Study On Geological Conditions And Evaluation Standards Of Shale Gas Of The Jurassic In Northern Margin Of Qaidam Basin

With the development of shale gas in the U.S,China is carrying out theoretical studies of shale gas,and trying to establish shale gas evaluation criteria for each study area.This paper is aimed at the Jurassic shele in the northern Qaidam Basin.The analysis is based on mass geologic,geophysical,geochemical data,including the distribution of the target shale,the geochemical properties of organic geochemistry and shale reservoir.The sequence framework is based on correlating well-seismic regional profiles,and analyzing outcrops and biostratigraphy to study the shale distribution.Three long-term base level cycles(3rd order sequences)and nine middle term base-level cycles(4th order sequences,from S1 to S9)are identified in the study area.Well correlation then traces the shale.There are five sedimentary facies in the Jurassic in the northern Qaidam Basin.The lacustrine and swamp facies are favorable for shale.In that case,shale is mainly distributing in the Xiaomeigou formation,and in the fifth and seventh section in Dameigou formation in middle Jrassic;The thickness of shale is thickening in the middle Jurassic from Qianxi to Yuqia.The average of single shale layer thickness of the Jurassic Dameigou formation in the Lenghu and the coal-bearing region is 43.66m;the total thickness is 55.33m.As the key standards in assessing shale gas potential,Total organic carbon(TOC)is about 5.28%in average in the lower Jurassic,and about 8.61%in the middle Jurassic;Ro is 0.77%,in average,in the lower Jurassic,and in the meddle Jurassic,Ro is 0.67%.The average clay content in shale is 46.16%that is similar to the content of brittle mineralsin shale,mainly for quartz,which is over 40%.The absorbed gas content from the adsorption isothermal experiment is 2.37-10.60 m3/t showing it that the samples have good adsorption properties.This paper evaluates the effectiveness of the shale gas accumulation and the operability in the exploitation,mainly based on nine indicators from two respects,hydrocarbon generation and reservoir evaluation:(1)TOC>1%;(2)Ro>0.8%;(3)Shale layer thickness>30m;(4)Single interlayer thickness<lm;(5)Total thickness of the sandwich accounts for the thickness of the shale layers<20%;(6)Content of the brittle minerals>30%;(7)Buried depth<4000m;(8)Area of shale distribution>50km2;(9)Gas content>1m3/t.The geologic and engineering sweet spot is carved out based on the nine standards,which are Lenghu structural belt in Xiaomeigou formation,and Yuqia,Hongshan,Delingha depressions in Dameigou formation.