川中磨溪—高石梯地区震旦系灯影组白云岩成因及与储层的关系

发布时间：2018-08-26 21:18

【摘要】：白云岩的成因、产状和分布等方面的研究是沉积盆地内白云岩型油气勘探的地质基础。四川盆地震旦系灯影组表现为大面积分布的白云岩,向东至盆地外围湖北三峡一带逐渐过渡为灰岩。大面积分布的厚层块状白云岩是灯影组储层发育的载体,灯影组储层的孔隙演化与其白云石化作用有着密不可分的关系。对于四川盆地震旦系灯影组白云岩的成因问题,一直来都是地质学家们争论的热点问题。因此,在对其储层形成机理研究过程中,需要从沉积学、岩石学以及地球化学等多方面对四川盆地内灯影组白云岩的成因进行研究。本论文在总结国内外相关研究技术及成果的基础上,结合研究区地质背景,主要采用岩矿鉴定技术以及多种地球化学分析技术(包括碳氧同位素、锶同位素、稀土元素、微量元素和包裹体等分析方法),对四川盆地川中磨溪-高石梯地区震旦系灯影组白云岩成因进行了较为深入的研究,并简要探讨了白云岩化与储层的关系。根据野外与岩心观察、薄片鉴定等研究,按照结构-成因对磨溪-高石地区灯影组白云岩类型及特征进行了较为系统的研究,认为该套地层主要发育蓝藻细菌白云岩、颗粒白云岩、晶粒白云岩及角砾白云岩四种类型,其中晶粒白云岩是重结晶作用的产物,角砾白云岩(包括风化残余角砾岩白云岩、岩溶角砾岩白云岩)是构造作用和表生岩溶作用共同影响下的产物。而蓝藻细菌白云岩、颗粒白云岩则是灯影组地层沉积时期所形成,保留了早期近地表环境下岩石的特征,这些特征为分析灯影组白云岩的成因提供了地质基础。碳、氧同位素分析表明,经受同位素分馏、地温及后期成岩作用改造较弱的岩石,其碳、氧同位素最为接近灯影组海水沉积时的海水碳、氧同位素值；而受后期成岩作用改造较为强烈的岩石,其氧同位素明显偏负、碳同位素略微有所偏负。锶同位素主要受到幔源(Sr86)和陆源(Sr87)的控制,其灯影组岩石锶同位素明显偏高,说明成岩流体中有放射性锶(Sr87)的混入。稀土元素分析显示,灯影组白云岩缺失海水Ce和Eu的负异常,而显示出明显的Ce和Eu正异常特征。此外,受成岩作用改造较弱、保存原始沉积组构、沉积构造、富含蓝藻细菌的岩石,其Mn和Fe含量相对较低。这一系列地化分析资料可以直接或间接证明四川盆地灯影组白云岩是在早期近地表环境下形成的,但后期成岩作用使得岩石碳氧同位素、锶同位素、微量元素、稀土元素等发生了变化。在近地表环境下,由于动力学障碍(SO42-的阻碍、Mg2+的束缚、CO32-浓度和活动性低)的存在,白云石是不能够从海水中直接沉淀的。除此之外,从结晶学和矿物学上讲,早期海水纤状白云石胶结物只能是交代文石和镁方解石的产物。四川盆地灯影组早期沉积物是富蓝藻细菌的碳酸盐物质,在硫酸盐还原细菌的作用下,地层中的蓝藻细菌席发生生物降解作用,导致沉积物中孔隙流体(当时的海水)的SO42-减少,增加了流体中CO32-的浓度及活动性,并使Mg2+从海水孔隙水和蓝藻细菌席中释放出来,从而克服白云石形成的动力学障碍,促进白云石化作用的进行。因此,结合上述灯影组白云岩地化资料及有机成因白云石化过程,证明灯影组白云岩是在早期近地表环境下交代而成。同时,白云岩化作用为白云岩储层发育提供了先决条件,但沉积、成岩、构造作用才是形成白云岩储层的关键因素。
[Abstract]:The genesis, occurrence and distribution of dolomite are the geological basis of dolomite-type oil and gas exploration in sedimentary basins. The Dengying Formation of Sinian in Sichuan Basin is characterized by large-area distribution of dolomite, which is gradually transformed into limestone from east to the periphery of the basin in the Three Gorges area of Hubei Province. The pore evolution of Dengying Formation reservoir is closely related to its dolomitization. The genesis of Dolomite in Dengying Formation of Sinian System in Sichuan Basin has always been a hot issue debated by geologists. The genesis of dolomite in the Dengying Formation in the Sichuan basin is studied in terms of chemistry and many other aspects.
On the basis of summarizing the related research technology and achievements at home and abroad, combined with the geological background of the study area, this paper mainly uses rock and mineral identification technology and a variety of geochemical analysis techniques (including carbon and oxygen isotopes, strontium isotopes, rare earth elements, trace elements and inclusions, etc.) to study the earthquakes in the Moxi-Gaoshiting area of Sichuan Basin. The genesis of dolomite in Dengying Formation of the Dengying Formation is studied in depth and the relationship between dolomitization and reservoir is briefly discussed.
