Delineation Of Shale Gas Preservation Units And Potential Analysis Of The Remnant Oblique Wufeng Formation-Longmaxi Formation In Qianbei Area

The Ordovician Wufeng Formation-Silurian Longmaxi Formation shale in Qianbei is the most favorable target formation for shale gas,and significant shale gas exploration breakthroughs have been achieved in the residual syncline areas of Daozhen and Anchang in the study area.In order to identify the differences in gas content and preservation conditions of this set of shales in the residual syncline areas in different fold zones in Qianbei region and to classify the preservation units,this thesis adopts the methods of field survey,profile measurement,geological data collection,oil and gas drilling data and seismic exploration data,and combines the organic geochemical characteristics and physical characteristics of this set of shales to dissect the tectonic style,tectonic background and syncline pattern in Qianbei region.We analyzed the quality and gas content of the shale of the Wufeng Formation-Longmaxi Formation in the residual syncline area,further studied the preservation conditions of the shale gas in this layer,and divided the preservation unit on this basis.(1)The distribution of high quality shales(organic-rich shales)of the Wufeng Formation-Longmaxi Formation in northern Qianjiang is relatively limited,mainly in the major remnant synclines in northern Qianjiang,with an overall distribution of "thin in the south and thick in the north";the burial depth of the Daozhen-Loulong syncline and the Chishui terrestrial basin can reach 2000-4000 m,while the burial depth of the Taibai syncline and the Dalong syncline is only When the burial depth of high quality shale exceeds 500 m,the Wufeng Formation-Longmaxi Formation has good oil and gas display and has good prospect of exploration and development.(2)The shale quality of the Wufeng Formation-Longmaxi Formation in the study area shows an overall trend of gradual deterioration from north to south.There are two main types of shale organic matter: putrefied amorphous body and putrefied clastic body,with average contents of 55.7% and 38.8% respectively;there are temporal and spatial variability in TOC content,with the lower part of the target layer better than the upper part,and the northern part better than the southern part;there is also variability in Ro,with Ro values of 1.51%-2.18% in the high-quality shale section,which is in the high maturity-overmaturity stage;The hydrocarbon generation intensity of shale shows a deteriorating trend from northwest to southeast.(3)The geological conditions in Qianbei area are special and the tectonic background is complex.The tectonic movement,tectonic style,fracture development,burial depth characteristics,top and bottom plate conditions,and syncline pattern all have influence on shale gas preservation conditions.The study area is divided into a blocking fold zone,a channel-blocking transition zone,and a channel-blocking fold zone from northwest to southeast.The results show that the structure,typical syncline pattern,and gas content of different folds in the study area are simple,with wide and gentle syncline,good gas content and intact hydrocarbon preservation conditions;the syncline in the trough conversion zone is wider and gentler,with good hydrocarbon display and better preservation conditions;the syncline in the trough folding zone is narrow and steep,and most of the syncline is retrograde-inverted syncline,and the internal fracture is developed,with poor gas content and high nitrogen content.The nitrogen content is high,reflecting the poor preservation conditions of shale gas.(4)The shale gas preservation conditions in the study area were divided into tectonically favorable zones.Combining the tectonic favorable zones and other preservation conditions evaluation indexes,the preservation conditions were comprehensively analyzed and evaluated,and the preservation conditions were divided into Class I favorable zones and Class II favorable zones.The favorable zones are mainly located in the residual syncline areas where the upper overburden is more stable,and the favorable zones avoid the fracture development areas as a whole,and the internal faults are less developed and smaller in scale,and from the geographical location,they are mainly located in the northern area of Zheng An-Yanhe.(5)The study area was divided into Type I and Type II preservation units according to the comprehensive evaluation index.Type I preservation units include Daozhen-Yuxi preservation unit,Yanhe-Shazi preservation unit,and Chishui terrestrial preservation unit;Type II preservation units are mainly located in the north of Sinan-Fenggang-Suiyang syncline area,with the overall depth of burial less than3,500 m.Type I preservation units can be used as key areas for shale gas exploration,and Type II preservation units can be used as exploration prospective areas.