The Accumulation Requisites Of Shale Gas In The Mesozoic Of Ordos Basin

By collecting the geological information,geochemical data and reservoir data as well as the gas-bearing property data,deposit facie and distribution,abundance,type,maturity,physical property,mineral composition,porosity type,diagenesis and diagenetic phase of shale of Chang7 member of Yanchang formation abundant in organic matters are well analyzed.And we can conclude as follows:The shale abundant in organic matters in the study area is mainly composed of Zhangjiatan shale in the bottom of Chang7 member and the shale in the lower and upper part of shale,among which shale at the bottom of Chang7 member widespread and are of good continuity and stable thickness,belonging to the deposit of deep-semi-deep lacustrine subfacie and the front delta subfacie.The thickness of shale abundant in organic matters is within range of 5 to 60m,and the average is 28m.And the larger value is in the southwest,the thickness of organic-matter-bearing shale in Ansai area varies from 5 to 20m,and the distribution area is relatively small with bad stability,in vertical,Zhangjiatan shale at the bottom of Chang7 member in the study area is large in distribution area with good stability,but shale distributes un stably in the lower and upper part with bad continuity and small area.According to the testing samples in the study area,the TOC value of organic-matter-bearing shale of Chang7 member of Yanchang formation in the Triassic of study area varies from 0.3%to 6.17%,whose average is 3.25%higher than the lower limit-2%,which is set by researchers at home and abroad,the organic matter type is oriented with type-I and type-II,mainly producing oil.Ro ranges from 0.51%to 1.01%,primarily concentrating between 0.5%and 0.7%,and is unapparent of gas generation in the maturity stage.According to the study of the reservoirs in the study area,the porosity of the reservoir in Chang 7 formation which is rich in organic matter ranges from 3.21%to 6.99%,the permeability of the matrix ranges from 0.004 × 10-3?m2 to 0.219 × 10-3?m2,whose average is about 0.053×10-3?m2.Compared with the production area of shale gas in the north of USA,the range of porosity agrees with it,and the permeability is apparently higher.In the study area,the shale is rich in clay minerals,which is unfavorable for artificial fracture,and the pore style are mainly composed of corrosion holes,and the denudation is the dominant diagenesis in the study area,which is in the A2 member.As mentioned above,the main control effect is the vitrinite reflection of organic matters,in lower maturity,not in gas window totally.