四川盆地南部志留系碳酸盐灰泥丘成因与储集性

灰泥丘与生物礁具有相似的地震反射特征,但两者的形成环境及内部组成完全不同。缓坡环境和较弱的水动力环境是灰泥丘形成的两个重要条件,灰泥丘主要由微生物所建造。川南志留系主要发育中—下志留统,自下而上划分为龙马溪组、石半栏组、韩家店组和秀山组,其中石牛栏组主要为碳酸盐岩,岩性较硬,上下地层相对为软岩层,构成“两软夹一硬”的地晏剖面结构。灰泥丘主要发育在石牛栏组,属于典型的开阔台地内缓坡泥丘。灰泥丘储层主要发育在丘翼和丘顶微相中,经历了胶结作用、重结晶作用、白云石化作用、压实作用和溶解作用等,其中胶结作用使丘翼储集物性变差,而白云石化作用使丘顶物性变好。因此丘翼灰岩被胶结后储集性能普遍较差,平均孔隙度1％～2％,渗透率(3～5)×10~(-3)μm~2;丘顶白云石化后储集物性普遍较好,孔隙度4％～5％,渗透率(6～8)×10~(-3)μm~2,构成了川南地区相对有利的天然气储层。

Origin and Reservoir Property of Silurian Carbonate Mud Mound in Southern Sichuan Basin

On the basis of the analysis of structures,sedimentary fa- cies and seismic sections,mud mounds built by microorganism have similar feature of seismic reflection with organic reefs but have distinct inner constitutes that formed under different envi- ronments.The main sedimentary facies include delta,chaotic sediment shelf,chaotic sediment open platform,carbonate open platform,and platform edge slope.The enwronment of a gentle slope with week hydrodynamics is the important condi- tion of mud mound formation The Lower-Middle Silurian strata in the southern Sichuan Basra can be divided upwards into Longmaxi,Shiniulan,Hanjiadian and Xiushan Formations,in which the Shiniulan Formation mainly consists of carbonate rock.The carbonate rock is hard but both the overlying and un- derlying beds are relatively soft,which forms a sandwich-liked bed texture The mud mound that exits in Shiniulan Formation is developed in the gentle slope of the open platform The mi- crofacies of the mud mound include mound core,mound flanks and mound roof The mound flanks and the roof are the good parts to be reservoirs,in which cementation,recrystallization, dolomitization,compaction and dissolution were undergone. Cementation brought in poor reservoir property in flank parts with the even porosity of 1%～2% and even permeability of 3 to 5×10~(-3)μm~2 but dolomitization resulted in better reservoir property in roof part with the even porosity of 4%～5% and even permeability of 6 to 8×10~(-3)μn~2.Combined with study of source rock and structures in this area,it is predicted that the mound flanks and roof will be the potential natural gas reser- voirs.