Quantitative Research On The Effect Of The Dolomite To The Reservior Brittleness Of Lacustrine Shale Formation

A considerable part of the oil and gas resources contained in continental lacustrine shale formation is related to lacustrine carbonate rocks in China,and many of the shale formation of the oil and gas reservoir develop numerous dolomites,even with the dolomite as the main reservoir.Thus,the content of dolomite has an important effect to the reservoir brittleness heterogeneity of lacustrine shale formation.Through a well-designed sampling scheme for shale reservoir samples,X-diffraction whole-rock analysis experiments and clay mineral analysis experiments were conducted on the down Es4 shale.On this basis,the single and three-axis compression experiments were performed on shale samples with different mineral contents.The deformation mechanism and failure characteristics of the sample were studied.According to the experimental data,static elastic parameters were calculated.At the same time,dynamic elastic parameters were calculated by using acoustic and density logging data,and the conversion relationship between dynamic and static elastic parameters was analyzed.According to Rickman's mineral division scheme,feldspar and pyrite are included in the quartz content,and calcite and siderite are included in the dolomite content.The mineral content and elastic parameter data were divided into four groups based on 10% clay mineral content,studying the functional relationship between dolomite content and brittleness index in each group separately,and the first calculation model was established.While,all the data were divided into six groups based on 10% quartz content,The functional relationship between dolomite content and brittleness index in each group was established and the second calculation model was established,and comparing the results of the two model operations.The experimental results show that the composition of the Es42 shale is from high to low: quartz,clay minerals,dolomite,feldspar,pyrite,calcite,and siderite.The deformation of the uniaxial compression test specimens is generally characterized by "elastic-short plasticity",which is both tensile and shear failure.The triaxial compression test is affected by the confining pressure.With the increase of confining pressure,the compressive strength of the specimen increases,and the axial strain increases during failure.The damage of the specimen is affected by the shearing effect.There is a good linear correlation between dynamic and static Young's modulus,and conversion relations have been established according to different lithologies.The definition of dolomite as a brittle mineral was clarified.By comparison,it is considered that the dolomite content in model 1 has a long span,and the function relationship with elastic parameters and brittleness index is mostly a cubic function relationship.The correlation coefficient is high and the fitting effect is good.It can be used as a quantitative relationship model of the impact of the dolomite content to reservoir brittleness for the terrestrial lake basin mud shale layer.