| The Qiangtang basin is one of the largest marine basin in Qinghai-Tibet Plateau. Late Triassic, Jurassic and Cretaceous formation are main stratum in the basin. It’s exploration field of the oil-gas. This paper discusses the relationship between the structure and the oil-gas preservation from basin’s tectonic background, sedimentary characteristics, structural characteristics, oil-gas preservation conditions. This paper accurately appraise oil-gas resource potential in the Qiangtang basin.1)The basic geological characteristicsQiangtang basin is located in the middle of the Tethys tectonic domain in the Qiangtang-Changdu block and sedimentary thickness excess 8000 m.By means of fully collected previous datum of the geological, petrolic, geophysical, geochemical and remote sensing,etc. On the basis of the subject of geological survey, the key blocks of detailed survey, geologic section survey in 5 years. We found the old metamorphic basement under the basin. Meanwhile the project team including author found metamorphic gneiss in the Ejiumai area. By the paper we know the burial depth of basement is different in the different regions, such as slightly deep in the North or South Qiangtang and shallow in the central uplift belt. By the basin stratigraphic division and comparison, we confirm that Middle-Early Triassic deposit mainly fill in North Qiangtang and in South Qiangtang only Late Triassic rock. But Middle and Late Jurassic are widely fill in Basin and each group is continuation. Early Jurassic rock mainly deposit in the South Qiangtang and volcanic rocks exist in northern Qiangtang. Through Collecting and measuring every stratum profiles, we comply the isopach map of main oil-gas formation, and analyse the distribution and features of reefs, dolomite and gypsum and other special lithology. By comparing the outcrops and drilling rock, we find the target layer to the deep metamorphic weakening and thickness increasing.The main tectonic line nearly EW and NWW, followed by the NE and SN in the basin. We analyze the basin boundary fault, composition and influence on basin formation; and study each tectonic structural characteristics; analyze the scale, stretch, width and composition of the fault and joint. The faults are mainly thrust faults and tectonic principal stress come from SN-NNE. Key research fold features such as form, style and trap form, give priority to with short fold and closed fold, the short fold forming composite fold. The basin is divided into 4 second tectonic units, 8 third tectonic units. Based on the above, the basin is divided 4 level transformation intensity area. The central uplift belt is the strongest area and most parts of South and North depression are weak areas.2)The petroleum geological characteristicsBy collecting and analysing the predecessors’ results, field investigation and lots of analysis data, this paper comprehensivly evaluate the source rock, reservoir rock, cap rock and source-reservoir-caprock association, and discuss the relationship of temporal and spatial distribution of the three.There are main source rocks in the basin,such as the coal-bearing of Triassic Xiaochaka group, the limestone of Middle Jurassic Buqu group, the shale of Middle Jurassic Xiali group and the micritic limestone of late Jurassic. Buqu group is best source rock. The thickness and high organic source rock mainly distribute in North Qaingtang depression and both sides of the central uplift belt. Organic matter type is II 1 and the type of the carbonate source rocks is better than politic rock at the same period. The surface organic matter of source rocks lie in high mature and over mature stage. The organic source rock in the drilling is higher than the surface’s, the type more than the surface’s, the thermal evolution degree lower than the surface’s.The reservoir is mainly distributed in the Xiaochaka group, Quemocuo group, Buqu group, Xiali group and Suowa group. Most reservoir distribute in Triassic. The lithologic types are fine-grained sandstone, grain limestone, reef limestone and dolomite etc. The physical characteristics is low porosity and low permeability. Drilling cores show reservoir cracks is less than the surface’s.The caprock is mainly distributed in the Xiaochaka group, Quemocuo group, Buqu group, Xiali group and Suowa group etc. The lithologic types are argillaceous rocks, gypsum rock, micrite, micritic limestone, lime mudstone. All of these have good sealing property.The research result shows that the source-reservoir-caprock association of Middle Jurassic Buqu group-Xili group is the best in the Qiangtang basin.3)Relation beween structure and oil-gas preservationOn the basis of geophysical exploration results, the paper analyzes 454 folds and more 740 large-mediumsize faults in the basin. Combining with the structure of the key block, we analyse tectonic style, formation time, structure strength, structure preserving condition of the basin. Tectonic line give priority to EW-NWW. The intensity transformation show strong in the south-north and weak in the middle, strong in the east and weak in the west. There are a lot of anticline traps and fault traps which form in the Late Yanshanian. These traps formation time is same or earlier the main hydrocarbon expulsion. This is advantageous to the oil and gas gathering.By the comprehensive study of the paper, Qiangtang basin has the basic geologic conditions for forming large and medium-sized oil-gas field and prospect is good. The exploration target layer are the sandstone of the Triassic Xiaochaka group and the granular limestone of the Middle Jurassic Buqu group. Considering various factors influence of the oil-gas and dividing the favorable zone and block.On the basis of comprehensive analysis of previous data, the paper study from the geophysical and geological investigation, source-reservoir-caprock association, structure analysis, trap conditions and preservation conditions, etc. we discuss and optimize the structural feature, relation between structure and oil-gas in Qiangtang basin. Moreover, providing the basis for petroleum exploration. |
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