致密天然气砂岩储层：成因和讨论

Tight sandstone gas reservoirs:their origin and discussion

致密天然气砂岩储层具有高的毛管压力和束缚水饱和度,孔隙类型以次生孔隙为主。根据致密成因可以将致密砂岩储层划分为4种类型:1)由自生粘土矿物的大量沉淀所形成的致密砂岩储层;2)由胶结物的晶出改变原生孔隙形成的致密砂岩储层;3)高含量塑性碎屑因压实作用形成的致密砂岩储层;4)粒间孔隙被碎屑沉积时的泥质充填形成的致密砂岩储层。有关致密砂岩储层次生孔隙的成因有多种理论,但是由于致密砂岩储层的复杂性,没有一种理论可以将所有的孔隙成因完全解释清楚,因此次生孔隙的成因机制仍然存在争议。尽管致密砂岩储层经历了复杂的成岩演化历史,成岩作用对储层的致密化起决定作用,然而沉积环境依然是控制致密砂岩储层形成的基本因素。深入了解沉积环境和成岩作用对致密砂岩储层的共同作用,有助于对致密天然气砂岩储层的预测和评价。

Tight gas reservoir sandstones are generally defined as gas-bearing and -yielding sandstones with porosity of less than 10 % and permeability to gas less than 0.1 md. Tight gas reservoir sandstones are characterized by high capillary entry pressures, high irreducible water saturations and predominance of secondary pores. This paper suggests that tight reservoir sandstones be classified into four types in terms of the origin： 1 ） tight sand formed by sedimentation of large amount of authigenic clay minerals; 2） tight sand resulted from crystallization and precipitation of cements altering primary pores ;3 ）tight sand caused by compaction of large amount of ductile clasts;4）tight sand formed by filling of intergranular pores by detrital clay matrix. To explain the origin of secondary pores in these sandstones, researchers have developed many theories. However, none of the theories are capable of clearing all the doubts due to the complexity of this kind of reservoirs. As a result, the issue is still in debate. Despite their complex burial history of tight reservoir sandstones and diagenesis being determinant in tightening of sandstones, original depositional environment is still a basic factor controlling the formation of tight sandstone reservoir. Deeper understanding of their depositional environment and the diagenetic history is critical to the prediction and evaluation of tight reservoir sandstones.