Geological Conditions And Forecast Of Favorable Areas Of The Middle-upper Ordovician Shale Gas In Wuzhong Area

Wuzhong area is located in the Ningxia Hui Autonomous Region.It belongs to the middle section of the western margin of the Ordos Basin in terms of regional structure and is a relatively low exploration area.Although the Middle Ordovician Uralike Formation is generally considered to be the main source rock of Ordovician natural gas in this area,the study of the Upper Ordovician shale gas in the study area is quite weak,and the current geological understanding has been difficult to satisfy the area.The actual needs of shale gas exploration and development.Based on the collection and arrangement of geological and seismic data,this paper makes full use of the geological conditions of the Upper Ordovician shale gas and the main controlling factors of rich gas in the study area of drilling,logging,logging,and organic geochemical data analysis and research,and uses seismic prediction technology In combination with geological and engineering factors,the forecast of favorable areas will be carried out in order to provide reliable geological basis for the next shale gas development.The Middle-Late Ordovician in Wuzhong area on the western margin of the Ordos Basin is dominated by deep-sea basin facies and is a favorable facies zone for the development of organic-rich shales.The dark mud shale in the study area has a large thickness and stable distribution.The paper found through lithology and X-ray diffraction analysis:The dark muddy rock series of the Uralike Formation are mostly gray-black mudstone,yellow-green silt mudstone,muddy siltstone,etc.The clastic components are mainly quartz,feldspar,etc.,and the brittle mineral content is relatively high.The dark mudstone of the Wulalik Formation has high organic matter content and great hydrocarbon generation potential,followed by the Lashzhong Formation.The organic carbon content of the Wulalik Formation is between 0.1%and 1.4%,with an average value of 0.65%.It is mainly composed of medium source rocks and good source rocks.The Types of organic matter are mainly type I and type II₁.Scanning electron microscope analysis found:The pore types of the Uralic and Rashizhong formations are mainly intergranular pores and intragranular dissolved pores.The porosity distribution range is from0.15%to 8.3%,mainly less than 1%,the average value is 1.23%,and the permeability is mainly less than 0.01×10^-3?m².The enrichment of shale gas is mainly affected by three aspects:source rocks,storage space,and preservation conditions.The thicker the shale,the higher the total organic carbon content,the more developed the pores and microfractures,and the easier the gas reservoir will be enriched.At the same time,mineral composition,brittle mineral content,proper burial depth and tectonic effect have direct or indirect effects on shale gas enrichment.Based on the analysis of the main controlling factors of the Middle-Upper Ordovician shale gas enrichment in the study area,seismic prediction technology is used as an important technical support for the prediction of favorable shale gas areas.Drawing on foreign shale gas prediction experience,the geological“sweet spots”prediction and engineering“sweet spots”prediction are carried out.Three types of favorable areas for the distribution of"sweet spots"of shale gas are comprehensively predicted:The area of Type I"sweet spots"is about 52km²,the area of Type II"sweet spots"is about 127km²,and the area of Type III"sweet spots"is about 156km².