致密砂岩储层微孔隙成因类型及地质意义——以库车前陆冲断带超深层储层为例

随着致密油气勘探开发的兴起,储层中微孔隙(孔隙直径小于30μm)对油气的赋存和运移起着越来越重要的作用。以库车前陆冲断带超深层(超过6 000 m)白垩系巴什基奇克组致密砂岩为例,通过选取代表性实验样品,在铸体薄片基础上进行激光共聚焦、扫描电镜、电子探针及高压压汞等序列平行表征实验,分析了储层微孔隙类型、特征、成因以及对储层物性的影响。结果表明:库车前陆冲断带超深层白垩系巴什基奇克组致密砂岩储层微孔隙总体表现为纳米级,其类型主要有粒间微孔、粒内微孔、晶间微孔,其中粒间微孔隙表现为微米—纳米级,粒内微孔和晶间微孔表现为纳米级。微孔隙以次生成因为主、原生成因为辅,溶蚀改造是次生成因的关键。研究区储层微孔隙度约为2.8%,微孔隙对储层孔隙度贡献率随孔隙度增大而减小。连接微孔隙的储层喉道为纳米级,孔喉半径为2~400nm,峰值为5~50nm,对渗透率起主要贡献的微孔喉半径为20~400nm,甲烷分子在微孔隙内主要表现为黏性流动,对天然气产能贡献大。不同微孔隙类型具有不同的尺度大小、几何形态、连通特点和气体赋存运移特征。

Genetic types and geological significance of micro pores in tight sandstone reservoirs: a case study of the ultra-deep reservoir in the Kuqa foreland thrust belt,NW China

With the exploration and development of tight oil and gas, mirco pores (pore diameter <30 μm) in reservoir play an increasingly important role in the occurrence and migration of oil and gas. Taking tight sandstone in the ultra-deep (>6 000 m) Cretaceous Bashijiqike Formation in Kuqa foreland thrust belt as an example, laser confocal microscopy, scanning electron microscopy, electron probe, high pressure mercury injection and other sequence parallel characterization experiments were conducted on the casting thin sections by selecting representative experimental samples, so as to analyze the types, characteristics and genesis of reservoir micro pore and its effect on reservoir property. The results show that nanoscale mirco pores are dominantly found in tight sandstone reservoirs of the ultra-deep Cretaceous Bashijiqike Formation in Kuqa foreland thrust belt, including intergranular micro pore, intragranular micro pore and intercrystalline micro pore. Intergranular mirco pores at the micro-nanoscale level are dominant, while intragranular and intercrystalline mirco pores are nanoscale ones. Mirco pores are formed mainly as a result of secondary origin with supplement of primary origin, and corrosion transformation is a key factor for secondary origin. Reservoir microporosity in the study area is about 2.8%, and the contribution rate of mirco pores to reservoir porosity decreases with increasing porosity. Reservoir throat connecting mirco pores is at the nanoscale, with the pore throat radius of 2-400 nm and peak value of 5-50 nm. The pore throat radius with the main contribution to permeability is 20-400 nm. Methane molecules form a viscous flow in mirco pores, making great contribution to gas productivity. Various types of mirco pores are different in scale dimensions, geometrical morphology, intercommunication and gas occurrence and migration characteristics.