Study On Shale Reservoir Characteristics Of Longmaxi Formation In Area A Of Northeastern Yunnan

Shale reservoirs characteristics of the Longmaxi formation in the northeast part of Chongqing were studied based on the macroscopic description of outcrops and cores,thin section authentication,SEM and experimental analysis of physical property,organic geochemistry and gas content ability.For the purpose of refined study in reservoirs features,the Longmaxi formation is divided into S1L1 and S1L2,and S1L1 is divided into S1L11 and S1L12.Moreover,S1L11 is divided into four sequences,namely S1L11a,S1L11b,S1L11c and S1L11d.The paper is focused on the S1L1.The TOC of shale in the study area is between 0.15%and 11%and the average is 3%.Meanwhile,the content of TOC is high and in normal distribution,which is majorly distributed in 2%to 6%.S1L11a shale has the highest TOC content and it decreases from S1L1a to S1L1.In the plane,the content of TOC decreases from the northwest to the southeast.Concluding from the type of kerogen in the adjacent regions,the kerogen in the study area are primarily I type and a few of them are ?1 kerogen.They are highly productive in producing hydrocarbon.The vitrinite reflectance and rock pyrolysis analysis reflect that the organic material in the most part of the study area is in the post-mature phase.Meanwhile,the vitrinite reflectance increases from the north to the south in the plane.Four kind of lithology are developed in the study area,which are shale,silt bearing shale,silty shale and tuffaceous shale.Shale is mainly distributed in S1L11a and S1L11c,silt bearing shale is majorly distributed in S1L11a,S1L11b and S1L11c,and silty shale and tuffaceous shale are principally distributed in S1L11d and S1L12.The color of shale from S1L11a to S1L12 is fading gradually,the content of silty increases little by little and shale's quality is getting worse.Furthermore,the mineral compositions of rocks are mainly quartz and clay,the content of quartz from the S1L11a upwards tapers off longitudinally.While the content of clay increases,carbonate minerals is less and pyrite is generally contained in the rock.The clay minerals are chief goeschwitzite,chlorite and mix layer of goeschwitzite and montmorillonite.The study of shale brittleness index shows that the index in longitudinal direction is relatively high generally and the tendency of shale brittleness index from S1L11a upwards is decreasing.The porosity of the study area is relatively low integrally and it is between 0.53%and 7.46%.The average porosity in S1L1 is merely 3%and the porosity from S1L11a to S1L12 decreases longitudinally.Moreover the permeability value of shale is low in common,the minimum value is 0.0000098 mD and the maximum value is 5.144 mD.What's more,higher permeability values in the test are caused by the development of fractures generally.Reservoir spaces are made up by fracture,mineral matrix pore and organic material pore.Fractures include structural fractures,subordinate horizontal fractures,bedding glide fractures and lithogenesis shrink fractures.Besides,observation and description of mineral matrix pore and organic material pore's morphologic characteristics are made under SEM.Meanwhile,gas content of shale samples in the study area is between 0.17m3/t and 5.66m3/t and the gas content is higher in S1L1 as a whole.The average of each sequence in S1L11 is higher than 3 m3/t and the gas content decreases significantly in S1L12(the average is only 0.58m3/t).The content of gas is gradually decreased from S1L11a to S1L12.The analysis suggests that the gas content is influenced by TOC and porosity principally and influence from mineral composition is limited.Reservoir assessment of the X2 well is made using TOC,porosity,gas content and brittleness index as classification and evaluation parameters.And results indicate that principal reservoir type is ? and ? and ? type reservoirs are less and thin.In general,the quality of reservoirs in the X2 well is poor.