The Relationship Between The Tectonic Characteristics Of Majiagou Submember5_1² And The Enrichment Of Natural Gas In Yan 115 Well Area Of Ordos Basin

Based on the fact that the drilling rate of industrial gas wells in the karst weathering crust of Majiagou Formation in Western Yan'an is low and the accumulation law of gas is complex,the characteristics and controlling effect of pre-Carboniferous top structure and present low-amplitude structure on natural gas in Majiagou submenber5_1² of Yan 115 well area are studied in this paper.In order to visually and quantitatively restore the pre-Carboniferous top structure,i.e.the karst paleogeomorphology of Majiagou Formation,this study uses the new method of karst paleogeomorphology restoration in Jingbian gas field for reference.Based on 1182 drilling data on the eastern side of central paleo-uplift,the top of Dongdayao limestone in Taiyuan Formation and the top of Majiagou submenber5 limestone are selected as the upper and lower datum planes to calculate the relative elevation of Paleogeomorphology Combining the analysis of data form and the plane distribution characteristics of elevation,the quantitative division criteria of the second and third grade geomorphic units are worked out.In the study area,karst highland and karst slope are divided into two secondary geomorphic units.The karst highland is subdivided into six three-level geomorphic units:highland,terrace,platform,residual platform,depression and buried pit.The karst slope is subdivided into four three-level geomorphic units:gentle slope,depression,trench and buried pit.According to the observation results of core?thin section and gas reservoir log interpretation,the physical characteristics of Majiagou submenber5_1² in each geomorphic unit are analyzed.It is concluded that the high parts of top structure in Karst period,such as highlands,terraces and platforms are the major developmental areas of karst weathering crust reservoirs,and their physical properties are obviously better than those of low-lying areas such as depressions,trenches and buried pits.At the same time,the existence of compact limestone in Majiagou submenber6 acts as a barrier to the occurrence of constructive karstification in underlying strata,so the compact limestone distribution area is a disadvantageous area for exploration.Considering the lack of coverage of seismic survey line in the study area,the quadratic trend surface analysis method was used to identify more than 10 low-amplitude structures at the top and bottom of Majiagou submenber5_1² in the study area,which are mainly small nose-like uplifts with NW-SE,E-W and NE-SW structural trends.Their development location is highly consistent with the distribution range of the high part of the pre-Carboniferous top structure,indicating that the high part of the pre-Carboniferous top structure evolved into positive low-amplitude uplift in the background of Westward dip monoclinic during the later tectonic evolution,and the present structure shows obvious inheritance to the pre-Carboniferous top structure.In summary,development area of the present positive low-amplitude structure in the study area indirectly represents the high-quality reservoir development area.In the late Stage of Yanshan,this area became the oil and gas migration direction area,and through good stratigraphic-lithological,stratigraphic-palaeogeomorphological trap conditions,it became the favorable enrichment area of natural gas.