川西南地区中二叠统栖霞组白云岩储层特征及成因

通过岩心观察、薄片鉴定、阴极发光和地球化学分析等方法，对川西南地区中二叠统栖霞组白云岩特征及成因进行了研究。研究结果表明：①研究区栖霞组基质白云岩主要是细—中晶白云岩，其次为粉—细晶白云岩，并伴生少量粗晶鞍形白云石。②研究区细—中晶白云岩与同期颗粒灰岩方解石胶结物的稀土配型模式一致，具有现代海水典型稀土配分特征，为半局限水体动荡环境下沉积的颗粒灰岩交代而成；粉—细晶白云岩具有∑REE含量较高、Ce微弱正异常、LREE整体抬升的平坦稀土配型形态特征，为半局限水体安静环境下沉积的泥晶灰岩或粒泥灰岩交代而成。不同类型白云石Fe，Mn元素含量随云化程度加深表现出协同性升高，表明其具有相同成因。③研究区沿坡折带发育的生屑滩滩间存在局部蒸发受限环境，形成浓缩咸化海水，发生准同生期回流渗透白云石化作用，主要形成粉—细晶白云岩；早成岩—浅埋藏期峨眉山大火成岩省岩浆活动提供异常高热，高温孔隙水与先期白云岩进一步进行水岩作用，形成细—中晶白云岩，具“雾心亮边”特征；埋藏期深部热液沿裂缝运移，在裂缝和围岩中沉淀出鞍形白云石。生屑滩导致的半局限环境与热源叠加是研究区白云岩形成的关键。

Reservoir characteristics and genesis of dolomite of Middle Permian Qixia Formation in southwestern Sichuan Basin

Through core observation,thin section identification,cathodoluminescence and geochemical analysis, the characteristics and genesis of dolomite of Middle Permian Qixia Formation in southwestern Sichuan Basin were studied. The results show that:(1)The matrix dolomites of Qixia Formation in the study area are mainly fine-medium crystalline dolomites,followed by powder-fine crystalline dolomites,accompanied by a small amount of coarse-grained saddle-shaped dolomites.(2)The fine-medium crystalline dolomites in the study area are consistent with the rare earth element(REE)matching model of calcite cement of granular limestone in the same period,with typical REE distribution characteristics of modern seawater,and it was formed by the replacement of granular limestone deposited in the turbulent environment of semi-restricted water body. The powder-fine crystalline dolomites have the morphological characteristics of flat REE matching with high ΣREE content,weak positive anomaly of CE and overall uplift of LREE. It was metasomatized by micritic limestone or granular marl deposited in semi-restricted water and quiet environment. The contents of Fe and Mn in different types of dolomites show a synergistic increase with the deepening of dolomitization,indicating that they have the same genesis. (3)There existed local evaporation restriction environment among the bioclastic shoals developed along the slope break zone in the study area,forming concentrated salted seawater,and penecontemporaneous reflux infiltration dolomitization resulted in the formation of powder-fine crystalline dolomites. From early diagenetic stage to shallow burial stage,the magmatic activity in the large Emei igneous province provided abnormally high heat, and the high-temperature pore water further interacted with the pre dolomite to form fine-medium crystalline dolomites,with the characteristics of fog core and bright edge. During the burial period,deep hydrothermal fluids migrated along the fractures and precipitated saddle-shaped dolomites in the fracture and surrounding rocks. The superposition of semi-restricted environment and heat source caused by bioclastic shoals is the key to the formation of dolomite in the study area.