川西北中二叠统超深层白云岩储层孔隙结构及控制因素

Pore structure and controlling factors of the ultra-deep Middle Permian dolomite reservoirs, northwestern Sichuan Basin

四川盆地西北部龙门山山前中二叠统白云岩储层埋藏深度大,经历了漫长的埋藏期成岩演化,现今仍有丰富的储集空间保存,并发现了孔隙式白云岩气藏,但白云岩储层质量的非均质性较强,不同类型的白云岩储层中储集空间的形成和保持机理还有待更深入的研究。结合物性测试、压汞实验和CT扫描,白云岩储层中可识别出3种孔喉结构类型,其中Ⅰ类孔喉配位数高、连通性较好,大孔粗喉型的孔喉组合占主导,Ⅱ类包括了大孔中喉型和中孔中喉型两套孔喉组合,孔喉非均质性强,Ⅲ类受压溶作用影响较强主要发育小孔细喉型孔喉组合。白云岩储层孔喉结构的差异性主要取决于先驱灰岩的岩石组构和后期多种成岩作用的叠加耦合。Ⅰ,Ⅱ类储层一般由多孔颗粒灰岩经白云化形成,白云化作用进行得较彻底,主要发育在平直自形-半自形白云岩中,但Ⅰ类储层比Ⅱ类储层受中晚期溶蚀作用的改造更强且过度白云化作用影响较弱;而残余结构白云岩和非平直他形白云岩由于抗压溶能力更弱和后期过度白云化作用更强因而仅能形成Ⅲ类储层。

The Middle Permian dolomite reservoirs in the piedmont of Longmen Mountain,northwestern Sichuan Basin,were deeply buried and experienced long-term diagenetic evolution during burial,but abundant reservoir space is still well preserved at present,and even porous dolomite gas reservoirs have been discovered in such ultra-deep reservoirs.However,these reservoirs feature high heterogeneity,and the formation and preservation mechanism of reservoir space still need further study.Three types of pore structures are recognized via physical property analysis,mercury injection porosimetry and CT scanning.TypeⅠis dominated by large pore and coarse throat with large pore-throat coordination number and high connectivity.TypeⅡhas two sets of pore-throat combinations(larger pore-medium throat and medium pore-medium throat),and shows higher pore throat heterogeneity.TypeⅢis characterized by small pore-fine throat combination resulting from strong pressure solution.The difference in pore-throat structures is mainly controlled by rock fabric of precursor limestone and the overprint of various kinds of diagenesis.TypeⅠandⅡreservoirs,usually originating from porous grainstones,went through thorough dolomitization,and were well developed in planar,automorphic-hypautomorphic dolomites.Nevertheless,in comparison with Type II reservoirs,TypeⅠreservoirs are stronger in modification by burial dissolution at middle-late stages but weaker in over-dolomitization.Furthermore,relict dolomite and non-planar,xenomorphic dolomite are most likely to form TypeⅢreservoirs due to their weaker anti-pressure-solution capacity and stronger late over-dolomitization.