四川盆地威远—高石梯地区中二叠统栖霞组台内薄层白云岩发育特征与成因

Development characteristics and genesis of thin layered dolomite of the Middle Permian Qixia Formation in the platform in Weiyuan-Gaoshiti area,Sichuan Basin

通过对四川盆地威远—高石梯地区取心和钻井老资料复查发现,中二叠统栖霞组发育规模性滩相薄层白云岩,且多口井在该套白云岩中获工业气流,因此厘清白云岩成因与分布规律对指导栖霞组下一步勘探具有重要意义。基于区内岩心和测井、录井资料,开展白云岩岩石学和分布特征研究,发现栖霞组薄层白云岩宏观产状上可分为豹斑状白云岩和层状白云岩两类,白云石晶粒多以细—中晶为主,残余生屑颗粒幻影明显,原岩主要为具微生物粘结灰泥组构的亮晶生屑灰岩。白云岩单层厚度薄,一般为1~4m,累计厚度可达3~16m;纵向发育受沉积旋回控制明显,主要分布在旋回中上部且具有多期叠置的特征。地球化学分析结果表明,研究区栖霞组白云石化流体以准同生期咸化海源流体为主,仅局部孔洞状白云岩受到一定程度的深部热液叠合改造。对比沉积期古地理隆坳格局与薄层白云岩钻遇井分布耦合关系,发现薄层白云岩往往分布于台内隆坳的坡折处,提出丘滩体叠置迁移和海水受限的环境促使渗透回流白云石化作用发生。台内多期叠置的丘滩体为白云岩形成奠定了良好的物质基础,准同生期岩溶作用有利于提升丘滩体孔渗条件并为白云石化流体提供优势运移通道。研究结果证实台内薄层白云岩具有环洼不连续带状规模性分布规律,为拓展栖霞组台内白云岩储层勘探领域提供了重要支撑。

The re-examination of core and well drilling data showed that the large-area beach facies thin layered dolomite was developed in the Middle Permian Qixia Formation in Weiyuan-Gaoshiti area in Sichuan Basin,and commercial gas flows were obtained from the dolomite reservoir in several wells.Therefore,it is of great significance to clarify the genesis and distribution law of dolomite for gas exploration of Qixia Formation in the near future.The petrology and distribution characteristics of dolomite are studied by using core,wireline logging and mud logging data.The results indicate that the thin layered dolomite is divided into two types of porphyritic dolomite and laminated dolomite in terms of the macroscopic occurrence,with fine-meso crystalline dominated of dolomite grains,and distinct phantoms of the residual bioclastic grains.The main protolith is sparry bioclastic limestone with microbial bond stucco fabric.Significantly controlled by the sedimentary cycle,the dolomite has a thin single layer thickness of 1-4 m,and cumulative thickness of 3-16 m,showing the characteristics of multi-stage superposition in vertical and mainly developed in the middle-upper part of the sedimentary cycle.The geochemical experiment results show that the dolomitization fluids of Qixia Formation were dominated by penecontemporaneous seawater,and only part of the vuggy dolomite was transformed by deep hydrothermal fluids to a certain extent.By analyzing the coupling relationship between the paleogeographic pattern of uplift and depression during the deposition period and the distribution of wells encountering the thin-layered dolomite,it is found that the thin-layered dolomite was generally developed in the slope break between uplift and depression in the platform,and new understanding is proposed that the superimposing and migrating microbial mound beach bodies and the restricted seawater environment promoted the infiltration reflux dolomitization.The multi-stage superimposed microbial mound beach bodies in the platform laid a good material foundation for the development of dolomite reservoirs.In addition,the penecontemporaneous karstification was conducive to the improvement of porosity and permeability of mound beach body and provided a dominant pathway for the migration of dolomitization fluids.The results confirm that the thin layered dolomite in the platform has a large-area discontinuous zonal distribution pattern in the circum subsag,which provides an important theoretical support for expanding the exploration field of the intra platform dolomite reservoir of Qixia Formation.