Based on field observation, core observation and thin section identification, the dolomite types and characteristics of Dengying Formation in Moxi-Gaoshi area are systematically studied according to their structure-genesis. It is considered that the dolomite is mainly composed of cyanobacterial dolomite, granular dolomite, granular dolomite and breccia dolomite. The product of crystallization, breccia dolomite (including weathered residual breccia dolomite, karst breccia dolomite) is the product of both tectonic and epigenetic karstification. Cyanobacterial dolomite and granular dolomite were formed during the deposition of the Dengying Formation, retaining the characteristics of rocks in the early near-surface environment. It provides geological basis for analyzing the genesis of dolomite in the Dengying Formation.
Carbon and oxygen isotope analysis shows that the rocks weakly reformed by isotope fractionation, geotemperature and later diagenesis are the closest to the carbon and oxygen isotope values of seawater deposited by the Dengying Formation, while the rocks strongly reformed by later diagenesis are obviously negative in oxygen isotope and slightly negative in carbon isotope. Strontium isotopes are mainly controlled by mantle (Sr86) and terrestrial (Sr87). Strontium isotopes of the Dengying Formation rocks are obviously high, indicating that there is a mixing of radioactive strontium (Sr87) in the diagenetic fluid. REE analysis shows that the Dengying Formation dolomite lacks negative Ce and Eu anomalies in the seawater and shows obvious positive Ce and Eu anomalies. This series of geochemical data can directly or indirectly prove that the Dolomite of Dengying Formation in Sichuan Basin was formed in the early near-surface environment, but the later diagenesis caused the carbon and oxygen isotopes and strontium isotopes of the rocks. Trace elements and rare earth elements have changed.
In the near-surface environment, dolomite can not be precipitated directly from seawater due to the existence of kinetic barriers (SO42-hindered, Mg2+ bound, CO32-concentration and low activity). In addition, the early marine fibrous dolomite cements can only be the products of metasomatic aragonite and magnesia calcite in terms of crystallography and mineralogy. The early sediments of the Dengying Formation were carbonate substances of cyanobacteria. Under the action of sulfate-reducing bacteria, the cyanobacteria mat in the formation biodegraded, resulting in the decrease of SO42-in the pore fluid (then seawater), increasing the concentration and activity of CO32-in the fluid, and making Mg2+ from seawater pore water and cyanobacteria fine. Therefore, combined with the geochemical data of the Dengying Formation dolomite and the process of organic dolomitization, it is proved that the Dengying Formation dolomite was formed in the early near-surface environment. Breeding provides a prerequisite, but sedimentation, diagenesis and tectonism are the key factors for dolomite reservoir formation.
【学位授予单位】：西南石油大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P618.13;P588.245

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