松辽盆地下白垩统沉火山碎屑岩优质储层特征和形成机理

松辽盆地王府断陷沉火山碎屑岩中发现了高丰度气藏,但对该类优质储层分布规律和形成机制的认识还不清楚。以王府断陷CS6井区沙河子组为例,利用铸体薄片、压氦法、压汞法和核磁共振方法研究了储集空间类型、储层物性、孔喉和孔隙直径,探讨了优质储层的形成机理和主控因素。研究结果表明优质储层岩性为沉集块岩和沉角砾岩,为向上变粗的序列。进一步识别出3类7种储集空间类型,其中具有丰富原生孔隙的岩屑内气孔是沉火山碎屑岩优质储层独有特征,为大孔隙-小喉道型储层;沉集块岩孔渗稍高于沉角砾岩、二者均明显高于沙河子组沉积岩,其孔隙度与埋深关系密切。沉火山碎屑岩储层形成机理为母岩挥发分逸出、颗粒支撑、溶蚀和构造改造作用,其中前两者为主控因素。研究区优质储层主要分布在沉火山碎屑岩扇体平面位置的中部,其中颗粒支撑、颗粒粗、岩屑内气孔碎屑含量高和埋深小于3000 m的沉火山碎屑岩最为有利。

Characteristics and formation mechanisms of high quality reservoir of volcaniclastic sedimentary rocks of Early Cretaceous in Songliao Basin

The prolific gas reservoir of volcaniclastic sedimentary rocks of Early Cretaceous of Wangfu fault sag (WFS) in Songliao Basin (SB) was found in 2011. However, the characteristics and formation mechanisms of high quality reservoirs in volcaniclastic sedimentary rocks of SB is not well understood. In this paper, volcaniclastic sedimentary rocks of Shahezi Formation of well block CS6 in WFS of SB is taken as an example of high quality reservoirs. The void space, porosity-permeability, pore throat and pore diameter are discussed using the following method; thin sections impregnated with blue epoxy resin, helium (He) injection, mercury (Hg) injection and nuclear magnetic resonance respectively. The formation mechanism and main controlling factors of high quality reservoirs are analyzed based on all the above methods. The result shows that the high quality reservoirs are reworked lapilli and breccia, and exhibits the coarsening-upward successions. Primary and secondary porosity and secondary fracture porosity has been identified. The intragranular vesicular of primary porosity are distinguished void space of high quality reservoirs. It has the characteristics of large pore and small throat. The porosity and permeability of reworked breccia is higher than reworked lapilli, and both display relatively higher porosity and permeability than the sedimentary rocks in Shahezi Formation. The porosity is correlated to burial depth. The formation mechanism of volcaniclastic sedimentary rocks reservoir are gas released in parent rocks, grain-support contacts, alteration by organic acid and tectonic deformation. The main controlling factors of reservoirs formation are gas released in parent rocks and the grain-support. The high quality reservoirs are distributed in the middle part of alluvial fan of volcaniclastic sedimentary rocks. Volcaniclastic sedimentary rocks with grain-support, coarse particle, highly percentage of volcaniclasts with intragranular vesicle and burial depth less than 3000 m are the favorable target of exploration.