Two-dimensional seismic structural interpretation and petroleum geological significance of the Cenozoic continental basin in Nyima of Tibet

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.