酸溶蚀模拟实验与致密砂岩次生孔隙成因机理探讨： 以鄂尔多斯盆地盒8段为例

鄂尔多斯盆地盒8段发现并探明了一批目前中国内陆最大的致密砂岩气田（藏），该致密砂岩气田的储层岩性主要为 一套“低长石、低杂基、高成分成熟度”的石英砂岩，储集空间类型以多类型的次生溶蚀孔隙为主。针对盒8段致密砂岩 次生孔隙的成因机理，本文在大量薄片观察、扫描电镜、场发射、CT扫描分析的基础上，开展了高温高压条件下真实岩心 的酸溶模拟实验，实验样品为鄂尔多斯盆地石盒子组绿灰色中粗粒含凝灰质长石石英砂岩、绿灰色中粗粒含凝灰质长石岩 屑砂岩、灰色中粗粒含凝灰质岩屑石英砂岩和灰色中粗粒含凝灰质石英砂岩等4种不同类型的砂岩岩心，采用地层水中普 遍存在的乙酸配置成0.5mol/L的反应介质，选用2mL/min流速，动态溶蚀100h后，不同类型砂岩的次生孔隙显著增加， 物性明显改善，通过对不同岩性实验前后样品的同位扫描、水-岩反应速度、离子浓度等测定分析，发现盒8段致密砂岩储 层次生孔隙的成因主要为凝灰质、长石等铝硅酸矿物和碳酸盐矿物的溶蚀，探讨了实验样品中不同矿物的溶蚀序列，凝灰 质、碳酸盐矿物主要在低温条件下发生溶蚀蚀变，高温条件下主要为长石的溶蚀。揭示凝灰质、碳酸盐岩、长石等矿物溶 蚀、次生孔隙形成、新生矿物的发育机理，结合埋藏史、热史分析，建立了盒8段致密砂岩储层埋藏-成岩-孔隙演化模 式，明确了鄂尔多斯盆地盒8段石英砂岩次生孔隙的成因机理。
Atpresent,anumberofgreatesttightsandstonegasfieldshavebeendiscoveredandprovedintheHe8MemberofOrdos Basin.Thetightsandstonereservoirsmainlyconsistofquartzsandstonewithlowfeldspar,lowmatrix,highcompositionalmaturity.Thetypesofreservoirspacearebasedonmultiplesecondaryerosionpores.Inordertostudytheoriginofvuggyporosityofquartz sandstone,acidolysissimulatingexperimentofopensystem,underthehightemperatureandhighpressureconditions,hasbeen conducted.Meanwhile,theanalysisisbasedonnumerouscastingslices,scanningelectronmicroscope,fieldemissionandCTscan. Experimental samples were selected from the Shihezi Formation of Perimian in Ordos Basin, containing green grey mediumcourse-grainedtuffaceousfeldspathicquartzsandstone,greengreymedium-course-grainedtuffaceousfeldspathiclitharenitesandstone, graymedium-course-grainedtuffaceouslithicquartzsandstoneandgraymedium-course-grainedtuffaceousquartzsandstone.In0.5mol/ Lreactionmediator,2ml/minflowrateanddynamiccorrosionfor100hexperimentalconditions,thesecondaryporosityfordifferent typesofsandstoneincreasedsignificantlyandpropertieswereobviouslyimproved.Bythemeansofisotopescanningofdifferent samplesbeforeandafterexperimentanalysis,thefluid-rockinteractionrateanalysisandplasmaemissionspectrumanalyisis,the authorsindicatethatthemainoriginofthesecondaryporesarethecorrosionoftuffaceous,calcareousandfeldsparmineraletc.The tuffaceous,calcareousoccurerosionundertheconditionoflowtemperature,andfeldsparcorrosionoccurunderhightemperature conditions.Thedevelopmentmechanismoftheerosionoftuffaceous,calcareousandfeldsparmineraletc,poreformationandnew mineralshasbeenrevealed.Combinedwiththeanalysisofburialhistoryandthermalhistory,thediageneticsequencesandporosity evolutionmodeloftightsandstoneofHe8Memberwasputforward.Allthestudiesmakethegenesisofsecondaryporesofquartz sandstoneclearly.