致密砂岩储层微裂缝发育特征及对物性的影响——以杭锦旗地区十里加汗区带为例

Development characteristics of microfractures in tight sandstone reservoir and its influence on physical properties: A case study of Shiligiahan zone in Hangjinqi

杭锦旗地区十里加汗区带二叠系下石盒子组盒1段砂岩平均孔隙度8.3%,平均渗透率0.89×10^(-3)μm^(2),为典型的致密砂岩储层,而微裂缝对改善致密砂岩储层物性起着重要作用,因此,开展微裂缝研究是该区储层研究的重点。通过微观薄片观察,结合压汞、岩心物性以及构造特征,对该区微裂缝的类型与成因、形成期次、控制因素以及微裂缝对物性的影响进行研究。结果表明,研究区盒1段致密砂岩按照接触关系可分为粒内缝、粒缘缝和穿粒缝,按照充填程度可分为未充填缝、不完全充填缝和完全充填缝。根据成因分析,研究区至少存在三期微裂缝。其发育程度受内因(砂岩粒度、岩石碎屑成分、岩屑和填隙物含量以及颗粒间的接触关系等)和外因(构造应力)的联合控制。微裂缝主要影响储层的渗透率,多因素影响是微裂缝性致密储层的一个基本特点,平均喉道半径对储层渗透率起主要控制作用。

The average porosity and permeability of sandstone of Lower Shihezi Formation of Permian system in Shilijiahan area of Hangjinqi area are of 8.3 % and 0.89×10^(-3)μm^(2) respectively, causing the reservior to be a typical tight sandstone reservoir.Meanwhile, the microfractures play an important role in improving the physical properties of tight sandstone reservoir. Therefore,the researches of the reservoir in this area focus on the microfractures. By the observation of the microscopic thin section, and combined with the mercury injection, core physical properties and structural characteristics, the types and genesis, formation stages, control factors and the influence of micro fractures on physical properties in this area have been studied. The results show that the tight sandstone of Lower Shihezi Formation in the study area can be divided into intragranular fracture, grain edge fracture and grain penetrating fracture according to the contact relationship of strata, and can be divided into unfilled fracture, incomplete filled fracture and complete filled fracture according to the filling degree. According to the genetic analysis, there are at least three stages of micro fractures in the study area. Their development degree are controlled by the combination of internal factors(sandstone particle size, rock debris composition, rock debris and interstitial content, contact relationship between particles, etc.)and external factors(tectonic stress). The microfractures mainly affect the permeability of the reservoir. The multi-factor influence is a basic feature of micro fractured tight reservoir. The average throat radius plays a major role in controlling the permeability of the reservoir.