Natural Gas Accumulation Mechanism And Models In The Upper Paleozoic Of Linxing Area,Ordos Basin

The gas production of coalbed methane well is lower in the eastern margin of Ordos basin. In order to improve the economic efficiency and reduce the exploration risk, the stereoscopic exploration between coalbed methane and sandstone gas can be realized. This article have studied on coal bearing strata in natural gas accumulation conditions, characteristics and main controlling factors of gas reservoir, and accumulation mechanism and model in Linxing area of Carboniferous and Permian at eastern margin of Ordos basin.There are favorable accumulation conditions such as gas generation, reservoir,preservation and so on in Linxing area. Coal, carbon mudstone and some dark mudstone are preferable source rocks in Carboniferous-Permian formation with high abundance of organic matter, wide distribution, stable thickness, mainly ? and ?2 kerogen and in moderate and high mature stage: all these show fine ability to produce natural gas. Multiple sets of laminated sand bodies are widely distributed with larger cumulative thickness. Barrier bar microfacies of the second member of Taiyuan formation and braided river delta front microfacies of the eighth member of Taiyuan formation are favorable reservoir facies belts. Porosity and permeability analysis showed that this region is mainly for tight sandstone reservoir. In terms of preservation,the nose structure belt controlled by magma diapir and stress field with developing high angle normal faults and smaller dense reverse faults indicates poor preservation conditions. However, in other regions have relatively good preservation conditions considering the thickness and the breakthrough pressure of mudstone caprock,multiple sets of thick mudstone continuous distribution and high salinity water.The overlapping method of three porosity curves and cross plot method of logging parameters are used to identify gas bearing layers qualitatively. Optimize key parameters of quantitative evaluation of gas content in reservoir can be used to calculate gas content quantitatively. Then the law is summarized as north higher than south and west higher than east of gas content, while there is a non-gas belt surrounding the Zijin mountain. The main controlling factors of gas content evaluated by linear regression and grey correlation are porosity and gas saturation of sandstone and breakthrough pressure of cap. Main types of gas reservoirs in this area are identified and there were lithologic gas reservoir, structural gas reservoir and structural-lithologic composite gas reservoir. Three laws of sandstone gas distribution were summarized: the distribution of gas reservoir controlled by the assembling of source, reservoir and cap; the distribution of lithologic gas reservoir controlled by porosity and permeability of reservoir; the enrichment of gas in high position and slope.Based on the comprehensive analysis of the burial history, thermal history,homogenization temperature of fluid inclusions and quantitative description of phase transform mechanism, it is identified that there are three stages of hydrocarbon accumulation and the mechanism of reservoir formation is explained. In the Triassic-early Jurassic, because of lower organic matter maturity, it mainly generates a limited number of hydrocarbon partly to meet coal adsorption and the small amount of gas migrate to reservoir in a long distance. Middle Jurassic to Early Cretaceous are the main hydrocarbon generating period affected by the abnormal geothermal gradient.Natural gas is driven by abnormal high pressure and migrate through micro-fractures in close distance and finally accumulate in tight sandstone. Under the abnormal geothermal field, the vapor and methane mixed with a single gas phase migrated and accumulated in face-like and suffusion style. The continuous uplift since the late Cretaceous resulted in the desorption from adsorbed gas in the coal bearing strata,which could be used as an important supplementary gas source and accumulate in the shallow reservoir. Consequently, based on the analysis of reservoir forming conditions and hydrocarbon accumulation mechanism, a "high quality reservoir" accumulation model under good preservation condition was establishedFinally, the amount of natural gas geological resources is totally 1645.36×108 m3,including the amount of trap resources about 601.21×108 m3 and the amount of lithologic trap resources about 1044.15×108 m3 by using area abundance method and data of the gas content of sandstone and the thickness, area and density of sand body.It is considered that the favorable areas are TB-29 well, LX-9 well, LX-4 well and LX-22 well.