富县地区长8致密砂岩储层孔隙结构特征及形成机理

Pore structure characteristics and formation mechanism of Chang 8 member tight sandstone reservoir in Fuxian area

鄂尔多斯盆地富县地区长8层段致密砂岩储层微观结构复杂,孔隙结构作为致密砂岩储层研究的关键内容,对致密砂岩油气藏勘探工作起着至关重要的作用。利用场发射扫描电镜、CT、压汞、核磁共振及薄片等分析方法,定性与定量化评价了富县地区长8层段致密砂岩在致密过程中孔隙结构的演化特征,并揭示致密砂岩储层孔隙结构的形成机理。研究结果表明:致密储层孔隙类型以溶孔、晶间孔和微裂缝为主,孔隙和喉道平均半径分别为1.2μm和0.1μm,为细孔-微细喉型储层;孔隙结构可划分为3类,分别是Ⅰ类(残余粒间孔与粒间溶孔)、Ⅱ类(长石粒内溶孔)和Ⅲ类(微米孔-纳米孔);储层致密过程受成岩作用控制明显,成岩综合系数2.75%,压实孔隙损失率62.35%,胶结孔隙损失率23.84%,溶蚀增孔率6.56%。其中压实作用是影响孔隙结构的主控因素,碎屑含量、颗粒粒度以及胶结物成分和产状是影响孔隙结构形成的次要因素;搭桥状伊利石、叶片状绿泥石、点状胶结方解石、钠长石伴生的自生石英晶体以及黑云母和泥质杂基为影响孔隙结构的主要岩石组分。

The microstructure of Chang 8 tight sandstone reservoir in Fuxian area of Ordos Basin is complex.As the key content of tight sandstone reservoir research,pore structure plays an important role in tight sandstone reservoir exploration.Field emission scanning electron microscope,CT,mercury injection,nuclear magnetic resonance and thin section analysis methods were used to qualitatively and quantitatively evaluate the evolution characteristics of pore structure of Chang 8 formation in Fuxian area,and reveal the formation mechanism of pore structure of tight sandstone reservoir.The results show that the pore types of tight reservoir are mainly dissolved pore,intergranular pore and microfracture,and the average radius of pore and throat is 1.2μm and 0.1μm,respectively.The pore structure can be divided into three types:class I(residual intergranular pore and intergranular dissolved pore),class II(intra granular dissolved pore)and class III(micro pore nanopore).The reservoir compaction process is controlled by diagenesis.The diagenetic comprehensive coefficient 2.75%,compaction porosity loss rate 62.35%,cemented pore loss rate 23.84%and dissolution pore increase rate 6.56%,respectively. Compaction is the main control factor affecting the pore structure,and the detrital content,particle size,cement composition and occurrence are the secondary factors affecting the formation of pore structure;bridging illite,leaflike chlorite and punctate gum are the secondary factors. The authigenic quartz crystals associated with calcite and albite,biotiteand argillaceous matrix are the main rock components affecting the pore structure.