不同孔隙结构碳酸盐岩的岩石物理响应特征及储层预测新方法——以鄂尔多斯盆地奥陶系马家沟组四段为例

Petrophysical response characteristics of carbonate rocks with different pore structures and new reservoir prediction method:A case study of the fourth member of Ordovician Majiagou Formation in the Ordos Basin

近年来,在鄂尔多斯盆地中东部地区下奥陶统马家沟组四段(以下简称马四段)的台内丘滩体钻遇了优质白云岩储层,但该套储层厚度薄、与围岩弹性参数差异较小,利用常规方法很难准确识别和预测。为了提高白云岩储层的预测精度,基于铸体薄片与岩心CT扫描,详细分析了马四段碳酸盐岩的特征,通过岩石物理弹性测试,结合物性测试及试采资料建立了马四段岩石弹性参数与孔隙结构的关系,提出了研究区一种变基质多孔微分等效介质模型的预测孔隙结构和储层的新方法,并进行了矿场验证。研究结果表明:(1)马四段碳酸盐岩发育孔隙型和裂缝型2类储层,其中孔隙型储层可细分为以晶间孔为主和以晶间溶孔+晶间孔为主2类;(2)晶间孔为白云岩化作用形成,孔隙纵横比较大,晶间溶孔由晶间孔经过溶蚀作用改造形成,孔隙纵横比变小;(3)裂缝主要为构造缝和溶蚀缝,孔隙纵横比最小;(4)沉积和成岩作用对碳酸盐岩的孔隙结构影响大,孔隙结构的差异会引起岩石弹性参数的较大变化,利用该差异可以有效预测白云岩优质储层。结论认为,该方法首先通过变基质骨架建模准确划分岩石的基质类型,然后利用多重孔微分等效介质模型预测岩石的孔隙结构类型,并定量计算不同孔隙类型孔隙的体积占比,为提高白云岩储层的预测精度奠定了技术基础,有助于碳酸盐岩优质储层的精确预测的深入研究。

In recent years,high-quality dolomite reservoirs have been encountered in the intra-platform mounds and shoals of the fourth member of Majiagou Formation of Lower Ordovician("Ma 4 member"for short)in the central-eastern Ordos Basin.However,this set of reservoir is thin and its elastic parameters is less different from that of its surrounding rock,so its accurate identification and prediction can be hardly realized by using conventional methods.In order to improve the prediction accuracy of dolomite reservoir,this paper analyzes the characteristics of Ma 4 member carbonate rock in detail based on casting thin section and core CT scanning data.Then,the relationship between rock elastic parameters and pore structures in the Ma 4 member is established through petrophysical elastic test,combined with physical property test and production test data.Finally,a new pore structure and reservoir prediction method based on the variable-matrix multi-pore differential equivalent medium model is proposed for the study area and verified in the field.And the following research results are obtained.First,the carbonate rocks of Ma 4 member develop into two types of reservoirs,i.e.,porous reservoir and fractured reservoir.The porous reservoir can be divided into two types.One is dominated by intercrystalline pores and the other is dominated by intercrystalline dissolved pores and intercrystalline pores.Second,intercrystalline pores are the products of dolomitization and their pore aspect ratio is larger,while intercrystalline dissolved pores are derived from intercrystalline pores by dissolution and their pore aspect ratio is smaller.Third,fractures are mainly structural fractures and dissolved fractures,with the smallest pore aspect ratio.Fourth,sedimentation and diagenesis have a great impact on the pore structure of carbonate rocks.The difference in pore structure can cause great changes in rock elastic parameter,so this difference can be used for the effective prediction of high-quality dolomite reservoir.In conclusion,in this method,the matrix types of rocks are firstly divided accurately by variable matrix framework modeling,then the pore structure types of rocks are predicted by using the multi-pore differential equivalent medium model,and the volume proportion of each type of pore is quantitatively calculated,so as to lay a technical foundation for improving the prediction accuracy of dolomite reservoir.The research results are conducive to the further study on the accurate prediction of high-quality carbonate reservoirs.