鄂尔多斯盆地姬塬地区长8致密储层溶蚀作用及其对储层孔隙的定量影响

鄂尔多斯盆地姬塬地区长8储层中长石含量较高，且普遍存在长石溶蚀现象，对致密储层物性的改善具有重要意义。在对长8致密储层特征的研究基础上，利用高温高压流体—岩石相互作用模拟实验，并结合偏光显微镜、扫描电镜（SEM）和X-射线衍射（XRD）等分析技术，模拟研究区长8储层岩石样品与有机酸的相互作用，分析了溶蚀作用类型及其特征，并解释了相关溶蚀作用机理，定量计算了溶蚀作用对储层孔隙度的影响。为使模拟实验更接近实际地质情况，流体采用0.15mol/L、pH=2.65的乙酸溶液，模拟温度为87~103℃，模拟压力为24.70~30.18MPa。研究表明：长8储层主要岩石孔隙类型为原生粒间孔和长石溶蚀孔，其中长石溶蚀孔较发育，占总孔隙的39%左右，计算得其视溶蚀率为37.8%~50.0%，呈中等溶蚀程度；长石类和碳酸盐类矿物在酸性条件下均能发生不同程度的溶蚀，碳酸盐类矿物的相对溶蚀率整体大于长石类矿物的相对溶蚀率；在95℃左右时矿物的溶蚀作用最为明显，依据地温梯度计算其对应的埋藏深度约为2 370~2 710m，该深度可能是长8储层中有利储层的主要分布区域；溶蚀作用导致长8致密储层的孔隙度增加约3.57%~3.69%，其平均孔隙度增加3.63%，溶蚀作用是研究区长8储层孔隙度增加的关键成岩作用类型，是储层“甜点”发育的主要控制因素。

Dissolution of Chang 8 tight reservoir and its quantitative influence on porosity in Jiyuan area,Ordos Basin

Chang 8 reservoir in Jiyuan area of Ordos Basin holds characteristics of high feldspar content and common feldspar dissolution phenomenon,which is important to improve the physical properties of tight reservoirs.On the basis of identifying the basic characteristics of Chang 8 tight reservoir,utilizing the simulation experiment of fluid-rock interaction with high temperature and high pressure,combined with polarized microscope and scanning electron microscope(SEM) and X-ray diffraction(XRD) analysis techniques,this paper simulates the interaction between Chang 8 reservoir rock samples and organic acid,analyzes the types and characteristics of dissolution,explains the dissolution mechanism and quantitatively calculates the influence of dissolution on porosity of reservoir.In order to make the simulation experiment more close to the actual geological conditions,0.15mol/L acetic acid solution with pH=2.65 is used as reaction fluid.The simulation temperature range is set to 87-103℃ and the simulated pressure range is set to 24.70-30.18MPa.The research result shows that intergranular pores and feldspar dissolved pores are the main pore types,of which the feldspar dissolved pores are more developed and accounts for 39% of the total porosity.The apparent solution ratio is 37.8% to 50.0% with moderate degree of dissolution.In addition,carbonate minerals can also be dissolved under acidic condition.The relative solution ratio of carbonate minerals generally exceeds feldspar.The dissolution degree is strongest at about 95℃ and the burial depth is about 2 370-2 710m by the geothermal gradient calculation,which should be the main distribution area of favorable reservoir of Chang 8.Overall,dissolution is the main diagenesis types to improve the porosity and result in the 3.57%-3.69% increase of porosityin tight reservoirs of Chang 8.Therefore,it is also the main controlling factor for the development of “dessert” in Chang 8 tight reservoir of western Jiyuan area.