sand body,and is negatively correlated with the physical properties;under the influence of the compaction resistance,dissolution conditions and various types of cement Under the influence of factors such as compaction resistance,dissolution conditions and the properties of various types of cement,high-quality reservoirs are mostly developed in medium and coarse-grained quartz sandstone.(4)In order to analyze in detail the relationship between the cement content and the longitudinal development of physical properties within the sandstone and the relationship between the two,the concept of "relative distance" between each sampling point and the middle of the sand body is proposed.The longitudinal development of the sand unit follows the distribution pattern of "the center of the sand body and the upper and lower sides".(5)Comprehensive secondary pore development conditions,it is presumed that the area with thicker sand in He 8 Membe and thicker coal seam in Shan 2 Membe or thicker carbonaceous mudstone in Shan 1 Membe is the secondary pore development area,which can be used as the key area for finding high-quality reservoirs.These research results enrich the research results of secondary pore space main control factors in this area,and have certain guiding significance for high-quality reservoir prediction and tight gas exploration and development.