四川盆地蜀南地区上三叠统须家河组低孔低渗储层特征及形成机理

Characteristics and Controlling Factors of Upper Triassic Xujiahe Tight Sandstone Reservoir in Southern Sichuan Basin

蜀南地区须家河组为一套辫状河三角洲平原分支河道沉积的岩屑砂岩、长石岩屑质石英砂岩和岩屑质石英砂岩。埋藏深度变化较大(0～2 673 m),大部分埋深800～2 000 m,平均孔隙度4.64%,平均渗透率0.147×10-3μm2,属于低孔低渗储层。造成低孔低渗的主要原因是:(1)在构造背景上,须家河组沉积时研究区靠近刚由海变陆而隆升的物源区,沉积物搬运距离较短,分异程度低,大量的泥岩、千枚岩等塑性岩屑及杂基被保存下来,同时碎屑颗粒分选磨圆程度较差,均不利于原生孔隙的形成;(2)在沉积条件上,由于辫状河河道水动力不稳定,侧向改道迅速,早期泥岩易被冲蚀,导致砂泥混杂,砂岩中含较多杂基,堵塞充填原始孔隙;(3)早期持续快速的埋藏过程,压实作用强烈,岩屑压实变形被挤入粒间孔隙中,同时普遍发育的石英次生加大使得砂岩越发致密,孔隙消失殆尽。(4)研究区须家河组远离沉积中心,缺乏烃源岩成熟产生的酸性流体溶蚀改造,导致次生孔隙不发育。

Xujiahe Formation in southern Sichuan Basin deposited in distributary channel of braided river delta plain and is dominated by lithic arenite, lithic quartz arenite. The Upper Triassic tight reservoirs with average porosity of 4.04% and average permeability of 0. 147 × 10.3 μm^2 are in a current depth of 0-2 673 m （mostly 800 - 2 000 m）. The causes of low porosity and permeability include four aspects. （ 1 ） The relative short transportation distance from provenance to deposition area resulted in large quantities of shale, phyllite and other plastic debris as well as poorly sorted grain pattern. （2） The early mudstone was easily eroded and mixed with sand grains because of the hydropower instability and quickly lateral diversion of braided river during the deposition process. （3） During the burial process, strong deformation of plastic particles caused by mechanical compaction combined with quartz cementation during the afterwards chemical compaction resulted in significant reduction of primary porosity. （4） The deficiency of acid fluid from the source rocks because the long distance from deposition center of basin resulting in weak dissolution after den- sification of sandstone. Hence, limited amount of secondary pores generated.