西藏尼玛新生代陆相盆地二维地震构造解释及油气地质意义
Two-dimensional seismic structural interpretation and petroleum geological significance of the Cenozoic continental basin in Nyima of Tibet
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摘要: 为研究尼玛新生代陆相盆地形成演化及其油气地质意义, 在野外地质调查工作基础上, 基于新采集的地震解释成果, 揭示了尼玛盆地具有“两凹一隆”构造格局。通过对新生代地层厚度横向变化以及地层剖面分析, 将尼玛盆地的构造演化分为4个阶段: 晚白垩世末, 随着雅江带向北俯冲, 班怒带内部中生代地层逆冲推覆, 形成间列的山间压陷盆地; 古新世—始新世中期为拗陷阶段, 受中央隆起带分隔, 尼玛盆地南北凹陷形成并各自接受沉积, 盆地广泛发育牛堡组一段、二段湖泊-三角洲沉积体系; 始新世末—渐新世初牛堡组三段—丁青湖组沉积早期为改造阶段, 盆地逐渐抬升, 湖水大幅度萎缩, 南北凹陷发育颜色较红、粒度较粗的河流-湖泊相沉积体系; 中新世末丁青湖组沉积末期为消亡阶段, 受青藏高原快速隆升影响, 构造活动强烈, 盆地结束沉积并抬升剥蚀。尼玛盆地北部凹陷牛堡组分布稳定且厚度较大,为主力生烃凹陷,主力烃源岩为牛堡组深湖相黑色油页岩,其次为半深湖相泥岩。据地震地质解释结果,尼玛盆地构造圈闭发育,地层不整合、地层超覆等地质现象丰富,发育三角洲相、湖泊相等多种沉积相类型,可形成多种类型油气藏。研究对准确评价尼玛盆地油气资源潜力及优选有利勘探目标具有重要意义。Abstract: To investigate the formation-evolution mechanisms and petroleum geological implications of the Cenozoic continental basin in Nyima, the authors in this study revealed that it featured a structural framework of two depressions separated by one uplift based on field geological surveys and newly acquired seismic interpretation data. The tectonic evolution of Nyima Basin was divided into four stages after analysis of lateral variations of the thickness of the Cenozoic strata and stratigraphic sections. At the end of the Late Cretaceous, the thrust nappe of Mesozoic strata was developed with northward subduction of Yarlung Tsangpo Belt, forming intermittent intermountain compressive depressions. The Paleocene-Middle Eocene was a sag stage, partitioned by the Central Uplift Belt. The northern and southern depressions of Nyima Basin formed and received sediments independently, with widespread lacustrine-delta sedimentary system of the first and second members of Niubao Formation. From the Late Eocene to the Early Oligocene, the third member of Niubao Formation and early deposition of Dingqinghu Formation entered a transformation stage, characterized by gradual uplift and drastic lake shrinkage. And fluvial-lacustrine deposits with reddish color and coarser grain sizes developed in both northern and southern depressions. The late deposition of Dingqinghu Formation at the end of the Miocene marked the basin's demise stage. Driven by the rapid uplift of Qinghai-Tibet Plateau, the intense tectonic activity terminated sedimentation, and triggered uplift and erosion of the basin. Niubao Formation strata were distributed steadily and with large thickness in the northern depression of Nyima Basin, making it the primary hydrocarbon-generating depression. The main source rocks were deep-lacustrine black oil shales of Niubao Formation, followed by semi-deep-lacustrine mudstones. The seismology interpretation indicated well-developed structural traps and abundant geological features such as stratigraphic unconformities and stratigraphic onlaps. Multiple sedimentary facies including deltaic and lacustrine facies were developed, enabling the formation of various types of hydrocarbon reservoirs. This study could provide significant guidance for accurately assessing the petroleum resource potential and optimizing favorable exploration targets in Nyima Basin.
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