Title
Pattern of hydrothermal dolomitization in the Middle Permian Maokou Formation, eastern Sichuan Basin, and its alteration on reservoirs herein
摘要
针对四川盆地东部中二叠统茅口组异常白云岩分布，利用岩心岩石学特征描述、地化测试、地震和测井资料综合解释等方法，分析了热液白云岩化作用的识别标志和发育时间，探讨了其成因模式及对储层改造。研究结果表明：不规则白云岩分布、中-粗晶基质白云岩、早期张性裂缝和溶洞中以充填鞍形白云石和石英-鞍形白云石是主要识别标志，白云石流体包裹体均一温度（115~145℃）高于地层温度，偏负的氧同位素值[δO（VPDB）为-5.4‰~-7.1‰]，高锶同位素比值（Sr/Sr为0.707 947~0.708 458），是识别热液的主要地化指标。热液白云岩化作用主要发生在张性或扭张性断裂系统附近，其成层性受原始沉积相控制，在断裂破碎带附近灰岩地层也被热液改造。热液改造主要时期是在茅口组经历浅埋藏、抬升暴露岩溶作用和吴家坪组沉积之后的晚二叠世-中三叠世。峨眉山玄武岩喷发为热液提供了热源，断裂、沉积相和热液侵入时间等因素在时间和空间上组合，造就了热液白云岩形成的条件和不同的分布形式。热液白云岩化作用对储层的改造具有复杂性，低渗透性灰岩层热液云化后新增孔隙空间不明显，具有初始空隙的高能滩体受热液云化适度改造后可能存在一定孔渗性。
Abstract
Based on the distribution of abnormal dolomites in the Maokou Formation, eastern Sichuan Basin, we analyzed the identification markers and timing of hydrothermal dolomitization (HTD), and discussed its genetic model and reservoir modification by means of petrological description of cores, geochemical testing, as well as comprehensive interpretation of seismic and logging data. The results indicate that the main identification markers include irregular dolomite distribution, medium-to-coarse crystalline matrix dolomite, saddle dolomite and quartz-saddle dolomite in the earlier tensile fractures and cavens. The homogenization temperature of dolomite fluid inclusions(115-145℃)being higher than the host formations, more negative oxygen isotope (δO(VPDB)-5.4‰-7.1‰), and increased strontium isotope (Sr/Sr 0.707 947-0.708 458) are the main geochemical indicators of hydrothermal activities. HTD mainly occurred near tensional or transtentional fault systems, and its stratification is controlled by original sedimentary facies. The limestone formations near the fracture/fault zones were also altered by hydrothermal fluids. The main stage of HTD alteration is the Late Permian-Middle Triassic after the shallower burial and karstification via uplifting and explosion of the Maokou Formation and the deposition of the Wujiaping Formation. The eruption of Emei basalt served as heat source for hydrothermal fluids, and the factors such as faults, sedimentary facies and the intrusion of hydrothermal fluids are coupled in time and space, resulting in the development and various distribution patterns of hydrothermal dolomites. The effect of HTD on reservoir modification is complex.The newly-generated pore space is not remarkable in lower-permeability limestones under HTD, while the high-energy beach facies with initial pores may preserve certain porosity and permeability under HTD.
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