Abstract: Lacustrine microbialites are widely distributed in the Paleogene-Neogene formations in Qaidam Basin, and constitute favorable oil and gas reservoir types. However, the small single-layer thickness, pronounced interlayer heterogeneity, complex sedimentary environment, and difficult-to-predict distribution patterns have constrained the identification of these reservoirs and the exploration and development of hydrocarbons in the reservoirs. Based on detailed observations of typical microbial mat and microbial mound profiles in the Paleogene outcrops of Xichagou area in the western front of Altun Mountain in Qaidam Basin, combined with cast thin-section petrography, whole-rock X-ray diffraction (XRD) mineral analysis, and integrated analyses of trace elements, rare earth elements (REEs), carbon and oxygen isotopes, this study systematically investigates the sedimentary sequences, structural characteristics, sedimentary environments, genetic mechanisms, and occurrence patterns of lacustrine microbialites. The results indicate that Paleogene lacustrine microbialites in Qaidam Basin predominantly occur in two morphologies: mat-like pattern (microbial mats) and mound-like pattern (microbial mounds). Microbial mats mainly develop peloidal textures and stromatolitic structures, exhibiting laterally persistent sedimentary sequences and development scales. They were deposited in aquatic environments marked by an arid climate, low water temperature, elevated salinity, and weak reducibility. In contrast, microbial mounds are characterized by the predominant development of concentric laminated structures along with a small proportion of peloidal textures. their sedimentary sequences and development scales exhibit pronounced lateral variations, and they formed in aquatic environments with a humid climate, higher water temperature, lower salinity, and stronger reducibility. Influenced by sedimentary paleogeomorphology and variations in water depth, the occurrence and distribution patterns of microbial mats and microbial mounds differ markedly. Microbial mats are mainly distributed as discontinuous sheet-like bodies parallel to the lakeshore, while exhibiting continuous distribution perpendicular to the shoreline. In contrast, microbial mounds show low-density, discontinuous distribution parallel to the lakeshore, and high-density, beaded distribution perpendicular to it. These findings provide important guidance and reference for predicting the distribution of high-quality lacustrine microbialite reservoirs in Qaidam Basin.

Key words: microbial mats, microbial mound, sedimentary sequence, formation mechanism, occurrence, Qaidam Basin

摘要： 湖相微生物岩在柴达木盆地古近系—新近系广泛分布,是良好的油气储层类型,但其单层厚度小、层间非均质性强、沉积环境复杂、分布规律难以预测,制约了该类油气储层的识别和内部油气的勘探与开发。基于柴达木盆地阿尔金山前西段西岔沟地区古近系典型微生物席和微生物丘剖面的观察描述、铸体薄片鉴定和X射线衍射全岩矿物分析以及微量元素、稀土元素和碳、氧同位素综合分析,系统探讨了湖相微生物岩的沉积序列、结构特征、沉积环境、成因机制和产状分布。研究结果表明,柴达木盆地古近系湖相微生物岩主要呈席状(微生物席)和丘状(微生物丘)2种形态,其中,微生物席主要发育球粒结构和叠层结构,其沉积序列和发育规模在横向上较为稳定,沉积于气候干旱、水温偏低、盐度较高、还原性较弱的水体环境。微生物丘主要发育圈层结构以及少量球粒结构,其沉积序列和发育规模在横向上变化大,沉积于气候湿润、水温偏高、盐度较低、还原性较强的水体环境。受沉积古地貌和水深变化影响,微生物席和微生物丘的产状及分布规律差异明显。微生物席平行湖岸线方向主要呈片状断续分布,而在垂直湖岸线方向上则呈连续分布;微生物丘在平行湖岸线方向呈低密度断续分布,而在垂直湖岸线方向上则呈高密度串珠状分布。研究认识对柴达木盆地湖相微生物岩优质储层的分布预测具有重要指导与借鉴意义。

关键词: 微生物席, 微生物丘, 沉积序列, 形成机制, 产状, 柴达木盆地