Forming Geological Features Of Jurassic CBM Reservoir And Evaluating Of Favorable Areas In The Western Margin Of Ordos Basin

According to complex fault block setting, forming geological features of CBM reservoir are analysed from the strcture, deposition and other macro-conditions, major geological control factors on reservoir forming are summarized. The study area is a foreland basin in the Meso-Cenozoic, and superimposed metamorphism was occurred in local areas, such as Ruqigou area, because of the magma activities in Yanshan period. There exist various structural styles, such as thrust nappe structure, inversion structure, shear structure and extensional structure, which result in a unfavorable geological conditions, but there are better structure configuring in some fault-blocks, such as synclinorium block, ramp triangular block and thrust monoclinal block, etc., which could be favorable exploration target. The study area is characterized by many coal seams, extensive and stable distribution area, and large thickness of coal seam. The turnaround surfaces of system tract are the best accumulation coal layers, and the turnaround surfaces of parasequence are also more favorable. The coal seams in the southwest and northwest basin have relatively high probability of mudstone development, which is easy to form good seal layer. The shallow groundwater (above 300m) is mainly a weak runoff opened to semi-opened belt, and the deep groundwater (below 300m) is mainly stagnation belt, which is adwantage to CBM preservation.Based on the analysis of the experiment data (coal petrology and quality, reservoir properties, isothermal adsorption), the reservoir characteristics of the western margin of Ordos Basin are summaried. The mainly coal lithotypes are semidurain and durain, the vitrinite content increases and the inertinite content reduces from the basin margin to basin center in the study area. The apparent porosity of coal reservior is higher and the mostly pore structures have the character of double peak, which mainly develop micropores and macropores, especially micropores, and mesopores are nearly absent. With the coal evolving, the density of microfissure in the coal is reducing, the permeability is also becoming bad, but the adsorption capacity is increasing. Based on the analysis of the gas bearing properties in coal bearing areas with different rank, the gas content increases with the increase of the coal rank and the burial depth of coal-bed. The gas content arrived at 0.25-21.06 m3/t in the high coal rank area of Erdaoling-Ruqigou area, the gas bearing zone above 250m is mixture gas zone of nitrogen-methane, and the average gas content of methane weathering zone is 6.5 m3/t; The gas content is lower in the low coal rank area of Binchang area, is mainly at 2.10-6.96 m3/t, the methane weathering zone is presumed to 300 m, and the average gas content of methane weathering zone is 3 m3/t.According to the characteristics of structure configuring, reservoir and gas bearing properties, four CBM reservior types are divided, including synclinorium block gas reservoir, ramp triangular block gas reservoir, thrust monoclinal block gas reservoir and ramp transition zone monocline gas reservoir. Based on the analysis of geological control factors on reservoir forming, synthetic evaluation index system for coalbed methane area in complex fault block setting is builted up by the fuzzy mathematics method. The results show: the Ruqigou is the most favorable area, the Henshanpu is the more favorable area, the Huating and Binchang are the favorable areas, and Suishijing and Lingwu are unfavorable areas.