鄂尔多斯盆地西部延长组长8～长6段致密储层微观孔隙特征差异

Differences of Microscopic Pore Characteristics among the Yanchang Formation Chang 8～Chang 6 Tight Reservoirs in the Western Ordos Basin

阐明鄂尔多斯盆地西部延长组长8~长6段致密储层微观孔隙特征的差异,对揭示致密油差异富集规律,实现有利储层的分级评价具有重要意义。运用铸体薄片、X射线衍射、场发射扫描电镜和高压压汞等技术手段,对长8~长6段致密储层样品进行测试,获取不同层段孔隙类型特征和孔隙结构特征。研究表明:从长8~长6段,致密储层面孔率先减小后增加,长8段以微裂缝和粒内溶孔为主,长7段和长6段以微裂缝和粒间孔为主,长6段杂基孔和粒内溶孔也较为发育。长8段孔喉半径最大,分选最好,歪度最粗,主要发育0.01~1μm的中孔喉,孔体积也最大。长7段和长6段歪度相近,均以1~10 nm的小孔喉为主,但长6段退汞效率最高,孔喉分选性和孔体积均好于长7段。结合毛细管压力曲线形态和孔隙结构参数,将孔隙结构分为Ⅰ中孔喉型、Ⅱ中细孔喉型、Ⅲ细孔喉型、Ⅳ微细孔喉型和Ⅴ微孔喉型5种类型。长8段和长6段孔隙结构分别以Ⅰ型和Ⅳ型为主,长7段以Ⅲ型和Ⅴ型为主。

Clarifying the differences of microscopic pore characteristics among the Yanchang Formation Chang 8 Chang 6 tight reserviors in the Western Ordos Basin is of great significance to revealing patterns of tight oil accumulation and making hierarchical evaluation on favorable reservoirs. By use of casting slice,X-ray diffraction,field Emission scanning electron microscope（ FE-SEM） and high pressure mercury intrusion pore measurement（ MIP）,pore type and pore structure characteristics of different members were systematically investigated from the samples of Chang 8 - Chang 6 tight reservairs. The results show that plane porosity firstly decreases and increases subsequently in chronological order. Chang 8 reservoir is mainly characterized by micro cracks and solution pores developed in feldspars. Chang 7 and Chang 6 reservoirs are dominated by micro cracks and intergranular pores respectively,meanwhile matrix pores and intragranular dissolved pores are well developed in Chang 6 reservoir. Chang 8 reservoir shows unique pore structure features characterized by maximum pore-throat diameter,maximum pore volumn,coarsest skewness,best pore sorting,and its dominant pore-throat radius ranges between 0. 01 - 1 μm. Chang 7 and Chang 6 reservoirs share a common feature characterized by fine skewness. The similarity in dominant porethroat radius between Chang 7 and Chang 6 reservoir is observed ranging from 1 ~ 10 nm,while Chang 6 reservoir shows pore structure features characterized by better pore sorting,larger pore volume and higher mercury withdrawal efficiency than Chang 7. Combined with morphological characteristics of capillary pressure curves and pore structural parameters,pore structures of Chang 8 ~ Chang 6 tight reservairs are divided into five types,respectively as medium pore-throat structure（ Type Ⅰ）,medium-fine pore-throat structure（ Type Ⅱ）,fine pore-throat structure（ Type Ⅲ）,micro-fine pore-throat structure（ Type Ⅳ） and micro pore-throat structure（ Type Ⅴ）. Type Ⅰ and Type Ⅳ are mainly developed in Chang 8 and Chang 6 reservoir respectively,and Type Ⅲ and Type Ⅴ dominantly exist in Chang 7 reservoir.