| Gas hydrate has been hailed as the most promising new energy in the 21 st century, because of its high energy density, small burning pollution, huge number, etc. Since gas hydrate samples was drilled in the qinghai-tibet plateau permafrost regions of qilian mountains in 2008, Sanlutian well field of Muli coal field has become one of research hotspots, however the research work is superficial about the control factors of gas hydrate accumulation, which hindered the exploration and exploitation of resources. Gas hydrate reservoiring conditions include temperature-pressure condition, gas source, sedimentary condition, structure condition and so on. Among them, the tectonic conditions is the most important controlling factors of gas hydrate rich integration.This thesis selected the Sanlutian field, as the research area, which belong to the Muli Coalfield Juhugeng coalmine area in southern margin of Qilian Mountains, natural gas hydrate as the research subject, control function of structure as the breakthrough point, based on the field geological investigation, data collection and processing, drilling core observation, detailed sample test method, used the structure geology, geochemistry, petrology and coal geology theories, studied the field structural framework, the existing property of samples, the gas source of nature gas hydrate, and the control effect of structure to nature gas hydrate, finally, summarized the reservoir model of nature gas hydrate under the conditions of tectonic.Muli Coalfield is located in the middle Qilian landmass of Xiyu block, in the north and south sides of the plate were South Qilian block and North Qilian Neoproterozoic Early Paleozoic suture zone, the main structure are NW-SE distribution; well-developed faults are closely gathered and long-term activited, the plate boundary fault and shell fracture were the edge of the active belt of continental block, basement faults controlled the sedimentary and basin types.Sanlutian well field is situated in the east of syncline at the south of Juhugeng coal field which is situated in the southwest of Muli coalfield, Overall like two monoclinal structure form; Fold structure is development in the central of field, which as Jurassic strata as the cores, fracture structure is often distributed in the field as intensive ribbon pattern in large area; fracture structure can be divided into NW-SE thrust faults and SN-NNE normal fault; based on the control and distribution characteristics of main fracture, well field can be divided into north, middle and south structural belts; according to tectonic development features and degree, well field can be divided into east, middle and west tectonic blocks; In conclusion, tectonic development degree is lower in the south and north, higher in middle; east and west is lower, higher in middle.Due to the regional tectonic movement, the tectonic of the basin evolution is closely related with the tectonic of the qilian Qilian continental block event, it can be roughly divided into three periods: during the late Triassic- late Jurassic, under the tension effect, the filed been transformed to graben basin; during the early cretaceous-paleogene period, tectonic stress turned into extrusion, formed a large number of thrust nappe fault; since Neogene, along with the rapid uplift of the Qinghai-tibet plateau, the study area been uplifted, formed imbricate structure, and showed up the after exhibition type characteristics.The samples of nature gas hydrate puts up a certain rule of distribution: the depth are about 110m-500 m, all of samples were located in the below of the permafrost, occurred in Jiangcang formation of Middle Jurassic, more than 90% samples in upper section of Jiangcang formation; most samples occurred in the fracture of low porosity sandstone, mudstone, oil shale, and so on; in the plane, exploration drilling for samples centrally locate in the South of F2 fault and west of F30 fault.Based on the previous experimental data and samples test, through the analysis of comparison, discusses hydrocarbon gas source of natural gas hydrate in Sanlutian. The samples is thermogenic gas, the characteristics are oil type gas and coal type gas. The coal series in middle Jurassic is a very good hydrocarbon source rock, humic kerogen, oil shale and mudstone are good hydrocarbon source rock, organic matter types were sapropelic-humic type kerogen; the coal were the mature-high mature stage of evolution, mudstone and oil shale were the mature stage, the three source rocks could produce oil and moisture; Isotopic characteristics of natural gas hydrate samples and the samples coal's pyrolysis and gas are very familiar. Accordingly, the characteristic of samples are oil type gas, but it are from Middle Jurassic coal.The main lithology of Jiang Cang formation is shale, mudstone and siltstone, is unconventional reservoir for extra-low permeability; but the fracture in Jiangcang formation provides enough space for nature gas hydrate. The main fissure are low-middle angle, taken percentage of 43.30% and 32.32% respectively, unfilled account for 75.34%, and the mainly fillings are calcite accounted for 30.65%. Therefore, according to the different space types of natural gas hydrate reservoir, it can be divided into fracture type, fissure + hole type and hole type, mainly samples are fracture type, their proportion are 65.22% 26.08% and 8.70% in turn, and reservoir thickness of total reservoir thickness are 67.25% 28.46% and 4.29%.Based on the field geological drilling, combined the geophysical prospecting and logging, proposed the structure-lithology model of nature gas hydrate in Sanlutian well filed. In summary, there are consist of source rock, reservoir and capping layer, and each has three types: source rocks can be divided into autochthonous Jiangcang group, affinis Muli group, and the distant deep Carboniferous dark marl, lower Permian limestone and dark mudstone in upper Triassic series; according to the occurrence of gas hydrate it can be divided into different fracture types, fracture and fracture-hole and hole; capping layer can be divided into sedimentary cycle, structure caprocks and the permafrost of Qinghai Tibet Plateau.Discussed the effects of structure to natural gas hydrate in Sanlutian from the following parts: tectonic framework, tectonic movement, tectonics and structural damage. Tectonic framework controls the distribution of the natural gas hydrate: The natural gas hydrate mainly enriches in the block of M1 and S1, and it tends to enrich in the deeper area. The peak of hydrocarbon generation of all hydrocarbon source rocks in mining field is the late early Cretaceous, but the formation of stable belt of temperature and pressure in Qilian mountain is later than 3.6 Ma ago, the gas pools been migrated into the stable belt of temperature and pressure and then converted into the natural gas hydrate accumulation. Faults and fissures provide channel for hydrocarbon gas migration and reservoir space, Structural trap makes gas gather. At the last, the destruction of structure is divided into two types: Deformation of the reservoir makes gas escaped or couldnot convert into gas hydrate; Due to the reservoir displacement, gas pool been exposure out the surface or buried deeper but not into the stable belt of temperature and pressure so that couldnot formed gas hydrate.According the different time of hydrocarbon gases moved into the stable belt, it can be divided into two stages: the first stage, hydrocarbon gas been saved into good traps, with the uplifted of the plateau into the stable zone, combined with water and converted into nature gas hydrate, not been corroded later, lastly, formed the first class natural gas hydrate; the second stage, due to the rapid uplift of the Qinghai Tibet Plateau, Sanlutian was more than 4000 meters above sea level, the permafrost was formed, and the suitable conditions of temperature and pressure was existed, hydrocarbon gas pools migrated along the channels from the deep to the temperature and pressure stability zone, combined with water, converted into natural gas hydrate, which was the second kinds of natural gas hydrate.According to the different stage of the formation of natural gas hydrate research, structural constraints into the reservoir is also divided into two categories: the first stage, the temperature and pressure condition has not yet formed, hydrocarbon gas formation must enter the good traps and enriched, so a good cover is a necessary condition for the first class of natural gas hydrate formation; the second stage, the permafrost has been formed, the suitable temperature and pressure conditions have existent, the gas in deep migrated into the effective space and formed the nature gas hydrate. To sum up, the above two types of gas hydrate deposits is not the same space: the first type enriched in the low permeability, the low inclination of the hole in the middle, however the second occurred in the high porosity and high permeability zone and fracture zone. |
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