川东南中二叠统茅口组白云岩特征与成因机理

Characteristics and formation mechanisms of dolomites in the Middle Permian Maokou Formation in southeastern Sichuan Basin

近期川东南多口钻井在中二叠统茅口组白云岩储层获得了油气突破,但具有很强的非均质性,白云岩化机理不明确,难以预测优质白云岩储层的展布规律。本次研究基于区域地质背景和峨眉山大火成岩省热事件,通过岩石学和地球化学等综合分析,系统总结了不同类型白云岩的特征及成因机理。研究认为:川东南茅口组白云岩以细-中晶白云岩为主,泥-粉晶白云岩和白云石胶结物次之;綦江地区主要发育层状细-中晶白云岩,白云石呈自形-半自形晶,多具有雾心亮边结构和明显的颗粒幻影,阴极发光下晶体内部呈棕色-红色,由内向外过渡为暗红色,细-中晶白云石的δ^(18)O_(V-PDB)、δ^(13)CV-PDB和^(87)Sr/^(86)Sr值均表明白云岩化流体主要为海水,白云石可能为浅埋藏期中等盐度回流渗透白云岩化作用生成,后期经历重结晶或海源流体的热调整改造;泰来地区主要发育斑块状泥-粉晶、细-中晶白云岩以及裂缝内充填的白云石胶结物,原始岩石较致密,晶粒结构不明显,白云石胶结物以具波状消光的鞍形白云石为主,阴极发光下可见多期环带状特征,δ^(18)O_(V-PDB)、δ^(13)CV-PDB和^(87)Sr/^(86)Sr值表明泥-粉晶、细-中晶白云岩主要形成于浅埋藏期海水环境,后期经历了来自或穿碎屑岩层热流体的直接影响,在裂缝内沉淀白云石胶结物及方解石。总体而言,准同生期溶蚀作用、浅埋藏期白云岩化作用、热流体调整改造作用和岩溶作用是控制川东南地区茅口组储层的关键因素,其中浅埋藏期回流渗透白云岩化作用是该区白云岩的主要成因。

Breakthroughs in oil and gas exploration have been made recently in dolomite reservoirs of the Middle Permian Maokou Formation in southeastern Sichuan Basin.However,reservoir heterogeneity in the Maokou Formation is high,which makes it difficult to identify high-quality reservoirs without a clear understanding of the dolomitization mechanisms.Based on sedimentary facies of the region and hydrothermal events associated with the Emeishan Large Igneous Province,dolomites in the Maokou Formation are characterized and their formation mechanisms are summarized through lithological and geochemical analyses.In the Maokou Formation fine-medium-crystalline dolomite is the main type,followed by micritic dolomite and dolomite cements.In the Qijiang area stratified,fine-medium-crystalline dolomite is mainly found.The euhedral to subhedral dolomite crystals appear cloudy at the center and bright around the edges,with obvious graining lines;under cathodoluminescence(CL)microscope they appear brown-red at the interior and dark-red toward the exterior.The δ^(18)O_(V-PDB),δ^(13)C_(V-PDB) and ^(87)Sr/^(86)Sr ratio values of fine-medium-crystalline dolomite indicate it was formed by seepage-reflux dolomitization at medium salinity during shallow burial and later underwent recrystallization and seawater-metasomatism.The Tailai area mainly occur porphyritic-micritic and fine-medium-crystalline dolomites,along with dolomite cements found in rock fractures.Dolomites are compact with no obvious grain structures,while dolomite cements are mainly saddle dolomite with wavy extinction,with multiphase ring-and-ribbon patterns visible under CL microscope.Isotopic data indicate dolomites in this area were formed during shallow burial under seawater environment,and late-stage thermal fluids flowing over clastic rocks directly contributed to the precipitation of dolomite cements and calcite.In general,the key reservoir-controlling factors of the Maokou Formation in southeastern Sichuan Basin are epigenetic dissolution,dolomitization during shallow burial,metasomatism and karstification;and seepage-reflux dolomitization during shallow burial is the main formation mechanism of dolomites.