川东北地区飞仙关组成岩晚期白云石溶解作用

Late diagenetic dissolution of dolomites in the Feixianguan Formation,northeastern Sichuan Basin

对川东北地区飞仙关组储层进行了岩石薄片观察及碳、氧、锶同位素和流体包裹体综合分析。结果发现,开江-梁平陆棚东北侧优质储层并不发育方解石白云石化后所应该产生的晶间孔,相反主要发育白云石粒间溶孔或粒间扩大溶孔。这些溶孔可与轻碳同位素的方解石紧密共生,并且分布在储层沥青与白云石晶体之间,应该是在沥青形成后白云石的溶解所致。而西南侧缺乏白云岩,这一成因的孔隙也不发育。在酸性条件下,东北侧的白云石与硬石膏或天青石发生溶解作用,产生MgSO4中间产物。而MgSO4易于被烃类所还原产生H2S,进一步促使白云石、硬石膏和天青石的溶解,沉淀了孔洞的充填巨晶嵌晶方解石,并在局部位置净增大了孔隙。这一成因机理很好地解释了东北侧与西南侧储层物性的差异,MgSO4很可能是自然界中硫酸盐热化学还原反应(TSR)的主要中间反应物。

Comprehensive analyses were performed on thin-section,carbon,oxygen and strontium isotopes and fluid inclusions of reservoirs in the Feixianguan Formation of northeastern Sichuan Basin.In the northeastern part of Kaijiang-Liangping shelf,intercrystalline pores resulting from dolomitization of calcite are absent in the high-quality reservoirs.In contrast,dolomite intergranular dissolution pores or intergranular enlarged dissolution pores are well developed in these reservoirs.Both two kinds of dissolution pores may coexist closely with calcites of light carbon isotope and occur in space between solid bitumen and dolomite crystals,thus they should have been produced by dolomite dissolution after solid bitumen precipitation.In the southern part of Kaijiang-Liangping shelf,dolostones are absent,thus pores of similar origin are also not developed.Under acidic conditions,dolomite may react with anhydrite or celestite,to produce MgSO4 in the northeastern part of Kaijiang-Liangping shelf.As the MgSO4 can be easily reduced by hydrocarbons to produce H2S,further promoting the dissolution of dolomite,anhydrite and celestite,the precipitation of giant crystal/poikilitic calcite,and the enlarging of secondary pores in local areas.This genetic mechanism can clearly explain the differences of reservoir physical properties between the northeastern and northwestern areas.The MgSO4 is likely to be the primary intermediate reactants of TSR.