塔北埃迪卡拉系奇格布拉克组多类型共生鲕粒特征及其成因探讨

Characteristics and genesis of various oolites in the Ediacaran Qigebulake Formation in northern Tarim Basin

碳酸盐岩中的鲕粒是古沉积环境重建的重要载体之一。塔里木盆地北部露头剖面、星火1井区埃迪卡拉系奇格布拉克组潟湖相及浅滩相沉积中发现了多种类型鲕粒及其共生现象。文中通过剖面详勘、岩心观察以及薄铸片、阴极发光、环境扫描电镜等研究方法,阐明了不同类型鲕粒的基本特征及分布,探讨了鲕粒成核、生长的特殊沉积环境和成岩改造等过程。按显微结构、特殊形态、组合以及成岩改造,依次将研究区奇格布拉克组鲕粒划分出12种类型,其中大气淡水、快速拟晶白云岩化、重结晶、交代白云岩化等成岩作用主要发现于中等水动力条件形成的同心(圈)鲕、同心—放射状鲕等。近源风暴引起的底流和涡流产生的撕裂、磨损及搬运作用为泥晶(大)鲕、薄皮鲕、放射状泥晶(正常或大)鲕、脑状鲕以及部分球(或细菌)鲕、复鲕的成核和微生物作用提供了基本条件,而文石—白云石海中低能环境下的黏性软底质中广泛发育的文石—高镁方解石胶结促进鲕粒的形成;前者发育于潮道或浅滩环境,后者发育于潮坪—潟湖环境,短暂风暴潮或底流是两者共生的可能原因。微生物参与泥晶成核、一定水动力条件下台阶状生长、差异性成岩改造是前寒武纪多种类型鲕粒发育的主控因素。鲕模孔、粒间(内)溶孔、晶间孔、有机微孔等孔隙具有一定储集意义。对奇格布拉克组鲕粒的研究,有助于深化对前寒武纪古海洋和大气组分、黏性软底质中成核与生长中的水动力及微生物作用、早期成岩改造及孔隙形成与保存机理等的认识。

Ooids in carbonate rocks are significant for the reconstruction of sedimentary environment. The co-occurrence of various ooids is found in the lagoon and shoal of the Ediacaran Qigebulake Formation in the Well Xinghuo 1 region located in the northern Tarim Basin. The characteristics and distribution of dif ferent types of oolites are studied based on the systematic methods including the description of field profile, observation of cores, thin sections, casting film, cathodoluminescence, environmental scanning electron microscope(SEM)etc. The specific sedimentary environments of the nucleation and growth of oolites, diagenetic transformation and other processes are discussed in detail. Twelve types of oolites in Qigebulake Formation of the study region are classified based on the microstructure, morphology, assemblage(single or polyphase)and diagenetic alteration. The fresh water metasomatism, rapid pseudo-dolomitization, recrystallization occur in the concentric ooids, radial ooids, etc. The tearing, wear and transport of the bottom currents and eddy currents caused by near-source storms provide the basic conditions for the nucleation of mud crystal ooids, thin-skin ooids, radial mud crystal ooids, brain-shaped ooids, and some spherical ooids, and compound ooids, and the microbial activities. The extensive development of aragonite-high magnesium calcite cementation in the soft substrate of the low-energy environment in the aragonite-dolomite sea promotes the formation of ooids. The former develops in tidal or shoal environments, while the latter develops in tidal-lagoon environments, and the storm surges or undercurrents caused the symbiotic combination of different types of ooids. Therefore, the micrite nucleation with the microbial participation, terrace-form crystal growth in the suspension and accretion process under a certain hydrodynamic condition, differential diagenetic alteration are the dominant factors for the development and occurrence of various types of ooids in the Ediacaran carbonate rocks. The oolitic mold pores, intergranular dissolved pores, intergranular pores, and micropores in the organic matters play the certain role of oil-gas reservoir. The study of the ooids in the Qigebulake Formation is helpful for the reconstruction of Precambrian palaeomarine and atmospheric components, hydrodynamic and microbial effects in nucleation and growth of oolites in soft substrates, early diagenetic transformation, and the formation and preservation mechanisms of pores.